Plant Physiology Preview. Published on September 15, 2009, as DOI:10.1104/pp.109.141267
A Genome-scale Metabolic Model of Arabidopsis
Mark Poolman School of Life Science,
Oxford Brookes University,
Gypsy Lane,
Headington,
Oxford,
OX3 OBP
Tel. +44 (0) 1865 483638
email: mgpoolman@https://www.doczj.com/doc/258285247.html,
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Copyright 2009 by the American Society of Plant Biologists
A Genome-scale Metabolic Model of Arabidopsis thaliana
and Some of its Properties
Mark G. Poolman a?, Laurent Miguet b, Lee J. Sweetlove b, David A. Fell a
a School of Life Science,
Oxford Brookes University,
Gypsy Lane,
Headington,
Oxford,
OX3 OBP
b Department of Plant Sciences,
University of Oxford,
South Parks Road,
Oxford,
OX1 3RB
? Corresponding author.
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Abstract
We describe the construction and analysis of a genome-scale metabolic model of Arabidopsis thaliana primarily derived from the annotations in the Aracyc database. We used techniques based on Linear Programming to demonstrate that: 1) the model is capable of producing biomass components (amino-acids, nucleotides, lipid, starch and cellulose) in the proportion observed experimentally in a heterotrophic
suspension culture ; 2) That, approximately, only 15 % of the available reactions are needed for this purpose and that the size of this network is comparable to estimates of minimal network size for other organisms ; 3) That reactions may be grouped
according to the changes in flux resulting from a hypothetical stimulus (in this case demand for ATP), and that this allows the identification of potential metabolic
modules ; 4) That total ATP demand for growth and maintenance can be inferred, and that this is consistent with previous estimates in prokaryotes and yeast. Nomenclature and abbreviations
With the exceptions noted here, all metabolite and reaction names are the
Aracyc unique identifiers. 2PG - 2-phosphoglycolate, 6PGdh - 6-phosphogluconate dehydrogenase, AconDHatase - aconitate dehydratase, αKG - alpha ketoglutarate, αKGdh - alpha ketoglutarate dehydrogenase, BPGA - glycerate 1,3-bisphosphate, CisAcon - cis-aconitate, Cit - citrate, CitSynth - citrate synthase, DHAP -
dihydroxyacetone phosphate, E4P - erythrose 4-phosphate, F6P - fructose 6-phosphate, FBP - fructose 1,6-bisphosphate, Fum - fumarate, G1P - glucose 1-phosphate, G6P - glucose 6-phosphate, G6Pdh - glucose 6-phosphate dehydrogenase, GAP - glyceraldehyde 3-phosphate, IsoCit - isocitrate, IsoCitDH - isocitrate dehydrogenase, Mal - malate, MalDH - malate dehydrogenase, NADOxid - generic NADH oxidase, OAA - oxaloacetate, PEP - phospho enol pyruvate, PGA - 3-phosphoglycerate, PyrDH - pyruvate dehydrogenase, R5P - ribose 5-phosphate,
Ru5P - ribulose 5-phosphate, Ru5Pk ribulose 5-phosphate kinase, RuBP - ribulose 1,5-bisphosphate, S7P - sedoheptulose 7-phosphate, SBP - sedoheptulose-1,7-bisphosphate, Suc - succiniate, SucThioK - succinyl thiokinase, X5P - xylulose 5-phosphate.
Transport proceses are denoted by the the suffix ‘_tx’ appended to the relevant metabolite name : ‘GLC_tx’ is the glucose transporter etc.
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Introduction
Historically, attempts to engineer plant metabolism for increased production of specific useful products have met with mixed success. Problems arise because of considerable metabolic redundancy that allows imposed genetic changes to be circumvented, because of an insufficiently detailed knowledge of the distribution of control of flux, and because the behaviour of plant metabolism at the network level is not well described. While increasingly complex metabolic networks are now being characterised with steady-state stable isotope labelling experiments (Libourel & Shachar-Hill2008, Schwender2008, Kruger & Ratcliffe2009) the resulting flux maps still only cover a small percentage of the total metabolic network. The construction of a comprehensive plant metabolic model that includes the complete repertoire of catalysed transformations represented within a specific genome (a genome-scale metabolic model) would represent a significant step forward in the development of a description of plant metabolic behaviour at the network level.
The aim of the current work is to describe a genome-scale structural model of Arabidopsis metabolism and to explore the utility of the model as a tool to characterise possible flux behaviour states of the metabolic network. Arabidopsis is the logical choice for this exercise because of its well annotated genome. Moreover, the translation of the Arabidopsis genome into a curated set of metabolic reactions is already well advanced and the reactions lists are available through the Aracyc database (Mueller et al.2003, Zhang et al.2005). While this is an excellent starting point for metabolic modelling, uncritical use of such reaction lists is likely to generate models exhibiting a number of problems, the most fundamental of which is violation of mass conservation. Thus, considerable effort is required to generate a useful genome scale metabolic model from these databases (Poolman et al.2006).
Once a stoichiometrically balanced structural model is achieved, the investigator is then faced with the challenge of how to analyse such a large model. We have previously introduced the concept of the reaction correlation coefficient (Poolman
et al.2007). Briefly, this is the value of Pearson’s correlation coefficient between a pair of fluxes over all possible steady-states of a system, and is calculated from the stoichiometry matrix. If a correlation matrix for all reactions is constructed, then a dendrogram (a metabolic tree) representing the relationship between fluxes in all reactions in the system can be generated. However, a potential drawback to this approach is that it does not take into account any information concerning flux values in a system, nor does it consider thermodynamic constraints. In this work we describe a refinement of the approach in which all reactions in a model of the system are assigned flux values, respecting reversibility criteria, on the basis of experimental observation and linear programming (LP).
Linear programming is an established method to explore flux states of large metabolic networks (Kauffman et al.2003, Price et al.2003), most commonly in
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microbes (Schilling et al.2002, Reed & Palsson2003) but more recently including primary metabolism in barley seeds (Grafahrend-Belau et al.2009). It is based on the optimisation of an objective function, subject to set of given constraints. Here the objective function is the minimisation of total reaction flux, and the constraints defined by the steady-state assumption and the biomass composition of a heterotrophic Arabidopsis thaliana cell suspension culture. This is, in essence, very similar to the commonly used objective of maximising efficiency of biomass production by (e.g.) Edwards & Palsson (2000) and Reed & Palsson (2003), with the difference that instead of pre-computing a vector in the direction of biomass production, we simply specify that all biomass components are produced at a defined rate. As demonstrated later, the solutions thus found are maximally efficient, at least with regard to glucose utilisation.
In this contribution, the properties of the solution space are then investigated by varying the demand for ATP in the model, and correlation coefficients between fluxes that vary as a response are used to generate a dendrogram, allowing the identification of groups of reactions with a common, or similar, response to a given metabolic demand. This approach also allows the identification of a minimal set of reactions needed for growth, and the indirect estimation of the ATP requirement for growth and maintenance.
The choice of a heterotrophic cell suspension system is a pragmatic one: the cell suspension system allows ready measurement of relevant constraints (growth rate, biomass composition) and avoids the problem of multiple cell types with different metabolic behaviours. Moreover, the Arabidopsis cell culture we use here (May & Leaver1993) has proved to be a useful model for the general molecular-biochemical behaviour of heterotrophic plant cells. As well as being used to analyze metabolic responses (Baxter et al.2007), the cell culture has been used as an experimental system for a range of different investigations including organellar proteomics (Lee
et al.2008) transcriptomic responses (Desikan et al.2001) and programmed cell death (McCabe & Leaver2000). Moreover, central aspects of metabolic behaviour are conserved between the cell culture and heterotrophic plant tissues such as roots (Lehmann et al.2009) although specific differences between differentiated cells types are likely and one should be cautious about generalising based on the analysis of a single cell type.
Results
Results obtained from the cell culture investigation were used to define the constraints used in the LP analysis of the metabolic model. We first present this experimental data, then the general properties of the model, and finally the results obtained by using the experimental observations to analyse the model.
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Experimental
Approximately 40% of glucose consumed was converted into biomass, a figure comparable to that found in other plant systems (e.g. in developing sunflower seeds, Alonso et al. (2007)).
The composition of the biomass in table I reveals that the bulk of biomass is comprised of cellulose and protein with minor contributions from lipid, starch and nucleic acids. Our measurements account for 65% of the total biomass. It is likely that the missing fraction consists of soluble metabolites and salts that have not been quantified (Williams et al.2008). The biomass composition is almost identical to independent determinations on the same cell culture suggesting that there is little variability in biomass composition of this cell culture. The rate of consumption of substrates and production of biomass were linear over the first 96h of cell growth (data not shown), suggesting that the cells were in a metabolic steady state over this period.
[Table 1 about here.]
Modelling
General model properties
The final model consisted of a total 1253 metabolites and 1406 reactions as summarised in table II.
[Table 2 about here.]
All internal metabolites were involved in at least two reactions, and thus capable of being balanced at steady-state. Examination of the null-space of the model showed no zero row-vectors, the presence of such a row indicating that the corresponding reaction is incapable of carrying steady-state flux (Heinrich & Schuster1996). We have recently described a more robust method of identifying these ‘dead’ reactions, based on consideration of reaction correlation coefficients (Poolman et al.2007), and the application of this showed that in fact 77 reactions were dead. This does not of itself indicate an error in the model, but shows that those reactions are not capable of contributing to biomass production from the nutrients specified here.
Linear programming results
Linear programming solutions showed that all biomass precursors could be synthesised in realistic proportions from glucose, nitrate and/or ammonia, phosphate and sulphate, and that the system was able to accommodate arbitrary changes in
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ATP demand.
232 of the 1406 available reactions in the model were assigned non-zero flux values. The fact that only a relatively small proportion of reactions were utilised springs from the fact that a) we are only exploring a limited subset of the organism’s biosynthetic potential and b) the LP objective of minimising total flux in the system also tends to minimise the total number of reactions employed.
[Table 3 about here.]
Responses to changing ATP demand
[Figure 1 about here.]
The results reveal that only 42 reaction fluxes (including those of ATPase and the mitochondrial equivalence reactions) vary in response to changing ATP demand with the remaining 185 maintaining constant flux. The varying flux values were used to construct a correlation tree, as described in the model analysis section of the methods, and presented in Fig. 1.
Three main sub-trees are discernible in Fig. 1, representing groups of reactions with distinct common responses to changes in ATP demand, and these fall, approximately, into three recognisable areas of central metabolism: reversible pentose phosphate reactions, glycolysis and the TCA cycle, and oxidative pentose phosphate metabolism.
