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第三章植物的光合作用Photosynthesis in Plant一、名词解释：1.光合作用（photosynthesis） 2 .光合膜（photosynthetic membrane）3.量子效率(quantum efficiency) 4.荧光现象与磷光现象（Fluorecence and phosphorecence）5.反应中心色素reaction centre pigment 6.聚光色素light-harvesting pigment或antenna pigment（天线色素) 7 Primary reaction 原初反应8.光合反应中心(Photochemical reaction centre) 9.红降(red drop) 10.爱默生效应（Emerson effect）11.光系统（photosystem）12.光合链（photosynthetic reaction）13.PQ循环（PQ cycle） 14.光合磷酸化photosynthetic phosphorylation or photophosphorylation 15. 希尔反应16. 磷酸运转器17.同化能力（assimilatory power）18.碳同化CO2 assimilation in photosynthesis 19.卡尔文循环（C3途径，还原戊糖途径）C3 photosynthetic pathway (Calvin cycle, RPPP) 20.C4途径C4 photosynthetic pathway 21.景天科酸代谢Crassulacean acid metabolism (CAM) pathway22.光呼吸（photorespiration） 23.光补偿点light compensation point(LCP) 24. light saturation point(LSP) 25.光合作用的光抑制Photoinhibition 26.二氧化碳补偿点CO2 compensation point27.二氧化碳饱和点CO2saturation point28.光合“午休现象”(midday depression of photosynthesis) 29.光能利用率Efficiency for solar energy utilization30.光合速率(photosynthetic rate)31.净光合速率(net photosynthetic rate，Pn)二、写出下列符号的中文名称PQ PC Fd NADP +RuBP PGAGAP DHAP FBP F6P G6P Ru5P PEPCAM TP HP OAA CF 1 - CF 0 PS ⅠPS ⅡBSC Mal FNR Rubico三、填空题1. 光合作用是一种氧化还原反应，在反应中被还原，被氧化。
Photosynthesis: Explanation and Process In the field of biology, photosynthesis refers to the process by which green plants, algae, and some bacteria convert light energy into chemical energy. This vital process allows organisms to produce glucose and oxygen from carbon dioxide and water. Photosynthesis is essential for the existence of life on Earth, as it sustains the intricate food webs and maintains the overall balance of atmospheric gases.The Process of PhotosynthesisPhotosynthesis can be divided into two stages: the light-dependent reactions and the light-independent reactions (also known as the Calvin cycle).Light-Dependent ReactionsThe first stage of photosynthesis, the light-dependent reactions, occurs in the thylakoid membranes of the chloroplasts. These reactions rely on the presence of light and primarily involve the following steps:1.Absorption of Light: Chlorophyll and other pigments in chloroplastscapture photons from sunlight.2.Electron Transport Chain: The energy from absorbed light isharnessed to generate ATP (adenosine triphosphate) and NADPH(nicotinamide adenine dinucleotide phosphate), which are energy-richmolecules.3.Splitting of Water: Water molecules are split, releasing oxygen as abyproduct and providing electrons for the next step.4.Electron Flow: High-energy electrons, derived from water, flowthrough an electron transport chain, ultimately leading to the synthesis of ATP.Light-Independent Reactions (Calvin Cycle)The light-independent reactions, also known as the Calvin cycle, take place in the stroma of chloroplasts. These reactions utilize the ATP and NADPH produced during the light-dependent reactions to convert carbon dioxide into glucose. The main steps of the Calvin cycle are as follows:1.Carbon Fixation: Carbon dioxide (CO2) combines with a five-carboncompound called RuBP (ribulose bisphosphate) to form an unstable six-carbon compound. This reaction is catalyzed by an enzyme called RuBisCO (ribulose bisphosphate carboxylase oxygenase).2.Reduction: The unstable compound formed in the previous step isconverted into two molecules of a three-carbon compound called PGA (3-phosphoglycerate). ATP and NADPH from the light-dependent reactions are used in this process.3.Regeneration of RuBP: Some PGA molecules are converted back intoRuBP using additional ATP, while others continue in the cycle.4.Glucose Formation: After several rounds of the Calvin cycle, thethree-carbon molecules are rearranged and combined to form glucose, which can be stored or used for energy by the organism.Significance of PhotosynthesisOxygen ProductionPhotosynthesis is responsible for the continuous supply of oxygen to the Earth’s atmosphere. During the light-dependent reactions, water molecules are split, releasing oxygen as a byproduct. This oxygen sustains aerobic respiration in organisms, enabling them to derive energy from glucose through the process of cellular respiration.Carbon Dioxide ReductionPhotosynthesis plays a crucial role in reducing the levels of carbon dioxide (CO2) in the atmosphere. Through the Calvin cycle, plants and other photosynthetic organisms utilize CO2 to produce glucose. This process helps in maintaining the balance of greenhouse gases, mitigating the impact of global climate change.Food ProductionPhotosynthesis is the primary source of energy for most ecosystems on Earth. Plants, algae, and photosynthetic bacteria serve as producers, converting light energy into chemical energy stored in glucose. This glucose provides the foundation of the food chain, as it is consumed by herbivores and subsequently transferred to carnivores and other higher trophic levels.Pharmaceutical and Industrial ApplicationsSeveral products obtained from photosynthetic organisms have significant pharmaceutical and industrial applications. Medicines, biofuels, and various natural products, such as rubber and dyes, are derived from plant or algal sources. Harnessing the processes and products of photosynthesis has the potential to contribute to sustainable development and the advancement of various industries.In conclusion, photosynthesis is a vital biological process that enables organisms to convert light energy into chemical energy, producing glucose and oxygen. Its role in oxygen production, carbon dioxide reduction, food production, and various applications underscores its significance in sustaining life on Earth.。
The Process of Photosynthesis in PlantsPhotosynthesis is a fundamental process that occurs in plants,enabling them to convert light energy from the sun into chemical energy stored in glucose.This process not only sustains the plants themselves but also supports life on Earth by producing oxygen and serving as the foundation of the food chain.Understanding the process of photosynthesis helps us appreciate the vital role that plants play in maintaining ecological balance and supporting life.In this essay,we will explore the key stages and components involved in photosynthesis.Photosynthesis takes place primarily in the leaves of plants,within specialized cell structures called chloroplasts.Chloroplasts contain a green pigment known as chlorophyll,which is essential for capturing light energy.The process of photosynthesis can be divided into two main stages:the light-dependent reactions and the light-independent reactions,also known as the Calvin cycle.The light-dependent reactions occur in the thylakoid membranes of the chloroplasts and require direct sunlight to proceed.When light strikes the chlorophyll molecules,it excites the electrons,raising them to a higher energy level.These high-energy electrons are then transferred through a series of proteins embedded in the thylakoid membrane, known as the electron transport chain.As the electrons move along the chain,their energy is used to pump protons(hydrogen ions)into the thylakoid lumen,creating a proton gradient.The energy from this proton gradient is harnessed by an enzyme called ATP synthase to produce ATP(adenosine triphosphate),a molecule that stores and transports energy within cells.Additionally,the excited electrons eventually combine with NADP+(nicotinamide adenine dinucleotide phosphate)to form NADPH,another energy-rich molecule. Both ATP and NADPH are crucial for the next stage of photosynthesis. During the light-dependent reactions,water molecules are also split(a process known as photolysis),releasing oxygen as a byproduct.This oxygen is then expelled into the atmosphere,contributing to the air we breathe.The light-independent reactions,or the Calvin cycle,take place in the stroma of the chloroplasts and do not require direct sunlight.Instead,they use the ATP and NADPH produced during the light-dependent reactions to convert carbon dioxide into glucose.The Calvin cycle can be broken down into three main phases:carbon fixation,reduction,and regeneration.In the carbon fixation phase,carbon dioxide molecules from the atmosphere are captured by an enzyme called ribulose-1,5-bisphosphate carboxylase/oxygenase,commonly known as RuBisCO.RuBisCO attaches the carbon dioxide to a five-carbon sugar molecule called ribulose-1,5-bisphosphate(RuBP),resulting in a six-carbon compound that quickly splits into two three-carbon molecules of3-phosphoglycerate(3-PGA).During the reduction phase,the3-PGA molecules are converted into glyceraldehyde-3-phosphate(G3P)using the energy from ATP and the reducing power of NADPH.G3P is a three-carbon sugar that serves as the building block for glucose and other carbohydrates.Some of the G3P molecules exit the Calvin cycle to be used in the synthesis of glucose and other organic compounds.The final phase,regeneration,involves the rearrangement of the remaining G3P molecules to regenerate RuBP,the molecule necessary for carbon fixation.This regeneration process requires additional ATP and ensures that the Calvin cycle can continue,allowing the plant to continuously capture carbon dioxide and produce glucose.The glucose produced through photosynthesis serves as an essential source of energy and building material for the plant.It can be used immediately for cellular respiration,stored as starch for later use,or converted into other organic compounds such as cellulose,which provides structural support to the plant.In conclusion,photosynthesis is a complex yet remarkably efficient process that enables plants to convert light energy into chemical energy, producing glucose and oxygen as vital end products.The light-dependent reactions capture and convert sunlight into ATP and NADPH, while the light-independent reactions,or Calvin cycle,use these energy-rich molecules to fix carbon dioxide and synthesize glucose.This process not only sustains plant life but also supports the entire biosphere by producing oxygen and forming the base of the food chain. Understanding photosynthesis highlights the incredible role that plantsplay in maintaining life on Earth and underscores the importance of protecting and preserving our natural environment.。
