Module 4 · Answers
Answers & explanations
Section A — Easy
01
carbon dioxide + water → glucose + oxygen. Needs light and chlorophyll.
02
glucose + oxygen → carbon dioxide + water (+ energy released)
03
Mitochondria.
04
(a) Nitrates (b) Magnesium
05
The flow of energy from the eaten organism to the eater.
Section B — Medium
06
Any three: broad & flat (large surface area for light), thin (short diffusion distance for gases), chlorophyll absorbs light, stomata for gas exchange, veins to transport water and glucose.
07
During the sprint, muscles can't get oxygen fast enough, so they switch to anaerobic respiration. This produces lactic acid which causes the ache. After the sprint, the body keeps breathing hard to repay the oxygen debt and break down the lactic acid — until it's cleared, the muscles still feel sore.
08
Rabbit population would rise (no foxes hunting them), then grass would be over-grazed, so eventually rabbits would run out of food and their numbers would fall.
09
Bioaccumulation: pesticide isn't broken down. Each animal eats many of the level below, so toxin concentrates higher up the food chain. Top predators receive the largest dose and are most affected.
10
Aerobic uses oxygen; anaerobic doesn't. Aerobic produces CO₂ + water; anaerobic produces lactic acid. Aerobic releases much more energy.
Section C — Hard
11
Magnesium deficiency (no chlorophyll → yellow leaves) or nitrate deficiency (poor protein production → yellowing and stunted growth). Test by giving one group of plants a magnesium supplement and another group a nitrate supplement; see which group recovers.
12
The yeast is doing anaerobic respiration on the sugar: glucose → ethanol + carbon dioxide. The CO₂ gas builds up pressure (bottle bulges) and the ethanol is the alcohol smell.
13
Plants respire all the time (every living cell needs energy). In daylight they also photosynthesise, and photosynthesis is faster than respiration — so net there's an output of oxygen and uptake of CO₂. At night there's no light, so photosynthesis stops; only respiration continues.
14
1) Underwater plants can't photosynthesise → they die. 2) Animals that ate those plants lose food → numbers fall. 3) Carnivores that depended on those animals also fall, and decomposing material reduces oxygen in the water, harming fish further.