In addition to these, and correlating most closely with oxidative pentose phosphate metabolism, are the reactions of Ru5Pk and rubisco carboxylase reaction, (the oxidase reaction of rubisco was also found in the solution, but maintained a fixed flux), and the NAD and NADP dependent variants of Icosanoyl-CoA synthase. This pair of reactions act together as a net transhydrogenase, producing NADPH. They form a reaction subset, and there is no production or consumption of other metabolites by these reactions. There are a number of other pairs of reactions capable of providing this function and the fact that these were ‘selected’ by LP is not thought to be of particular significance (ie that this particular pair of reactions was selected could be regarded as and artefact of the algorithm - as long as at least one pair of reactions providing a net transhydrogenase activity is available, other results would be unaffected.)
[Figure 2 about here.]
The reactions exhibiting variable flux form a single connected component as shown in Fig. 2. Although the reactions associated with photorespiration carry fixed flux in these results, the initial and final metabolites of this component reside within the block of variable reactions, and so have been included in this figure.
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[Figure 3 about here.]
Typical responses of the variable reactions to changing ATP demand are shown in Fig. 3. These also show the levels of ATP demand at which one or more reactions start or stop carrying flux as shown in table III.
It can be seen that these transitions occur at only two points, which results in the system existing in one of three states, corresponding to low, medium and high ATP demand. Inflections in the flux response curves only occur at these transitions. Furthermore, only a small number of reactions become active or inactive at transitions. At the first transition one reaction is inactivated and three are activated, at the second transition three reactions are inactivated and one reaction is activated. Rubisco carboxylase can be seen to be active only at the lowest of ATP demands, falling to zero flux at the first transition. Ru5Pk follows a parallel course but remains constant after the first transition, this being equal to the flux of the rubisco oxygenase reaction.
Flux in the oxidative limb of the oxidative pentose phosphate pathway and the Icosanoyl-CoA synthase reactions, while being more closely correlated with
Ru5Pk/rubisco exhibit subtly different behaviour. Flux falls steeply until the first transition, and then more gently until the second transition, at which point it becomes constant. The oxidative pentose phosphate pathway flux becomes zero at this point while the Icosanoyl-CoA synthase reactions do not, indicating a constant transhydrogenase flux.
Reversible pentose phosphate reactions all exhibit the same trajectory, remaining constant until the first transition, increasing steeply until the second transition, becoming constant thereafter.
Fluxes in glycolysis/TCA cycle also show very similar responses, remaining constant (in some cases zero) until the first transition and increasing linearly with ATP demand after that. An exception to this is the generic NADH oxidase: although flux across this reaction is more closely correlated with glycolysis/TCA fluxes, it remains at zero until the second transition, and then increases linearly. After the second transition, all of these fluxes increase linearly in response to further increases in ATP demand.
In addition to supplying energy to the rest of metabolism, this variable block of reactions is also the carbon source for all biomass precursors. A number of carbon compounds are also recycled back into this set of reactions, as detailed in table IV. [Table 4 about here.]
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Calculation of ATP requirements
In the absence of any additional ATP demand, the LP calculates that total ATP requirement for biomass precursors is 8.7 × 10-3 mol (g DW)-1. Varying ATP demand in the LP (using a bisection search) to determine the level at which glucose uptake in the solution matches the experimentally observed glucose uptake, results in an a figure for ATPase flux of 7.9 × 10-3 mol.(L h)-1, approximately 4 times higher the greatest ATPase flux used in the results shown in Fig. 3. Although it is not possible to achieve separate estimates of growth and maintenance demand from the present data, if we assume that the growth demand is 65 × 10-3 mol.(g DW)-1 (see discussion), and that growth is linear over the observed time, the maintenance demand can be estimated as 7.1 × 10-3 mol.(g DW h)-1.
Discussion
Analysis of the model identifies a core of metabolism required for synthesis of the main biomass components (ie all monomeric precursors). Only a relatively small subset of reactions (227 out of 1406) are required to realise this function, and can be regarded as a (possibly non-unique) minimal core of metabolism.
Although, the model did not include separate compartments for plastid and cytosol (see methods), the scheme shown in Fig. 2 is consistent with a standard view of metabolic compartmentation between plastid and cytosol, given the presence of transporters for hexose and triose phosphate and 2PG in the plastid membrane (Neuhaus & Wagner2000, Neuhaus & Emes2000). That is, reactions shown in green and blue are those present in the plastid, while those in red (between G6P and PEP) are present in the cytosol. Indeed, there is no reason to assume that the glycolytic reactions between G6P and PGA could not be occurring simultaneously in both the plastidic and cytosolic compartments. Consequently the lack of of a separate plastidic compartment can be seen to be justifiable, for heterotrophic plant cells under the conditions described here, although this does not, of course, justify such an assumption under other conditions, and especially not autotrophic conditions.
Within the core reactions, only 42 have altered flux when the demand for ATP is varied, and only 4 of these undergo on-off transitions. While the identity of the reactions that vary is not entirely surprising (they are connected to processes such as glycolysis and TCA cycle that are known to be involved in ATP generation), the fact that the network can accommodate a large range of ATP synthesis rates with minimal disturbance to most fluxes demonstrates the inherent robustness of the metabolic network. Metabolism is known to be robust to gene deletions (Blank
et al.2005, Gerdes et al.2006, Behre et al.2008) and steady state isotope labelling experiments in plants have revealed an inherent robustness to both genetic intervention (Spielbauer et al.2006) and altered environment (Williams et al.2008). This work can now extend that observation of robustness from the tens of reactions
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amenable to quantification from isotope labelling experiments, to some 185 reactions of primary metabolism, and that this robustness is conferred by the structure of the network, independently of kinetic or genetic control (because these were not included in the model).
The choice of objective function to analyse was based on simplicity, and the fact that it minimised the number of a priori assumptions as to what the the cultured cells were “optimised for”. However, the model solutions can be seen to be operating at the maximum possible efficiency with respect to glucose consumption. At low ATP demand, 100% of the carbon in glucose is recovered as biomass, and as ATP demand increases the P/O ratio (of the whole model, not just the mitochondrion) asymptotically approaches a value of 2.7 (Fig. 4).
[Figure 4 about here.]
This compares with a proposed maximum P/O ratio of 2.58 calculated by
Brand (1994), whose calculations included some additional transport costs not considered here.
Coordination of flux in primary metabolism and a novel role for rubisco
A surprising aspect of the model was an active rubisco reaction in a non-photosynthetic cell suspension. A function for rubisco in recycling CO2 during lipid synthesis has been shown in certain oilseeds (Schwender et al.2004), and the reaction scheme described by these authors between G6P and PGA is almost identical to the low energy demand solutions described here. Here, in the absence of photosynthesis, rubisco is active in two contexts. First, at low ATP demands (before the first transition), the carboxylase reaction of rubisco operates without the Calvin cycle. In this state, the demand for reducing equivalents exceeds the demand for ATP, and this is satisfied by rubisco, acting in conjunction with G6Pdh, lactonase,
6PGdh and Ru5Pk to catalyse the net oxidation of G6P to two molecules of PGA, reducing two NADP at the expense of one ATP (reactions shown in green in Figs. 1 – 3).
After the first transition (note that up to this point glucose consumption remains constant - Fig. 3), more carbon is routed into glycolysis and the TCA cycle (reactions shown in red in Figs. 1 – 3) producing concomitantly more reducing equivalents, the contribution from oxidative reactions above is diminished, and some ATP is saved by no longer operating the rubisco carboxylase reaction. It is at this point that
CO2 export starts to increase (not shown).
At the second transition, demand for ATP and reducing equivalents are exactly balanced, and once ATP demand exceeds this point excess NADH is simply oxidised by the NADOxid reaction, which, until this point carries no flux (Fig. 3). It can also be
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seen that there is a point of inflection in the glucose uptake curve at this point, indicating that in this state the yield of ATP/unit glucose is slightly lower than previously.
Over all these states, the rubisco oxidase reaction carries a constant flux, and variation in demand for Ru5P is accommodated by variation in the fluxes of the reversible reactions of pentose phosphate metabolism (blue reactions in Figs. 1 – 3). In these results the fate of 2PG is the generation of αKG via glycine aminotransferase, the reactions in the TCA cycle from malate (see the black and solid red reactions in Fig. 2) and the photorespiratory reactions. These are ultimately responsible for supplying ≈ 33 % of the αKG demand, with the remainder of the demand being made up with relatively minor and approximately equal fluxes from 13 other reactions involved in amino acid metabolism. The only reaction consuming
αKG is glutamate dehydrogenase, and thus the role for photorespiration in these results is to support NH3 assimilation.
It is relevant to note that both Rubisco and photorespiratory enzymes have been identified in proteomic studies of heterotrophic Arabidopsis cells (Baerenfaller
et al.2008) and Miguet & Sweetlove, (unpublished data). However, in the absence of further experimental study it would be somewhat ambitious to propose that the behaviour described here is an exact mirror of the in vivo reality. This is especially so given that the fluxes in the rubisco carboxylase and oxygenase cannot be independent of one another, and the fact that at high ATP demand there is higher production of CO2 and consumption of O2, which would be expected to favour the carboxylase over the oxygenase reaction.
Nonetheless, these results show that fluxes in a reaction network have the potential to respond to changes in demand in surprisingly subtle and elegant ways, and that these responses can only be identified when the network is considered as a whole, rather than a collection of semi-independent and somewhat arbitrarily defined
‘pathways’. The concept of groups of reactions acting in concert as metabolic
‘modules’ is an attractive one, but there is little agreement as to their constitution or identification - see the discussion in Poolman et al. (2007). It could be argued that the groups of reactions identified here as having a common response also constitute metabolic modules, and that by taking variation in fluxes into account these modules are a closer realisation of the biological function than those methods that rely on a purely structural analysis of the network.
Energy cost of cell maintenance
Another feature of the flux distribution obtained here is that, in the absence of an additional ATP demand, the cell can generate sufficient ATP and reductant for complete biosynthesis of all the main biomass precursors with relatively low flux through glycolysis and without a complete oxidative TCA cycle (the reactions between αKG and fumarate carry no flux in the absence of an imposed ATP
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demand). However, it is known from isotope labelling experiments that in the same Arabidopsis cell suspension culture, glycolytic and TCA fluxes are relatively high and that a complete TCA cycle operates (Williams et al. 2008). In the model, introduction of an additional ATP demand leads to activation of the reactions between αKG and fumarate such that a full TCA cycle flux operates and the fluxes of glycolysis and TCA cycle increase linearly, bringing the flux distribution into line with what was seen experimentally (Williams et al. 2008)
This implies that a relatively small proportion of the actual ATP consumed by the cell is used to generate biosynthetic precursors, and that the combined ATP requirement for polymerisation and maintenance is relatively high.