Antisense Inhibition of Rubisco Activase IncreasesRubisco Content and Alters the Proportion ofRubisco Activase in Stoma and Thylakoids inChloroplasts of Rice Leaves1Jin Jiang HongSonghengCollege of Life Sciences Institute of BiotechnologyUniversityZhejiangZhejiangUniversityHangzhou, P. R. China 310029 Hangzhou, P. R. China 310029Li Dean Jiang*XueqinCollege of Life Sciences College of Life SciencesUniversityZhejiangUniversityZhejiangHangzhou, P. R. China 310029 Hangzhou, P. R. China 310029dajiang@AbstractRibulose-1,5-bisphosphate carboxylase/oxygenase (Rubsico) activase (RCA) is anuclear-encoded chloroplast protein that modifies the conformation of Rubisco,releases inhibitors from the active sites, and increases enzymatic activity. It appears tohave other functions, which are related to its distribution within the chloroplast. Theaim of this research was to resolve uncertainty about the localization of RCA, and todetermine whether the distributions of Rubisco and RCA were altered when RCAcontent was reduced. Gas exchange and Rubisco were measured, and the sub-cellularlocations of Rubisco and RCA were determined using immunogold-labeling electronmicroscopy, in wild-type and antisense rca rice plants. Net photosynthetic rate and theinitial Rubisco activity in the antisense rca plants decreased much less than RCAcontent in the antisense plants. Immunocytolocalization showed that Rubisco in1Support by the the Doctoral Foundation of Education Department (20020335043) of P. R. China. and National Natural Science Foundation (30471051)wild-type and antisense plants was localized in the stroma of chloroplasts. However,the amount of Rubisco in the antisense rca plants was greater than in the wild typeplants. RCA was detected in both the stroma and in the thylakoid membranes ofwild-type plants. We show that the percentage of RCA labeling in the thylakoidmembrane was substantially decreased, while the fraction in the stroma was increased,by the antisense rca treatment. From the changes in RCA distribution and alterationsin Rubisco activity, RCA in stroma of chloroplast probably contributes to theactivation of Rubisco, and RCA in thylakoids compensates for the reduction of RCAin the stroma, allowing steady-state photosynthesis to be maintained when RCA isdepleted. RCA may also have a second role in protecting membranes againstenvironmental stresses as a chaperone.Keywords:Oryza sativa L., Rubisco, Rubsico activase, Cellular localization1 IntroductionIn green plants, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) catalyzes the irreversible carboxylation of ribulose-1,5-bisphosphate and CO2 to form two 3-phosphoglyceric acid molecules. However, the rate of the reaction is extremely slow, and Rubisco must be activated and carbamylated to become catalytically competent. Activation is achieved by Rubisco activase (RCA), which can remove inhibitors from Rubisco’s catalytic sites, alter the conformation, and activate Rubisco in vivo[1]. RCA is a nuclear-encoded chloroplast protein, and is essential for plants [2]. Komatsu and coworkers [3] reported that a giberillin-binding protein in rice was homologous to RCA. Sharma et al. [4], using an in-gel protein kinase assay, suggests that RCA was associated with Ca2+-dependent protein kinases in gibberillin signaling. These studies suggest some additional role for RCA beyond Rubisco regulation [5]. Possibly, the role of RCA depends on its location within the chloroplast, as protein functions may be related to their cellular localization. Immunogold labeling for electron microscopy have been widely used to localize macromolecules in plant tissues. Therefore, one aim of this study was to establish where RCA is located in higher plants.A second aim of this study was to determine whether Rubisco and RCA contents were altered, or the proportions in different parts of the chloroplast changed, when RCA content was reduced by use of antisense rca: this is made possible by using genetically modified rice plants with antisense-RNA to RCA [6-7]. The photosynthetic rate of such plants was largely unaffected by RCA concentration until it was reduced below approximately 35% of that of wild-type plants [6]. These results were similar to results obtained in transgenic tobacco [8-10]. and Arabidopsis thaliana[11-12]. In these plants, modest reduction (49% – 32%) in Rubisco activation did not mirror the large decrease (0.02 – 0.0025) in RCA/Rubisco ratio that occurred [13]. Although the reduced amount of RCA in the anti-activase plants might be partially compensated by an increase in ATP or in the ATP/ADP ratio in vivo, in no case was compensation by ATP sufficient to explain the relative insensitivity of photosynthesis to loss of RCA. Other factors, such asincreased amount of Rubisco protein, or re-location of sequestered RCA to maintain Rubisco activity might also explain the insensitivity of photosynthesis to loss of RCA. We therefore hypothesize that changes in amount and distribution of RCA and Rubisco are responsible for the discrepancy between changes in RCA and gas exchange and Rubisco activity, and test this by examining the localization of Rubisco and RCA in the wild type and antisense rca rice plants.2 Materials and Methods2.1 Plant materials and growthTransgenic rice with reduced amounts of RCA was grown from seed collected from selfed R1 progeny of rice (Oryza sativa L. cv. ZhongHua 11) transformed with an antisense gene directed against RCA by the CaMV35S promoter using the Agrobacterium tumefaciens system [7]. The R1 seeds of the transformant with 30% of wild-type RCA were used for this study to test the effects of a large but not damaging change in RCA. Untransformed cv. ZhongHua11 rice plants were used as controls. About 50 R1 seeds and 30 wild type seeds were germinated, and their seedlings were grown in paddy soil in 15 L pots in a shaded greenhouse with natural sunlight during the day (maximum of 800 µmol photons m-2 s-1) at the Huajiachi Campus of the Zhejiang University. The greenhouse temperature was 28±3ºC during the day and 25±2ºC at night.2.2 Gas exchange measurementsThe gas exchange was determined with a portable photosynthesis system (LiCor-6400; LiCor Inc. Lincoln, Nebraska, USA) and a LED light source, 6400-02. This experiment was conducted at a light intensity of 1500 µmol m-2s-1, a leaf temperature of 28ºC, and CO2 of 380±5 µmolCO2 mol-1 in the sample chamber. Measurements were made on fully expanded uppermost leaves of the main stem of 50 day-old plants of antisense rca and wild-type rice plants, and were repeated at least six times on each. After measurements, the leaves were excised, frozen in liquid N2 and stored at –80 °C for Rubisco and RCA assays. There were at least six replications for each plant-type.2.3 Measurements of Rubisco content and activityAbout 6-7 cm2 (0.10 g) of frozen rice leaves were ground to a powder using a chilled mortar and pestle with liquid N2, a small amount of quartz sand and insoluble polyvinylpolypyrrolidone (PVP), then homogenized with 1.9 mL cooled extraction buffer containing 50 mM Tris-HCl (pH 7.5), 1mM EDTA, 10mM MgCl2, 12% (v/v) glycerol, 0.1% (v/v) β-mercaptoethanol and 1% (w/v) PVP-40 (soluble PVP) at 0-4°C. The homogenate was centrifuged at 15,000 × g for 15 min at 4°C. The supernatant was used to determine the concentration and activity of Rubisco. The Rubisco concentrations were measured with the single radial immunodiffusion method as described [14].The Rubisco activity in the supernatant was assayed according to Sawada et al. [15] with minormodifications. The initial Rubisco activity was measured at 30°C by adding 100 µl of supernatant into 900 µl of assay buffer containing 50 mM HEPES-KOH (pH 8.0), 1 mM EDTA-2Na, 20 mM MgCl2, 2.5 mM dithioerythritol (DTT), 10 mM NaHCO3, 5 mM ATP, 0.15 mM NADH, 5 mM creatine phosphate, 0.6 mM RuBP, 10 units of phosphocreatine kinase, 10 units of glyceraldehydes-3- phosphate dehydrogenase and 10 units of phosphoglycerate kinase. The total Rubisco activity was assayed by adding 100 µl of the Rubisco-containing supernatant into 200 µl of an activation medium containing 33 mM Tris-HCl (pH 7.5), 0.67 mM EDTA-2Na, 33 mM MgCl2 and 10 mM NaHCO3, and then incubating the sample at 30°C temperature for 10 min prior to measurements.2.4 Quantification of RCA contentThe amount of RCA was quantified by the immumo-diffusion method using rabbit serum antibody and the purified RCA from rice leaves as a standard [16].2.5 Fixation and ImmunolocalizationSub-cellular protein distribution was analysed by electron microscopy, on sections from the middle portions (about 10 mm taken at 100 mm away from the leaf tip) of fully expanded uppermost leaves of the main stem, similar to those used for gas exchange. Three leaves from different plants of each type were collected, and two pieces (1×3 mm) of each were taken and used for analysis. Small pieces of the leaves were fixed with 0.1M phosphate buffer (pH 7.2) containing 3% (v/v) paraformaldehyde and 1% (v/v) glutaraldehyde for 2 h at 4°C and washed in the buffer. The segments were dehydrated in an ethanol series and embedded in Lowicryl K4M according to the following protocol: 100% ethanol/resin 1:1 (v/v) for1 h, 100% ethanol/resin 1:2 (v/v) for 1h, pure resin for 12 h at -20°C. The embedded samples