5.1 - Photosynthesis

Question 1

Where do the light-dependent & light-independent reactions occur in plants?

Answer 1

light-dependent: in the thylakoids of chloroplasts
light-independent: stroma of chloroplasts

Question 2

Explain the role of light in photoionisation.

Answer 2

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 3

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

Answer 3

1. electron transfer chain 2. chemiosmosis

Question 4

What happens in the electron transfer chain (ETC)?

Answer 4

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 5

How is a proton concentration gradient established during chemiosmosis?

Answer 5

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

Question 6

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

Answer 6

H+ ions (protons) move down their concentration gradient from the thylakoid space into the stroma via the channel protein ATP synthase.
ATP synthase catalyses ADP + Pi → ATP.
https://bit.ly/pmt-edu

Question 7

Explain the role of light in photolysis.

Answer 7

Light energy splits molecules of water 2H2O → 4H+ + 4e- + O2

Question 8

What happens to the products of the photolysis of water?

Answer 8

● 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 9

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

Answer 9

● NADP + 2H+ + 2e- → reduced NADP. ● Catalysed by dehydrogenase
enzymes.
● Stroma of chloroplasts.

Question 10

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

Answer 10

● H+ ions: photolysis of water
● Electrons: NADP acts as the final
electron acceptor of the electron transfer chain

Question 11

Name the 3 main stages in the Calvin cycle.

Answer 11

1. Carbon fixation 2. Reduction
2. Regeneration

Question 12

What happens during carbon fixation?

Answer 12

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

Question 13

What happens during reduction (in the Calvin cycle)?

Answer 13

● 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 14

Outline the sequence of events in the light-independent reaction (Calvin cycle).

Answer 14

check picture

Question 14

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

Answer 14

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

Question 15

What happens during regeneration (in the Calvin cycle)?

Answer 15

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

Question 16

State the roles of ATP & (reduced) NADP in the light-independent reaction.

Answer 16

● ATP: reduction of GP to TP & provides phosphate group to convert RuP into RuBP.
● (reduced) NADP: coenzyme transports electrons needed for reduction of GP to TP.

Question 17

State the roles of ATP & (reduced) NADP in the light-independent reaction.

Answer 17

● ATP: reduction of GP to TP & provides phosphate group to convert RuP into RuBP.
● (reduced) NADP: 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 (envelope).
● 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

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

Question 21

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

Answer 21

● ATP synthase channels within granal membrane.
● large surface area of thylakoid membrane for ETC.
● photosystems position chlorophyll to enable
maximum absorption of light.

Question 22

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

Answer 22

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

Question 23

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

Answer 23

● Light intensity (light-dependent stage).
● CO2 levels (light-independent stage).
● Temperature (enzyme-controlled steps).
● Mineral/ magnesium levels (maintain
normal functioning of chlorophyll).

Question 24

Define ‘limiting factor’

Answer 24

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

Question 25

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

Answer 25

● Artificial light, especially at night. ● Artificial heating.
● Addition of CO2 to greenhouse
atmosphere.

Question 26

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

Answer 26

● 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 CO2 is 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 between origin and centre of pigment spot / distance between origin and solvent front.