Chapter 6 // Pt3: Light Independent Reactions 

Light Independent Reactions

Light independent reactions take place in the stroma

Product: Sugars

• Makes sugars (glucose)

• Use CO₂ to make carbon backbone of sugar

• Carbon fixation - taking carbon atoms from inorganic molecules (CO₂) to attach them to organic molecules (sugar)

Light Independent Reactions (AKA Calvin Cycle)

1. 3 CO₂ molecules enter chloroplast. Enzyme rubisco attaches to CO₂ to ribulose biphosphate (RuBP). CO₂ and RuBP have an unstable reaction. Immediately splits into two separate molecules called phosphoglycerate (PGA)

2. PGA’s receive phosphate group from ATP. also receive Hydrogen ion and electrons from NADPH. Creates phosphoglyceraldehyde (PGAL).

   1. ADP and NADP+ are recycled back to stroma to be used in light-dependent reaction again

3. Six molecules of PGALs are made. Simple sugar product

4. Five molecules PGALs recycled. Regenerates more RuBP. restarts reaction (cyclic)

5. Remaining one molecule PGAL is exported from chloroplast to cell’s cytoplasm

6. In the cytoplasm PGAL can be converted to other molecules the plant needs. Assembled to various carbohydrates. Most combine to make sucrose - main sugar in plants.

What happens to the sugar?

• Sucrose is loaded into vascular tissues (veins of plant) and transported to other parts of the plant

Excess Sucrose

• Sunny days = lots of light dependent reactions running (light independent running too)

• Some PGAL left in chloroplasts - assembled into starch instead

• Starch disassembled at night to make sucrose

• Sustains plant metabolism and growth at night

Photorespiration

Not good for the plant

The Cuticle

Controls gas exchange

• Thin waterproof layer covering plant

• Keeps plant from losing water

• Also keeps gasses from moving in and out of the plant

So what about the O₂ that leaves and the CO₂ that enters?

• Stomata control gas exchange

  • Tiny closable pores

  • Open to let CO₂ in and O₂ out

• Close stomata to conserve water on hot and dry days = gas exchange stops

C3 Plants

• Fix carbon only by Calvin Cycle

• Both stages of photosynthesis run during the day

• Closed stomata - O₂ level in plant rises, CO₂ level declines

• Reduces efficiency of sugar production

• 85% of modern plants

Photorespiration in C3 Plants

• Rubisco initiates photorespiration by attaching O₂ to RuBP

• Produces CO₂ and ammonia

• Very inefficient way to make sugar

• C3 plants make a lot of rubisco

  • Most abundant protein on earth

CO₂ needs rubisco for Calvin Cycle but O₂ also likes rubisco and wants to steal it from CO₂ = photorespiration

Alternative Pathways in Plants

C4 plants

• Also close stomata on hot dry days

• But keeps making sugar

• Fix carbon twice in two different cells

• 3% of modern plants

  • Begin in mesophyll cells

  • Carbon fixed by enzyme that does not use oxygen - malate

  • Rubisco fixes carbon second time when enters Calvin Cycle in chloroplast

  • High CO₂ and low O₂ means no competition (minimizes photorespiration)

CAM Plants

• Also fix carbon twice

• Stomata open at night when water less likely to evaporate

• Desert plants - conserve water

• 12% of modern plants

Light dependent reaction = light reactions

Light independent reactions, Calvin Cycle = dark reactions