Lec19/20 - Photosynthesis

Question 1

What is photosynthesis?

Answer 1

A process (series of reactions) that converts light into chemical energy - this chemical energy is then used to convert atmospheric CO2 into carbohydrates

Question 2

What are the two fundamental stages of photosynthesis?

Answer 2

The light-dependent reactions: Solar energy is captured and converted into chemical energy as ATP and NADPH

The light-independent reactions: ATP and NADPH are used to convert CO2 to sugar phosphates (and then to carbohydrates)

Question 3

Where does the light-dependent reaction of photosynthesis take place?

Answer 3

The thylakoid membrane: H+ are pumped from choroplast stroma to thylakoid space/lumen

Question 4

Name the 8 (main) structures found in the thylakoid membrane and the role that each of them plays

Answer 4

1. PSII - contains chlorophyll
2. Plastoquinone (PQ) - electron carrier to Cytbf
3. Cytbf - links PSII and PSI
4. Plastocyanin (PC) - electron carrier
5. PSI - contains chlorophyll
6. Ferredoxin (Fd) - electron carrier
7. NADP+reducatase: catalyses reduction of NADP+ to NADPH
8. ATP Synthase

Question 5

Describe what happens in PSII (up to the special pair)

Answer 5

Several antenna pigments capture light energy (wavelength < 680 nm) and transfer it amongst themselves, until it reaches a special pair of chlorophyll-a molecules in the reaction centre

Question 6

Name the important structural feature of chlorophyll molecules (same answer for chl a and b)

Answer 6

Cyclic tetrapyrrole: planar, single-double bond alternation allows electron delocalisation and makes it sensitive to light

Question 7

Why does water become involved once the P680 pair is ionised (to P680+)?

Answer 7

Oxidised chlorophyll is unstable and needs to gain an electron back, which it does from water

Question 8

Describe how Plastoquinol (PQH2) is produced at PSII

Answer 8

P680 pair absorbs a photon and is excited, then passes an electron to pheophytin -> then to plastoquinone (PQ) at site Qa (fixed) -> then plastoquinone at site Qb (mobile)

Second electron reduces mobile plastoquinone to plastoquinol (PQH2)

Question 9

Describe the role of water at PSII

Answer 9

Oxidised chlorophyll is unstable and needs to gain an electron back, which it does from water;
H2O needs to lose an electron TWICE, by the same method, to give 1/2 O2;
At the oxygen evolving centre, water molecules (bound to Ca and Mn4) are oxidised and linked to form a molecule of O2, which is released

Question 10

Compare Plastoquinone and Ubiquinone

Answer 10

Plastoquinone (found in thylakoid membrane) and Ubiquinone (in mitochondrial inner membrane) have a similar structure - both can travel freely in the membrane due to the hydrophobic region

Question 11

Describe the Oxygen Evolving Centre

Answer 11

It is an Oxygen-Manganese-Calcium cluster near the special pair, where H2O binds and is oxidised one electron at a time;
It contains one Ca2+ ion and four Mn2+ ions;
Manganese is useful as it can have several different oxidation states

Question 12

Describe what PQH2 does in photosynthesis

Answer 12

After receiving the 2 electrons from chlorophyll, mobile PQH2 diffuses into the membrane, and carries electrons between PSII and cytochrome bf

Question 13

Describe the process and stoichiometry of proton translocation at PSII

Answer 13

• Two electrons needed to reduce each PQ to PQH2 (2e- then carried to cyt bf)
• Four electrons needed to generate one molecule of O2, thus 4 electrons transferred (by 2PQH2)
• Four protons from water released into thylakoid lumen; four protons from stroma transferred to PQH2 (these 4 are then released into thylakoid lumen once PQH2 is oxidised by cyt bf)

Question 14

Describe what happens once PQH2 reaches cyt bf

Answer 14

Cyt bf receives electrons from PQH2 (oxidises it); two protons released into thylakoid lumen
Two additional protons are transferred from stroma to thylakoid lumen via Q-cycle (4 total at Cyt bf)

Question 15

Describe what happens after Cyt bf in the LDR of photosynthesis

Answer 15

PLASTOCYANIN (PC) plays a similar role to cyt c in the ETC;
It is reduced at the copper atom, one electron at a time, while PQH2 is oxidised one electron at a time;
PC then carries electrons between Cyt bf and PSI

Question 16

Describe what happens at PSI

Answer 16

(Similar mechanism to PSII, different wavelength)
- Special pair at active centre absorb photon of 700nm (P700 -> P700)
- After excitation, P700 transfer electron to Chlorophyll A0, then to phyloquinone, then to three iron-sulphur clusters, finally to Ferredoxin (Fd)
- Ionised P700+ recovers electron from plastocyanin

Question 17

Describe what happens after PSI transfers electron to Ferredoxin

Answer 17

Ferredoxin carries electron to Ferredoxin-NADP+ reducatase.

Fd-NADP+r catalyses the reduction of NADP+ to NADPH, and the oxidation of Fd (meanwhile the FAD part of the enzyme is reduced one electron at a time, then oxidised again when it reduces NADP+)

Question 18

What letter is used to describe the overall electron flow in photosynthesis and why?

Answer 18

“Z-scheme” - the reduction potential increases significantly twice (when the P680 and P700 are excited) - i.e. these two steps are endergonic, but every other step is a small decrease in reduction potential. Overall, it looks a bit like a “Z”

Question 19

Is the overall process from H2O to NADPH endergonic or exergonic?

