Photosynthetic Carbohydrate Synthesis - PEOI

13/09/2006 · How are carbohydrates synthesized in cells ..

Photosynthetic Carbohydrate Synthesis


Overview: This lesson will detail the biochemical mechanisms that are affected by herbicides which inhibit a plant’s ability to synthesize amino acids. The significance of amino acids and proteins will also be described. The herbicide, glyphosate (i.e. Roundup), will be studied at length, including the advances made by biotechnology.

Objectives:

At the completion of this lesson, students will be able to:

1. Explain the importance of amino acid biosynthesis in plant growth and development.
2. Understand that amino acid biosynthesis depends on a sub-group of proteins called enzymes.
3. Describe how a class of herbicides can inhibit the biosynthesis of aromatic amino acids.
4. Outline how plants can develop resistance to the herbicide family which inhibits the biosynthesis of aromatic amino acids.

Plants are especially versatile in handling carbohydrates, for several reasons

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Triose phosphates produced by the Calvin cycle inbright sunlight, as we have noted, may be stored temporarilyin the chloroplast as starch, or converted to sucroseand exported to nonphotosynthetic parts of theplant, or both. The balance between the two processesis tightly regulated, and both must be coordinated withthe rate of carbon fixation. Five-sixths of the triosephosphate formed in the Calvin cycle must be recycledto ribulose 1,5-bisphosphate; if more thanone-sixth of the triose phosphate is drawn out of thecycle to make sucrose and starch, the cycle will slow orstop. However, insufficient conversion of triose phosphateto starch or sucrose would tie up phosphate, leavinga chloroplast deficient in Pi, which is also essentialfor operation of the Calvin cycle.
The flow of triose phosphates into sucrose is regulatedby the activity of fructose 1,6-bisphosphatase(FBPase-1) and the enzyme that effectively reverses itsaction, PPi-dependent phosphofructokinase.These enzymes are therefore critical points fordetermining the fate of triose phosphates produced byphotosynthesis. Both enzymes are regulated by fructose2,6-bisphosphate (F2,6BP), which inhibits FBPase-1and stimulates PP-PFK-1. In vascular plants, the concentrationof F2,6BP varies inversely with the rate ofphotosynthesis (Fig. 20–26). Phosphofructokinase-2,responsible for F2,6BP synthesis, is inhibited by dihydroxyacetonephosphate or 3-phosphoglycerate andstimulated by fructose 6-phosphate and Pi. During activephotosynthesis, dihydroxyacetone phosphate isproduced and Pi is consumed, resulting in inhibition ofPFK-2 and lowered concentrations of F2,6BP. This favorsgreater flux of triose phosphate into fructose 6-phosphate formation and sucrose synthesis. With thisregulatory system, sucrose synthesis occurs when thelevel of triose phosphate produced by the Calvin cycleexceeds that needed to maintain the operation of thecycle.
The key regulatory enzyme in starch synthesis isADP-glucose pyrophosphorylase (Fig. 18–14); it isactivated by 3-phosphoglycerate (which accumulatesduring active photosynthesis) and inhibited by Pi (whichaccumulates when light-driven condensation of ADPand Pi slows). When sucrose synthesis slows, 3-phosphoglycerateformed by CO2 fixation accumulates, activatingthis enzyme and stimulating the synthesis ofstarch.

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FIGURE 18–2 (a) Amyloplasts filled with starch (dark granules) arestained with iodine in this section of Ranunculus root cells. Starchgranules in various tissues range from 1 to 100 μm in diameter.(b) Plastids: their origins and interconversions. All typesof plastids are bounded by a double membrane, and some (notablythe mature chloroplast) have extensive internal membranes. The internalmembranes can be lost (when a mature chloroplast becomes aproplastid) and resynthesized (as a proplastid gives rise to a pregranalplastid and then a mature chloroplast). Proplastids in nonphotosynthetictissues (such as root) give rise to amyloplasts, which containlarge quantities of starch. All plant cells have plastids, and these organellesare the site of other important processes, including the synthesisof essential amino acids, thiamine, pyridoxal phosphate, flavins,and vitamins A, C, E, and K.


Carbohydrates: Synthesis, Mechanisms, and …

Because the primary role of protein is as the building blocks for muscles, bone, skin, hair, and other tissues, relying on protein for energy (by failing to take in adequate carbohydrate) can limit your ability to build and maintain tissues. Additionally, this stresses the kidneys because they have to work harder to eliminate the byproducts of this protein breakdown.

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FIGURE 18–12 Starch synthesis. Starch synthesis proceeds by a twositeinsertion mechanism, with ADP-glucose as the initial glucosyldonor. The two identical active sites on starch synthase alternate indisplacing the growing chain from each other, and new glucosyl unitsare inserted at the reducing end of the growing chain.

Carbohydrates in diversity-oriented synthesis: …

Adequate carbohydrate intake also helps prevent protein from being used as energy. If the body doesn’t have enough carbohydrate, protein is broken down to make glucose for energy.

Schematic of photosynthesis in plants

FIGURE 18–13 Sucrose synthesis. Sucrose is synthesized fromUDPglucose and fructose 6-phosphate, which are synthesized from triosephosphates in the plant cell cytosol. The sucrose 6-phosphate synthase of most plantspecies is allosterically regulated by glucose 6-phosphate and Pi.