Bioenergetics 7: Photosynthesis

Question 1

What are the two reactions in photosynthesis and their products and the overall reaction of photosynthesis

Answer 1

The overall reaction is 6CO2 + 2H2O –> Light–> C6H12O6 + 6O2.

1. There light dependent reactions split water= O2, reduce NADP and generate the proton gradient.
2. Calvin cycle fixes CO2 from air to produces 1 Glyceraldehyde3Phosphate

Question 2

Explain the structure of the chloroplast and the

molecules involved in light absorption

Answer 2

Photosynthesis occurs in plants, bacteria and algae. In the leaves that have stomata, mesophyll cells that have chloroplasts with double membrane. Inside they have a thylakoid membranes in stacks called grana surrounded by stroma fluid.

Question 3

Where does the O2 produced from photosynthesis come from in the equation

Answer 3

the splitting of water/ not CO2

Question 4

Thylakoid membrane impermable to most ions but not

Answer 4

Mg and Cl

Question 5

How is ATP made from the light reactions

Answer 5

1. 2H+ from splitting water and the 4H+ pumped into the thylakoid space by the cytochrome complex develops a proton gradient.
2. This goes through an ATP synthase (monomer) with 14 C subunits in the rotor back to the stroma to make ATP

Question 6

Compare how efficient the chloroplast ATP synthase is with mitochondria

Answer 6

1. Is in monomer so doesn’t have cristaeing effect to make negative charges more dense
2. Has 14 subunits in rotor so requires more H+ to turn rotor
3. thylakoid membrane is permeable to Cl- and Mg+ so this equilibrates the charge and takes away the “electro” force (mp)
4. Chloroplasts are therefore more reliant on the pH gradient so need big pH difference

Question 7

What is a chromophore and how are they arranged in photosystems

Answer 7

Chromophore is a pigment molecule that absorbs visible (and near infrared) light. Within photosystems the chromophores are chlorophyll and these are arranged in Antennae.

Question 8

What happens when light hits a chlorophyll

Answer 8

Light hits chlorophyll and most of the spectrum is absorbed (esp Blue and red) but the green light is reflected. As the light hits the chlorophyll it excites the electron from its orbital, giving out infrared (heat and fluorescence - red light).

Question 9

Describe what happens at the first photosystem (PS2) up to the electron carrier

Answer 9

1. Light energy is absorbed by chlorophylls in PS2 causing them to resonate.
2. Within the photosystem there is transfer of energy between chlorophyls by Quantum tunnelling until chophyl a molecules in the reaction centre (P680)
3. Electrons in P680 reaction centre are excited to give to 1’ acceptor.
4. Reaction centre is now very strong oxidant so can take 2 e- from water (electron donor), producing O2
5. 1’ acceptor transfers e- to Plastoquinone (-> pqol)
6. Pq transfers e to cytochrome complex where through the Q cycle it pumps 4H+ into the thylokoid space from the stroma (indirectly ATP making)

Question 10

Describe what happens at the second photosystem (PS2) from Cyt c up to the electron carrier

Answer 10

1. Light excites chlorophylls in the PS1 and this causes them to resonate and transfer energy through quantum tunnelling
2. Energy reaches reaction center P700 chlorophyls. This causes them to excite and reduce the 1- acceptor
3. The reaction centre steals the e- from electron donor Plastocyanin. Plastocyanin got its electrons from the cytochrome complex.
4. The electrons from the 1’ acceptor get passed through Fe-S to Ferrodoxin and then to NADP+ reductase which reduces NADP+ to NADPH + H +

Question 11

Where does the light dependent reactions vs Calvin cycle take place and why

Answer 11

Light dependent reactions take place on the thylakoid membrane whereas the Calvin cycle takes place in the Stroma.

Question 12

What are the 3 Steps of the calvin cycle

Answer 12

1. Carbon fixation
2. Reduction
3. Regeneration

Question 13

Describe the fixation phase

Answer 13

1. 3CO2 + 3Ribulose biphosphate (c5) are added together by Rubisco enzyme to make 3 x C6 compound which is then split to 6 x C3 compound called PGA (phosphoglyceric acid)

Question 14

Describe the reduction phase

Answer 14

The 6 x 3-phosphoglycerate has a phosphate added to it from 6ATP. Then 6 electrons from the 6NADPH reduces PGA to make 6 G3P ! (NADPH oxidised to go to 6NADP+)

Question 15

Describe the regeneration phase

Answer 15

One of the 6 Gyceraldehyde3Phosphate is stolen and therefore there are 5 x 3C G3Ps left. This is turned back to 3 RB5P and then then 3 ATPs are used to rearrange into 3 C5 mulcules

Question 16

What are the inputs and outputs of the calvin cycle

Answer 16

input of 9 atp and 6 NADH and 3 CO2

Question 17

What is on the thylakoid membrane in the grana vs the stroma lamellae. Why

Answer 17

Grana : PS2 and sort of Cytochrome complex.
Stroma lamellae: PS1 and F ATPase. This is because cyclic electron flow happens at PS1 so it separates the two pathways- linear and cyclic

Question 18

What is cyclic electron flow. What is result

Answer 18

Electrons from PS1 can jump up and go to 1’ acceptor but then are passed to Fd to Pq which goes back to Cytochrome complex and then Plastocyanin to be donor again.
This pumps protons in the thylakoid space which ultimately produces ATP only.

Question 19

What are the cons of RuBisCo

Answer 19

It works very slow and it also binds O2 in the process photorespiration making useless compound- consuming O2 not Co2.
As temperature increases it becomes more error prone.

Why: Rubisco didn’t evolve with high O2 world, and therefore they don’t need high affinity carboxylase.

Question 20

What is photorespiration

Answer 20

When O2 is combined into the Calvin cycle and therefore not fixing CO2.

Question 21

What is difference between C3 and C4 plants

Answer 21

C3 plants needs a lot of water because it has problems with photorespiration. It has a thicker leaf and photosynthesis is happening in all layers so will produce O2 fast to compete with CO2.

C4 plants in hot climates can concentrate CO2 so less O2 around so doesn’t need as much water.
It has a thinner leaf with bundle sheath cells (where CC is) and mesophyll cells.

Question 22

What is the required input for the Calvin cycle to make 1 glucose

Answer 22

3CO2, 9ATP and 6NADPH for 1 G3P, and 2 G3P that makes glucose, so overall need 6CO2, 18ATP and 6NADPH

Question 23

What are ways that Calvin cycle is regulated by the Light reactions

Answer 23

1. CC works faster in an alkaline environment and the Light reactions make the pH in the stroma rise.
2. Ferrethioreductase stabilises RuBisCo through reducing and needs ATP and NADPH from light reactions to function

Question 24

How does C4 plant work

Answer 24

1. In mesophyll cell:
   C4 plant uses CO2 from outside, and adds it to PEP to make oxaloacetate.
2. Oxaloacetate is converted to Malate
3. Malate enters the Bundle sheath cell where it is dexcarboxylated back to pyruvate and CO2. CO2 enters the Calvin cycle.
4. The pyruvate is transported back to mesophyll cell and turned into PEP requiring investment of ATP