Lecture 19 and 20 - Photosynthesis

Question 1

What organisms contribute what % of global photosynthesis?

Answer 1

40% plants
60% algae/bacteria

Question 2

Overall photosynthesis equation

Answer 2

3CO₂ + 9ATP + 6NADPH + 6H⁺ ⇌ 3-phosphoglycerate + 9ADP + 6NADP + 9Pᵢ + 3H₂O

Question 3

Photosynthesis process

Answer 3

Light hits photosystem 2, knocking off some electrons which move down the ETC and eventually gives them to photosystem 1 (electron balance is restored by the hydrolysis of water)

Light hits photosystem 1 and as electrons are passed even more down an ETC, even more reducing power is generated which is used to synthesise ATP (electron balance is restored by photosystem 2)

Question 4

Plant cell structure: C, C, F, G, L, M, N, P, P, P, R, R, S, and V

Answer 4

Cell wall - protects the cell
Chloroplast - Convert sunlight into glucose
Filamentous cytoskeleton - made of MTs, actin, and IF, forming the cytoskeleton and cell shape
Golgi apparatus - protein sorting/processing
Leukoplast - Starch formation and storage
Mitochondria - generate ATP/energy from glucose
Nucleus - contains DNA
Peroxisome - H₂O₂ for detoxification/metabolism
Plasma membrane - controls what enters/exits
Plasmodesmata - small channels between cells
rER - synthesis of proteins for secretion
Ribosomes - protein synthesis
sER - storage/steroids storage/creation
Vacuole - Water storage that also allows turgidity

Question 5

Chloroplast structure

Answer 5

Nucleoid - DNA rings
Stroma - Liquid space
Thylakoids - Used to synthesise ATP
Grana - Stacks of thylakoids
Plastoglobulus - Lipid synthesis, thylakoid storage

Question 7

Thylakoid membrane

Answer 7

PSII - Starts the photosystem chain
PSI- Passes electrons through the ETC
Cytochrome bf - links both photosystems

Question 8

Photosystem 2

Answer 8

Enormous transmembrane protein (>20 subunits) which responds to wavelengths shorter than 680nm and has several antenna pigments which capture and transfer light until a special pair of chlorophyll molecules (which can be ionised - energy trap) at the reaction centre of the photosystem is reached

Requires light with shorter wavelengths (higher energy) than PSI
Also known as P680 as the maximum frequency (?) it can absorb is 680nm

Question 9

Chlorophyll a and b

Answer 9

Planar structure for easy ionisation, allows for easy electron delocalisation

A - better at absorbing lower wavelengths of light
B - better at absorbing higher wavelengths of light

Question 10

Transfer of electrons in PSII

Answer 10

P680 get excited -> pheophytin -> plastoquinone at fixed site (Qa) -> plastoquinone at mobile site (Qb)

Excited P680 extracts electrons from water bound at a manganese centre, forming O₂

Qb plastoquinone is reduced from plastoquinone (Q) into plastoquinol (QH₂)

Question 11

Oxygen evolution centre of PSII

Answer 11

Composed of 4 manganese atoms, 1 calcium atom, and oxygen atoms

Oxidised by one electron at a time

Attaches water molecules to the manganese and calcium atoms and combines them to form O₂

Question 12

Plastoquinone electron transfer

Answer 12

Plastoquinone (PQ) is reduced into plastoquinol (PQH₂) using hydrolysis of water

PQH₂ can diffuse through the membrane (into the middle) while carrying its two electrons

Once next to cytochrome bf, it gets oxidised and causes 2 protons to be released into the lumen

Question 13

Cytochrome bf electron transfer: what does it do, where does it pass its electrons to, and what is the overall equation?

Answer 13

Once it oxidises plastoquinol, the Q cycle causes two more protons to be released into the lumen

The electrons are then passed to plastocyanin (PC), the final electron carrier, along with the 4 protons (2 from plastoquinol and two from the stroma) being passed into the lumen

PQH₂ + 2PC(ox) + 2H⁺ → PQ + 2PC(red) + 2H⁺ + 2H⁺

Question 14

Plastocyanin: what is its structure, how does reduction work, and what is the equation?

Answer 14

Blue copper protein that is soluble and resides in the thylakoid lumen

Reduction occurs one electron at a time at the copper atom

PQH₂ + 2PC-Cu2⁺ → PQ + 2PC-Cu⁺

Question 15

PSI: what is its structure, how does it capture light, and what type of light does it capture?

Answer 15

Large transmembrane assembly (>100 cofactors with 14 polypeptide chains) with a core made of two subunits psaA and psaB (~82kDa each)

Antenna complex contains several chlorophylls
and carotenoids on two proteins, capture light
at different wavelengths, (<700 nm)

Special pair of chlorophyll a molecules in the reaction centre of P700, absorb light at 700 nm

Question 16

PSI electron transfer

Answer 16

P700* -> chlorophyll A₀ -> phylloquinone -> 3 iron-sulphur clusters -> ferredoxin

P700* then is restored to normal by receiving electrons from plastocyanin

Question 17

Fd-NADP⁺ reductase: what does it do and what is the mechanism behind the process?

Answer 17

Reduces NADP⁺ to NADPH

(H⁺ + Fdᵣₑ -> Fdₒₓ) occurs in two separate steps, the first step occurs and causes FAD to be turned into the semiquinone intermediate (FADH⁺) which is then turned into FADH₂ after the reaction occurs again

Question 18

The Z-scheme: what is it, what is the process behind it, and why is this mechanism needed?

