Photosynthesis

Question 1

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

Answer 1

Light-dependent - in thylakoid of chloroplasts
Light-independent - stroma of chloroplasts

Question 2

Role of light in photoionisation

Answer 2

Chrolophyll molecules absorb energy from photons of light
this ‘excites’ 2 electrons, causing them to be released from chlorophyll

Question 3

What happens in the electron transfer chain (ETC)?

Answer 3

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

Question 4

How is proton gradient established during chemiosmosis?

Answer 4

Some energy released from ETC is coupled to active transport of H+ ions from stroma in to thylakoid space

Question 5

How does chemiosmosis produce ATP in light-dependent stage?

Answer 5

H+ ions move down their conc. gradient from thylakoid space into stroma via channel protein ATP synthase
ATP syntahse catalysys ADP + Pi - ATP

Question 6

What happens to products of photolysis of water?

Answer 6

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

Question 7

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

Answer 7

• NADP + 2H+ + 2E- = NADPH
• Catalysed by dehydrogenase enzymes
• Stroma of chloroplasts

Question 8

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

Answer 8

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

Question 9

What happens during carbon fixation?

Answer 9

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

Question 10

What happens during reduction (In Calvin Cycle)?

Answer 10

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

Question 11

What happens during regeneration (in Calvin Cycle)?

Answer 11

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

Question 12

State roles of ATP & NADPH in light-independent reaction

Answer 12

• 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 13

Structure of chloroplast

Answer 13

• 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 14

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

Answer 14

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

Question 15

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

Answer 15

• Own DNA & ribosomes for synthesis of enzymes (e.g. rubisco)
• Conc. of enzymes & substrates in stroma is high

Question 16

4 environmental factors that can limit rate of photosynthesis

Answer 16

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

Question 17

What are Rf values? How can they be calculated?

Answer 17

• Ratios that allow comparison of how far molecules have moved in chromatograms
• Rf values= distance between origin and centre of pigment spot/ distance between origin and solvent front

Question 18

Define ‘limiting factor’

Answer 18

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

Question 19

Common agricultural practices used to overcome effect of limiting factors in photosynthesis

Answer 19

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

Question 20

State purpose & principle of Paper Chromatography

Answer 20

Molecules in mixture separated based on relative attraction to mobile phase (running solvent) vs stationary phase (chromatography paper)

Question 21

Outline how Paper Chromatography can be used to separate photosynthetic pigments

Answer 21

1. Use 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 line)
   3., Allow solvent to run util it almost touches other end of paper. Pigments move different distances