LECTURE 14- carbon fixation

what occurs during photosynthesis

1. chloroplasts sit in mesophyll cells

2. thylakoids membranes have lumen inside

3. thylakoid captures light energy. produces oxygen and high energy molecules for CO2 fixation

4. calvin cycle occurs in stroma

what happens during carbon fixation

1. light reactions produce ATP and NADPH: high energy molecules

2. CO2 is binded to carbon to make carbon molecule. Energy is required for binding

what is the carbon fixation process

1. 3 carbon molecule (CO2) + energy enter reaction

2. single 3-carbon molecule comes out

3. starts with 3 molecules of 5 carbons (15 total)

4. cycle churns 3 times to make one molecule of glyceraldehyde 3-phosphate

5. CO2 attaches to carbons to make three 6-carbon molecules (unstable)

6. 6 carbons split to make 6 molecules 3-phosphoglycerate molecules (three 6-carbon molecules = 18 total)

7. 3-phosphoglycerate transforms to 3 × 6  1,3 bisphosphoglycerate. requires energy and electrons from NADPH to make glyceraldehyde 3-phosphate (PGAL) molecules (six 3-carbon molecules)

8. PGAL removes from calvin cycle to make five 3-carbon molecules

9. changes from five 3-carbons to three 5-carbons using ATP

OVERALL: 3 carbon molecules synthesize into one 3-carbon molecules

what are the 3 stages of carbon fixation process

stage 1: fixation

stage 2: reduction

stage 3: regeneration of acceptor

whats the most abundant enzyme in the world

ribulose 1,5 bisphosphate (RuBP)

what are the 2 activities that ribulose 1,5 bisphosphate has

1. carboxylase activity

2. oxygenase activity

what occurs during carboxylase activity

CO2 and water are added to molecules to create two phosphoglycerate molecules (creates 6 C molecules)

CO2 is fixed to the C molecules and goes into the calvin cycle

what occurs during oxygenase activity

uses oxygen + energy, but doesn’t produce fixed carbon

RuBP has 5 atoms, and either binds CO2 or oxygen

what happens when RuBP binds oxygen (oxygenase activity)

splits molecule, doesn’t add CO2 to RuBP. ends with a 2-C molecule and 3-C molecule.

goes from 5C to 3C + 2C, and calvin cycle can’t use it

• Phosphoglycolate is removed to make glycolate

what is photorespiration

wasteful process of plants where oxygen is removed and costs ATP to keep the process going, but doesn’t cost any important compounds due to recycling

RuBP grabs oxygen instead of CO2 and reduces photosynthesis efficiency

what’s the process of photorespiration

1. RuBP uses oxygen to make phosphoglycerate and glycolate

2. glycolate isn’t used in chloroplasts, so it moves to the peroxisome

3. in peroxisome, glycolate → glyoxylate, and oxygen is used to produce hydrogen peroxide

4. glyoxylate → glycine, and moves to mitochrondria

5. glycine has some energy and transfers electrons to NADH to create serine

6. serine is transported back to peroxisome to become glycerate (process costs energy)

7. glycerate → phosphoglycerate (costs energy)

how do c$ plants reduce photorespiration

by using PEP carboxylase

• PEP binds to CO2 to form oxaloacetate

• PEP carboxylate doesn’t bind oxygen = more efficient in capturing CO2

• Oxaloacetate transforms into malate (costs energy)

• Malate is moved to different kinds of cells

why is high CO2 levels important in mesophyll cells

because CO2 is required to capture more CO2, and CO2 levels need to be high in the place where its captured

• oxygen levels also need to be high ; oxygen is required for CO2 to be present (but oxygen inhibits RuBP carbon fixation)

what is the PEP carboxylase process

process fixes carbon, but doesn’t use oxygen

1. CO2 → malate, and moved to different bundle sheath cells (oxygen not needed in process)

2. malate → pyruvate tha releases CO2

3. pyruvate takes CO2 from mesophyll cells and releases it to bundle sheath cells

4. high levels of CO2 are are made in bundle sheath, that doesn’t require high oxygen levels

why is oxygen not required in bundle sheath cells

because light reaction doesn’t occur in bundle sheaths = no oxygen needed

what does PEP carboxylase mean for C4 plants

RuBP becomes more efficient in the calvin cycle of C4 plants, and better at carbon fixation instead of respiration

• C4 plants are better carbon fixers

where does light reaction take place in the maize cell

mesophyll cells

where does carbon fixation take place in the maize cell

bundle-sheath cells