Answer 18

The transfer of electrons from light hitting PSII all the way to the final electron carrier

Electron flow from H₂O to NADPH is an
endergonic process (ΔG > 0) that is made possible by the absorption of 2 photons of light at PSII (P680) and PSI (P700)

Absorption of light energy converts special
pairs of chlorophylls P680 and P700 (poor
reducing agents) to excited molecules (P680* and
P700* good reducing agents) which are also powerful oxidising agents (particularly P680⁺) after losing their electrons

Question 19

Chloroplast ATP synthase

Answer 19

Very similar to ATP synthase (e.g. β subunits from human complex V and corn chloroplasts are >60% identical)

Structure with knob (CF1) and stalk (CF0)

• CF0: 12 subunits/rotations that span the membrane, forming a proton channel for H⁺ travel from the thylakoid lumen to the stroma
• CF1: protrudes into the stroma, containing the catalytic subunits for ATP synthesis from ADP + Pᵢ

Question 20

Overall photosynthesis stoichiometry: PSII, PSI, overall photosystem, ATP synthesis, and overall ATP synthesis

Answer 20

Photosystem II:
ET = PQ/PQH2 –> Cyt bf –> PC
4 photons, 4e⁻ produces one O₂
8 protons from stroma -> thylakoid lumen
4 protons from H₂O -> thylakoid lumen
12 protons transferred into the thylakoid lumen

Photosystem I:
ET = Fd –> NADP-reductase
4 more photons, 4e⁻ to reduce 2NADP⁺ along with 2 protons from stroma to make NADPH

Overall photosystem reaction:
2H₂O + 2NADP⁺ + 10H⁺ stroma + 4H⁺ water→ O₂ + 2NADPH + 12H⁺ lumen

CF0-CF1 ATP synthase:
12 protons flow back to the stroma in one rotation
3 ATP molecules are generated per full rotation

Overall reaction including ATP:
2NADP+ + 3ADP + 3Pᵢ+ 4H⁺ → O₂ + 2NADPH + 3ATP + H₂O

• 8 photons required to yield 3 ATP (~2.7 photons per ATP)

Question 21

What happens if there is no CO₂ or NADP⁺ available or ferredoxin cannot give an electron?

Answer 21

Cyclic phosphorylation

Plastocyanin is used to restore PSI to normal

(not sure of the process)

Net result: pumping of 8 protons to lumen per 4
photons at PSI, ATP is synthesized, but no O₂ or NADPH is produced

Yield ~ 2 photons per ATP

Question 22

Photosynthesis - light-independent reactions

Answer 22

Energy from sunlight is transformed into ATP
and reducing power (NADPH)

Electrons are obtained from H₂O oxidation and
used to reduce NADP⁺ to NADPH

Proton gradient is created using the free energy of electron transport. The gradient is later used to synthesize ATP

3Pᵢ + 3ADP → 3ATP + 3H₂O
2H₂O + 2NADP⁺ + 8 photons → 2NADPH + 2H⁺ + O₂

Question 23

Calvin cycle: where does it take place, what does it use, and what does it do>

Answer 23

Reactions taking place in the stroma of chloroplasts powered by ATP and NADPH

Converts CO₂ (fully oxidized carbon atoms) into
carbohydrates (more reduced state) using NADPH

Question 24

Calvin cycle: the three steps to it

Answer 24

1 - Fixation of atmospheric CO₂ by RuBisCO by using ribulose 1,5-bisphosphate (5C) to form two molecules of 3-phosphoglycerate (2×3C)

2 - Reduction of 3-phosphoglycerate using NADPH, forming glyceraldehyde 3-phosphate (3C) which can be converted to hexoses (6C) or used in step 3

3 - Regeneration of ribulose 1,5-bisphosphate
(5C) from 2G3P (2×3C) for more CO₂ fixation

Question 25

RuBisCO: what does it stand for, what does it do, and what are its key characteristics?

Answer 25

Ribulose bisphosphate carboxylase oxygenase

Fixes CO₂ from the atmosphere and uses it with ATP and 1,5-ribulose bisphosphate to form 3-phosphoglycerate

• Very inefficient (slow) enzyme fixes ~3CO2 per second
• Bad selectivity - occasionally it binds O₂ instead of CO₂ and carries out an oxygenase reaction instead of the correct carboxylase reaction
• RuBisCo is the most abundant protein in plants and likely on the Planet too

Question 26

The conversion of 3-phosphoglycerate into hexoses

Answer 26

3-phosphoglycerate (3PG) is an intermediate in glycolysis and gluconeogenesis

3PG is phosphorylated to 1,3- bisphosphoglycerate (1,3BPG) with consumption of ATP and it is then reduced to glyceraldehyde 3-phosphate (G3P) with oxidation of NADPH and G3P can be used to either regenerate ribulose bisphosphate or form hexoses

Question 27

The requirements for the formation of one glucose molecule

Answer 27

Making one molecule of glucose requires
6CO2 (atmosphere),18ATP/12NADPH (light reactions)

Question 28

What can plants convert glucose/fructose into?

Answer 28

1- Sucrose (in the cytosol)
2 - Starch (in the chloroplast)
3 - Cellulose (in the cell wall)