lecture 11 Photosynthesis: The Light-Independent Reactions

calvin cycle

the light-independent (dark) reactions of photosynthesis that take place in the storm of the chloroplast

Three phases:

1. Carbon Fixation

2. Reduction

3. Regeneration of RuBP

Carbon fixation

first phase of the Calvin Cycle; a single chemical reaction that produces two three-carbon molecules (3-PG) from 1 CO₂ molecule combining 1 five-carbon molecule (RuBP)

Ribulose-1,5,-bisphosphate (RuBP) combines with CO₂ to produce an unstable six-carbon intermediate that ultimately splits into two 3-phosphoglycerate molecules.

must take place minimum 3x to produce one G3P molecule used in glucose synthesis.

Reduction

the second phase of the Calvin Cycle; two redox chemical reactions that produce glyceraldehyde-3-phosphate (G3P)

required minimum 2 G3P molecules

• One G3P molecule is produced 

• One ATP molecule is used 

• One NADPH molecule is oxidized

Regeneration of RuBP

the third phase of the Calvin Cycle; two chemical reactions that utilize some of the G3Pproduced to reproduce RuBP (other molecules outside of the Calvin Cycle are used to produce glucose

requires minimum 2 G3P molecules to regenerate 1 RuBP molecule, allowing the cycle to continue

mycorrhizae

a class of fungi that forms symbiotic relationships with many plant species; found beneath the soil among plant roots

excellent at carbon sequestration, sequestering carbon from the atmosphere and soil, which is provided to the plant host to aid in light-independent reactions of photosynthesis

photorespiration

a process that occurs in plants when the enzyme RuBisCO oxygenates RuBP instead of carboxylating it, leading to a decrease in photosynthetic efficiency

wasteful because it used energy produced form the light-independent reactions

• the cell also has to expend energy to clean up the mess from photorespiration

transpiration

process of plants in exchanging gases (specifically, O₂ and CO₂), as well as release H₂O through small pores located on the underside of their leaves called stomata (plural of stoma

occurs in C3 plants; stroma will open if:

• High humidity 

• Heat (often a result of direct sunlight) 

• Low atmospheric CO₂ concentrations

C3 plants lack mechanisms for mitigating photorespiration

spatial and temporal mechanisms for reducing the likelihood of photorespiration occurring

since the Calvin Cycle in C3 plants takes place in leaf cells known as mesophyll, if the stomata of the leaves are open and gas exchange can occur, CO₂ and O₂ can enter/ exit the leaf, meaning that the Calvin Cycle and photorespiration are possible. 

C4 plants have a spatial mechanism to mitigate photorespiration

helps prevent CO₂ loss via stomata, which augments overall concentration of CO₂ for carbon fixation of the Calvin Cycle

the enzyme PEP carboxylase catalyze the reaction of producing oxaloacetate from phosphoenolpyruvate (PEP) and incoming CO₂

Oxaloacetate is then converted to malate and transported into another cell type, bundle sheath cells, where the Calvin Cycle can take place. 

Pyruvate is also produced from the catabolism of malate, and this pyruvate can then be recycled to reproduce PEP.

CAM plants employ temporal mechanism to mitigate photorespiration

generally desert/tropical plants; have a temporal mechanism to reduce CO₂ loss via stomata

• During daytime, the stomata are closed and CO₂ (accumulated over the course of the night) can be fixed in the Calvin Cycle. 

• During nighttime, the stomata are open and PEP carboxylase catalyzes the same reaction observed in the mesophyll cells of C4 plants to accumulate more CO₂ for carbon fixation during the day.