Conventionally, biological ATP demand is assumed to be distributed between growth and maintenance demands (
Stephanopoulos et al. 1998):
(1) where r ATP is total ATP requirement(mol.(g DW h)-1) , YxATP is ATP required to generate biomass (mol.(g DW)-1), μ is the specific growth rate (h -1), and m ATP the maintenance requirement (mol.(g DW h)-1). Estimates of these values have been previously reported by a number of workers as summarised in table V . Using the mean value for YxATP in table V of 65 × 10-3 mol.(g DW)-1, the estimated value of m ATP as 7.1 × 10-3 mol.(g DW h)-1 compares favourably with the mean value of 6.3 × 10-3 mol.(g DW h)-1 for m ATP reported in table V .
[Table 5 about here.]
Minimal metabolic network
There is considerable interest in the identification of the minimal set of cellular
components required to sustain life (e.g. Fraser et al. 1995, Mushegian &
Koonin 1996, Gil et al. 2004, Glass et al. 2006, Forster & Church 2006). Such work is centered on the analysis of prokaryotic organisms, in particular Mycoplasma
genitalium which has the smallest known genome, and the size of which is thus assumed to place an upper limit on that of the minimal genome. Furthermore, such work tends to pay relatively scant attention to metabolism, and is predicated on organisms growing in a nutrient-rich environment. The details of techniques used to determine these minimal genomes are beyond the scope of the current work (see Gil et al. (2004) for a review), but tend to rely either on gene disruption or the
identification of common genes between phylogenetically distant organisms with small genomes. Recently, Suthers et al. (2009) reported a metabolic reconstruction of M. genitalium (but without reference to minimality) which yielded a network of 380 reactions, 186 internal and 87 external metabolites. The relevant results are
summarised in table VI .
Although the goal of the current work is not primarily to identify a minimal network for A. thaliana, the objective function used, minimisation of total flux, will also tend minimise the number of reactions used, and so the solution obtained here sets an upper limit in the size of the minimal metabolic network. The size of the core A. thaliana network is larger than that of proposed minimal prokaryotic networks, although it is smaller than the average network size of 294 reactions required for growth reported in a constraint based analysis of a metabolic model E.coli (Reed & Palsson2004). This is to be expected given the greatly increased functionality of the A. thaliana network, but the size is still of a comparable order of magnitude to the minimal prokaryotic networks, and directly comparable to the whole M.
genitalium network. Furthermore, approximately 400 genes would be required to encode the A. thaliana core network, a number falling into the range of minimal genomes shown in table VI.
[Table 6 about here.]
However, despite these differences a certain amount of comparison with the current work is possible. The original description of M. genitalium (Fraser et al.1995) proposed 470 genes of which 64 were associated with metabolism. A subsequent comparative study of M. genitalium and Haemophilus influenzae (Mushegian & Koonin1996) suggested a hypothetical minimal genome of 256 genes, to which 80 were ascribed metabolic function.
It is also noteworthy that the LP optimisation method employed here generated a demonstrably functional metabolic network, and that this was achieved without any a priori assumptions as to which reactions would be present in the solution. Although there is clearly scope to further investigate this aspect of the study, we propose that the metabolic network we have described is likely to be close to the minimal metabolic network for an organism with single carbon and nitrogen sources. Recently, a hand-built model of primary metabolism in barley seeds has been described (Grafahrend-Belau et al.2009) which also contains a similar number of reactions. However the metabolic requirements of the barley seed model and the methods of model construction and analysis were sufficiently different from those of the current model, to render detailed comparison of the two beyond the scope of this discussion.
Conclusion
Construction of a metabolic model of A. thaliana, based on reactions reported as being present on the basis of genome sequence data, and with scrupulous curation of reaction stoichiometries to ensure conservation of C, N, P and S, has resulted in a network demonstrably capable of reproducing, in at least a semi-quantitative fashion, the experimentally observed behaviour of a heterotrophic culture of A. thaliana in a
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minimal medium. Only about 15 % of the total network was needed to achieve this, and both the size of the whole and minimal networks are comparable to those previously proposed for microbial metabolism.
A novel approach to incorporating experimental data with model analysis has enabled the identification of a number of metabolic modules involved in the response to varying energy demand, which, although comprised of groups of reactions previously known to be associated on the basis of their biochemistry, suggest new ways in which their activity may be coordinated. This is especially the case for reactions involving pentose phosphate species. The same technique also provides a means estimating total cellular ATP requirement and this estimate is consistent with previously published values for microbial species.
A number of refinements to the present work suggest themselves. In particular the assignment of reaction reversibility is somewhat arbitrary, and the recent group assignment method of Jankowski et al. (2008) provides the potential to improve this aspect.
The models and techniques developed here are readily applicable to other situations and are hereby made freely available, (ie on acceptance of this ms) and it is hoped that these will prove to be useful resource for the wider community.
Materials and Methods
Experimental
Plant material
Cell suspensions of A. thaliana ecotype Landsberg erecta were maintained and subcultured as described elsewhere (Millar et al.2001). Briefly, 15 mL of a 168 h old, light-grown cell culture was transferred into 90 mL fresh growth medium and grown in the dark at 21°C for 96 h before cells were harvested for biomass measurements. Biomass Analysis
Growth rate of cell suspensions was determined as the weight of freeze-dried cells harvested at regular intervals during the 96 h growth cycle.
Cell wall was extracted by repeated washing of a known mass of ground lyophilized tissue with a mixture of phenol, acetic acid, and water in the ratio 2:1:2 (Sriram
et al.2006) The remaining insoluble material was washed with distilled water, freeze dried, and weighed.
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Starch content was determined by enzymatic digestion and spectrophotometric assay of the resultant glucose.
Lipids were extracted from a known mass of ground lyophilized tissue using hexane and isopropanol according to an established protocol (Hara & Radin1978, Mhaske
et al.2005). Solvent was removed by gentle heating and lipids quantified by weight.
Soluble protein extracted with phosphate buffered saline was quantified using the Bradford assay. The amino acid content of protein hydrolysates (6N HCl, 110°C) was determined by HPLC (Bruckner et al.1995), amino acids were derivitized with O-phthaldialdehyde, separated using a reverse phase C18 column, and quantified by fluorescence using standard curves.
Nucleic acids were extracted and quantified from lyophilized tissue using standard methods (for RNA, TRIzol followed by DNAse; for DNA phenol/chloroform) and quantified spectrophotometrically.
Glucose consumption was determined by measuring the glucose content of the growth medium at regular intervals using a spectrophotometric assay, as described by Sweetlove et al. (1996)
Model construction
Data sources
A model of A. thaliana metabolism was constructed from the Aracyc database (version 4.5) (Zhang et al. (2005), https://www.doczj.com/doc/258285247.html,/biocyc/), using the ScrumPy metabolic modelling package (Poolman2006) such that:
1. Wherever possible, reactions are taken directly from the Aracyc database.
2. All reactions are atomically balanced with respect to C, N, P and S.
3. All reactions are capable of carrying steady-state flux.
4. All metabolites are balancable (ie are both consumed and produced by at least
one flux-carrying reaction).
5. The model is capable of producing all amino acids, nucleotide bases, lipid
(assuming linoleate to be a “generic lipid”), starch and cellulose, using
glucose, NO3 (and/or NH4), SO4 and P i as sole input material.
ScrumPy has a modular model definition language (ie, a model can be defined as a nested set of independent sub-models) which is particularly convenient when data for a model is drawn from several different sources. The reactions taken from
Aracyc were included in a single module which was incorporated with a number of additional modules, described below, to produce the whole model.
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The Aracyc module
The list of all reactions involving small metabolites in Aracyc was extracted and subsequently modified as follows:
Reactions were the checked for atomic balance with respect to C, N, S and P. A small number were found to be unbalanced but most of these proved relatively easy to correct. For example an incorrect empirical formula for synapoyl alcohol resulted in a number of reactions of lignin syntheses becoming unbalanced,
‘UROPORIIIMETHYLTRANSA-RXN’ (2.1.1.107) was stoichiometrically incorrect with respect to adenosylmethionine. These were submitted to Aracyc and have been incorporated into the current Aracyc release (Pfeifen Zhang, personal communication). The small remaining number of unbalanced reactions are identified as such in Aracyc, and were omitted from the model.
The treatment of hydrogen and oxygen is more problematic: protons and water are frequently omitted from reaction stoichiometries and the atomic proportion of hydrogen in a compound depends upon its pK A and the intra-cellular pH. A total of 693 reaction were identified as being unbalanced with respect to H+, of which 374 were also unbalanced with respect to O. This was deemed to many to be practical to correct manually and reactions that were only unbalanced towards H+ or to H+ and
H2O were left unaltered, and H+ and H2O defined as external metabolites.
This may appear to be a dangerously laissez faire strategy, with the apparent potential to violate both the laws of conservation of mass and of energy (the latter because O imbalances might lead to the generation of oxidation potential effectively from nothing). It was therefore verified that solutions obtained from the LP analysis (below) had no mass imbalance not attributable to H+, or O, and that furthermore the overall solutions were not capable of synthesising ATP or NADH in the absence of an oxidisable carbon source. Consequently, although these assumptions are not desirable, there is no evidence of them leading to undue consequences in the results obtained.
As there is relatively little information concerning reaction reversibility in Aracyc, a conservative approach was taken whereby all reactions were initially assumed to be irreversible, and a subset of these were subsequently made reversible. The reversible list includes all isomerases and amino transferases. A number of reactions were also identified has having been defined in the non-physiological direction, for example phosphoglycerate kinase was defined in the anabolic, not the catabolic direction.
A number of reactions are reported as utilising NAD(P)H. These were replaced with pairs of reactions, one utilising NADH, the other NADPH.
Metabolites, and associated reactions, with ambiguous atomic composition were
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removed. These included generic reactants (e.g. ‘Carboxylates’) and those reported as having an ‘R’ group as part of their chemical formula (e.g. ALDOSE).
Finally all isostoichiometric reactions were removed, i.e. so that each unique
stoichiometry was represented exactly once in the model. Although it is debatable as to whether or not the presence of such reactions represents a biological error, it is certainly undesirable from a modelling point of view, as it increases the complexity of any analysis and subsequent interpretation, without generating any new results.
At the end of this process the Aracyc module was composed of 1231 metabolites and 1336 unique reactions.
The transport module
A fundamental consideration in the construction of any metabolic model is the
identification of those metabolites whose concentrations are to be assumed to be unaffected by the action of the system under investigation (source and sink
metabolites). These commonly include species freely available in the environment (e.g. oxygen), and are called the external metabolites, while all others are called internal . Following this logic, any reaction that interconverts internal and external metabolites is deemed to be a transporter.
However, it should be realised that, in this context, the terms internal and external do not necessarily refer to any particular physical location (eg cytosol or other sub-
cellular compartment) and that metabolites denoted as external in this sense may none the less be physically located in the cell. An important class of such compounds are polymeric species, which cannot be given a meaningful concentration, but which can be considered to be sources and/or sinks of their monomeric sub-units. Hence reactions involved with polymers are included in the transport module.
