Photosynthesis

Question 1

Where does the light dependent reaction occur?

Answer 1

In the thylakoid membrane of the chloroplasts

Question 2

Where does the light independent reaction occur?

Answer 2

In the stroma of the chloroplasts

Question 3

Explain the role of light in photoionisation

Answer 3

Chlorophyll molecules absorb energy from photons of light. This ‘excites’ 2 electrons (raises them to a higher energy level), causing them to
be released from the chlorophyll.

Question 4

Name the 2 main stages involved in ATP
production in the light-dependent
reaction.

Answer 4

Electron Transfer Chain and Chemiosmosis

Question 5

What happens in the electron transfer

chain (ETC)?

Answer 5

Electrons released from chlorophyll move down a series of carrier proteins embedded in the thylakoid membrane & undergo a series of redox reactions, which releases energy.

Question 6

How is a proton concentration gradient

established during chemiosmosis?

Answer 6

Some energy released from the ETC is coupled to the active transport of H+ ions (protons) from the stroma into the thylakoid space.

Question 7

How does chemiosmosis produce ATP in the light-dependent stage?

Answer 7

H+ ions move down their concentration gradient from the thylakoid space into the stroma via the channel protein ATP synthase. ATP synthase catalyses ADP + Pi → ATP

Question 8

Explain the role of light in photolysis.

Answer 8

Light energy splits molecules of water

2H2O → 4H+ + 4e-+ O2

Question 9

What happens to the products of the

photolysis of water?

Answer 9

H+ ions: move out of thylakoid space via ATP synthase & are used to reduce the coenzyme NADP.
e-: replace electrons lost from chlorophyll.
O2: used for respiration or diffuses out of leaf as waste gas.

Question 10

How and where is reduced NADP
produced in the light-dependent
reaction?

Answer 10

NADP + 2H+ + 2e- → reduced NADP.

Catalysed by dehydrogenase enzymes in the stroma of chloroplasts.

Question 11

Where do the H+ ions and electrons used to reduce NADP come from?

Answer 11

H+ ions: photolysis of water

Electrons: NADP acts as the final electron acceptor of the electron transfer chain

Question 12

Name the 3 main stages in the Calvin

cycle.

Answer 12

1. Carbon fixation
2. Reduction
3. Regeneration of RuBP

Question 13

What happens during carbon fixation?

Answer 13

Reaction between CO2 & ribulose
bisphosphate (RuBP) catalysed by rubisco.
Forms unstable 6C intermediate that
breaks down into 2x glycerate 3-phosphate
(GP)

Question 14

What happens during reduction (in the

Calvin cycle)?

Answer 14

2 x GP are reduced to 2 x triose phosphate (TP)
Requires 2 x reduced NADP & 2 x ATP
Forms 2 x NADP & 2 x ADP

Question 15

How does the light-independent reaction
result in the production of useful organic
substances?

Answer 15

1C leaves the cycle (i.e. some of the TP
is converted into useful organic
molecules).

Question 16

What happens during regeneration (in

the Calvin cycle)?

Answer 16

After 1C leaves the cycle, the 5C compound RuP forms
RuBP is regenerated from RuP using 1x ATP
Forms 1x ADP

Question 17

State the roles of ATP & NADPH in the light-independent reaction.

Answer 17

ATP: reduction of GP to TP & provides phosphate group to convert RuP into RuBP.
NADPH: coenzyme transports electrons needed for reduction of GP to TP.

Question 18

State the number of carbon atoms in

RuBP, GP & TP

Answer 18

RuBP: 5
GP: 3
TP: 3

Question 19

Describe the structure of a chloroplast

Answer 19

Usually disc-shaped.
Double membrane.
Thylakoids: flattened discs stack to form grana.
Intergranal lamellae: tubular extensions attach thylakoids in adjacent grana.
Stroma: fluid-filled matrix.

Question 20

How does the structure of the chloroplast maximise the rate of the light-dependent reaction?

Answer 20

-ATP synthase channels within granal membrane =ATP synthesis +proton gradient
-Large surface area of thylakoid membrane for ETC and the attachment of chlorophyll, electrons and enzymes
-photosystems position chlorophyll to enable maximum absorption of light.
-Chloroplasts contain DNA and ribosomes allowing them to synthesise proteins needed in
the LDR

Question 21

How does the structure of the chloroplast maximise the rate of the light-independent reaction?

Answer 21

Own DNA & ribosomes for synthesis of enzymes e.g. rubisco.
Concentration of enzymes & substrates in stroma is high.

Question 22

Define ‘limiting factor’.

Answer 22

Factor that determines maximum rate of a reaction, even if other factors change to become more favourable.

Question 23

Name 4 environmental factors that can limit the rate of photosynthesis.

Answer 23

Light intensity (LDR).
CO2 levels (LIR).
Temperature (enzyme-controlled steps).
Mineral/ magnesium levels (maintain
normal functioning of chlorophyll).

Question 24

Outline some common agricultural practices used to overcome the effect of limiting factors in photosynthesis

Answer 24

Artificial light.
Artificial heating.
Addition of CO2 to greenhouse atmosphere.

Question 25

Why do farmers try to overcome the effect of limiting factors?

Answer 25

To increase yield.
Additional cost must be balanced with
yield to ensure maximum profit.

Question 26

Suggest how a student could investigate the effect of a named variable on the rate of photosynthesis.

Answer 26

dependent variable: rate of O2 production/ CO2 consumption

1. Use a potometer
2. Place balls of calcium alginate containing green algae in hydrogencarbonate indicator (colour change orange → magenta as CO2is consumed & pH
   increases) .

Question 27

State the purpose and principle of paper

chromatography.

Answer 27

Molecules in a mixture are separated based on their relative attraction to the mobile phase (running solvent) vs the stationary phase (chromatography paper).

Question 28

Outline a method for extracting photosynthetic pigments.

Answer 28

Use a pestle and mortar to grind a leaf

with an extraction solvent e.g. propanone.

Question 29

Outline how paper chromatography can
be used to separate photosynthetic
pigments.

Answer 29

1. Use a capillary tube to spot pigment extract onto pencil ‘start line’ (origin) 1 cm above bottom of paper.
2. Place chromatography paper in solvent (origin should be above solvent level).
3. Allow solvent to run until it almost touches the other
   end of the paper. Pigments move different distances.

Question 30

What are Rf values? How can they be

calculated?

Answer 30

Ratios that allow comparison of how far
molecules have moved in chromatograms.
Rf value = distance from origin/ distance between origin and solvent front.