Respiration — Science Sage

Releasing energy from food

Cellular respiration is the process that releases the chemical energy stored in food (glucose) so that cells can use it. Carbon dioxide is produced as a waste product by all living organisms during respiration.

Aerobic respiration

Aerobic respiration requires oxygen and takes place in two stages: glycolysis (in the cytoplasm) followed by the Krebs cycle and oxidative phosphorylation (in the mitochondria). It's far more efficient than anaerobic respiration, releasing much more energy per glucose molecule.

glucose + oxygen → carbon dioxide + water + energy (large amount)

Anaerobic respiration

When the body can't supply oxygen fast enough — such as during intense exercise — muscle cells switch to anaerobic respiration, producing lactic acid instead. This releases energy much faster, but far less efficiently, and the build-up of lactic acid contributes to muscle fatigue.

glucose → lactic acid + energy (small amount)

	Aerobic	Anaerobic
Oxygen needed?	Yes	No
Product	Carbon dioxide + water	Lactic acid
Energy released	Large amount	Small amount

Anaerobic respiration by yeast (fermentation) is used industrially in beer brewing and bread making.

Exercise & Respiration Simulator

Increase exercise intensity and watch the body switch from aerobic to anaerobic respiration as oxygen demand outpaces supply.

Muscle cell energy source

Exercise intensity Resting

AEROBIC

Plenty of oxygen available — muscles respire aerobically, releasing energy efficiently.

Answer in your exercise book.

Question 1 · (2 marks)

Define cellular respiration.

Question 2 · (2 marks)

Name the two locations in a cell where aerobic respiration takes place.

Question 3 · (4 marks)

Compare aerobic and anaerobic respiration in terms of oxygen requirement, product formed, and relative energy released.

Question 4 · (3 marks)

Explain why muscles produce lactic acid during intense exercise, and how this relates to muscle fatigue.

Question 5 · (2 marks)

Give ONE industrial use of anaerobic respiration (fermentation) by yeast.

Question 6 · (6 marks)

Two students, A and B, performed the same strenuous exercise. Their pulse rate was then recorded every minute during recovery:

Time after exercise stopped (min)	Student A pulse (bpm)	Student B pulse (bpm)
0	160	165
1	140	150
2	120	138
3	100	128
4	85	120
5	72	112

(a) Calculate the total decrease in pulse rate, from 0 to 5 minutes, for each student. (2 marks)
(b) Calculate each student's average rate of recovery (decrease in bpm per minute) over this 5-minute period. Show your working. (2 marks)
(c) Based on the data, which student is likely to be more physically fit? Explain your answer. (2 marks)