The stoichiometries of these reactions were re-defined so as to ensure that all
reactions involved with a given polymeric species are consistent in terms of the monomeric subunit(s). For example, “MALTODEXGLUCOSID-RXN” (Maltase) is recorded in Aracyc as having the stoichiometry:
This was replaced with:
whereas RXN0-5181 (3.2.1.1) is reported as having the stoichiometry
which was replaced with:
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i.e. glucose is assumed to be the monomeric subunit of 1-4-alpha-D-Glucan. Taken together the two new stoichiometries imply, correctly, that even in the absence of empirical formulae, that MALTOHEXAOSE must contain six times as many carbon atoms as ALPHA-GLUCOSE. This is in contrast to the uncorrected stoichiometries which implies that they would be the same. This is an important consideration as, if not corrected, it can lead to the possibility of sequences of reactions, using a
polymeric species as an intermediate, with a net stoichiometry that violates the law of mass conservation.
CO 2 and all internal metabolites assumed to be biomass components (point 5,
above) were assigned an external counterpart and an explicit transporter. The
advantage of this approach is that transport reactions in the model can be maintained separately from those automatically generated from the database, and individual rates of production and consumption of a given metabolite in the model can be
determined or assigned by the manipulation of a single transport step. Likewise, media components assumed to be the ultimate biomass precursors (Glc, NH 3, NO 3, P i and SO 4) were also assigned external counterparts and transporters.
Stoichiometries of transport reactions were defined in a consistent fashion such that negative flux indicates production (ie loss from the system) and positive flux indicates consumption of the external metabolite.
The mitochondrial module
Aracyc contains only very sparse information concerning protein location and
membrane transports. For this reason the model is not fully compartmented.
Although this certainly represents a significant approximation, subsequent results, do not suggest that it is an unreasonable one (see discussion).
The one exception to this approximation concerns mitochondrial metabolism, more specifically the TCA cycle, electron transport chain and, especially, oxidative
phosphorylation, which cannot be validly represented without a proper distinction between cytosolic and mitochondrial H +.
To this end, relevant mitochondrial metabolism was included in a separate module, and the equivalent reactions removed from the Aracyc module. These reactions were comprised of the TCA cycle, electron transport chain and oxidative phosphorylation. The substrates and products of these reactions were defined as separate species from their cytosolic counterparts, but “pseudo reactions” transporting metabolites between the mitochondria and cytosol were defined to allow mitochondrial species to take part in amphibolic reactions. Mitochondrial protons are treated as distinct from cytosolic, and in particular were defined as internal, thus imposing the requirement
that mitochondrial protons must balanced by reactions of the electron transport chain.
This module was examined as an independent model, and when using pyruvate and CO2 as sole carbon source and sink, had a single elementary mode, producing ATP with a P/O ratio of 2.5.
Other reactions
In addition to these, the following reactions were added:
1. For convenience, a generic fatty acid synthase, producing linoleate from
AcCoA, ATP and NADPH in stoichiometrically correct ratio (In order to avoid
including a large number of reactions that have a single net function, and
whose intermediates are not used elsewhere). All lipid in biomass was
assumed, in this model, to be in the form of linoleate.
2. dATP diphosphatase, needed for the synthesis of deoxy nucleotides (metacyc
reaction RXN0-384), missing from Aracyc.
3. A generic ATPase to allow investigation of the model’s response to changing
energy demand.
4. A generic NADH oxidase to balance NADH production.
Reaction reversibility
Once all these modules were assembled, LP was used to examine the capability of the model to produce each product in turn(a trivial modification of the LP described below). If the model was not capable of producing a particular product, all reactions were made temporarily reversible, and the LP re-solved. Reactions that had been previously irreversible and that subsequently carried negative flux were identified as candidates to be defined as reversible. On-line databases (nist -
https://www.doczj.com/doc/258285247.html,/enzyme_thermodynamics/enzyme_thermodynamics_data.html and brenda https://www.doczj.com/doc/258285247.html,/) were then examined to establish whether or not these candidates could reasonably made reversible. If such a candidate could not be made reversible, this was added as a constraint to the LP, and the process repeated until all products could be synthesised with a reasonable set of reversible reactions.