were transferred to 0.5 ml of tubes filled with resin and polymerized completely under UV-radiation at -20°C for 72 h, followed by 24 h at room temperature. Ultrathin sections (70-90 nm) were cut with a diamond knife and placed on nickel grids.Two sections from each small piece were analysed. The sections were washed with distilled water for 15 min, and then incubated in blocking buffer (0.05M phosphate-buffered saline (PBS) with 1% (w/v) BSA, 0.02% PEG20000, 0.1M NaCl, 1% (w/v) NaN3) for 1 h at room temperature. The sections were then incubated for 1 h at room temperature with anti-Rubisco, or anti-RCA serum applied at dilutions of 1:1000 and 1:200, respectively, in blocking buffer. For control sections, antiserum was replaced with non-immune serum. After washing with blocking buffer, the sections were incubated in blocking buffer containing protein A conjugated with 15-nm colloidal gold particles for 1 h, and then were washed in PBS and in deionized distilled water. Finally, the sections were stained with uranyl acetate and lead citrate, observed and photographed with an electron microscope (JEM-1200EX, JEOL, Japan) at 80 kV.The labeling density was determined by counting the gold particles on electron micrographs at 20 000× magnification and calculating the number per unit area (µm2). Between 7 and 10 individual cells from different immunolabelled sections for each cell type were examined. The areas occupied by starch grainswere omitted from the calculation of chloroplast area. No significant labeling for Rubisco or RCA was present in the vacuole, cell wall, mitochondria or cytosol, and these labeling densities were taken as the background value. To obtain the proportions of Rubisco and RCA in the stroma and thylakoids, the average density of immunogold particles in the background was subtracted from the average density within the stroma and thylakoids.2.6 AntibodiesRabbit antibodies to whole molecule of rice Rubisco and to rice RCA, which recognized both forms of RCA [17], were used in this work.2.7 Statistical methodsData were analysed statistically by ANOV A, using the Student’s t-test for comparison of means. For these analyses we used SPSS 10.0 software (SPSS Inc., Chicago, IL, USA), and set the statistical significance level at P < 0.05.3 ResultsThe RCA content of the antisense plants was about 30% of that of the wild-type. However, the antisense plants possessed much more (1.8-fold) Rubisco in their leaves (Table 1). The net photosynthetic rate (Pn) and the initial activity of Rubisco in the antisense plants were reduced by about 50%, as compared to those in the wild type plants (Table 1). Thus, the magnitude of the decrease in the initial Rubisco activity and Pn was much less than that in RCA content. Nevertheless, the total activity of Rubisco in antisense plants exposed to light was significantly higher than that in the wild type (P < 0.05), correlating with the measured Rubisco concentrations (Table 1). The intercellular CO2 concentrations (Ci) of the antisense rca rice were higher than that of controls (P<0.05), while there were no changes in stomatal conductance (g s) between the antisense and wild-type plants (Table 1), indicating that the reductions of photosynthetic rate of the selected antisense plants were not due to stomatal conductance.In the wild-type and antisense rca rice plants, when the thin sections were treated with antibody directed against Rubisco of rice, almost all of the immunogold was in the stroma of chloroplasts (Fig 1A,B). However, the labeling density in the stroma was related to the RCA content; in the antisense plants small RCA content resulted in a higher densities of particles in the stroma (Table 2), consistent with the increased Rubisco content measured in vitro (Table 1). There were only a few particles over the thylakoid in both types of plant (Fig 1A, B; Table 2).Table 1. The net photosynthetic rate (Pn), intercellular CO 2 concentrations (C i ), stomatal conductance (g s ), RCA contents, Rubisco contents, initial and total Rubisco activity of the leaves of the wild type andantisense rca plants.Parameter Wild type Antisense % ChangePn (µmol m -2s -1) 18.9±0.7 a 9.68±0.5 b –48.8C i (µmol mol -1) 311±8 b 369±5 a +18.6g s (mol m -2s -1) 0.46±0.09 a 0.44±0.07 a –4.0RCA (mg m -2) 20.1±0.6 a 6.11±0.9 b –69.2Rubisco (g m -2) 1.68±0.20 b 2.97±0.57 a +76.8Initial activity (µmol m -2s -1) 32.7±3.3 a 17.2±2.58 b –47.4Total activity (µmol m -2s -1) 42.4±6.4 b 70.8±13.84 a +67.0Fig. 1 Immunogold labeling of Rubisco (A,B) and RCA (C,D) in mesophyll cell chloroplasts of leaves of wild type (A,C) and rca1 (B,D) rice plants. T – thylakoid; S – stroma. Bars = 0.1 µm.When sections of rice leaves were treated with antibody directed against rice RCA, most of the A C ST D Bimmunogold particles in the wild type were heavily concentrated in the stroma (dark part), and some in the thylakoid membranes (white part) (Fig 1C). Gold particle densities were 389±62 µm -2 in the stroma, and 132±36 µm -2 in the thylakoid membranes (Table 2). In contrast, we observed less RCA labeling in the chloroplasts of antisense rca plants than in the wild-type, which correlates with the RCA concentrations previously observed in vivo (Table 1). The densities of gold particles in the antisense plants were 161±47 and 12±4 µm -2 in the stroma and thylakoid membrane of chloroplasts (Table 2), respectively. Interesting, the percentage of the immunogold RCA labeling in the chloroplast stroma depended on the RCA concentration. There were a significantly higher percentage of immunogold particles in the rca1 chloroplast stroma than in the wild type (93 compared to 74%) (P < 0.05) (Fig 1D, Table 2). It is clear that almost all of the RCA was in the chloroplast stroma of antisense plants. When sections were incubated with non-immune serum, they showed only non-specific and negligible labeling with gold particles (data not shown).Table 2 Immunogold labeling of Rubisco and RCA in the chloroplast organelles of the mature leaves of the wild type and antisense rca plants grown in a greenhouse.Label density of Rubisco(µm 2) Label density of RCA (µm 2) Strain Stroma ThylakoidRubisco instroma (%) d Stroma Thylakoid RCA in stroma (%) e Wild type 736±82 b 29±7 c 96.2 389±62 a 132±36 b 74.7Anti-rca 867±97 a 33±11 c 96.3 161±47 b 12±4 c 93.84 DiscussionAlthough RCA is an important enzyme activating Rubisco in vivo [5, 18], the cellular localization of RCA is not well established. Anderson et al. [19] found that RCA was in the stroma of chloroplasts in pea plants. Recently, Hong et al. [20] demonstrated that RCA was in the chloroplast of the bundle sheath and mesophyll cells in the C 4 plant, Amaranthus tricolor . We now show that RCA is mainly in the stroma, and to a smaller extent in the thylakoid membranes, of chloroplasts (Fig 1C). Rokka et al [21] reported that most RCA in spinach was sequestered in the thylakoid membranes region during heat treatment, and that the amount of RCA associated with the thylakoid membrane increased with the temperature and duration of the heat treatment. Interestingly, a reduction of RCA in tobacco plants increased sensitivity of photosynthesis to heat [22]. Therefore, RCA appears to be is a multifunction enzyme, regulating Rubisco activity and photosynthetic rate, and also involved in protection against heat damage in chloroplasts. The RCA in stroma of chloroplasts contributes to the activation of Rubisco, a process dependent on ATP hydrolysis [5, 18] and it is possible that the RCA in thylakoid membranes has a second, protective role there.RCA has been likened to a molecular chaperone [23]. Neuwald et al. [24] reported that RCA is related to an AAA family of proteins, a class of chaperone-like ATPases associated with a variety of cellular activities. They are a novel type of molecular chaperone, typically acting as disruptors of molecular or macromolecular structures [25]. This describes the role of RCA in disrupting Rubisco-inhibitor complexes. AAA+ modules are also often linked covalently to other protein domains that mediate transport and position in cellular membranes. AAA+ modules are also often linked covalently to other protein domains that mediate transporting to, and positioning in, cellular membranes. It is also possible that RCA sequestered to the thylakoid membrane is due to its redox regulation, since the activity of the large RCA isoform is regulated by redox changes via the ferredoxin/thioredoxin system at physiological ATP/ADP ratios [12, 26]. Reduction of RCA by thioredoxin could lead to inactivation and sequestration in thylakoid membranes.RCA facilitates the dissociation of inhibitors from Rubisco, so it must bind to Rubisco and induce a conformational change at the active site. RCA was chemically cross-linked to the Rubisco [27] and co-immunoprecipitation of the two proteins was reported [23, 28]. Portis [5] suggested that RCA encircles the Rubisco molecule. However, no RCA-Rubisco complex has been isolated. A double immunogold labeling study suggested that most of RCA is associated with Rubisco, forming complexes in the chloroplast, while a large part of the Rubisco is not associated [29]. In our study, approximately 75% of RCA was in the stroma, and 25% in the thylakoids (Talbe 2). In contrast, 96% or more of the Rubisco was in the stroma (Talbe 2). These results imply that 75% or less of the RCA interacts with Rubisco.Rubisco occurs in the chloroplasts of higher plants [30-32] with most immunogold particles from anti-Rubisco antibodies in the stroma. In the antisense rca rices, we found that the Rubisco content was substantially increased (Table 1), although its location was unaltered (Fig 1 and Table 2). This effect of reduced RCA content upon Rubisco concentration is consistent with work using tobacco plants [8-9]. The observed increase in Rubisco concentration tempts us to speculate that Rubisco accumulation counteracts the RCA deficit. Therefore, an increased requirement for RCA in the stroma was required to activate the increased Rubisco. We demonstrated that labeling for RCA decreased, as did the percentage of RCA labeling in thylakoid membranes (Fig 1D, Talbe 2). Mate et al.