CO2 fixation, C3/C4 Plants, Chapter 20 (Biochem Final)

(Pi-triose phosphate antiport system) Pi required for what is moved into the stroma?

Pi required for photophosphorylation is moved into the stroma

Four Calvin cycle enzymes are subject to a special type of regulation by light. (Ribulose 5-phosphate kinase, fructose 1,6-bisphosphatase, sedoheptulose 1,7-bisphos- phatase, and) glyceraldehyde 3-phosphate dehydrogenase are activated by light-driven reduction of disulfide bonds between two residues of?

between two Cys residues of thioredoxin, (Ribulose 5-phosphate kinase, fructose 1,6-bisphosphatase, sedoheptulose 1,7-bisphosphatase, and glyceraldehyde 3-phosphate dehydrogenase) all are activated by light-driven reduction of a critical disulfide bond to a pair of sulfhydryls (-SH)

In C4 plants, the Calvin cycle occurring in

bundle-sheath cells (special cells around the leaf veins)

Oxaloacetate is then converted to malate, that can be transported in to the

bundle-sheath cells thru plasmodesmata (inside the bundle sheath, malate breaks down)

(carbon-fixation reaction) The first stage in the assimilation of CO2 into biomolecules is the carbon-fixation reaction: condensation of CO2 with a five-carbon acceptor? to form two molecules of?

condensation of CO2 with a five-carbon acceptor, ribulose 1,5-bisphosphate, to form two molecules of 3-phosphoglycerate

Triose phosphate isomerase then interconverts glyceraldehyde 3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). Most of the triose phosphate thus produced is used to regenerate ribulose 1,5-bisphosphate; the rest is, if stay in chloroplast?

converted to starch in the chloroplast and stored for later use

(G3P and DHAP) (Most of the triose phosphate thus produced is used to regenerate ribulose 1,5-bisphosphate) the rest if immediately exported to the cytosol and converted to

converted to sucrose for transport to growing regions of the plant

C3 plants are the most common and the most efficient at photosynthesis in what climates?

cool, wet climates

In other situations the glyceraldehyde 3-phosphate is converted to dihydroxyacetone phosphate, which leaves the chloroplast via a specific transporter and, in the cytosol, can be (2)

degraded glycolytically to provide energy (developing leaves), or used to form Fructose 1,6-bisphosphate then fructose 6-phosphate and hence sucrose

At nightfall, the Cys residues in the four enzymes are reoxidized to their disulfide forms, the enzymes are inactivated, and ATP is not expended in CO2 assimilation. Instead, starch synthesized and stored during the day- time is

degraded to fuel glycolysis at night

(A small fraction of the "extra" glyceraldehyde 3-phosphate may be used immediately as a source of energy, but most is converted to sucrose for transport or is stored in the chloroplast as starch) In the latter case, glyceraldehyde 3-phosphate condenses with what in the stroma?

dihydroxyacetone phosphate (DHAP)

In the latter case, glyceraldehyde 3-phosphate condenses with DHAP in the stroma to form? precursor of?

fructose 1,6-bisphosphate, a precursor of starch

Chemical energy in the form of ATP and NADH or NADPH generated by

generated by the light-dependent reactions of photosynthesis

Stromal isozymes of the glycolytic enzymes catalyze reduction of 3-phosphoglycerate to? each molecule reduced requires 2 molecules?

glyceraldehyde 3-phosphate (G3P); (also called triose phosphate) each molecule reduced requires one ATP and one NADPH, the net product of photosynthesis

Third stage of CO2 assimilation. The starting materials are (2)

glyceraldehyde 3-phosphate and dihydroxyacetone phosphate (Black dots represent the number of carbons in each compound)

Four enzymes of the Calvin cycle are activated indirectly by light and are inactive in the dark, so that hexose synthesis does not compete with

glycolysis—which is required to provide energy in the dark

C3 plants combat photorespiration how?

have no special features to combat photorespiration

(carbon-fixation reaction) The sixth molecule of triose phosphate (G3P), the net product of photosynthesis, can be used to make hexoses for (2)? sucrose? or starch for

hexoses for fuel and building materials, sucrose for transport to nonphotosynthetic tissues, or starch for storage.