In reality no reaction is truly irreversible, and there may be a good argument to take the exact opposite approach: start with all reactions reversible, and eliminate only those carrying a completely unrealistic negative flux. Although attractive, this is not a terribly practical strategy to execute as it would require examination of many hundreds of reaction fluxes and respective thermodynamics, the latter being particular hard to determine for such large numbers of reactions. However, the end result of both strategies should be the same: a set of reactions capable of generating all products whilst respecting thermodynamic constraints.
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Model availability
The complete model, including the model representing the LP solutions described in the results, in ScrumPy and SBML format, is available as supplementary material. Model analysis
Model analysis was by LP as described below, using the Gnu Linear Programming Kit (GPLK https://www.doczj.com/doc/258285247.html,/software/glpk) and a Python module to act as an interface between GLPK and ScrumPy. For the purposes of LP, reversible reactions were split into irreversible forward and reverse components.
The linear program was defined with the objective of minimising the total flux of all reactions, given that the flux in some of these is fixed (assumed or based on observation). It should be noted that this is a purely computational objective, and should not be taken as an assumption of a biological objective. Rather it is a simply defined objective that allows investigation of the behaviour of the system under a range of different scenarios. Formally the linear program was defined:
(2)
Where v is the flux vector, N the stoichiometry matrix, reactions i..j are the transport steps, t is the vector of these rates, v a is the ATPase reaction and J a the (imposed) flux it carries. The first part defines the objective function, the second part imposes the steady-state constraint, the third part specifies that reactions i..j carry fixed flux, and the fourth part defines an imposed ATP demand.
This program was utilised in three ways: Firstly, elements in t corresponding to export of biomass precursors were set to -1 (in this model negative transport rate indicates export from the system), to ensure that the model represents a system capable producing all biomass precursors, secondly the same elements of t were set to the molar ratios derived from experimental observation and shown in table I, in order to verify that the system represented by the model was capable of generating biomass precursors in biologically realistic proportions. Finally, the second program was solved repeatedly with increasing flux set in the ATPase reaction, to investigate the response of the system to varying energy demand.
Secondary analysis
Here, we refine the metabolic tree approach by using the flux evaluations from the
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8字模型与飞镖模型模型1：角的8字模型 如图所示，AC 、BD 相交于点O ，连接AD 、BC ． 结论：∠A ＋∠D ＝∠B ＋∠C ． O D C B A 模型分析 证法一： ∵∠AOB 是△AOD 的外角，∴∠A ＋∠D ＝∠AOB ．∵∠AOB 是△BOC 的外角， ∴∠B ＋∠C ＝∠AOB ．∴∠A ＋∠D ＝∠B ＋∠C ． 证法二： ∵∠A ＋∠D ＋∠AOD ＝180°，∴∠A ＋∠D ＝180°－∠AOD ．∵∠B ＋∠C ＋∠BOC ＝180°， ∴∠B ＋∠C ＝180°－∠BOC ．又∵∠AOD ＝∠BOC ，∴∠A ＋∠D ＝∠B ＋∠C ． （1）因为这个图形像数字8，所以我们往往把这个模型称为8字模型． （2）8字模型往往在几何综合题目中推导角度时用到． 模型实例 观察下列图形，计算角度： （1）如图①，∠A ＋∠B ＋∠C ＋∠D ＋∠E ＝________； 图图① F D C B A E E B C D A 图③ 2 1O A B 图④ G F 12 A B E 解法一：利用角的8字模型．如图③，连接CD ．∵∠BOC 是△BOE 的外角， ∴∠B ＋∠E ＝∠BOC ．∵∠BOC 是△COD 的外角，∴∠1＋∠2＝∠BOC ． ∴∠B ＋∠E ＝∠1＋∠2．（角的8字模型），∴∠A ＋∠B ＋∠ACE ＋∠ADB ＋∠E ＝∠A ＋∠ACE ＋∠ADB ＋∠1＋∠2＝∠A ＋∠ACD ＋∠ADC ＝180°． 解法二：如图④，利用三角形外角和定理．∵∠1是△FCE 的外角，∴∠1＝∠C ＋∠E ．
∵∠2是△GBD 的外角，∴∠2＝∠B ＋∠D ． ∴∠A ＋∠B ＋∠C ＋∠D ＋∠E ＝∠A ＋∠1＋∠2＝180°． （2）如图②，∠A ＋∠B ＋∠C ＋∠D ＋∠E ＋∠F ＝________． 图② F D C B A E 312图⑤ P O Q A B F C D 图⑥ 2 1 E D C F O B A （2）解法一： 如图⑤，利用角的8字模型．∵∠AOP 是△AOB 的外角，∴∠A ＋∠B ＝∠AOP ． ∵∠AOP 是△OPQ 的外角，∴∠1＋∠3＝∠AOP ．∴∠A ＋∠B ＝∠1＋∠3．①（角的8字模型），同理可证：∠C ＋∠D ＝∠1＋∠2．② ，∠E ＋∠F ＝∠2＋∠3．③ 由①＋②＋③得：∠A ＋∠B ＋∠C ＋∠D ＋∠E ＋∠F ＝2（∠1＋∠2＋∠3）＝360°． 解法二：利用角的8字模型．如图⑥，连接DE ．∵∠AOE 是△AOB 的外角， ∴∠A ＋∠B ＝∠AOE ．∵∠AOE 是△OED 的外角，∴∠1＋∠2＝∠AOE ． ∴∠A ＋∠B ＝∠1＋∠2．（角的8字模型） ∴∠A ＋∠B ＋∠C ＋∠ADC ＋∠FEB ＋∠F ＝∠1＋∠2＋∠C ＋∠ADC ＋∠FEB ＋∠F ＝360°．（四边形内角和为360°） 练习： 1．（1）如图①，求：∠CAD ＋∠B ＋∠C ＋∠D ＋∠E ＝ ； 图 图① O O E E D D C C B B A A 解：如图，∵∠1=∠B+∠D ，∠2=∠C+∠CAD ， ∴∠CAD+∠B+∠C+∠D+∠E=∠1+∠2+∠E=180°． 故答案为：180° 解法二：
美国常青藤名校的由来 以哈佛、耶鲁为代表的“常青藤联盟”是美国大学中的佼佼者，在美国的3000多所大学中，“常青藤联盟”尽管只是其中的极少数，仍是许多美国学生梦想进入的高等学府。 常青藤盟校(lvy League)是由美国的8所大学和一所学院组成的一个大学联合会。它们是：马萨诸塞州的哈佛大学，康涅狄克州的耶鲁大学，纽约州的哥伦比亚大学，新泽西州的普林斯顿大学，罗德岛的布朗大学，纽约州的康奈尔大学，新罕布什尔州的达特茅斯学院和宾夕法尼亚州的宾夕法尼亚大学。这8所大学都是美国首屈一指的大学，历史悠久，治学严谨，许多著名的科学家、政界要人、商贾巨子都毕业于此。在美国，常青藤学院被作为顶尖名校的代名词。 常青藤盟校的说法来源于上世纪的50年代。上述学校早在19世纪末期就有社会及运动方面的竞赛，盟校的构想酝酿于1956年，各校订立运动竞赛规则时进而订立了常青藤盟校的规章，选出盟校校长、体育主任和一些行政主管，定期聚会讨论各校间共同的有关入学、财务、援助及行政方面的问题。早期的常青藤学院只有哈佛、耶鲁、哥伦比亚和普林斯顿4所大学。4的罗马数字为“IV”，加上一个词尾Y，就成了“IVY”，英文的意思就是常青藤，所以又称为常青藤盟校，后来这4所大学的联合会又扩展到8所，成为现在享有盛誉的常青藤盟校。 这些名校都有严格的入学标准，能够入校就读的学生，自然是品学兼优的好学生。学校很早就去各个高中挑选合适的人选，许多得到全国优秀学生奖并有各种特长的学生都是他们网罗的对象。不过学习成绩并不是学校录取的惟一因素，学生是否具有独立精神并且能否快速适应紧张而有压力的大一新生生活也是他们考虑的重要因素。学生的能力和特长是衡量学生综合素质的重要一关，高中老师的推荐信和评语对于学生的入学也起到重要的作用。学校财力雄厚，招生办公室可以完全根据考生本人的情况录取，而不必顾虑这个学生家庭支付学费的能力，许多家境贫困的优秀子弟因而受益。有钱人家的子女，即使家财万贯，也不能因此被录取。这也许就是常青藤学院历经数百年而保持“常青”的原因。 布朗大学(Brown University) 1754年由浸信会教友所创，现在是私立非教会大学，是全美第七个最古老大学。现有学生7000多人，其中研究生近1500人。 该校治学严谨、学风纯正，各科系的教学和科研素质都极好。学校有很多科研单位，如生物医学中心，计算机中心、地理科学中心、化学研究中心、材料研究实验室、Woods Hole 海洋地理研究所海洋生物实验室、Rhode 1s1and反应堆中心等等。设立研究生课程较多的系有应用数学系、生物和医学系、工程系等，其中数学系海外研究生占研究生名额一半以上。 布朗大学的古书及1800年之前的美国文物收藏十分有名。 哥伦比亚大学(Columbia University) 私立综合性大学，位于纽约市。该校前身是创于1754年的King’s College，独立战争期间一度关闭，1784年改名力哥伦比亚学院，1912年改用现名。