[8], Eckardt et al.[11] and Hammond et al. [10] showed that RCA was largely saturating for the steady-state concentration of active Rubisco. This suggests that the changes in subcellular RCA distribution in the antisense plants might be related to compensation for the loss of RCA in the stroma and to the increase in Rubisco. This process may be linked to the redox regulation of large RCA isoform. This may explain why steady-state photosynthesis is largely unaffected until RCA concentration is reduced substantially. RCA in the antisense plants may then be transferred from the thylakoid membranes to the stroma to compensate for the reduced RCA, which activates Rubisco. However, transgenic plants with reduced RCA [10, 33] have a slower rate of photosynthetic induction following a rapid increase in light intensity, suggesting that RCA was not present in excess under those conditions; whether RCA in thylakoid membrane is transferred to the stroma is unclear. Changes in protein distribution following a stress have already been reported. For instance, mechanical stimulation induces achange in the location of GPXle-1 protein from the wall to the cytosol in collenchyma [34]. To our knowledge, a change of protein location induced by reduction of protein content has not previously been reported. However, we did not rule out a decrease in the binding affinity of the protein for the thylakoid membrane in the antisense rca plants: more detailed biochemical characterization of these changes will be required to establish if this occurs.References[1] Andrews, T.J., Hudson, G.S., Mate, C.J., von Caemmerer, S., Evans, J.R., Arvidsson, Y.B.C. (1995) Rubisco: theconsequences of altering its expression and activation in transgenic plants. Journal of Experimental Botany 456: 1293–1300.[2] Portis, A.R. Jr., (1995) The regulation of Rubisco by Rubisco activase, Journal of Experimental Botany 46:1285–1291.[3] Komatsu, S., Masuda, T., Hirano, H. (1996) Rice gibberellin-binding phosphoprotein structurally related toribulose-1,5-bisphosphate carboxylase-oxygenase activase. FEBS Letters 384: 167–171.[4] Sharma, A., Komatsu, S. (2002) Involvement of a Ca2+-dependent protein kinase component downstream to thegibberellin-binding phosphoprotein, RuBisCO activase, in rice. Biochemical and Biophysical Research Communications 290: 690–695.[5] Portis, A.R. Jr. (2003) Rubisco activase – Rubisco’s catalytic chaperone. Photosynthesis Research 75: 11–27.[6] Jin, S., Jiang, D., Li, X., Sun, J. (2004a) Characteristics of photosynthesis in rice plants transformed with anantisense Rubisco activase gene. Journal of Zhejiang University Science 5: 897–899.[7] Jin, S.H., Weng, X.Y., Wang, N.Y., Li, X.Q., Mao, W.H., Jiang, D.A. (2004b) Construction of expressionvector with antisense Rubisco activase gene and its genetic transformation in rice. Hereditas (Beijing) 26: 881–886.[8] Mate, C.J., Hudson, G.S., von Caemmerer, S., Evans, J.R., Andrews, T.J. (1993) Reduction of ribulosebisphosphate carboxylase activase levels in tobacco (Nicotiana tabacu m) by antisense RNA reduces ribulose bisphosphate carboxylase carbamylation and impairs photosynthesis. Plant Physiology 102: 1119–1128.[9] Mate, C.J., Hudson, G.S., von Caemmerer, S., Evans, J.R., Andrews, T.J. (1996) The relationship betweenCO2-assimilation rate, Rubisco carbamylation and Rubisco activase content in activase-deficient transgenic tobacco suggests a sample model of activase action. Planta 198: 604-613.[10] Hammond, E.T., Andrews, T.J., Mott, K.A. (1998) Regulation of Rubisco activation in antisense plants oftobacco containing reduced levels of Rubisco activase. The Plant Journal 4: 101–110.[11] Eckardt, A.N., Snyder, G.W., Portis, A.R., Ogren, W.L. (1997) Growth and photosynthesis under high and lowirradiance of Arabidopsis thaliana antisense mutants with reduced ribulose-1,5-bisphosphate carboxylase/oxygenase activase content. Plant Physiology 113: 575–586.[12] Zhang, N., Kallis, R.P., Ewy, R.G., Portis, A.R. Jr. (2002) Light modulation of Rubisco in Arabidopsis requires acapacity for redox regulation of the larger Rubisco activase isoform. Proceedings of the National Academy of Sciences USA 99: 3330–3334.[13] He, Z., von Caemmerer, S., Hudson, G.S., Price, G,D., Badger, M.R., Andrews, T.J. (1997)Ribulose-1,5-bisphosphate carboxylase/oxygenase activase deficiency delays senescence of ribulose-1,5-bisphosphate carboxylase/oxygenase but progressively impairs its catalysis during tobacco leaf development. 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(2000) The regulation of Rubisco carboxylation activityand photosynthetic rate by Rubisco activase during leaf senescence in rice, Journal of Zhejiang University (Agriculture and Life Sciences) 26: 119-124.[18] Spreitzer, R.J., Salvucci, M.E. (2002) Rubisco: structure, regulatory interactions, and possibilities for a betterenzyme. Annual Review of Plant Biology 53: 449–485.[19] Anderson LE, Gibbons JT, Wu WX. 1996. Distribution of ten enzymes of carbon metabolism in pea (Pisumsativum) chloroplasts. International Journal of Plant Sciences 157: 525–538.[20] Hong, J., Jiang, D.A., Weng, X.Y, Wang, W.B., Hu, D.W. (2005) Leaf anatomy, chloroplast ultrastructure andRubisco and Rubisco activase cellular localization in Amaranthus tricolor L. Photosynthetica 43: 519–528. [21] Rokka, A., Zhang, L., Aro, E.N. (2001) Rubisco activase: an enzyme with a temperature-dependent dualfunction. The Plant Journal 25: 463–471.[22] Sharkey, T.D., Badger, M.R., von Caemmerer, S., Andrews, T.J. (2001) Increased heat sensitivity ofphotosynthesis in tobacco plant with reduced Rubisco activase. Photosynthesis Research 67: 147–156.[23] Sánchez de Jiménez, E., Medrano, L., Martínez-Barajas, E. (1995) Rubisco activase, a possible new member ofthe molecular chaperone family. Biochemistry 34: 2826–2831.[24] Neuwald, A.F., Aravind, L., Spouge, J.L., Koonin, E.V. (1999) AAA +: A class of chaperone-like ATPasesassociated with the assembly, operation, and disassembly of protein complexes. Genome Research 9: 27–43. [25] Ogura, T., Wilkinson, A.J. (2001) AAA+ superfamily ATPases: common structure – diverse function. Genes toCells 6: 575–597.[26] Zhang, N., Portis, A.R. Jr. (1999) Mechanism of light regulation of Rubisco activase isoform involving reductiveactivation by thioredoxin-f, Proceedings of the National Academy of Sciences USA 96: 9438–9443.[27] Yokota, A., Tsujimoto, N. (1992) Characterization of ribulose-1,5-bisphosphate carboxylase/oxygenase carryingribulose-1,5-bisphosphate at its regulatory sites and the mechanism of interaction of this form the enzyme with ribulose-1,5-bisphosphate carboxylase/oxygenase activase. European Journal of Biochemistry 204: 901–909. [28] Zhang, Z.L., Komatsu, S. (2000) Molecular cloning and characterization of cDNAs encoding two isoforms ofribulose-1,5-bisphosphate carboxylase/oxygenase activase in rice (Oryza sativa L.). Journal of Biochemistry 128: 383–389[29] Anderson, L.E., Carol, A.A. (2004) Enzyme co-localization with Rubisco in pea leaf chloroplasts.。
光合作用英文介绍Photosynthesis is a fundamental process in the life of green plants and certain types of bacteria. It is a complex biochemical reaction that utilizes sunlight, carbon dioxide, and water to produce glucose and oxygen. This process plays a crucial role in the ecosystem as it provides the primary source of energy for almost all living organisms.1. Overview of PhotosynthesisPhotosynthesis can be divided into two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle). In the light-dependent reactions, light energy is absorbed by chlorophyll and other pigments in the chloroplasts of plant cells. This energy is used to split water molecules into oxygen, protons, and electrons. The oxygen is released as a byproduct, while the electrons are used to generate ATP and NADPH.2. Light-Dependent ReactionsThe light-dependent reactions take place in the thylakoid membranes of the chloroplasts. The absorbed light energy excites electrons in the chlorophyll molecules, which are then passed along an electron transport chain. This process generates a proton gradient across the thylakoid membrane, which drives the synthesis of ATP through chemiosmosis.Meanwhile, water molecules are split by the enzyme complex known as Photosystem II, releasing oxygen and protons. The electrons extracted from water are used to replace the ones lost by chlorophyll, ensuring the continuity of the electron transport chain.3. Calvin CycleThe ATP and NADPH generated in the light-dependent reactions are utilized in the Calvin cycle, the light-independent phase of photosynthesis. In this cycle, carbon dioxide is captured by the enzyme RuBisCO and incorporated into a 5-carbon compound, forming a 6-carbon intermediate that quickly breaks down into two molecules of 3-phosphoglycerate.Subsequent reactions convert 3-phosphoglycerate into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar that can be further processed into glucose or used in other metabolic pathways. Some of the G3P molecules are regenerated to restart the Calvin cycle, ensuring a continuous supply of glucose precursors.4. Importance of PhotosynthesisPhotosynthesis is essential for the survival of almost all life on Earth. Green plants, algae, and certain bacteria are capable of photosynthesis, providing food andoxygen for animals and other organisms. Additionally, photosynthetic organisms act as primary producers in the food chain, converting solar energy into chemical energy that fuels the entire ecosystem.Moreover, photosynthesis plays a crucial role in the global carbon cycle by sequestering atmospheric carbon dioxide and releasing oxygen. This process helps regulate the Earth’s climate and maintain a balance between oxygen and carbon dioxide levels in the atmosphere.In conclusion, photosynthesis is a remarkable biological process that sustains life on our planet. By harnessing the power of sunlight, plants and other photosynthetic organisms are able to convert inorganic molecules into organic compounds, supporting the diverse web of life that thrives on Earth.。
