Calvin Cycle: Photosynthesis

Calvin Cycle

products of the light reactions (ATP & NADPH) are released into the stroma

Where does the Calvin Cycle take place?

stroma

Carbon Fixation

“fixes” carbon dioxide into a solid form by combining it with a five-carbon sugar to create a six-carbon sugar with RuBisCo

RuBisCo

ezyme in carbon fixation

Reduction

ATP & NADPH are used to turn low energy 3-PGA into high energy G3P that is then used to make glucose; must run twice to make 1 mol of glucose

glucose

stable, storable form of chemical energy

Regeneration

1 G3P molecule is produced to make a 6-carbon glucose; ATP is used to generate the other five G3P molecules into three RuBP molecules

RuBP

5 carbon sugar

Calvin Cycle

begins with carbon fixation, incorporating CO₂ into organic molecules; uses ATP and the reducing power of electrons that are carried by NADPH; regenerates its starting material (RuBP) with energy from ATP

C3 Photosynthesis 

carbon fixation produces 3-carbon molecule; accounts for majority of earth’s photosynthesis; driven by RuBisCo enzyme

RuBisCo challenges

slow; uses O₂ instead of CO₂

How does organism respond to RuBisCo being slow?

it is plentiful

How does organism respond to RuBisCo using O₂ instead of CO₂?

photorespiration

Photorespiration

process in plants where the enzyme RuBisCo mistakenly uses oxygen instead of carbon dioxide during photosynthesis; 20-50% yield penalty

What does RuBisCo bind to in low temperatures?

CO₂

What does RuBisCo bind to in high temperatures?

O₂

O₂ + RuBP

3-carbon sugar & 2-carbon sugar that is toxic to plants and useless

RuBisCo evolved in

an atmosphere without oxygen

C3 photosynthesis works in

cool, humid environments

Why does C3 photosynthesis only work in cool, humid environments?

photosynthesis produces O₂ that builds up inside leaf; stomata open and allow O₂ to escape the leaf but also H₂O evaporation; risk of dehydration in hotter, dry environments

C4 Photosynthesis

physically separate carbon fixation from Calvin cycle into different cells; CO₂ is fixed to 4-C intermediate (malate) with PEPC as enzyme in mesophyll cells; in bundle sheath cells, 4-C intermediate is split into CO₂ for Calvin Cycle and 3-C compound (pyruvate) to regenerate PEP

Mesophyll Cell has

chloroplasts

Mesophyll Cells in C4 Photosynthesis

CO₂ is fixed to form a 4-C intermediate, typically malate; PEPC catalyzes fixation reaction

PEPC

enzyme for C4 Photosynthesis; higher attraction to CO₂ than O₂ (better than RuBisCo)

Malate is actively pumped into

bundle sheath cell

Bundle Sheath Cells in C4 Photosynthesis

4-C intermediate is split into CO₂ for Calvin Cycle and a 3-C compound (pyruvate) ot regenerate PEP

C4 Photosynthesis Environment

warmer but some precipitation; between humid & dry and cold & hot

C3 photosynthesis environment

cool, humid

CAM Photosynthesis environment

dry

CAM Photosynthesis

separate the Calvin Cycle fomr the C₄cycle temporally; open stomata in the night and fix CO₂ using C4 pathway; closed stomata in day to prevent water loss