(carbon-fixation reaction) The sixth molecule of triose phosphate (G3P), the net product of photosynthesis, can be used to make (3)

hexoses, sucrose, starch

C4 plants are most efficient at photosynthesis in what climates?

hot, sunny climates

CO2 is fixed to oxaloacetate occurs in the cytosol of leaf mesophyll cells for which the substrate is HCO3- , not CO2. The oxaloacetate thus formed is either reduced to? or converted to?

malate at the expense of NADPH, or converted to aspartate by transamination

Photosynthesis in the leaves of C4 plants involves two cell types

mesophyll and bundle-sheath cells

The cost of fixing three CO2 into one triose phosphate is how much ATP and NADPH?

nine ATP and six NADPH, which are provided by the light-dependent reactions of photosynthesis.

Stoichiometry of CO2 assimilation in the Calvin cycle. For every three CO2 molecules fixed, one molecule of what is produced and how much ATP and NADPH are consumed?

one molecule of triose phosphate (glyceraldehyde 3-phosphate) is produced and nine ATP and six NADPH are consumed

(C4 photosynthesis) First, atmospheric CO2 is fixed in the mesophyll cells to form a simple carbon organic acid

oxaloacetate

In plants that use aspartate as the CO2 carrier, aspartate arriving in bundle- sheath cells is transaminated to form

oxaloacetate and reduced to malate, then the CO2 is released

Stromal enzymes, including transketolase and aldolase, rearrange the carbon skeletons of triose phosphates, generating intermediates of three, four, five, six, and seven carbons and eventually yielding

pentose phosphates

Another process in plants that consumes O2 and produces CO2 and is driven by light. This process

photorespiration

Some plants can escape the worst effects of photorespiration. The C4 and CAM pathways are two adaptations that allow certain species to minimize

photorespiration

Biosynthesis occurs primarily in this membrane-bounded organelles unique to photosynthetic organisms, and the movement of intermediates between cellular compartments is an important aspect of metabolism.

plastids - any of a class of small organelles, such as chloroplasts, in the cytoplasm of plant cells, containing pigment or food

This process isn't without its energetic price: ATP must be expended to return the what from the bundle sheath cell?

pyruvate from the bundle sheath cell and get it ready to pick up another molecule of atmospheric CO2

In the bundle-sheath cells, malate is oxidized and decarboxylated to yield (3)

pyruvate, CO2, and NADPH by the action of malic enzyme (reducing NADP+)

For every molecule of triose phosphate removed from the chloroplast, one Pi is transported into the chloroplast, providing the ninth Pi, to be used in

regenerating ATP

Sucrose synthesis releases how many Pi molecules from the four triose phosphates required to make sucrose?

releases four Pi molecules

(Third stage of CO2 assimilation) The first reaction in the assimilation of CO2 into triose phosphates consumes ribulose 1,5-bisphosphate and, for continuous flow of CO2 into carbohydrate, ribulose 1,5-bisphosphate must be

ribulose 1,5-bisphosphate must be constantly regenerated. This is accomplished in a series of reactions

The pentose phosphates are converted to ribulose 5-phosphate, then phosphorylated to (complete the Calvin cycle)

ribulose 1,5-bisphosphate to complete the Calvin cycle

Rubisco is activated by covalent modification (carbamoylation of Lys201) catalyzed by rubisco activase and is inhibited by a natural transition-state analog, the concentration of this inhibitor rises in the (light/dark?)

rises in the dark and falls during daylight, Rubisco activity is dependent on light activation and dark inactivation.