Carbon dioxide,often abbreviated as CO2,is a colorless,odorless gas that is essential for life on Earth.It is a natural component of the atmosphere and plays a crucial role in the carbon cycle,which is the process by which carbon is exchanged between the atmosphere,oceans,soil,plants,and animals.Sources of Carbon Dioxide:Carbon dioxide is produced through various natural processes such as respiration, volcanic eruptions,and the decay of organic matter.However,human activities have significantly increased the levels of CO2in the atmosphere.The primary sources of anthropogenic CO2emissions include the burning of fossil fuels like coal,oil,and natural gas for energy production,deforestation,and industrial processes.Role in Photosynthesis:Plants absorb CO2during photosynthesis,a process in which they convert sunlight, carbon dioxide,and water into glucose and oxygen.This process is vital for plant growth and is the foundation of the food chain,supporting all life on Earth.Greenhouse Gas:Carbon dioxide is a greenhouse gas,which means it traps heat in the Earths atmosphere. While the greenhouse effect is necessary for maintaining the Earths temperature,an excess of greenhouse gases,including CO2,leads to global warming and climate change. This can result in rising sea levels,more frequent and severe weather events,and disruptions to ecosystems.Carbon Sequestration:To mitigate the effects of CO2emissions,various carbon sequestration techniques are being explored.These include afforestation,which involves planting trees to absorb CO2, and carbon capture and storage CCS technologies,which aim to capture CO2emissions from industrial sources and store them underground.Reducing CO2Emissions:Individuals and governments can take steps to reduce CO2emissions.This includes using energyefficient appliances,reducing reliance on fossil fuels,promoting renewable energy sources like solar and wind power,and supporting policies that encourage sustainable practices.Conclusion:Understanding the role of carbon dioxide in the environment is crucial for developing strategies to combat climate change.While CO2is a necessary component for life,its excessive accumulation in the atmosphere poses a significant threat to the planetsecosystems and human societies.By adopting sustainable practices and supporting technological innovations,we can work towards a more balanced and environmentally friendly future.。
光合作用英文介绍Photosynthesis: The Life-Sustaining ProcessIntroductionPhotosynthesis is a vital process that occurs in plants, algae, and some bacteria. It is the process by which these organisms convert light energy from the sun into chemical energy in the form of organic compounds, such as glucose. This process is essential for the survival of most life forms on Earth, as it provides the primary source of energy and organic matter needed to sustain life. In this article, we will explore the intricacies of photosynthesis, including its history, the role of chlorophyll, the light-dependent reactions, the Calvin cycle, and the importance of photosynthesis in the global carbon cycle.History of PhotosynthesisThe discovery of photosynthesis can be traced back to the 17th century, when a Belgian physician named Jan Baptista van Helmont conducted experiments on a willow tree. He observed that the tree gained weight over time, even though it was only watered and not fertilized. This led him to concludethat the tree was somehow absorbing nutrients from the air. However, it was not until the 19th century that scientists began to understand the true nature of photosynthesis.In 1864, a German botanist named Julius Robert Mayer proposed that plants convert solar energy into chemical energy through a process he called "photochemical transformation." This theory was further developed by two other German scientists, Hermann von Helmholtz and Hermann Schlegel, who independently discovered the role ofchlorophyll in photosynthesis.Chlorophyll: The Pigment of LifeChlorophyll is a green pigment found in the chloroplasts of plant cells. It is responsible for absorbing light energy from the sun and converting it into chemical energy that can be used by the plant. Chlorophyll is composed of a complex molecule called a porphyrin ring, which contains a magnesium ion at its center. This magnesium ion gives chlorophyll its characteristic green color and allows it to absorb light energy efficiently.There are several different types of chlorophyll, but the most common ones are chlorophyll a and chlorophyll b. Chlorophyll a is found in all plants and algae, while chlorophyll b is only found in some species. Both types of chlorophyll absorb light energy in the blue and red regions of the spectrum, but they reflect green light, giving plants their characteristic color.Light-Dependent ReactionsThe light-dependent reactions of photosynthesis occur in the thylakoid membranes of chloroplasts. These reactions involve the transfer of electrons from water molecules to electron carriers, such as NADP+ and ATP. This process generates oxygen gas as a byproduct and releases energy that can be used to power the Calvin cycle.The light-dependent reactions can be divided into two main stages: photosystem I (PSI) and photosystem II (PSII). PSII absorbs light energy and uses it to split water molecules into oxygen and hydrogen ions. The oxygen is released into the atmosphere, while the hydrogen ions are used to generate ATP and NADPH. PSI then uses the energy from these compounds to power the synthesis of organic molecules, such as glucose.Calvin CycleThe Calvin cycle, also known as the dark reaction orlight-independent reaction, occurs in the stroma ofchloroplasts. This cycle involves a series of chemical reactions that use the energy from ATP and NADPH to convert carbon dioxide into organic molecules, such as glucose.The Calvin cycle can be divided into three main stages: carbon fixation, reduction, and regeneration. In the first stage, carbon dioxide is combined with a five-carbon sugar called ribulose bisphosphate (RuBP) to form a six-carbon compound called 3-phosphoglycerate (3PG). In the second stage, ATP and NADPH are used to convert 3PG into glyceraldehyde 3-phosphate (G3P), a three-carbon sugar that can be used to synthesize glucose and other organic molecules. Finally, inthe third stage, RuBP is regenerated from G3P so that thecycle can continue.Importance of PhotosynthesisPhotosynthesis plays a crucial role in maintaining the delicate balance of life on Earth. It is the primary sourceof energy and organic matter for most organisms, includinghumans. Without photosynthesis, there would be no food, no oxygen, and no biodiversity.Photosynthesis also plays a key role in the global carbon cycle. Plants absorb carbon dioxide from the atmosphere during photosynthesis and release oxygen as a byproduct. This helps to regulate the levels of these gases in the atmosphere and prevent the buildup of harmful greenhouse gases.ConclusionIn conclusion, photosynthesis is a complex andfascinating process that has been essential for the survival of life on Earth for millions of years. From its discovery in the 17th century to our current understanding of itsintricate mechanisms, photosynthesis has captivatedscientists and laypeople alike. By harnessing the power of the sun, photosynthesis provides us with the energy and organic matter we need to survive and thrive. As we continue to learn more about this remarkable process, we may find newways to harness its power for the benefit of humanity and the planet.。
The Process of PhotosynthesisPhotosynthesis is a vital process for the survival of most living organisms on Earth. It is a complex biochemical reaction that occurs in the chloroplasts of plant cells. Through photosynthesis, plants and other photosynthetic organisms convert light energy into chemical energy in the form of glucose, which serves as the primary source of energy for all living things. Understanding the process of photosynthesis is essential for comprehending the intricate relationship between plants and the environment.1. Chloroplasts: The Site of PhotosynthesisPhotosynthesis takes place within specialized organelles called chloroplasts. Chloroplasts contain pigments known as chlorophyll, which are responsible for capturing light energy from the sun. The chloroplast is divided into two main compartments: the grana and the stroma. The grana are stacks of disc-like structures called thylakoids, where the light-dependent reactions of photosynthesis occur. The stroma, on the other hand, is the fluid-filled region where the light-independent reactions take place.2. Light-Dependent ReactionsThe first stage of photosynthesis is the light-dependent reactions, which require light energy to proceed. During this phase, light energy is absorbed by chlorophyll and other pigments in the thylakoid membranes. The absorbed light energy is used to split water molecules into oxygen, protons, and electrons. This process, known as photolysis, releases oxygen as a byproduct and generates ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate) molecules, which serve as energy carriers for the subsequent stage.3. Light-Independent Reactions (Calvin Cycle)The light-independent reactions, also known as the Calvin Cycle, occur in the stroma of the chloroplast and do not directly require light. In this stage, carbon dioxide molecules from the atmosphere are fixed into a three-carbon compound called 3-phosphoglycerate. This process, catalyzed by the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase), is known as carbon fixation. The 3-phosphoglycerate molecules are then converted into glucose through a series of enzymatic reactions, utilizing the ATP and NADPH generated in the light-dependent reactions.4. Factors Affecting PhotosynthesisSeveral factors influence the rate of photosynthesis, including light intensity, carbon dioxide concentration, temperature, and water availability. Optimalconditions for photosynthesis typically include moderate light intensity, sufficient carbon dioxide levels, and a suitable temperature range. Any deviation from these conditions can limit the efficiency of photosynthesis and subsequently impact plant growth and productivity.5. Importance of PhotosynthesisPhotosynthesis plays a crucial role in the Earth’s ecosystems by provi ding oxygen for respiration and serving as the foundation of the food chain. Plants are not only primary producers but also essential for maintaining atmospheric balance and biodiversity. Additionally, photosynthesis helps mitigate the effects of climate change by absorbing carbon dioxide from the atmosphere and storing it in plant biomass.In conclusion, photosynthesis is a complex yet essential process that sustains life on Earth. By converting light energy into chemical energy, plants produce the organic compounds necessary for their growth and development. Understanding the intricacies of photosynthesis is key to appreciating the interconnectedness of living organisms and the environment.。
10PhotosynthesisConcept Outline10.1What is photosynthesis?The Chloroplast as a Photosynthetic Machine.Thehighly organized system of membranes in chloroplasts isessential to the functioning of photosynthesis.10.2Learning about photosynthesis: An experimentaljourney.The Role of Soil and Water.The added mass of agrowing plant comes mostly from photosynthesis. In plants, water supplies the electrons used to reduce carbon dioxide.Discovery of the Light-Independent Reactions.Photosynthesis is a two-stage process. Only the first stagedirectly requires light.The Role of Light.The oxygen released during greenplant photosynthesis comes from water, and carbon atomsfrom carbon dioxide are incorporated into organic molecules.The Role of Reducing Power.Electrons released fromthe splitting of water reduce NADP+; ATP and NADPHare then used to reduce CO2and form simple sugars. 10.3Pigments capture energy from sunlight.The Biophysics of Light.The energy in sunlight occursin “packets” called photons, which are absorbed by pigments.Chlorophylls and Carotenoids.Photosyntheticpigments absorb light and harvest its energy.Organizing Pigments into Photosystems.Aphotosystem uses light energy to eject an energized electron.How Photosystems Convert Light to Chemical Energy.Some bacteria rely on a single photosystem to produceATP. Plants use two photosystems in series to generateenough energy to reduce NADP+and generate ATP.How the Two Photosystems of Plants Work Together.Photosystems II and I drive the synthesis of the ATP andNADPH needed to form organic molecules.10.4Cells use the energy and reducing power capturedby the light reactions to make organic molecules.The Calvin Cycle.ATP and NADPH are used to buildorganic molecules, a process reversed in mitochondria.Reactions of the Calvin Cycle.Ribulose bisphosphatebinds CO2in the process of carbon fixation.Photorespiration.The enzyme that catalyzes carbonfixation also affects CO2release.L ife on earth would be impossible without photosyn-thesis. Every oxygen atom in the air we breathe was once part of a water molecule, liberated by photosynthesis. The energy released by the burning of coal, firewood, gasoline, and natural gas, and by our bodies’ burning of all the food we eat—all, directly or indirectly, has been cap-tured from sunlight by photosynthesis. It is vitally impor-tant that we understand photosynthesis. Research may en-able us to improve crop yields and land use, important goals in an increasingly crowded world. In the previous chapter we described how cells extract chemical energy from food molecules and use that energy to power their activities. In this chapter, we will examine photosynthesis, the process by which organisms capture energy from sun-light and use it to build food molecules rich in chemicalenergy (figure 10.1).FIGURE 10.1Capturing energy.These sunflower plants, growing vigorously in the August sun, are capturing light energy for conversion into chemical energy through photosynthesis.18310.1What is photosynthesis?Stoma Bundle sheathChloroplastsInnermembraneGranumFIGURE 10.2Journey into a leaf.A plant leaf possesses a thick layer of cells (the mesophyll) rich in chloroplasts. The flattened thylakoids in the chloroplast are stacked into columns called grana (singular, granum). The light reactions take place on the thylakoid184Part III Energeticssis takes place in three stages: (1) capturing energy from sunlight; (2) using the energy to make ATP and reducing power in the form of a compound called NADPH; and (3)using the ATP and NADPH to power the synthesis of organic molecules from CO2in the air (carbon fixation).The first two stages take place in the presence of light and are commonly called the light reactions.The third stage, the formation of organic molecules from atmos-pheric CO2, is called the Calvin cycle.As long as ATP and NADPH are available, the Calvin cycle may occur in the absence of light.The following simple equation summarizes the overall process of photosynthesis:6 CO2+ 12 H2O + light —→C6H12O6+ 6 H2O + 6 O2 carbon water glucose water oxygen dioxideInside the ChloroplastThe internal membranes of chloroplasts are organized into sacs called thylakoids,and often numerous thylakoids are stacked on one another in columns called grana.The thy-lakoid membranes house the photosynthetic pigments for capturing light energy and the machinery to make ATP. Surrounding the thylakoid membrane system is a semiliq-uid substance called stroma.The stroma houses the en-zymes needed to assemble carbon molecules. In the mem-branes of thylakoids, photosynthetic pigments are clustered together to form a photosystem.Each pigment molecule within the photosystem is capa-ble of capturing photons,which are packets of energy. A lat-tice of proteins holds the pigments in close contact with one another. When light of a proper wavelength strikes a pigment molecule in the photosystem, the resulting excita-tion passes from one chlorophyll molecule to another. The excited electron is not transferred physically—it is the en-ergy that passes from one molecule to another. A crude analogy to this form of energy transfer is the initial “break”in a game of pool. If the cue ball squarely hits the point of the triangular array of 15 pool balls, the two balls at the far corners of the triangle fly off, but none of the central balls move. The energy passes through the central balls to the most distant ones.Eventually the energy arrives at a key chlorophyll mole-cule that is touching a membrane-bound protein. The en-ergy is transferred as an excited electron to that protein, which passes it on to a series of other membrane proteins that put the energy to work making ATP and NADPH and building organic molecules. The photosystem thus acts as a large antenna, gathering the light harvested by many indi-vidual pigment molecules.The reactions of photosynthesis take place withinthylakoid membranes within chloroplasts in leaf cells.Chapter 10Photosynthesis185 SunlightLight reactionsH2OPhotosystemThylakoidFIGURE 10.2 (continued)membrane and generate the ATP and NADPH that fuel the Calvin cycle. The fluid interior matrix of a chloroplast, the stroma, contains the enzymes that carry out the Calvin cycle.The Role of Soil and WaterThe story of how we learned about photosynthesis is one of the most interesting in science and serves as a good intro-duction to this complex process. The story starts over 300 years ago, with a simple but carefully designed experiment by a Belgian doctor, Jan Baptista van Helmont (1577–1644). From the time of the Greeks, plants were thought to obtain their food from the soil, literally sucking it up with their roots; van Helmont thought of a simple way to test the idea. He planted a small willow tree in a pot of soil after weighing the tree and the soil. The tree grew in the pot for several years, during which time van Helmont added only water. At the end of five years, the tree was much larger: its weight had increased by 74.4 kilograms. However, all of this added mass could not have come from the soil,because the soil in the pot weighed only 57 grams less than it had five years earlier! With this experiment, van Helmont demonstrated that the substance of the plant was not produced only from the soil. He incorrectly concluded that mainly the water he had been adding accounted for the plant’s increased mass.A hundred years passed before the story became clearer. The key clue was provided by the English scientist Joseph Priestly, in his pioneering studies of the properties of air. On the 17th of August, 1771, Priestly “accidentally hit upon a method of restoring air that had been injured by the burning of candles.” He “put a [living] sprig of mint into air in which a wax candle had burnt out and found that, on the 27th