Photosynthesis 5.1

Question 1

Describe the light (In)dependent reaction

Answer 1

• Chlorophyll absorbs light energy & excites electrons
• Electrons are removed (by oxidation of chlorophyll) via photoionisation;
• Electrons move along carriers/electron transport chain releasing ENERGY (series of redox reactions)
• Energy released (by electrons) used to form proton gradient;
• H+ ions move through ATP synthase;
• Providing energy to join ADP to Pi (forming ATP)
• Photolysis of water produces 2 protons, 2 electrons and 1/2 oxygen.
• NADP reduced by electrons/protons/hydrogen.

Question 2

Name the two products of the light-dependent reaction that are required for the light-independent reaction

Answer 2

1. ATP
2. Reduced NADP

Question 3

Describe the light Independent reaction

Answer 3

1. Carbon dioxide combined with Ribulose Biphosphate (RuBP)
2. Produces 2 glycerate (3-) phosphate (GP) using rubisco enzyme
3. GP reduced to triose phosphate;
4. Using reduced NADP;
5. Using energy from ATP
6. Triose phosphate converted to glucose/hexose/RuBP.

Question 4

Where is rubisco found in a cell?

Answer 4

Stroma

Question 5

Explain why scientists measure the rate of production of oxygen in this investigation (Rate of Photosynthesis)

Answer 5

1. Oxygen produced in light-dependent reaction;
2. The faster the oxygen is produced, the faster the light-dependent reaction.

Question 6

Explain why plants that have more chlorophyll will grow faster than plants with less chlorophyll

Answer 6

1. Have faster production of ATP and reduced NADP;
2. So have faster/more light-independent reaction;
3. So produce more sugars that can be used in respiration;
4. So have more energy for growth;
5. Have faster/more synthesis of new organic materials.

Question 7

ATP is produced in the light dependent reaction, suggest why this is not the plant’s only source of ATP

Answer 7

• Plants don’t photosynthesise in the dark;
• Not all parts of the plant photosynthesise e.g root hair cells
• Plants require MORE ATP than is produced in the light dependent reaction;
• ATP used in active transport (in phloem)

Question 8

Describe the effect of introducing a herbicide/inhibitor on the electron transport chain (4)

Answer 8

1. Reduced transfer of protons/proton gradient across thylakoids membrane
   //
   reduced chemiosmotic gradient across thylakoid membrane.
2. So less ATP produced
3. So less NADP produced
4. So light-independent reaction slows/stops
   // Less reduction of GP to Triose phosphate

Question 9

Describe what happens during photoionisation in the light-independent reaction

Answer 9

• Chlorophyll absorbs light // light excites electrons in chlorophyll

• Electrons are LOST to transport chain / Chlorophyll becomes positively charged

Question 10

When producing a chromatogram, explain why the origin is marked using pencil rather than ink

Answer 10

• Ink and leaf pigments will mix

• Origin line in pencil still visible in same position
// Origin line in pen in different position

Question 11

While making a chromatogram, describe the method used to separate the pigments after the solution of pigment had been applied to the origin.

Answer 11

• Level of solvent below origin line;
• Remove before solvent reaches top/end.

Question 12

Suggest and explain the advantage to plants of having different colour pigments in leaves

Answer 12

• Absorb more different wavelengths of light for photosynthesis

Question 13

Explain the relationship between stomatal opening and photosynthesis (2)

Answer 13

1. Stomata allow uptake of CO2
2. Carbon dioxide used in photosynthesis

Question 14

Describe the need for plants to both photosynthesise AND respire

Answer 14

• In the dark, no photosynthesis so no ATP produced;
• Some tissues e.g root hair cells don’t photosynthesise;
• ATP cannot be moved from cell-cell or stored;
• Plant uses MORE ATP than is produced;
• ATP for active transport (translocation) AND for synthesis (of named substance!)

Question 15

How is the chloroplast adapted for maximising the rate of photosynthesis in the stroma? (5 max)

Answer 15

• Large number of thylakoids/grana (so many chlorophyll)
• Short diffusion distances
• Ribosomes (for protein production e.g ATP Synthase)
• Aqueous
• Abundance of RuBisCO (for LIR)

Question 16

Describe what would happen to the rate of photosynthesis when light intensity decreases? (5 max)

Answer 16

• Less photoionisation of chlorophyll (so fewer electrons lost)
• Less photolysis to replace electrons lost;
• Less ATP and NADP (for LDR);
• Less energy to activate proton pump in ETC;
• Less H+ pumped into thylakoid space (so smaller chemiosmotic gradient formed)
RATE IS SLOWER

Question 17

Describe what would happen to the rate of photosynthesis if there is no ATP/NADP present (2)

Answer 17

• Less ADP/energy available (as ATP is needed to regenerate ADP)
• So ATP Synthase cannot produce any more ATP by photo-phosphorylation

• Less reduction of NADP so the LIR cannot occur

Question 18

State what chemicals are needed for the light-dependent reaction to occur (1)

Answer 18

NADP, ADP, Pi and Water

Question 19

The concentration of CO2 in the air at different heights above the ground in a forest changes over a period of 24 hours. Explain why (5)

Answer 19

1. High CO2 conc linked with night darkness;
2. No photosynthesis in dark/at night;
3. In dark plants/other organisms respire;
4. In light, rate of photosynthesis is greater than rate of aerobic resp;
5. Decrease in CO2 with height;
6. At ground level fewer leaves/photosynthesising tissue.