Photorespiration, is a costly side reaction of photosynthesis, a result of the lack of specificity of the enzyme

rubisco

Photorespiration is a wasteful pathway that occurs when the Calvin cycle enzyme rubisco acts on what?

rubisco acts on oxygen rather than carbon dioxide, (take up oxygen in the light and give out some carbon dioxide, contrary to the general pattern of photosynthesis)

The CO2 (bundle- sheath cells) is then fixed by what and made into sugars via the

rubisco, Calvin cycle (exactly as in C3 photosynthesis)

C4 minimize photorespiration by

separating initial CO2 fixation and the Calvin cycle in space, performing these steps in different cell types

Crassulacean acid metabolism (CAM) plants minimize photorespiration and save water by

separating these steps in time, between night and day

(carbon-fixation reaction) Overall (1st + 2nd stage), three molecules of CO2 are fixed to three molecules of ribulose 1,5-bisphosphate to form

six molecules of glyceraldehyde 3-phosphate (18 carbons) in equilibrium with dihydroxyacetone phosphate.

(Once CO2 is fixed into 3-phosphoglycerate in the bundle-sheath cells, the other reactions of the Calvin cycle take place) Thus in C4 plants, mesophyll cells carry out CO2 assimilation by the C4 pathway and bundle-sheath cells synthesize (2)

starch and sucrose by the C3 pathway

The chloroplast stroma contains all the enzymes necessary to convert the triose phosphates produced by CO2 assimilation (glyceraldehyde 3-phosphate and dihydroxyacetone phosphate) to

starch, which is temporarily stored in the chloroplast as insoluble granules

Photorespiration, It uses up fixed carbon, wastes energy, and tends to happens when plants close their

stomata (leaf pores) to reduce water loss. High temperatures make it even worse.

The products of photosynthetic carbon assimilation are thus moved into the cytosol where they serve as a starting point for what biosynthesis?

sucrose biosynthesis

This antiporter simultaneously moves Pi into the chloroplast, where it is used in photophosphorylation, and moves triose phosphate into the cytosol, where it can be used to synthesize

sucrose, the form in which the fixed carbon is transported to distant plant tissues

Sucrose synthesis in the cytosol and starch synthesis in the chloroplast are the major pathways by which what from photosynthesis is "harvested"

the excess triose phosphate from photosynthesis is "harvested"

(CO2-assimilating reactions, the Calvin cycle, ATP and NADPH are used to reduce CO2 to triose phosphates) These reactions occur in three stages

the fixation reaction itself, catalyzed by rubisco; reduction of the resulting 3-phosphoglycerate to glyceraldehyde 3-phosphate; and regeneration of ribulose 1,5-bisphosphate from triose phosphates

C4 plants involves two cell types why? (mesophyll and bundle-sheath cells)

the light-dependent reactions and the Calvin cycle are physically separated

In C4 plants (the light-dependent reactions and the Calvin cycle are physically separated) with the light-dependent reactions occurring in the

the light-dependent reactions occurring in the mesophyll cells (spongy tissue in the middle of the leaf)

The 3-phosphoglycerate formed in stage 1 is converted to glyceraldehyde 3-phosphate in two steps that are essentially the reversal of the corresponding steps in glycolysis, with one exception

the nucleotide cofactor for the reduction of 1,3-bisphosphoglycerate is NADPH rather than NADH

(Third stage of CO2 assimilation) The product of the first assimilation reaction (3-phosphoglycerate) thus undergoes transformations that regenerate ribulose 1,5-bisphosphate. The intermediates in this pathway include how many carbon sugars?

three-, four-, five-, six-, and seven-carbon sugars

(C3 plant) The first step of the Calvin cycle is the fixation of carbon dioxide by rubisco, and plants that use only this "standard" mechanism of carbon fixation to produce?

three-carbon compound (3-Phosphoglycerate) the reaction produces

(carbon-fixation reaction) In the second stage, the 3-phosphoglycerate is reduced to what type of molecule?

triose phosphates (G3P: glyceraldehyde 3-phosphate and DHAP)

Photosynthesis in vascular plants takes place in chloroplasts. In the CO2-assimilating reactions (the Calvin cycle), ATP and NADPH are used to reduce CO2 to

triose phosphates.

Anabolic pathways

use chemical energy in the form of ATP and NADH or NADPH to synthesize cellular components from simple precursor molecules. Anabolic pathways are generally reductive rather than oxidative.

CAM plants are adapted to avoid what during photosynthesis?

water loss during photosynthesis so they are best in deserts.