第四章景观模型制作 第一节主要工具的使用方法 —、主要切割材料工具的使用方法 （—）美术刀 美术刀是常用的切割工具，一般的模型材料（纸板，航模板等易切割的材料）都可使用它来进行切割，它能胜任模型制作过程中，从粗糙的加工到惊喜的刻划等工作，是一种简便，结实，有多种用途的刀具。美术刀的道具可以伸缩自如，随时更换刀片;在细部制作时，在塑料板上进行划线，也可切割纸板，聚苯乙烯板等。具体使用时，因根据实际要剪裁的材料来选择刀具，例如，在切割木材时，木材越薄越软，刀具的刀刃也应该越薄。厚的刀刃会使木材变形。 使用方法：先在材料商画好线，用直尺护住要留下的部分，左手按住尺子，要适当用力（保证裁切时尺子不会歪斜），右手捂住美术刀的把柄，先沿划线处用刀尖从划线起点用力划向终点，反复几次，直到要切割的材料被切开。 （二）勾刀 勾刀是切割切割厚度小于10mm的有机玻璃板，ABS工程塑料版及其他塑料板材料的主要工具，也可以在塑料板上做出条纹状机理效果，也是一种美工工具。 使用方法：首先在要裁切的材料上划线，左手用按住尺子，护住要留下的部分，右手握住勾刀把柄，用刀尖沿线轻轻划一下，然后再用力度适中地沿着刚才的划痕反复划几下，直至切割到材料厚度的三分之二左右，再用手轻轻一掰，将其折断，每次勾的深度为0.3mm 左右。 （三）剪刀 模型制作中最常用的有两种刀：一种是直刃剪刀，适于剪裁大中型的纸材，在制作粗模型和剪裁大面积圆形时尤为有用；另外一种是弧形剪刀，适于剪裁薄片状物品和各种带圆形的细部。 （四）钢锯 主要用来切割金属、木质材料和塑料板材。 使用方法：锯材时要注意，起锯的好坏直接影响锯口的质量。为了锯口的凭证和整齐，握住锯柄的手指，应当挤住锯条的侧面，使锯条始终保持在正确的位置上，然后起锯。施力时要轻，往返的过程要短。起锯角度稍小于15°，然后逐渐将锯弓改至水平方向，快钜断时，用力要轻，以免伤到手臂。 （五）线锯 主要用来加工线性不规则的零部件。线锯有金属和竹工架两种，它可以在各种板材上任意锯割弧形。竹工架的制作是选用厚度适中的竹板，在竹板两端钉上小钉，然后将小钉弯折成小勾，再在另一端装上松紧旋钮，将锯丝两头的眼挂在竹板两端即可使用。 使用方法：使用时，先将要割锯的材料上所画的弧线内侧用钻头钻出洞，再将锯丝的一头穿过洞挂在另一段的小钉上，按照所画弧线内侧1左右进行锯割，锯割方向是斜向上下。 二、辅助工具及其使用方法 （一）钻床 是用来给模型打孔的设备。无论是在景观模型、景观模型还是在展示模型中，都会有很多的零部件需要镂空效果时，必须先要打孔。钻孔时，主要是依靠钻头与工件之间的相对运动来完成这个过程的。在具体的钻孔过程中，只有钻头在旋转，而被钻物体是静止不动的。 钻床分台式和立式两种。台式钻床是一种可以放在工台上操作的小型钻床，小巧、灵活，使
O D C B A 图12图E A B C D E F D C B A O O 图12图E A B C D E D C B A H G E F D C B A 第一章 8字模型与飞镖模型 模型1 角的“8”字模型 如图所示，AB 、CD 相交于点O ， 连接AD 、BC 。 结论：∠A+∠D=∠B+∠C 。 模型分析 8字模型往往在几何综合 题目中推导角度时用到。 模型实例 观察下列图形，计算角度： （1）如图①，∠A+∠B+∠C+∠D+∠E= ； （2）如图②，∠A+∠B+∠C+∠D+∠E+∠F= 。 热搜精练 1．（1）如图①，求∠CAD+∠B+∠C+∠D+∠E= ； （2）如图②，求∠CAD+∠B+∠ACE+∠D+∠E= 。 2．如图，求∠A+∠B+∠C+∠D+∠E+∠F+∠G+∠H= 。
D C B A M D C B A O 135E F D C B A 105O O 120 D C B A 模型2 角的飞镖模型 如图所示，有结论： ∠D=∠A+∠B+∠C 。 模型分析 飞镖模型往往在几何综合 题目中推导角度时用到。 模型实例 如图，在四边形ABCD 中，AM 、CM 分别平分∠DAB 和∠DCB ，AM 与CM 交于M 。探究∠AMC 与∠B 、∠D 间的数量关系。 热搜精练 1．如图，求∠A+∠B+∠C+∠D+∠E+∠F= ； 2．如图，求∠A+∠B+∠C+∠D = 。
O D C B A O D C B A O C B A 模型3 边的“8”字模型 如图所示，AC 、BD 相交于点O ，连接AD 、BC 。 结论：AC+BD>AD+BC 。 模型实例 如图，四边形ABCD 的对角线AC 、BD 相交于点O 。 求证：（1）AB+BC+CD+AD>AC+BD ； （2）AB+BC+CD+AD<2AC+2BD. 模型4 边的飞镖模型 如图所示有结论： AB+AC>BD+CD 。
描写常青藤优美句段写常青藤作文散文句子 描写常青藤优美句段写常青藤作文散文句子第1段： 1.睁开朦胧的泪眼，我猛然发觉那株濒临枯萎的常春藤已然绿意青葱，虽然仍旧瘦小，却顽强挣扎，嫩绿的枝条攀附着窗格向着阳光奋力伸展。 2.常春藤是一种常见的植物，我家也种了两盆。可能它对于很多人来说都不足为奇，但是却给我留下了美好的印象。常春藤属于五加科常绿藤本灌木，翠绿的叶子就像火红的枫叶一样，是可爱的小金鱼的尾巴。常春藤的叶子的长约5厘米，小的则约有2厘米，但都是小巧玲珑的，十分可爱。叶子外圈是白色的，中间是翠绿的，好像有人在叶子上涂了一层白色的颜料。从叶子反面看，可以清清楚楚地看见那凸出来的，一根根淡绿色的茎。 3.渴望到森林里探险，清晨，薄薄的轻雾笼罩在树林里，抬头一看，依然是参天古木，绕着树干一直落到地上的常春藤，高高低低的灌木丛在小径旁张牙舞爪。 4.我们就像马蹄莲，永不分开，如青春的常春藤，紧紧缠绕。 5.我喜欢那里的情调，常春藤爬满了整个屋顶，门把手是旧的，但带着旧上海的味道，槐树花和梧桐树那样美到凋谢，这是我的上海，这是爱情的上海。 6.当我离别的时候，却没有你的身影；想轻轻地说声再见，已是人去楼空。顿时，失落和惆怅涌上心头，泪水也不觉悄悄滑落我伫立很久很久，凝望每一条小路，细数每一串脚印，寻找你
的微笑，倾听你的歌声――一阵风吹过，身旁的小树发出窸窸窣窣的声音，像在倾诉，似在安慰。小树长高了，还有它旁边的那棵常春藤，叶子依然翠绿翠绿，一如昨天。我心头不觉一动，哦，这棵常春藤陪伴我几个春秋，今天才惊讶于它的可爱，它的难舍，好似那便是我的生命。我蹲下身去。轻轻地挖起它的一个小芽，带着它回到了故乡，种在了我的窗前。 7.常春藤属于五加科常绿藤本灌木，翠绿的叶子就像火红的枫叶一样，是可爱的小金鱼的尾巴。常春藤的叶子的长约5厘米，小的则约有2厘米，但都是小巧玲珑的，十分可爱。叶子外圈是白色的，中间是翠绿的，好像有人在叶子上涂了一层白色的颜料。从叶子反面看，可以清清楚楚地看见那凸出来的，一根根淡绿色的茎。 8.常春藤是多么朴素，多么不引人注目，但是它的品质是多么的高尚，不畏寒冷。春天，它萌发出嫩绿的新叶；夏天，它郁郁葱葱；秋天，它在瑟瑟的秋风中跳起了欢快的舞蹈；冬天，它毫不畏惧呼呼作响的北风，和雪松做伴常春藤，我心中的绿色精灵。 9.可是对我而言，回头看到的只是雾茫茫的一片，就宛如窗前那株瘦弱的即将枯死的常春藤，毫无生机，早已失去希望。之所以叫常春藤，可能是因为它一年四季都像春天一样碧绿，充满了活力吧。也许，正是因为如此，我才喜欢上了这常春藤。而且，常春藤还有许多作用呢！知道吗？一盆常春藤能消灭8至10平
一、常青藤大学 目录 联盟概述 联盟成员 名称来历 常春藤联盟（The Ivy League）是指美国东北部八所院校组成的体育赛事联盟。这八所院校包括：布朗大学、哥伦比亚大学、康奈尔大学、达特茅斯学院、哈佛大学、宾夕法尼亚大学、普林斯顿大学及耶鲁大学。美国著名的体育联盟还有太平洋十二校联盟（Pacific 12 Conference）和大十联盟（Big Ten Conference）。常春藤联盟的体育水平在美国大学联合会中居中等偏下水平，远不如太平洋十校联盟和大十联盟。 联盟概述 常春藤盟校（Ivy League）指的是由美国东北部地区的八所大学组成的体育赛事联盟（参见NCAA词条）。它们全部是美国一流名校、也是美国产生最多罗德奖学金得主的大学联盟。此外，建校时间长，八所学校中的七所是在英国殖民时期建立的。 美国八所常春藤盟校都是私立大学，和公立大学一样，它们同时接受联邦政府资助和私人捐赠，用于学术研究。由于美国公立大学享有联邦政府的巨额拨款，私立大学的财政支出和研究经费要低于公立大学。 常青藤盟校的说法来源于上世纪的50年代。上述学校早在19世纪末期就有社会及运动方面的竞赛，盟校的构想酝酿于1956年，各校订立运动竞赛规则时进而订立了常青藤盟校的规章，选出盟校校长、体育主任和一些行政主管，定期聚会讨论各校间共同的有关入学、财务、援助及行政方面的问题。早期的常青藤学院只有哈佛、耶鲁、哥伦比亚和普林斯顿4所大学。4的罗马数字为"IV"，加上一个词尾Y，就成了"IVY"，英文的意思就是常青藤，所以又称为常青藤盟校，后来这4所大学的联合会又扩展到8所，成为如今享有盛誉的常青藤盟校。 这些名校都有严格的入学标准，能够入校就读的学生，必须是品学兼优的好学生。学校很早就去各个高中挑选合适的人选，许多得到全国优秀学生奖并有各种特长的学生都是他们网罗的对象。不过学习成绩并不是学校录取的惟一因素，学生是否具有独立精神并且能否快速适应紧张而有压力的大一新生生活也是他们考虑的重要因素。学生的能力和特长是衡量学生综合素质的重要一关，高中老师的推荐信和评语对于学生的入学也起到重要的作用。学校财力雄厚，招生办公室可以完全根据考生本人的情况录取，而不必顾虑这个学生家庭支付学费的能力，许多家境贫困的优秀子弟因而受益。有钱人家的子女，即使家财万贯，也不能因
幻想之旅角色模型制作流程 1.拿到原画后仔细分析角色设定细节，对不清楚的结构、材质细节及角色身高等问题与 原画作者沟通，确定对原画理解准确无误。 2.根据设定，收集材质纹理参考资料。 3.开始进行低模制作。 4.制作过程中注意根据要求严格控制面数（以MAX为例，使用Polygon Counter工具查 看模型面数）。 5.注意关节处的合理布线，充分考虑将来动画时的问题。如有疑问与动作组同事讨论咨 询。 6.由于使用法线贴图技术不能使用对称复制模型，可以直接复制模型，然后根据具体情 况进行移动、放缩、旋转来达到所需效果。 7.完成后，开始分UV。 分UV时应尽量充分利用空间，注意角色不同部位的主次，优先考虑主要部位的贴图（例如脸，前胸以及引人注意的特殊设计），为其安排充分的贴图面积。使用Relax Tool 工具确保UV的合理性避免出现贴图的严重拉伸及反向。 8.低模完成后进入法线贴图制作阶段。 现在我们制作法线贴图的方法基本上有三种分别是： a.在三维软件中直接制作高模，完成后将低模与高模对齐，然后使用软件工具生成法 线贴图。 b.将分好UV的低模Export成OBJ格式文件，导入ZBrush软件。在ZB中添加细节 制作成高模，然后使用Zmapper插件生成法线贴图。 c.在Photoshop中绘制纹理或图案灰度图，然后使用PS的法线贴图插件将灰度图生成 法线贴图。 （具体制作方法参见后面的制作实例） 建议在制作过程中根据实际情况的不同，三种方法结合使用提高工作效率。 9.法线贴图完成后，将其赋予模型，查看法线贴图的效果及一些细小的错误。 10.进入Photoshop,打开之前生成的法线贴图，根据其贴在模型上的效果对法线贴图进行 修整。（例如边缘的一些破损可以使用手指工具进行修补，或者在绿色通道中进行适当的绘制。如需加强某部分法线贴图的凹凸效果可复制该部分进行叠加可以起到加强