of the same month, another candle could be burned in this same air.” Somehow, the vegetation seemed to have restored the air! Priestly found that while a mouse could not breathe candle-exhausted air, air “restored” by vegetation was not “at all inconvenient to a mouse.” The key clue was that living vegetation adds something to the air.How does vegetation “restore” air? Twenty-five years later, Dutch physician Jan Ingenhousz solved the puzzle. Working over several years, Ingenhousz reproduced and significantly extended Priestly’s results, demonstrating that air was restored only in the presence of sunlight, and only by a plant’s green leaves, not by its roots. He proposed that the green parts of the plant carry out a process (which we now call photosynthesis) that uses sunlight to split carbon dioxide (CO2) into carbon and oxygen. He suggested that the oxygen was released as O2gas into the air, while the carbon atom combined with water to form carbohydrates. His proposal was a good guess, even though the later step was subsequently modified. Chemists later found that the proportions of carbon, oxygen, and hydrogen atoms in car-bohydrates are indeed about one atom of carbon per mole-cule of water (as the term carbohydrate indicates). A Swiss botanist found in 1804 that water was a necessary reactant. By the end of that century the overall reaction for photo-synthesis could be written as:CO2+ H2O + light energy —→(CH2O) + O2 It turns out, however, that there’s more to it than that. When researchers began to examine the process in more detail in the last century, the role of light proved to be un-expectedly complex.Van Helmont showed that soil did not add mass to agrowing plant. Priestly and Ingenhousz and others then worked out the basic chemical reaction. Discovery of the Light-Independent ReactionsIngenhousz’s early equation for photosynthesis includes one factor we have not discussed: light energy. What role does light play in photosynthesis? At the beginning of the previous century, the English plant physiologist F. F. Blackman began to address the question of the role of light in photosynthesis. In 1905, he came to the startling conclu-sion that photosynthesis is in fact a two-stage process, only one of which uses light directly.Blackman measured the effects of different light inten-sities, CO2concentrations, and temperatures on photo-synthesis. As long as light intensity was relatively low, he found photosynthesis could be accelerated by increasing the amount of light, but not by increasing the tempera-ture or CO2concentration (figure 10.3). At high light in-tensities, however, an increase in temperature or CO2 concentration greatly accelerated photosynthesis. Black-man concluded that photosynthesis consists of an initial set of what he called “light” reactions, that are largely in-dependent of temperature, and a second set of “dark” re-actions, that seemed to be independent of light but lim-ited by CO2. Do not be confused by Blackman’s labels—the so-called “dark” reactions occur in the light (in fact, they require the products of the light reactions); their name simply indicates that light is not directly in-volved in those reactions.Blackman found that increased temperature increases the rate of the dark carbon-reducing reactions, but only up to about 35°C. Higher temperatures caused the rate to fall off rapidly. Because 35°C is the temperature at which many plant enzymes begin to be denatured (the hydrogen bonds that hold an enzyme in its particular catalytic shape begin to be disrupted), Blackman concluded that enzymes must carry out the dark reactions.Blackman showed that capturing photosynthetic energy requires sunlight, while building organic moleculesdoes not.186Part III Energetics10.2Learning about photosynthesis: An experimental journey.The Role of LightThe role of light in the so-called light and dark reactions was worked out in the 1930s by C. B. van Niel, then a graduate student at Stanford University studying photosynthesis in bacteria. One of the types of bacteria he was studying, the purple sulfur bacteria, does not release oxygen during photosynthesis; instead, they convert hydrogen sulfide (H2S) into globules of pure elemental sulfur that accumulate inside themselves. The process that van Niel observed wasCO2+ 2 H2S + light energy →(CH2O) + H2O + 2 S The striking parallel between this equation and Ingenhousz’s equation led van Niel to propose that the generalized process of photosynthesis is in factCO2+ 2 H2A + light energy →(CH2O) + H2O + 2 A In this equation, the substance H2A serves as an electron donor. In photosynthesis performed by green plants, H2A is water, while among purple sulfur bacteria, H2A is hydrogen sulfide. The product, A, comes from the splitting of H2A. Therefore, the O2produced during green plant photosyn-thesis results from splitting water, not car-bon dioxide.When isotopes came into common use in biology in the early 1950s, it became possible to test van Niel’s revolu-tionary proposal. Investigators examined photosynthesis in green plants supplied with 18O water; they found that the 18O label ended up in oxygen gas rather than in carbohy-drate, just as van Niel had predicted:CO2+ 2 H218O + light energy —→(CH2O) + H2O + 18O2In algae and green plants, the carbohydrate typically pro-duced by photosynthesis is the sugar glucose, which has six carbons. The complete balanced equation for photosynthe-sis in these organisms thus becomes6 CO2+ 12 H2O + light energy —→C6H12O6+ 6 O2+ 6 H2O.We now know that the first stage of photosynthesis, the light reactions, uses the energy of light to reduce NADP (an electron carrier molecule) to NADPH and to manufac-ture ATP. The NADPH and ATP from the first stage of photosynthesis are then used in the second stage, the Calvin cycle, to reduce the carbon in carbon dioxide and form a simple sugar whose carbon skeleton can be used to synthesize other organic molecules.Van Niel discovered that photosynthesis splits watermolecules, incorporating the carbon atoms of carbondioxide gas and the hydrogen atoms of water intoorganic molecules and leaving oxygen gas. The Role of Reducing PowerIn his pioneering work on the light reactions, van Niel had further proposed that the reducing power (H+) generated by the splitting of water was used to convert CO2into organic matter in a process he called carbon fixation. Was he right?In the 1950s Robin Hill demonstrated that van Niel was indeed right, and that light energy could be used to generate reducing power. Chloroplasts isolated from leaf cells were able to reduce a dye and release oxygen in response to light. Later experiments showed that the electrons released from water were transferred to NADP+. Arnon and coworkers showed that illuminated chloroplasts deprived of CO2accu-mulate ATP. If CO2is then introduced, neither ATP nor NADPH accumulate, and the CO2is assimilated into organic molecules. These experiments are important for three rea-sons. First, they firmly demonstrate that photosynthesis oc-curs only within chloroplasts. Second, they show that the light-dependent reactions use light energy to reduce NADP+ and to manufacture ATP. Thirdly, they confirm that the ATP and NADPH from this early stage of photosynthesis are then used in the later light-independent reactions to reduce carbon dioxide, forming simple sugars.Hill showed that plants can use light energy to generate reducing power. The incorporation of carbon dioxideinto organic molecules in the light-independentreactions is called carbon fixation.Chapter 10Photosynthesis187 sFIGURE 10.3Discovery of the dark reactions. (a) Blackman measured photosynthesis rates under differing light intensities, CO2concentrations, and temperatures. (b) As this graph shows, light is the limiting factor at low light intensities, while temperature and CO2 concentration are the limiting factors at higher light intensities.The Biophysics of LightWhere is the energy in light? What is there in sunlight that a plant can use toreduce carbon dioxide? This is themystery of photosynthesis, the one fac-tor fundamentally different fromprocesses such as respiration. To an-swer these questions, we will need toconsider the physical nature of light it-self. James Clerk Maxwell had theo-rized that light was an electromagnetic wave—that is, that light movedthrough the air as oscillating electric and magnetic fields. Proof of this camein a curious experiment carried out in alaboratory in Germany in 1887. A young physicist, Heinrich Hertz, was attempting to verify a highly mathe-matical theory that predicted the exis-tence of electromagnetic waves. To see whether such waves existed, Hertz de-signed a clever experiment. On oneside of a room he constructed a powerful spark generatorthat consisted of two large, shiny metal spheres standingnear each other on tall, slender rods. When a very high sta-tic electrical charge was built up on one sphere, sparkswould jump across to the other sphere.After constructing this device, Hertz set out to investigate whether the sparking would create invisible electromagneticwaves, so-called radio waves, as predicted by the mathemati-cal theory. On the other side of the room, he placed theworld’s first radio receiver, a thin metal hoop on an insulat-ing stand. There was a small gap at the bottom of the hoop,so that the hoop did not quite form a complete circle. WhenHertz turned on the spark generator across the room, he sawtiny sparks passing across the gap in the hoop! This was thefirst demonstration of radio waves. But Hertz noted anothercurious phenomenon. When UV light was shining acrossthe gap on the hoop, the sparks were produced more readily.This unexpected facilitation, called the photoelectric effect,puzzled investigators for many years.The