BISC-220 Lecture 10 Notes

What are the two types of photophosphorylation?

Cyclic and noncyclic photophosphorylation.

What are the products of noncyclic photophosphorylation?

ATP and NADPH. Both are used in the light-independent reactions of photosynthesis. [Image: 01-noncyclic-photophosphorylation.png]

Under what conditions is noncyclic photophosphorylation common in plant cells?

When sufficient water is available and photosystem II is functioning.

Where does the Calvin cycle take place?

In the stroma of the chloroplast.

What are the three phases of the Calvin cycle?

Carbon fixation, reduction, and regeneration of RuBP. [Image: 02-calvin-cycle.png]

What happens during carbon fixation?

CO₂ combines with the five-carbon molecule RuBP, forming an unstable six-carbon intermediate that splits into two three-carbon molecules of 3-phosphoglycerate (3-PG).

Which enzyme catalyzes the first reaction of the Calvin cycle?

RuBisCo.

What happens when RuBisCo combines RuBP with CO₂?

An unstable six-carbon intermediate forms and splits into two molecules of 3-phosphoglycerate (3-PG).

Why must carbon fixation occur repeatedly?

Repeated fixation supplies enough 3-PG to make G3P for sugar synthesis and to regenerate RuBP so the cycle continues.

What happens during the reduction phase of the Calvin cycle?

3-phosphoglycerate is converted through redox reactions into glyceraldehyde-3-phosphate (G3P).

For each 3-phosphoglycerate molecule processed in the reduction phase, what is used or produced?

One ATP is used, one NADPH is oxidized, and one G3P is produced.

What is the role of G3P made by the Calvin cycle?

Some G3P leaves the cycle and can be used to make sugars such as glucose; some is used to regenerate RuBP.

How many G3P molecules are needed to make one glucose molecule?

Two G3P molecules.

What happens during the regeneration phase of the Calvin cycle?

Some G3P is rearranged to regenerate RuBP, using ATP. [Image: 02-calvin-cycle.png]

What is photorespiration?

A process that occurs when RuBisCo binds O₂ instead of CO₂, diverting materials from the Calvin cycle. [Image: 03-photorespiration.png]

When is RuBisCo more likely to bind O₂ and trigger photorespiration?

When the O₂ concentration is higher than the CO₂ concentration around the enzyme.

Why is photorespiration considered wasteful?

It uses energy from the light-dependent reactions but does not produce a useful Calvin-cycle intermediate; the cell also spends energy recovering the intermediates.

What are mycorrhizae?

Fungi that form symbiotic relationships with many plant species, commonly associated with plant roots beneath the soil.

How can mycorrhizae support plants?

They sequester carbon, and some of that carbon can be supplied to the plant to support light-independent photosynthesis.

What are stomata, and what do they allow?

Small pores, often on the underside of leaves, that allow gas exchange (including CO₂ and O₂) and water-vapor release through transpiration.

Where does the Calvin cycle occur in C₃ plants, and what consequence does this have?

It occurs in mesophyll cells. When stomata are open, CO₂ and O₂ can move through the leaf, so both the Calvin cycle and photorespiration can occur. [Image: 04-c3-pathway.png]

Do C₃ plants have a specialized mechanism to reduce photorespiration?

No spatial or temporal mechanism is described for C₃ plants in these notes. [Image: 04-c3-pathway.png]

What kind of mechanism do C₄ plants use to reduce photorespiration?

A spatial mechanism: initial CO₂ capture happens in mesophyll cells, while the Calvin cycle happens in bundle-sheath cells. [Image: 05-c4-pathway.png]

What enzyme initially fixes CO₂ in C₄ plants, and what does it make?

PEP carboxylase in mesophyll cells combines CO₂ with phosphoenolpyruvate (PEP) to form oxaloacetate.

How is CO₂ delivered to the Calvin cycle in C₄ plants?

Oxaloacetate is converted to malate and transported to bundle-sheath cells, where CO₂ is released for the Calvin cycle.

What happens to pyruvate produced from malate in C₄ plants?

It is recycled to help regenerate PEP.

What kind of mechanism do CAM plants use to reduce CO₂ loss?

A temporal mechanism: they take up CO₂ at night and use it in the Calvin cycle during the day. [Image: 06-cam-pathway.png]

What happens to CAM plant stomata at night and during the day?

They open at night to take up CO₂ and close during the day to limit water loss; stored CO₂ is then used in the Calvin cycle.

How do CAM plants initially capture CO₂ at night?

PEP carboxylase fixes CO₂ into organic acids (including malate), storing carbon for daytime use.

What is the key difference between the C₄ and CAM strategies?

C₄ separates initial CO₂ capture and the Calvin cycle by location (spatial separation); CAM separates them by time (temporal separation).

Noncyclic photophosphorylation

Calvin cycle

Photorespiration

C₃ pathway

C₄ pathway

CAM pathway