https://www.doczj.com/doc/258285247.html, 有意向申请美国大学的学生，大部分听过一个名字，常青藤大学联盟。那么美国常青藤大学盟校到底是怎么一回事，又是由哪些大大学组成的呢？下面为大家介绍一下美国常青藤大学联盟。 立思辰留学360介绍，常青藤盟校(lvy League)是由美国的七所大学和一所学院组成的一个大学联合会。它们是：马萨诸塞州的哈佛大学，康涅狄克州的耶鲁大学，纽约州的哥伦比亚大学，新泽西州的普林斯顿大学，罗德岛的布朗大学，纽约州的康奈尔大学，新罕布什尔州的达特茅斯学院和宾夕法尼亚州的宾夕法尼亚大学。这8所大学都是美国首屈一指的大学，历史悠久，治学严谨，许多著名的科学家、政界要人、商贾巨子都毕业于此。在美国，常青藤学院被作为顶尖名校的代名词。 常青藤由来 立思辰留学介绍，常青藤盟校的说法来源于上世纪的50年代。上述学校早在19世纪末期就有社会及运动方面的竞赛，盟校的构想酝酿于1956年，各校订立运动竞赛规则时进而订立了常青藤盟校的规章，选出盟校校长、体育主任和一些行政主管，定期聚会讨论各校间共同的有关入学、财务、援助及行政方面的问题。早期的常青藤学院只有哈佛、耶鲁、哥伦比亚和普林斯顿4所大学。4的罗马数字为“IV”，加上一个词尾Y，就成了“IVY”，英文的意思就是常青藤，所以又称为常青藤盟校，后来这4所大学的联合会又扩展到8所，成为现在享有盛誉的常青藤盟校。 这些名校都有严格的入学标准，能够入校就读的学生，自然是品学兼优的好学生。学校很早就去各个高中挑选合适的人选，许多得到全国优秀学生奖并有各种特长的学生都是他们网罗的对象。不过学习成绩并不是学校录取的惟一因素，学生是否具有独立精神并且能否快速适应紧张而有压力的大一新生生活也是他们考虑的重要因素。学生的能力和特长是衡量学生综合素质的重要一关，高中老师的推荐信和评语对于学生的入学也起到重要的作用。学校财力雄厚，招生办公室可以完全根据考生本人的情况录取，而不必顾虑这个学生家庭支付学费的能力，许多家境贫困的优秀子弟因而受益。有钱人家的子女，即使家财万贯，也不能因此被录取。这也许就是常青藤学院历经数百年而保持“常青”的原因。
动画精度模型制作与探究 Animation precision model manufacture and inquisition 前言 写作目的：三维动画的制作，首要是制作模型，模型的制作会直接影响到整个动画的最终效果。可以看出精度模型与动画的现状是随着电脑技术的不断发展而不断提高。动画模型走精度化只是时间问题，故精度模型需要研究和探索。 现实意义：动画需要精度模型，它会让动画画面更唯美和华丽。游戏需要精度模型，它会让角色更富个性和激情。广告需要精度模型，它会让物体更真实和吸引。场景需要精度模型，它会让空间更加开阔和雄伟。 研究问题的认识：做好精度模型并不是草草的用基础的初等模型进行加工和细化，对肌肉骨骼，纹理肌理，头发毛发，道具机械等的制作更是需要研究。在制作中对于层、蒙版和空间等概念的理解和深化，及模型拓扑知识与解剖学的链接。模型做的精，做的细，做的和理，还要做的艺术化。所以精度模型的制作与研究是很必要的。 论文的中心论点：对三维动画中精度模型的制作流程，操作方法，实践技巧，概念认知等方向进行论述。 本论 序言：本设计主要应用软件为Zbrsuh4.0。其中人物设计和故事背景都是以全面的讲述日本卡通人设的矩阵组合概念。从模型的基础模型包括整体无分隔方体建模法，Z球浮球及传统Z球建模法（对称模型制作。非对称模型制作），分肢体组合建模法（奇美拉，合成兽），shadow box 建模和机械建模探索。道具模型制作，纹理贴图制作，多次用到ZBURSH的插件，层概念，及笔刷运用技巧。目录： 1 角色构想与场景创作 一初步设计：角色特色，形态，衣装，个性矩阵取样及构想角色的背景 二角色愿望与欲望。材料采集。部件及相关资料收集 三整体构图和各种种类基本创作 2 基本模型拓扑探究和大体模型建制 3 精度模型大致建模方法 一整体无分隔方体建模法 二Z球浮球及传统Z球建模法（对称模型制作。非对称模型制作） 三分肢体组合建模法（奇美拉，合成兽） 四shadow box 建模探索和机械建模 4 制作过程体会与经验：精度细节表现和笔刷研究 5 解剖学，雕塑在数码建模的应用和体现（质量感。重量感。风感。飘逸感）
2019年美国常春藤八所名校排名享有盛名的常春藤盟校现在是什么情况呢？接下来就来为您介绍一下！以下常春藤盟校排名是根据2019年美国最佳大学进行的。接下来我们就来看看各个学校的状态以及真实生活。 完整的常春藤盟校名单包括耶鲁大学、哈佛大学、宾夕法尼亚大学、布朗大学、普林斯顿大学、哥伦比亚大学、达特茅斯学院和康奈尔大学。 同时我们也看看常春藤盟校是怎么样的？也许不是你所想的那样。 2019年Niche排名 3 录取率5% 美国高考分数范围1430-1600 财政援助：“学校选择美国最优秀的学生，想要他们来学校读书。如果你被录取，哈佛会确保你能读得起。如果你选择不去入学的话，那一定不是因为经济方面的原因。”---哈佛大三学生2019年Niche排名 4 录取率6% 美国高考分数范围1420-1600 学生宿舍：“不可思议！忘记那些其他学校的学生宿舍吧。在耶鲁，你可以住在一个豪华套房，它更像是一个公寓。一个公寓有许多人一起住，包括一个公共休息室、洗手间和多个卧室。我再不能要求任何更好的条件了。这个套房很大，很干净，还时常翻修。因为学校的宿舍深受大家喜爱，现在有90%的学生都住在学校！”---耶鲁大二学生
2019年Niche排名 5 录取率7% 美国高考分数范围1400-1590 综合体验：“跟任何其他学校一样，普林斯顿大学有利有弊。这个学校最大的好处也是我选择这个学校的主要原因之一就是它的财政援助体系，任何学生想要完成的计划，它都会提供相应的财政支持。”---普林斯顿大二学生 2019年Niche排名 6 录取率9% 美国高考分数范围1380-1570 自我关心：“如果你喜欢城市的话，宾夕法尼亚大学是个不错的选择。这里对于独立的人来说也是一个好地方，因为在这里你必须学会自己发展。要确保进行一些心理健康的训练，因为这里的人通常会过量工作。如果你努力工作并且玩得很嗨，二者都会使你精疲力尽，所以给自己留出点儿时间休息。”---宾夕法尼亚大一学生 2019年Niche排名7 录取率7% 美国高考分数范围1410-1590 综合体验：“学校的每个人都很关心学生，包括我们的身体状况和学业成绩。在这里，你可以遇到来自世界各地的多种多样的学生。他们在学校进行的安全防范教育让我感觉受到保护。宿舍生活非常精彩，你会感觉跟室友们就像家人一样。总之，能成为学校的一员我觉得很棒，也倍感荣幸！”---哥伦比亚大二学生2019年Niche排名9 录取率9% 美国高考分数范围1370-1570 学术点评：“新的课程培养学术探索能力，在过去的两年中我
我国很多学子都想前往美国的常青藤大学就读于研究生，所以美国常青藤大学研究生申请条件都有哪些? 美国常青藤大学研究生申请条件： 1、高中或本科平均成绩(GPA)高于3.8分，通常最高分是4分，平均分越高越好; 2、学术能力评估测试I(SAT I，阅读+数学)高于1400分，学术能力评估测试II(SAT II，阅读+数学+写作)高于2000分; 3、托福考试成绩100分以上，雅思考试成绩不低于7分; 4、美内国研究生入学考试(GRE)成绩1400分以上，经企管理研究生入学考试(GMAT)成绩700分以上。 大学先修课程(AP)考试成绩并非申请美国大学所必需，但由于大学先修课程考试对于高中生来说有一定的挑战性及难度，美国大学也比较欢迎申请者提交大学先修课程考试的成绩，作为入学参考标准。
有艺术、体育、数学、社区服务等特长者优先考容虑。获得国际竞赛、辩论和科学奖等奖项者优先考虑，有过巴拿马国际发明大赛的得主被破例录取的例子。中国中学生在奥林匹克数、理、化、生物比赛中获奖也有很大帮助。 常春藤八所院校包括：哈佛大学、宾夕法尼亚大学、耶鲁大学、普林斯顿大学、哥伦比亚大学、达特茅斯学院、布朗大学及康奈尔大学。 新常春藤包括：加州大学洛杉矶分校、北卡罗来纳大学、埃默里大学、圣母大学、华盛顿大学圣路易斯分校、波士顿学院、塔夫茨大学、伦斯勒理工学院、卡内基梅隆大学、范德比尔特大学、弗吉尼亚大学、密歇根大学、肯阳学院、罗彻斯特大学、莱斯大学。 纽约大学、戴维森学院、科尔盖特大学、科尔比学院、瑞德大学、鲍登学院、富兰克林欧林工程学院、斯基德莫尔学院、玛卡莱斯特学院、克莱蒙特·麦肯纳学院联盟。 小常春藤包括：威廉姆斯学院、艾姆赫斯特学院、卫斯理大学、斯沃斯莫尔学院、明德学院、鲍登学院、科尔比学院、贝茨学院、汉密尔顿学院、哈弗福德学院等。
1 O D C B A 图1 2图E A B C D E F D C B A O O 图12图E A B C D E D C B A 第一章 8字模型与飞镖模型 模型1 角的“8”字模型 如图所示，AB 、CD 相交于点O ， 连接AD 、BC 。 结论：∠A+∠D=∠B+∠C 。 模型分析 8字模型往往在几何综合 题目中推导角度时用到。 模型实例 观察下列图形，计算角度： （1）如图①，∠A+∠B+∠C+∠D+∠E= ； （2）如图②，∠A+∠B+∠C+∠D+∠E+∠F= 。 热搜精练 1．（1）如图①，求∠CAD+∠B+∠C+∠D+∠E= ； （2）如图②，求∠CAD+∠B+∠ACE+∠D+∠E= 。
2 H G E F D C B A D C B A M D C B A O 135 E F D C B A 2．如图，求∠A+∠B+∠C+∠D+∠E+∠F+∠G+∠H= 。 模型2 角的飞镖模型 如图所示，有结论： ∠D=∠A+∠B+∠C 。 模型分析 飞镖模型往往在几何综合 题目中推导角度时用到。 模型实例 如图，在四边形ABCD 中，AM 、CM 分别平分∠DAB 和 ∠DCB ，AM 与CM 交于M 。探究∠AMC 与∠B 、∠D 间的数量关系。
3 105O O 120 D C B A O D C B A 热搜精练 1．如图，求∠A+∠B+∠C+∠D+∠E+∠F= ； 2．如图，求∠A+∠B+∠C+∠D = 。 模型3 边的“8”字模型 如图所示，AC 、BD 相交于点O ，连接AD 、BC 。 结论：AC+BD>AD+BC 。
2021中考数学易错题飞镖模型8字模型探究试题模型一：角的飞镖模型基础 结论：C + ∠ ∠ = ∠ B + A BDC∠ 解答： ①方法一：延长BD交AC于点E得证 ②方法二：延长CD交AB于点F得证 ③方法三：延长AD到在其延长方向上任取一点为点G得证 总结： ①利用三角形外角的性质证明
模型二：角的8字模型基础结论：D ∠ ∠ = + + C B A∠ ∠
解答： ①方法一：三角形内角和得证 ②方法二：三角形外角【BOD 】的性质得证总结： ①利用三角形内角和等于 180证明 推出 ②利用三角形外角的性质证明
角的飞镖模型和8字模型进阶 【例1】如图，则= ∠E D B A + C + + ∠ ∠ ∠ + ∠ 解答： ①方法一：飞镖ACD得证 ∠E + D C A B ∠ ∠ = 180 ∠ + + ∠ +
②方法二：8字BECD得证 + ∠ ∠E B A + C D ∠ = + 180 + ∠ ∠ 【例2】如图，则= E ∠F + D C A B ∠ ∠ ∠ + + ∠ ∠ + + 解答：飞镖ABF+飞镖DEC得证 ∠F + ∠ E D B + A C ∠ = ∠ + 210 ∠ ∠ + + 【例3】如图，求= E D ∠F B A + C ∠ + ∠ + ∠ ∠ + ∠ + 解答：8字模型得证 ∠F + ∠ E D A B C + 360 + = ∠ ∠ ∠ + ∠ + 【例4】如图，求= ∠D C A + B ∠ + ∠ + ∠
解答：连接BD得飞镖BAD+飞镖DBC得证 + ∠D A ∠ C B = + ∠ 220 + ∠ 【例5】如图，求= ∠H G ∠ F + D A C + E B + ∠ + ∠ ∠ + + ∠ + ∠ ∠ 解答：飞镖EHB+飞镖FAC得证 ∠H ∠ + + ∠ G F A B C D E ∠ + + = 360 ∠ ∠ ∠ + + ∠ + 模型三：边的飞镖模型基础 结论：CD + > AC BD AB+