Answer 19

ENDERGONIC - the free energy is provided by the 2 photons

Question 20

State the overall pathway taken by electrons as they are transferred in the LDR

Answer 20

H2O -> Mn centre -> P680() -> Ph -> PQ -> Cyt bf -> PC -> P700() -> PhQ -> FeS -> Fd -> NADP+ reductase -> NADPH

Question 21

What is the end result of the LDR?

Answer 21

The formation of a H+ gradient, which is used to synthesise ATP.

(Also the formation of NADPH)

Question 22

Describe the structure of ATP Synthase in chloroplasts

Answer 22

Similar to Complex V; structure with a knob (CF1) and stalk (CF0)

CF0 spans the membrane and forms a proton channel for H+ travel from thylakoid lumen to stroma

CF1 protrudes into the stroma and contains the catalytic subunits for ATP Synthesis from ADP and Pi

Question 23

Describe the overall stoichiometry from PSII to PC

Answer 23

4 photons, 4e- to produce one O2

8 protons from stroma to thylakoid lumen

4 more protons from H2O released into thylakoid lumen (12 protons total)

Question 24

Describe the overall stoichiometry from PSI to NADP+reductase

Answer 24

4 more photons, 4e- to reduce 2NADP+ to 2NADPH

2 protons from stroma to make NADPH

Question 25

Describe the overall stoichiometry of the LDR excluding ATP

Answer 25

2H2O + 2NADP+ + 10H+(stroma) -> O2 + 2NADPH 12H+(lumen)

Question 26

Describe the overall stoichiometry of the LDR including ATP

Answer 26

2NADP+ + 3ADP + 3Pi + H2O -> 2NADPH + 3ATP + O2 + 2H+

Question 27

How many photons are required for a full rotation of ATP Synthase in the chloroplast, and how many ATP are produced (and therefore how many photons PER ATP)?

Answer 27

8 photons per full rotation; yields 3 ATP (2.7 photons per ATP)

Question 28

When is cyclic phosphorylation used?

Answer 28

It is an alternative pathway for when NO CO2 is available OR NO NADP+ is available to accept e- from Fd

Question 29

Describe how electrons are actually transferred in cyclic phosphorylation

Answer 29

After P700 is excited and electrons are passed along the pathway to PhQ, FeS, Fd, they are then passed FROM FD to CYT BF, then to PC, then BACK TO P700

Question 30

State the net result of Cyclic Phosphorylation, and why it is not normally used

Answer 30

Net result: pumping of 8 protons to lumen per 4 photons are PSI; ATP is synthesised but NO O2 OR NADPH; yield is 2 photons per ATP

Although more ATP per photon, not normally used because it does not produce NADPH (which is needed for sugar synthesis in LIR)

Question 31

What are the three alternative names for the LIR

Answer 31

Calvin-Benson Cycle, Calvin Cycle, Pentose Phosphate Cycle

Question 32

State the fundamental principle of the LIR

Answer 32

Conversion of CO2 (fully oxidised carbon) into carbohydrates (more reduced state) - this is why NADPH is needed (reducing power)

Question 33

Name and describe the 3 stages of the Calvin Cycle

Answer 33

1. FIXATION of CO2 by Ribulose 1,5-bisphosphate to form 2 molecules of 3-phosphoglycerate
2. REDUCTION of 3-phosphoglycerate to form glyceraldehyde-3-phosphate (G3P) which can be converted to hexoses
3. REGENERATION of ribulose 1,5-bisphosphate from 2G3P, so that more CO2 can be fixed

Question 34

Describe the carbon fixation stage of the LIR

Answer 34

R1,5-bisphosphate (5C+2P) combines with CO2 to form a 6C+2P intermediate, which breaks into 2 molecules of 3-phosphoglycerate (3C+1P). This is catalysed by RuBisCO on the stromal surface of the thylakoid membrane

Question 35

Which reaction is the limiting step in hexose synthesis (in Photosynthesis)?

Answer 35

The carbon fixation step of the LIR

Question 36

State the downsides of RuBisCo as an enzyme

Answer 36

It is a slow, inefficient and poorly selective enzyme, which fixes 3CO2 per second (“pathetic”)
It sometimes binds O2 instead of CO2 and carries out oxygenase reaction

Question 37

Describe the reduction stage of the LIR

Answer 37

3-phosphoglycerate is phosphorylated to 1,3-bisphosphoglycerate with consumption of ATP
1,3-BPG is then reduced to glyceraldehyde-3-phosphate (G3P) with oxidation of NADPH
These two reactions REVERSE two steps from glycolysis

Question 38

Describe the regeneration stage of the LIR (lots of stoichiometry info, sorry)

Answer 38

In 3 cycle turns, 3 molecules of CO2 (3C) are combined with 3 molecules of R1,5-BP (15) to make 6 molecules of 3PG/G3P (18C)

3 molecules of R1,5BP needs to be regenerated for the cycle to continue

5 of the 6 G3P molecules are used to regenerate 3 molecules of R1,5BP, producing ONE NET MOLECULE OF G3P

TWO molecules of G3P can be combined to make glucose, therefore 6 turns of the cycle allows production of one molecule of glucose

Question 39

How many turns of the Calvin Cycle are required for 1 molecule of glucose to be produced?

Answer 39

6

Question 40

How much CO2, ATP and NADPH is required in total per molecule of glucose synthesised?

Answer 40

6CO2 (6x1), 18ATP (6x3), 12NADPH (6x2)

All x6 because 6 cycle turns needed per glucose

Question 41

What 3 things do plants convert sugars into?

Answer 41

Sucrose (in cytosol)
Starch (in chloroplast)
Cellulose (in cell wall)

Question 42

State the structure and function of sucrose

Answer 42

Disaccharide of glucose and fructose; mobile form of carbohydrate which can circulate throughout plant