photosynthesis SLOP

Question 1

Describe what is meant by the term compensation point

Answer 1

When the rate of photosynthesis is equal to the rate of respiration. This is when there is no net gain or loss of carbohydrate

Question 2

Describe the structure of a photosystem

Answer 2

A light-harvesting complex that channels light towards a reaction centre (containing a primary pigment called chlorophyll a); accessory pigments channel photons towards the reaction centre

Question 3

Name one primary pigment and 3 accessory pigments

Answer 3

Primary - chlorophyll A; accessory - xanthophylls, carotenoids, chlorophyll b

Question 4

Describe the differences between photosystem I and photosystem II

Answer 4

PSI contains chlorophyll a that absorbs light at a peak of 700nm (P700). PSII contains chlorophyll a that absorbs light at a peak of 680 nm. Electrons are excited to a higher energy level in PSI than PSII

Question 5

Describe the light absorption of carotenoids, xanthophylls and chlorophylls

Answer 5

Carotenoids absorb blue light; xanthophylls absorb blue and green light; chlorophylls absorb blue and red light

Question 6

In which part of the chloroplast does the light-dependent reaction take place?

Answer 6

Thylakoid membrane

Question 7

In which part of the chloroplast does the light-independent reaction take place

Answer 7

Stroma

Question 8

Suggest why the Rf values of some pigments show a range rather than a single figure

Answer 8

There are different types of these, with slightly different molecular masses/sizes of molecules/solubilities in the solvent

Question 9

Rf values for the different pigments differ according to the solvent used. Suggest why this is the case

Answer 9

The molecules of pigment will have different solubilities in different solvents.

Question 10

What 4 processes occur in the light-dependent reaction?

Answer 10

Light harvesting at the photosystems; photolysis of water; photophosphorylation (production of ATP in the presence of light); formation of reduced NADP

Question 11

Outline the role of water in photosynthesis

Answer 11

Electron donor; source of protons/hydrogen ions; source of by-product, oxygen; keeps cells turgid so they can function (all metabolic reactions need to be in solution).

Question 12

Suggest why a lack of iron in soil may reduce growth in plants

Answer 12

Iron is needed for electron carriers in the chain (on thylakoid membranes) and for ferredoxin. A reduction of electron carriers could reduce rate of photosynthesis.

Question 13

What is photolysis and where exactly does it take place?

Answer 13

The enzyme-catalysed splitting of water molecules, in the presence of light. It takes place in PSII on thylakoid membranes of chloroplasts.

Question 14

Describe the differences between non-cyclic and cyclic photophosphorylation

Answer 14

NC - involves PSI and PSII; produces ATP, oxygen and reduced NADP (NADP is final electron acceptor).

C - involves PSI only; produces ATP only as electrons are recycled back to PSI and NADP is not reduced

Question 15

Describe how a proton gradient develops between the thylakoid lumen and the stroma

Answer 15

Energy from electrons moving through chain of electron carriers is used to actively pump protons across the thylakoid membrane into the thylakoid lumen

Question 16

Describe the purpose of the proton gradient

Answer 16

Protons can diffuse down their gradient through ATP synthase channels (using proton motive force), causing ADP to combine with inorganic phosphate to form ATP

Question 17

Describe what happens to protons that have moved through ATP synthase

Answer 17

They are accepted along with electrons from the electron transport chain, by NADP. This is facilitated by NADP reductase (NADP has been reduced to NADPH)

Question 18

Describe how an electron moves through the electron transport chain

Answer 18

A photon excites an electron from PSII, the electron rising to a higher energy level and getting accepted by an electron acceptor. This energy level is unstable so the electron moves through a chain of electron carrier proteins, losing energy as it moves (this energy is transferred to pump protons). It is accepted by chlorophyll a in PSI and re-excited by another photon of light to a higher energy level than before.

Question 19

Explain why chloroplasts in guard cells only contain PSI

Answer 19

Produces ATP which actively pumps potassium ions into the cell, lowering the water potential so water follows by osmosis. This causes the guard cell to become turgid and the stoma to open.

Question 20

Describe the differences in the fate of electrons released from chlorophyll in both non-cyclic and cyclic photophosphorylation

Answer 20

Cyclic - pass via electron carriers back to PSI (little ATP generated); Non-cyclic - from PSII, pass via electron transport chain to PSI, PSI electrons accepted by NADP

Question 21

Suggest why there are always only low levels of RuBP in the stroma of chloroplasts

Answer 21

It is being continually regerated and then combined with CO2

Question 22

Describe the role of carbon dioxide in the Calvin cycle

Answer 22

Combines with 5C-RuBP to form a 6C intermediate. This is unstable and so breaks into 2 x 3C-GP molecules. This reaction is catalysed by RuBisCO

Question 23

Describe the conversion of GP into TP in the Calvin cycle

Answer 23

3C-GP is converted into 3C-TP using ATP and the H atom from reduced NADP

Question 24

Describe how RuBP is regenerated in the Calvin cycle

Answer 24

5/6 of TP is converted back into RuBP through reshuffling of the arrangement of carbon atoms. This conversion also uses ATP (RuP –> RuBP)

Question 25

Describe the fate of triose phosphate

Answer 25

1/6 leaves the Calvin cycle and is converted into sucrose, starch, cellulose, amino acids, glycerol, fatty acids, some is respired in glycolysis. The other 5/6 is used to regerate RuBP

Question 26

State 2 products of the light dependent reaction used in the Calvin cycle

Answer 26

ATP and reduced NADP

Question 27

Describe how plants respond to water stress. Explain how this affects their abiity to photosynthesise

Answer 27

Leaves lose more water by transpiration than can be replaced by uptake at roots. Cells lose water/become plasmolysed. Tissues become flaccid. Leaves wilt. Roots are unable to take in water and secrete abscisic acid, which travels in xylem to leaves causing stomata to close to prevent further water vapour loss. Closed stomata reduce gaseous exchange so not enough carbon dioxide for photosynthesis. Plasmolysed cells cannot function as enzymes cannot work. Reduced water availability for photosynthesis – as source of electrons and protons.

Question 28

Describe the effects of reducing the light intensity on the Calvin cycle

Answer 28

Light dependent reaction cannot occur, so less ATP/NADPH; less GP –> TP; TP levels fall and GP accumulates; if TP levels fall then RuBP cannot be regenerated

Question 29

Describe the effects of reducing carbon dioxide levels in the Calvin cycle

Answer 29

RuBP cannot accept it and RuBP accumulates; GP cannot be made; TP cannot be made