Next, NADPH donates electrons in a reduction catalyzed by the chloroplast-specific isozyme, producing (2)

(chloroplast-specific isozyme of glyceraldehyde 3-phosphate dehydrogenase) producing glyceraldehyde 3-phosphate and Pi

In the first step of stage 2, the stromal (3-phosphoglycerate kinase) catalyzes the transfer of a phosphoryl group from ATP to 3-phosphoglycerate, yielding

1,3-bisphosphoglycerate

The high concentrations of NADPH and ATP in the chloroplast stroma allow this thermodynamically unfavorable pair of reactions to proceed in the direction of

1,3-bisphosphoglycerate

Glycolate pathway, converts two molecules of 2-phosphoglycolate to (2 molecule)

2-phosphoglycolate to a molecule of serine (three carbons) and a molecule of CO2

Rubisco condenses CO2 with ribulose 1,5-bisphosphate, forming an unstable hexose bisphosphate that splits into two molecules of

3-phosphoglycerate

An antiporter in the inner chloroplast membrane exchanges Pi from the cytosol for (2 molecules)

3-phosphoglycerate or dihydroxyacetone phosphate produced by CO2 assimilation in the stroma.

Oxidation of dihydroxyacetone phosphate in the cytosol generates (2 reducing equivalents )

ATP and NADH, thus moving ATP and reducing equivalents from the chloroplast to the cytosol.

Pi-triose phosphate antiport can transport 3-phosphoglycerate and acts in the shuttle for exporting (2) what from chloroplast?

ATP and reducing equivalents (The inner chloroplast membrane is impermeable to most phosphorylated compounds)

A "normal" plant—one that doesn't have photosynthetic adaptations to reduce photorespiration—is called a

C3 plant

3-phosphoglycerate is an early intermediate in photosynthesis. The many plants in which this three-carbon compound is the first intermediate are called

C3 plants (in contrast to the C4 plants)

Thus C4 plants need how many ATP molecules to assimilate one molecule of CO2, whereas C3 plants need?

C4 plants need five ATP molecules to assimilate one molecule of CO2, C3 plants need only three (nine per triose phosphate)

CO2 fixation (or carbon fixation), terms for the specific reaction in which

CO2 is incorporated (fixed) into a three-carbon organic compound (the triose phosphate) 3-phosphoglycerate

(C4 and CAM pathways) These pathways work by ensuring that Rubisco always encounters high concentrations of

CO2 making it unlikely to bind to O2

Source of ATP and NADPH. ATP and NADPH produced by the light reactions are essential substrates for the reduction of

CO2. The photosynthetic reactions that produce ATP and NADPH are accompanied by movement of protons (red) from the stroma into the thylakoid

The Pi-triose phosphate antiport system of the inner chloroplast membrane. This transporter facilitates the exchange of cytosolic Pi for stromal

Exchange of cytosolic Pi for stromal dihydroxyacetone phosphate. The inner chloroplast membrane is impermeable to most phosphorylated compounds.

(carbon-fixation reaction) In the third stage, (1) how many of the six molecules of triose phosphate (15 carbons) are used to regenerate (2) how many molecules of ribulose 1,5-bisphosphate (15 carbons), the starting material?

Five of the six molecules of triose phosphate (15 carbons) are used to regenerate three molecules of ribulose 1,5-bisphosphate

Glycolate is transported to the peroxisome where molecular oxygen further oxidizes it to glyoxylate. Product is hydrogen peroxide (H2O2). Glycolate pathway reverse photorespiration (rubisco grabs O2 rather than CO2) how?

Hydrogen peroxide, H2O2 is rapidly broken down by catalase to water and oxygen.

(C4 plant: CO2 to oxaloacetate) This step is carried out by a non-rubisco enzyme that has no tendency to bind O2

PEP carboxylase (has biotin)

The pathway of CO2 assimilation has a greater energy cost in C4 plants than in C3 plants. For each molecule of CO2 assimilated in the C4 pathway, a molecule of what must be regenerated from pyruvate (coming back from Bundle-sheath cell)? at the expense of?

PEP must be regenerated at the expense of two phosphoanhydride bonds in ATP