□所需要的设备有：电脑，设计软件AutoCAD，雕刻机，工作台，油漆喷枪等。 □所需要的原材料有：各种厚度的有机玻璃板，各种厚度的PVC板，普通海绵，大孔海绵，背胶纸，各色绒线末，粗鱼线，铜丝电线，0.5mm漆包线，涂料，各色油漆，绒面墙纸，三氯甲烷，干花，发胶，小彩灯等。 □所需要的工具有：美工刀、锯条刀、木工工具、电工工具等。 一、沙盘台子 首先，要将顾客交付持房地产平面布置图和施工图纸研究透，组装部根据平面布置图及沙盘的比例来制作沙盘的台子。台子一般做成台球桌状，如果是大型的沙盘，要做成几个小台子，拼到一起。 二、PVC板喷漆 喷漆部根据楼房图纸的设色调出相应颜色的油漆来，喷在相应的PVC板上，送到设计部进行雕刻。 三、雕刻楼房部件 设计部根据施工图按比例设计出楼房的结构，并在电脑上分解成不同的板块，按施工的要求设计出墙面的花纹、房顶的瓦棱、窗子等，然后发送到雕刻机在PVC板上雕刻出楼房的板块，送到制作部制作。 四、组合楼房 制作部根据设计部送来的楼房板块，根据说明和粘合方式，用三氯甲烷将PVC板块粘合成楼房的大致形状。窗子的形状是直接雕刻在PVC板上的，用薄而透明的有机玻璃板粘在内部窗子的位置作为窗子的玻璃。 五、置景 置景部根据组装部所作的台子和平面布置图，在台子上划分出平面布局，用绿色绒面墙纸作为草地粘在绿化区，大孔海绵浸上绿色油漆晾干，裁成长条作为绿化带粘在小灌木区。如果布局中有水和湖泊，可以用波纹面的有机玻璃板，背面喷湖蓝色漆，裁成河流或湖泊的形状放在相应的位置。若是有高地，可将有机玻璃板或PVC板层层堆积并修整成形，再抹上涂料填充缝隙，晾干后覆上草地。用灰色的背胶纸粘成公路，用白色背胶纸刻成公路线标粘在上面。 六、制作配件 制作部将铜丝电线剥皮，将铜丝拧成树干的形状，喷上漆。普通海绵浸漆，晾干后粉碎，将树干的枝丫浸胶，粘上碎海绵，做成树。若是绿树，海绵可浸绿漆，若是秋天的树，可浸橙色漆。柳树可用0.2mm的漆包线拧成树干与树枝，然后在树枝上粘上绿色绒线末。松树是将粗鱼线剪成细段，用夹子夹住，再将两根0.5mm的漆包线夹住绞动，松开夹子，就成了松树的形状，修剪一下，粘上绿色绒线末即可。其它的花草可以用干花剪下来染色来制作。用医用棉签或牙签做成路灯。泡沫塑料可以用刀片雕刻成假山石的形状，喷上漆。 七、整体组合 置景部将制作部送来的花草树木及楼房按布置粘在相应的地方。组装部根据每栋楼房所在的位置，打孔并装上小彩灯，使楼房模型内部能发光，如同开灯的效果，并接好线路。
留学美国常春藤八大院校 美国常春藤声誉： 几乎所有的常春藤盟校都以苛刻的入学标准著称，近年来尤其如此：在过去的10多年里常春藤盟校的录取率正在下降。很多学校还在 特别的领域内拥有极大的学术声誉，例如： 哥伦比亚大学的法学院、商学院、医学院和新闻学院； 康乃尔大学的酒店管理学院和工程学院； 达特茅斯学院的塔克商学院（TuckSchoolofBusiness）； 哈佛大学的商学院、法学院、医学院、教育学院和肯尼迪政府学院； 宾夕法尼亚大学的沃顿商学院、医学院、护理学院、法学院和教 育学院； 普林斯顿大学的伍德鲁·威尔逊公共与国际事务学院； 耶鲁大学的法学院、艺术学院、音乐学院和医学院； 美国常春藤八大名校【哈佛大学】 哈佛大学（HarvardUniversity）是一所位于美国马萨诸塞州波 士顿剑桥城的私立大学，常春藤盟校成员之一，1636年由马萨诸塞州 殖民地立法机关立案成立。 该机构在1639年3月13日以一名毕业于英格兰剑桥大学的牧师 约翰·哈佛之名，命名为哈佛学院，1780年哈佛学院更名为哈佛大学。直到19世纪，创建了一个半世纪的哈佛学院仍然以英国的牛津大学、 剑桥大学两所大学为模式，以培养牧师、律师和官员为目标，注重人 文学科，学生不能自由选择课程。19世纪初，高等教育课程改革的号
角在哈佛吹响了，崇尚“学术自由”和“讲学自由”。“固定的学年”和“固定的课”的老框框受到冲击，自由选修课程的制度逐渐兴起。 哈佛大学是一所在世界上享有顶尖大学声誉、财富和影响力的学校， 被誉为美国政府的思想库，其商学院案例教学也盛名远播。作为全美 的大学之一，在世界各研究机构的排行榜中，也经常名列世界大学第 一位。 美国常春藤八大名校【耶鲁大学】 耶鲁大学（YaleUniversity）是一所坐落于美国康涅狄格州纽黑 文（纽黑文市CityofNewHaven）的私立大学，创于1701年，初名“大学学院”（CollegiateSchool）。 耶鲁起初是一所教会学校，1718年，英国东印度公司高层官员伊莱休·耶鲁先生向这所教会学校捐赠了9捆总价值562英镑12先令的 货物、417本书以及英王乔治一世的肖像和纹章，在当时对襁褓之中的耶鲁简直是雪中送炭。为了感谢耶鲁先生的捐赠，学校正式更名为 “耶鲁学院”，它就是今日耶鲁大学的前身。18世纪30年代至80年代，耶鲁在伯克利主教、斯泰尔斯牧师、波特校长等的不懈努力下， 逐渐由学院发展为大学。至20世纪初，随着美国教育的迅猛发展，耶 鲁大学已经发展到了惊人的规模，在世界的影响力也达到了新的高度。耶鲁大学是美国历建立的第三所大学（第一所是哈佛大学，第二所是 威廉玛丽学院），该校教授阵容、学术创新、课程设置和场馆设施等 方面堪称一流，与哈佛大学、普林斯顿大学齐名，历年来共同角逐美 国大学和研究生院前三的位置。哈佛大学注重闻名于研究生教育，威 廉玛丽学院闻名于本科生教育，耶鲁则是双脚走路，都非常，在世界 大学排名中名列前茅。 美国常春藤八大名校【宾夕法尼亚】 宾夕法尼亚大学(UniversityofPennsylvania)是一所私立大学， 是在美国开国元勋本杰明·富兰克林的倡导下于1740年建立起来的。 它是美国东北部常春藤大学之一，坐落于合众国的摇篮——费城，独
建筑制作项目流程 1、制作前期策划 根据甲方提供的平面图、立面图、效果图及模型要求，制定模型制作风格。 2、模型报价预算 预算员根据[1]、模型比例大小、材料工艺及图纸深度确定模型收费、签订制作服务订单。 3、制作组织会审 技术人员将核对分析图纸，确定模型材质、处理工艺、制作工期及效果要求。 (1)建筑制作进程： 建筑制作师根据甲方提供的图纸施工制作，效果以真实、美观为原则。所有建筑均采用AutoCAD绘图，电脑雕刻机切割细部、建筑技师手工粘接的流水线作业法，既保证了各部件的质量又保证了工期。 (2)环境景观设计制作进程： 总体环境将由专业景观设计师进行把控。专业制作人员结合图纸进行设计制作。原则是根据甲方的设计图纸再现设计师的设计意图。切不可胡乱操作，自由发挥。同时使用仿真树木、小品、雕塑等进行点缀，使得整个景观部分美观精致。 (3)建筑环境灯光组装： 灯光系统根据甲方要求进行设计制作，体现沙盘的夜景效果。 4、制作完工检验 质检部经理及项目负责人对照图纸，进行细部检查和调整。 5、模型安装调试 模型服务人员在模型展示地现场调试安装清洁，达到甲方满意后离开。 编辑本段 建筑模型分类 黏土模型 黏土材料来源广泛取材方便价格低廉经过“洗泥”工序和“炼熟过程其质地更加细腻。黏土具有一定的粘合性可塑性极强在塑造过程中可以反复修改任意调整修刮填，补比较方便。还可以重复使用是一种比较理想的造型材料,但是如果黏土中的水分失去过多则容易使黏土模型出现收缩龟裂甚至产生断裂现象不利于长期保存。另外，在黏土模型表面上进行效果处理的方法也不是很多,黏土制作模型时一定要选用含沙量少，在使用前要反复加工，把泥和熟，使用起来才方便。一般作为雕塑、翻模用泥使用。 油泥模型 油泥是一种人造材料。凝固后极软，较软，坚硬。油泥可塑性强，黏性、韧性比黄泥（黏土模型）强。它在塑造时使用方便，成型过程中可随意雕塑、修整，成型后不易干裂，可反复使用。油泥价格较高，易于携带，制作一些小巧、异型和曲面较多的造型更为合适。一般像车类、船类造型用油泥极为方便。所以选用褐油泥作为油泥的最外层是很明智的选择。油泥的材料主要成分有滑石粉62%、凡士林30%、工业用蜡8%。 石膏模型 石膏价格经济，方便使用加工，用于陶瓷、塑料、模型制作等方面。石膏质地细腻，成型后易于表面装饰加工的修补，易于长期保存，适用于制作各种要求的模型，便于陈列展示。 塑料模型 塑料是一种常用制作模型的新材料。塑料品种很多，主要品种有五十多种，制作模型应
美国常青藤名校的由来
美国常青藤名校的由来 以哈佛、耶鲁为代表的“常青藤联盟”是美国大学中的佼佼者，在美国的3000多所大学中，“常青藤联盟”尽管只是其中的极少数，仍是许多美国学生梦想进入的高等学府。 常青藤盟校(lvy League)是由美国的8 所大学和一所学院组成的一个大学联合会。它们是：马萨诸塞州的哈佛大学，康涅狄克州的耶鲁大学，纽约州的哥伦比亚大学，新泽西州的普林斯顿大学，罗德岛的布朗大学，纽约州的康奈尔大学，新罕布什尔州的达特茅斯学院和宾夕法尼亚州的宾夕法尼亚大学。这8所大学都是美国首屈一指的大学，历史悠久，治学严谨，许多著名的科学家、政界要人、商贾巨子都毕业于此。在美国，常青藤学院被作为顶尖名校的代名词。 常青藤盟校的说法来源于上世纪的50年代。上述学校早在19世纪末期就有社会及运动方面的竞赛，盟校的构想酝酿于1956年，各校订立运动竞赛规则时进而订立了常青藤盟校 的规章，选出盟校校长、体育主任和一些行政主管，定期聚会讨论各校间共同的有关入学、财务、援助及行政方面的问题。早期的常青藤学院只有
哈佛、耶鲁、哥伦比亚和普林斯顿4所大学。4的罗马数字为“IV”，加上一个词尾Y，就成了“IVY”，英文的意思就是常青藤，所以又称为常青藤盟校，后来这4所大学的联合会又扩展到8所，成为现在享有盛誉的常青藤盟校。 这些名校都有严格的入学标准，能够入校就读的学生，自然是品学兼优的好学生。学校很早就去各个高中挑选合适的人选，许多得到全国优秀学生奖并有各种特长的学生都是他们网罗的对象。不过学习成绩并不是学校录取的惟一因素，学生是否具有独立精神并且能否快速适应紧张而有压力的大一新生生活也是他们考虑的重要因素。学生的能力和特长是衡量学生综合素质的重要一关，高中老师的推荐信和评语对于学生的入学也起到重要的作用。学校财力雄厚，招生办公室可以完全根据考生本人的情况录取，而不必顾虑这个学生家庭支付学费的能力，许多家境贫困的优秀子弟因而受益。有钱人家的子女，即使家财万贯，也不能因此被录取。这也许就是常青藤学院历经数百年而保持“常青”的原因。 布朗大学(Brown University)
课题： 5-4建筑模型制作步骤 教学目标：（结合岗位知识、能力、素质目标确定） 1.通过讲授建筑模型的具体制作步骤，让学生了解建筑模型制作过程中各环节所需要具备的能力，培养学生的模型制作技能。 教学重难点分析： 1.教学重点： （1）建筑模型制作步骤 （2）建筑模型制作技能点 2.教学难点： 通过课程讲解，加深对建筑模型成型特点、以及技术介绍，并让学生熟悉建筑模型分类及其作用，培养学生对建筑模型类别和作用的掌握。 教学过程： 1.导课： 由多媒体PPT展示建筑模型制作过程的图片，从建筑模型图片引入课程，全方位观察建筑模型的制作全过程，导入课堂新课程——建筑模型制作步骤。 2.教学内容： （1）建筑模型制作步骤： 1.绘制建筑模型的工艺图： 首先确定建筑模型的比例尺寸，然后按比例绘制出制作建筑模型所需要的平面图和立面图。 2.排料画线： 将制作模型的图纸码放在已经选好的板材上，仵图纸和板材之间夹一张复印纸，然后用双面胶条固定好图纸与板材的四角，用转印笔描出各个而板材的切割线。需要注意的是图纸在板材上的排列位置要计算好，这样可以节省板料。
3.加工镂空的部件 制作建筑模型时，有许多部位，比如门窗等是需要进行镂空工艺处理的。可先在相应的部件上用钻头钻好若干小孔，然后穿入钢丝，锯出所需要的形状。锯割时需要留出修整加工的余量。 4.精细加工部件 将切割好的材料部件，夹放在台钳上，根据大小和形状选择相宜的锉刀进行修整。外形相同的部件，或者是镂空花纹相同的部件，可以把若干块夹在一起，同时进行精细的修整加工，这样可以很容易地保证花纹的整齐。 5.部件的装饰 在各个立面黏结前，先将仿镜面幕墙及窗格子处理好，再进行黏结。 6.组合成型 将所有的立面修整完毕后，对照图纸精心地黏结。