TOPIC 5 MARK SCHEME SHIT

Question 1

Explain how nitrate may cause the death of fish in fresh water (5)

Answer 1

1. Growth of algae/algal bloom blocks light
2. No PHOTOSYNTHESIS so plants die
3. SAPROBIOTIC microorganisms/bacteria
4. Aerobically respire
5. Less oxygen for fish to respire

Question 2

Describe the role of microorganisms in producing nitrate from the remains of dead organisms (3)

Answer 2

1. Saprobiotic bacteria/microorganisms break down remains/dead material/protein/DNA into ammonia/ammonium
2. Ammonia/ammonium ions into nitrite and then into nitrate
3. By nitrifying bacteria/nitrification

Question 3

Explain decrease in gross productivity as woodland matures (2)

Answer 3

1. Less light
2. Reduced photosynthesis

Question 4

Use your knowledge of net productivity to explain why biomass shows little increase after 100 years (2)

Answer 4

1. Net productivity = gross productivity minus respiratory loss
2. Decrease in gross productivity/photosynthesis/increase in respiration

Question 5

Explain why an increase in shoot biomass can be taken as a measurement of net primary productivity (2)

Answer 5

1. Represents dry mass/mass of carbon
2. Represents gross production minus respiratory losses

Question 6

Explain why CO2 uptake in plants is a measure of net productivity (1)

Answer 6

Shows photosynthesis/productivity minus respiration/more carbon dioxide used in photosynthesis than produced in respiration

Question 7

Large areas of tropical forest are still found on some Caribbean islands. The concentration of carbon dioxide in the air of these forests changes over a period of 24 hours and at different heights above ground. Use your knowledge of photosynthesis and respiration to describe and explain how the concentration of carbon dioxide in the air changes:
- over a period of 24 hours
- at different heights above ground. (5)

Answer 7

1. High concentration of / increase in carbon dioxide linked with respiration at night / in darkness;
2. No photosynthesis in dark / night / photosynthesis only in light / day;
3. In light net uptake of carbon dioxide / use more carbon dioxide than produced / (rate of) photosynthesis greater than rate of respiration;
4. Decrease in carbon dioxide concentration with height;
5. (At ground level) less photosynthesis / less photosynthesising tissue / more respiration /
   more micro-organisms / micro-organisms produce carbon dioxide.

Question 8

Suggest and explain why the rate of photosynthesis was low between 525 nm and
575 nm wavelengths of light. (2)

Answer 8

1. Less absorption/more reflection (of these wavelengths of light)
2. Light required for LDR/photolysis
3. Green light/colour of chlorophyll

Question 9

Describe what happens during photoionisation in the light-dependent reaction. (2)

Answer 9

1. CHLOROPHYLL ABSORBS LIGHT
2. Electrons are lost

Question 10

In muscles, pyruvate is converted to lactate during prolonged exercise. Explain why converting pyruvate to lactate allows the continued production of ATP by anaerobic respiration. (2)

Answer 10

1. Regenerates/produced NAD OR oxidises reduced NAD
2. So glycolysis continues

Question 11

Explain why the trained mice (regular exercise for 6 weeks) could exercise for a longer time period than the control mice (no exercise). (3)

Answer 11

1. More aerobic respiration produces more ATP
2. Anaerobic respiration delayed
3. Less/no lactate

Question 12

At the end of a sprint race, a runner continues to breathe rapidly for some time. Explain the advantage of this (2)

Answer 12

1. lactate/lactic acid has built up
2. oxygen used to break down lactate/convert it back to pyruvate/glucose/glycogen

Question 13

Explain why respiration produces more ATP per molecule of glucose in the presence of oxygen than it does when oxygen is absent (2)

Answer 13

1. Oxygen is final electron acceptor
2. Oxidative phosphorylation provides most ATP/only glycolysis occurs without oxygen/no Krebs/no link reaction

Question 14

In muscles, some of the lactate is converted back to pyruvate when they are well supplied with oxygen. Suggest one advantage of this (1)

Answer 14

(Pyruvate used) in aerobic respiration/(lactate/lactic acid) is toxic/harmful/causes cramps/(muscle) fatigue

Question 15

Describe the process of glycolysis (4)

Answer 15

1. Phosphorylation of glucose using ATP
2. Oxidation of triose phosphate to pyruvate
3. Net gain of ATP
4. NAD is reduced

Question 16

Denitrification requires anaerobic conditions. Ploughing aerates the soil. Explain how ploughing would affect the fertility of the soil (2)

Answer 16

1. (Fertility increased as) more nitrate formed/less nitrate removed/broken down
2. Less/no denitrification/fewer denitrifying bacteria

Question 17

Suggest two ways in which crop rotation may lead to higher crop yields (2)

Answer 17

1. Grow crops/plants with nitrogen fixing bacteria
2. Different minerals/salts/nutrients/ions
3. Different pests/pathogens/diseases

Question 18

Describe the role of saprobionts in the nitrogen cycle (2)

Answer 18

1. (They use enzymes to) decompose proteins/DNA/RNA/urea
2. Producing/releasing ammonia/ammonium compounds/ions

Question 19

In muscles, pyruvate is converted to lactate during prolonged exercise. Explain why converting pyruvate to lactate allows the continued production of ATP by anaerobic respiration. (2)

Answer 19

1. Regenerates/produces NAD OR oxidises reduced NAD
2. So glycolysis continues

Question 20

Define negative feedback (1)

Answer 20

Where a change triggers a response which reduces the effect of a change

Question 21

Explain how ADH increases the movement of water from the lumen of the collecting duct into the blood (4)

Answer 21

1. ADH causes vesicles containing aquaporins to be inserted into membrane/collecting duct wall
2. Water enters cells through aquaporins
3. Via osmosis
4. (From cell) to capillary
5. Via interstitial fluid

Question 22

Explain why it’s important that neurotransmitters are transported back out of synapses (2)

Answer 22

1. (If not removed) keeps binding (to receptors)
2. Keeps causing action potentials/depolarisation (in postsynaptic membrane)