photoelectric effect was finally explained using aconcept proposed by Max Planck in 1901. Planck devel-oped an equation that predicted the blackbody radiationcurve based upon the assumption that light and other formsof radiation behaved as units of energy called photons. In1905 Albert Einstein explained the photoelectric effect uti-lizing the photon concept. Ultraviolet light has photons ofsufficient energy that when they fell on the loop, electronswere ejected from the metal surface. The photons hadtransferred their energy to the electrons, literally blastingthem from the ends of the hoop and thus facilitating the passage of the electric spark induced by the radio waves.Visible wavelengths of light were unable to remove the electrons because their photons did not have enough en-ergy to free the electrons from the metal surface at the ends of the hoop.The Energy in PhotonsPhotons do not all possess the same amount of energy (fig-ure 10.4). Instead, the energy content of a photon is in-versely proportional to the wavelength of the light: short-wavelength light contains photons of higher energy than long-wavelength light. X rays, which contain a great deal of energy, have very short wavelengths—much shorter than visi-ble light, making them ideal for high-resolution microscopes.Hertz had noted that the strength of the photoelectric effect depends on the wavelength of light; short wave-lengths are much more effective than long ones in produc-ing the photoelectric effect. Einstein’s theory of the photo-electric effect provides an explanation: sunlight contains photons of many different energy levels, only some of which our eyes perceive as visible light. The highest energy photons, at the short-wavelength end of the electromag-netic spectrum (see figure 10.4), are gamma rays, with wavelengths of less than 1 nanometer; the lowest energy photons, with wavelengths of up to thousands of meters,are radio waves. Within the visible portion of the spectrum,violet light has the shortest wavelength and the most ener-getic photons, and red light has the longest wavelength and the least energetic photons.188Part IIIEnergetics10.3Pigments capture energy from sunlight.FIGURE 10.4The electromagnetic spectrum.Light is a form of electromagnetic energy convenientlythought of as a wave. The shorter the wavelength of light, the greater its energy. Visible light represents only a small part of the electromagnetic spectrum between 400 and 740nanometers.Ultraviolet LightThe sunlight that reaches the earth’s surface contains a significant amount of ultraviolet (UV) light, which, because of its shorter wavelength, possesses considerably more en-ergy than visible light. UV light is thought to have been an important source of energy on the primitive earth when life originated. To-day’s atmosphere contains ozone (derived from oxygen gas), which absorbs most of the UV photons in sunlight, but a considerable amount of UV light still manages to pene-trate the atmosphere. This UV light is a po-tent force in disrupting the bonds of DNA, causing mutations that can lead to skin can-cer. As we will describe in a later chapter, loss of atmospheric ozone due to human ac-tivities threatens to cause an enormous jump in the incidence of human skin cancers throughout the world.Absorption Spectra and Pigments How does a molecule “capture” the energy of light? A photon can be envisioned as a very fast-moving packet of energy. When it strikes a molecule, its energy is either lost as heat or absorbed by the electrons of the mol-ecule, boosting those electrons into higher energy levels. Whether or not the photon’s energy is absorbed depends on how much energy it carries (defined by its wavelength) and on the chemical nature of the molecule it hits. As we saw in chapter 2, electrons occupy discrete energy levels in their orbits aroundatomic nuclei. To boost an electron into a different energy level requires just the right amount of energy, just as reach-ing the next rung on a ladder requires you to raise your foot just the right distance. A specific atom can, therefore, absorb only certain photons of light—namely, those that correspond to the atom’s available electron energy levels. As a result, each molecule has a characteristic absorption spectrum,the range and efficiency of photons it is capable of absorbing.Molecules that are good absorbers of light in the visible range are called anisms have evolved a vari-ety of different pigments, but there are only two general types used in green plant photosynthesis: carotenoids and chlorophylls. Chlorophylls absorb photons within narrow energy ranges. Two kinds of chlorophyll in plants, chloro-phylls a and b,preferentially absorb violet-blue and red light (figure 10.5). Neither of these pigments absorbs pho-tons with wavelengths between about 500 and 600 nanometers, and light of these wavelengths is, therefore, reflected by plants. When these photons are subsequently absorbed by the pigment in our eyes, we perceive them as green.Chlorophyll a is the main photosynthetic pigment and is the only pigment that can act directly to convert light en-ergy to chemical energy. However, chlorophyll b,acting as an accessory or secondary light-absorbing pigment, com-plements and adds to the light absorption of chlorophyll a. Chlorophyll b has an absorption spectrum shifted toward the green wavelengths. Therefore, chlorophyll b can absorb photons chlorophyll a cannot. Chlorophyll b therefore greatly increases the proportion of the photons in sunlight that plants can harvest. An important group of accessory pigments, the carotenoids, assist in photosynthesis by cap-turing energy from light of wavelengths that are not effi-ciently absorbed by either chlorophyll.In photosynthesis, photons of light are absorbed bypigments; the wavelength of light absorbed dependsupon the specific pigment.Chapter 10Photosynthesis189FIGURE 10.5The absorption spectrum of chlorophyll.The peaks represent wavelengths of sunlight that the two common forms of photosynthetic pigment, chlorophyll a(solid line) and chlorophyll b(dashed line), strongly absorb. These pigments absorb predominately violet-blue and red light in two narrow bands of the spectrum and reflect the green light in the middle of the spectrum. Carotenoids (not shown here) absorb mostly blue and green light and reflect orange and yellow light.Chlorophylls and CarotenoidsChlorophylls absorb photons by means of an excitation process analogous to the photoelectric effect. These pigments contain a complex ring structure, called a porphyrin ring,with alternating single and double bonds. At the center of the ring is a magnesium atom. Photons absorbed by the pigment molecule excite electrons in the ring, which are then chan-neled away through the alternating carbon-bond system. Sev-eral small side groups attached to the outside of the ring alter the absorption properties of the molecule in different kinds of chlorophyll (figure 10.6). The precise absorption spectrum is also influenced by the local microenvironment created by the association of chlorophyll with specific proteins.Once Ingenhousz demonstrated that only the green parts of plants can “restore” air, researchers suspected chlorophyll was the primary pigment that plants employ to absorb light in photosynthesis. Experiments conducted in the 1800s clearly verified this suspicion. One such experiment, per-formed by T. W. Englemann in 1882 (figure 10.7), serves as a particularly elegant example, simple in design and clear in outcome. Englemann set out to characterize the action spectrum of photosynthesis, that is, the relative effective-ness of different wavelengths of light in promoting photo-synthesis. He carried out the entire experiment utilizing a single slide mounted on a microscope. To obtain different wavelengths of light, he placed a prism under his micro-scope, splitting the light that illuminated the slide into a spectrum of colors. He then arranged a filament of green algal cells across the spectrum, so that different parts of the filament were illuminated with different wavelengths, and allowed the algae to carry out photosynthesis. To assess how fast photosynthesis was proceeding, Englemann chose to monitor the rate of oxygen production. Lacking a mass spectrometer and other modern instruments, he added aerotactic (oxygen-seeking) bacteria to the slide; he knew they would gather along the filament at locations where oxygen was being produced. He found that the bacteria ac-cumulated in areas illuminated by red and violet light, the two colors most strongly absorbed by chlorophyll.All plants, algae, and cyanobacteria use chlorophyll a as their primary pigments. It is reasonable to ask why these photosynthetic organisms do not use a pigment like retinal (the pigment in our eyes), which has a broad absorption spectrum that covers the range of 500 to 600 nanometers.The most likely hypothesis involves photoefficiency.Al-though retinal absorbs a broad range of wavelengths, it does so with relatively low efficiency. Chlorophyll, in con-trast, absorbs in only two narrow bands, but does so with high efficiency. Therefore, plants and most other photo-synthetic organisms achieve far higher overall photon cap-ture rates with chlorophyll than with other pigments.190Part III EnergeticsmembraneThylakoidGranumChlorophyll molecules embedded in a protein complex in the thylakoid FIGURE 10.6Chlorophyll.Chlorophyllmolecules consist of a porphyrin head and ahydrocarbon tail that anchors the pigment molecule to hydrophobic regions of proteins embedded within the membranes of thylakoids. The only difference between the two chlorophyll molecules is the substitution of a —CHO(aldehyde) group in chlorophyll b for a —CH 3(methyl) group in chlorophyll a.。
