Respiration

Respiration 呼吸作用 is the release of energy 能量 from food, and it happens in every living cell, all the time. (It is not the same as breathing.) Cells use this energy for many jobs:

• muscle 肌肉 contraction (movement).
• protein synthesis 蛋白质合成 (building proteins).
• cell division 分裂 (making new cells).
• active transport 主动运输.
• growth.
• the passage of nerve impulses 神经冲动.
• keeping a constant body temperature 温度.

Cells use the energy from respiration for many different jobs

You can investigate respiration in yeast 酵母: warmer yeast respires faster (up to its best temperature), giving off bubbles of carbon dioxide 二氧化碳.

Aerobic respiration releases energy steadily for sustained exercise.

Aerobic respiration 有氧呼吸 uses oxygen 氧气 to break down nutrient molecules 营养物质 (mainly glucose 葡萄糖) and release energy.

Word equation:

(Supplement) Balanced chemical equation:

Aerobic respiration releases a lot of energy from each glucose molecule.

A hard sprint relies on anaerobic respiration, building up lactic acid.

Anaerobic respiration 无氧呼吸 breaks down glucose to release energy without oxygen. It releases much less energy from each glucose molecule than aerobic respiration, because the glucose is not fully broken down.

Aerobic respiration needs oxygen and gives a lot of energy; anaerobic gives much less

In yeast

The alcohol 酒精 made by yeast is used to make bread rise and to brew drinks.

In muscles (Supplement)

During hard exercise your muscles cannot get enough oxygen, so they respire anaerobically:

The lactic acid 乳酸 builds up in the muscles and the blood. This creates an oxygen debt 氧债 — the extra oxygen the body will need later to break that lactic acid down.

After you stop exercising, the oxygen debt is repaid:

• your heart rate 心率 stays high, carrying the lactic acid in the blood from the muscles to the liver 肝脏.
• you keep breathing deeply and quickly, taking in extra oxygen.
• the liver uses this oxygen to break the lactic acid down by aerobic respiration.

Lactic acid from anaerobic respiration is broken down later, using extra oxygen

Worked example. A runner sprints hard for 20 seconds, then breathes deeply for several minutes after stopping. Explain why. During the sprint the muscles cannot get oxygen fast enough, so they respire anaerobically and lactic acid builds up, creating an oxygen debt. The deep breathing afterwards takes in the extra oxygen the liver needs to break that lactic acid down by aerobic respiration, while a high heart rate carries the acid from the muscles to the liver. The breathing repays a debt built up during the sprint - it is not simply "because the muscles are still working".

• Respiration releases energy in all living cells, all the time — it is not breathing.
• Aerobic: glucose + oxygen → carbon dioxide + water, and it gives a lot of energy. Learn the balanced equation too.
• Anaerobic (no oxygen) gives much less energy. In yeast: glucose → alcohol + carbon dioxide. In muscles: glucose → lactic acid.
• Lactic acid causes an oxygen debt, repaid by a fast heart rate and deep breathing, with the liver breaking the lactic acid down.