C4 Pathway - Plant bio

[lifeisgood]

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I tried searching for a thread on this but could not find one. Has anyone seen or heard of anything about the C4 pathway for plant photsynthesis on the DAT? It wants to park itself in my brain but the lot is full.

 

[lifeisgood]

Nevermind. I just studied it anyway.

 

[dat_student]

I tried searching for a thread on this but could not find one. Has anyone seen or heard of anything about the C4 pathway for plant photsynthesis on the DAT? It wants to park itself in my brain but the lot is full.

Most test prep books (Kaplan etc) only talk about the C3 pathway. I've only seen the C4 pathway in "AP Biology" books. I studied the C4 pathway but didn't see it on the DAT.

For those who want to know the difference between the two:

C3 PATHWAY
C3 carbon fixation is a metabolic pathway for carbon fixation in photosynthesis. This process converts carbon dioxide and ribulose bisphosphate (RuBP, a 5-carbon sugar) into 3-phosphoglycerate through the following reaction:
6 CO2 + 6 RuBP → 12 3-phosphoglycerate

This reaction occurs in all plants as the first step of the Calvin cycle. In C4 plants, carbon dioxide is drawn out of malate and into this reaction rather than directly from the air.

Plants that survive solely on C3 fixation (C3 plants) tend to thrive in areas where sunlight intensity is moderate, temperatures are moderate, carbon dioxide concentrations are around 200 ppm or higher, and ground water is plentiful. The C3 plants, originating during Mesozoic and Paleozoic era, predate the C4 plants and still represent approximately 95% of Earth's plant biomass.

The isotopic signature of C3 plants shows higher degree of 13C depletion than the C4 plants.


C4 PATHWAY:
C4 carbon fixation is a metabolic pathway found in some land plants (C4 plants). They have a competitive advantage over plants possessing the more common C3 carbon fixation pathway under conditions of drought, high temperatures and nitrogen limitation. The C4 plants possess a characteristic leaf anatomy. Their vascular bundles are surrounded by two rings of cells. The inner ring, called Bundle Sheath Cells, contain starch-rich chloroplasts lacking grana which differ from those in mesophyll cells present as the outer ring. Hence, the chloroplasts are called dimorphic. This peculiar anatomy is called Kranz Anatomy (Kranz-Crown/Halo). The C4 cycle allows for a spacial separation of carbon uptake from carbon fixation, thus allowing C4 plants to increase concentration of CO2 within their leaves. This increases the amount of photosynthesis and decreases the chances of photorespiration, a harmful process in which organic material and energy is lost from the plant due to high concentrations of oxygen. This was proved by two Australian scientists Hatch and Slack in 1966. Therefore, it is also called Hatch-Slack pathway. It is called "C4" because the product, oxaloacetate, contains four carbon atoms. It occurs in the mesophyll of the leaf, specifically in the mesophyll cells and the bundle sheath cells. The chemical equation is:
PEP carboxylase + PEP + CO2 → oxaloacetate

The product is usually converted to malate, a simple organic compound that gives up its CO2 to the Calvin cycle after being shipped off to bundle sheath cells surrounding a nearby vein. After losing the CO2, it becomes pyruvate, and can be phosphorylated into PEP at the cost of a phosphorus group and one ATP. It can then be reused in the above equation. Since every CO2 molecule has to be fixed twice, the C4 pathway is more energy consuming than the C3 pathway. The C3 pathway requires 18 ATP for the synthesis of one molecule of glucose while the C4 pathway requires 30 ATP. But since otherwise tropical plants lose more than half of photosynthetic carbon in photorespiration, the C4 pathway is an adaptive mechanism for minimizing the loss.

C4 carbon fixation has evolved on several occasions in different groups of plants, so is an example of convergent evolution. Plants which use C4 metabolism include sugarcane, maize, sorghum, Eleusine, Amaranthus, and ... .

C4 plants are known only since the Cenozoic and did not became common until the Miocene. Today they represent about 5% of Earth's plant biomass.

The isotopic signature of C4 plants shows lower degree of 13C depletion than the signature of C3 plants.