C1.2 Cell respiration. Practice questions with markscheme.

• D: aerobic respiration requires oxygen and has a much higher ATP yield, producing carbon dioxide and water; anaerobic respiration in human muscle needs no oxygen, yields far less ATP, and produces lactate;

• B: ATP cannot be stored in quantity and is used constantly, so it must be regenerated continuously;

• C: ATP is soluble in water, so it moves readily through the cytoplasm to wherever energy is needed; it cannot pass freely through membranes and is not a long-term store;

• A: 30 × 30 kJ = 900 kJ is transferred to ATP, and 900 ÷ 2870 × 100 = 31 %; the other 69 % is released as heat; ADP and phosphate hold less energy than ATP, since energy must be supplied to convert them to ATP, so the rest cannot be stored in them;

• independent variable: temperature of the yeast and glucose mixture, set at at least five values from about 15 °C to 55 °C using water baths;
• dependent variable: rate of carbon dioxide production, e.g. volume of gas collected in a gas syringe per minute (accept number of bubbles per minute);
• the mixture is sealed, or covered with a layer of oil, so that respiration is anaerobic;
• controlled variables: volume and concentration of the glucose solution, volume and concentration of the yeast suspension (same culture and age), pH, and the total time over which gas is collected;
• replication: at least three trials at each temperature, with a mean rate calculated;
• a control tube containing boiled/killed yeast shows that gas production is due to living yeast;

Replication must be quantified. Award marks only where a variable is clearly identified as independent, dependent or controlled.

• synthesis of macromolecules (anabolism), e.g. building proteins or nucleic acids;
• movement of the whole cell or of cell components, e.g. muscle contraction, movement of chromosomes during cell division;
• activation of molecules by phosphorylation, e.g. phosphorylating glucose at the start of glycolysis;

Accept any two correct, distinct examples of energy-requiring cell processes.

• anaerobic yields (only) 2 ATP per glucose, whereas aerobic yields (about) 30–38 / many more;
• anaerobic supplies ATP faster/immediately (no oxygen delivery needed), whereas aerobic is slower but far more efficient;
• (so) anaerobic suits short bursts, aerobic suits sustained activity, OWTTE;

Award converse statements.

• (a) [2]
  • mean distance = 24 mm;
  • rate = 2.4 mm min⁻¹;

  Award ECF from an incorrect mean. Units required for the second mark.

• (b) [2]
  • absorbs the carbon dioxide released by respiration;
  • (so) the movement of the liquid is due to oxygen consumption only, OWTTE;
• (c) [1]
  • control (for changes in temperature/pressure not caused by respiration), OWTTE;
• (d) [1]
  • rate increases with temperature (over the range tested);
• (e) [3 max]
  • (active) transport of ions/molecules across membranes;
  • synthesis of macromolecules / anabolic reactions (proteins/DNA);
  • movement (of the whole cell) / muscle contraction / movement of components within the cell;
  • any other valid use, e.g. maintaining body temperature in endotherms, OWTTE;

  Content pivot. Mark the first three answers only.

• ADP + phosphate is converted to ATP using energy (released) from cell respiration;
• hydrolysis of ATP to ADP + phosphate releases energy;
• the released energy drives cell activities e.g. active transport / synthesis of macromolecules / movement;
• each molecule is recycled many times / interconversion is continuous and rapid, so only a small pool of ATP/ADP is needed, OWTTE;

• occurs in the cytoplasm (no oxygen required);
• glucose is phosphorylated using (2) ATP;
• the (6C) hexose bisphosphate is split (lysis) into two 3C (triose phosphate) molecules;
• the triose phosphates are oxidized, reducing NAD (to NADH);
• ATP is produced (by substrate-level phosphorylation): 4 made, net gain 2;
• end products: 2 pyruvate (+ 2 net ATP + 2 NADH), OWTTE;

• (a) [2]
  • volume of oxygen consumed = 44 (mm) × 1.0 (mm²) = 44 mm³ (in 10 minutes by 5.0 g);
  • rate = 44 ÷ (5.0 × 10) = 0.88 mm³ g⁻¹ min⁻¹;

  Accept 0.88 to 0.9. Award ECF for a correct method using a wrongly read distance. Units are required for the second mark.

• (b) [2 max]
  • rate increases with temperature from 10 °C to 30 °C (12 mm to 58 mm);
  • rate reaches a maximum at (about) 30 °C;
  • above 30 °C the rate falls, steeply between 35 °C and 40 °C (52 mm to 21 mm), OWTTE;

  Figures from the table are needed for at least one point.

• (c) [2]
  • 25 °C versus 30 °C: significant, because the ranges of mean ± SE (41.7 to 46.3 and 55.1 to 60.9) do not overlap;
  • 30 °C versus 35 °C: not significant, because the ranges of mean ± SE (55.1 to 60.9 and 47.4 to 56.6) overlap, OWTTE;

  Both the conclusion and the overlap reason are required for each mark. Accept "error bars would overlap / not overlap".

• (d) [2 max]
  • oxygen is used in aerobic respiration (glucose + oxygen → carbon dioxide + water);
  • aerobic respiration releases energy from glucose/carbon compounds, which is used to produce ATP;
  • so the faster oxygen is consumed, the faster substrate is respired aerobically and the faster ATP is produced (assuming little anaerobic respiration), OWTTE;
• (e) [1]
  • individual larvae differ in their tolerance of high temperature, so some are damaged / stressed more than others, giving more variable results, OWTTE; OR larvae are more active / move about at high temperature, increasing variation between respirometers, OWTTE;

  Do not accept "human error" or "the equipment was less accurate" without a link to temperature.

• (f) [2 max]
  • the reduction in rate caused by 40 °C is not reversed on return to 25 °C / the damage is permanent, because the treated larvae still respire more slowly than the control at 25 °C;
  • longer exposure causes more damage, because 60 minutes reduced the rate (to 9 mm) more than 10 minutes (27 mm);
  • the differences are significant, because the mean ± SE ranges of the control and the treated groups do not overlap, OWTTE;

  Accept reference to denaturation of respiratory enzymes as a reason for the permanence.

• (g) [3 max]
  • cell respiration is a (series of) chemical reactions inside cells, whereas gas exchange is the physical process of diffusion of gases across a surface;
  • cell respiration releases energy from carbon compounds (glucose / fatty acids) to produce ATP, whereas gas exchange does not involve energy release / ATP production;
  • cell respiration is catalysed by enzymes (in the cytoplasm and mitochondria), whereas gas exchange occurs by passive diffusion without enzymes;
  • cell respiration consumes oxygen and produces carbon dioxide inside cells, whereas gas exchange moves oxygen into the organism and carbon dioxide out (across the body surface / tracheae / lungs);
  • anaerobic respiration can occur without gas exchange of oxygen, whereas gas exchange of oxygen is only useful if aerobic respiration is occurring, OWTTE;

  Content pivot. Award marks only for explicitly paired statements. Do not accept "respiration is breathing".