Respiration
Visão Geral
Every second you are alive, the cells in your body are quietly taking glucose apart and using what is released to keep you going. Lift a bag, hold a thought, stay warm on a cold morning: all of it is paid for by respiration. It is the one chemical process that never stops, in every living cell, in every organism, from the moment life begins.
In this lesson you will follow the energy from a glucose molecule to a contracting muscle. You will write both equations for aerobic respiration, work out what your body changes when you start to run, and see what happens when your muscles cannot get oxygen fast enough: the switch to anaerobic respiration and the lactic acid that builds up behind it. Two sentences of this topic are printed in bold in the specification, which means they belong to the Extension Tier alone, and this note shows you exactly where that line falls so that neither tier wastes time on the wrong side of it.
Objetivos
- Respiration in cells can take place aerobically (using oxygen) or anaerobically (without oxygen), to transfer energy.
- During aerobic respiration chemical reactions occur that use glucose (a sugar) and oxygen and transfer energy.
- Aerobic respiration is summarised by the equations: glucose + oxygen → carbon dioxide + water (+ energy) C6H12O6 + 6O2 → 6CO2 + 6H2O (+ energy)
- Aerobic respiration takes place continuously in both plants and animals.
- Most of the reactions in aerobic respiration take place inside mitochondria.
- The energy that is transferred during respiration may be used by the organism in a variety of ways: to build larger molecules from smaller ones; in animals, to enable muscles to contract; in mammals and birds, to maintain a steady body temperature in colder surroundings; in plants, to build up sugars, nitrates and other nutrients into amino acids, which are then built up into proteins.
- During exercise the human body needs to react to the increased demand for energy. A number of changes take place: the heart rate increases, increasing blood flow to the muscles; the rate and depth of breathing increase; glycogen stored in the muscles is converted back to glucose.
- These changes increase the supply of glucose and oxygen to, and increase the rate of removal of carbon dioxide from, the muscles.
- If insufficient oxygen is reaching the muscles, energy is transferred by anaerobic respiration. glucose → lactic acid C6H12O6 → 2C3H6O3
- Anaerobic respiration in muscles is the incomplete breakdown of glucose, which causes a build-up of lactic acid. An oxygen debt needs to be repaid to oxidise the lactic acid to carbon dioxide and water.
- As the breakdown of glucose is incomplete, much less energy is transferred in anaerobic respiration than during aerobic respiration.
- During long periods of vigorous activity muscles become fatigued and stop contracting efficiently. One cause of muscle fatigue is the build-up of lactic acid in the muscles. Blood flowing through the muscles eventually removes the lactic acid. Required practical: Investigate the effects of exercise on the human body.
- Anaerobic respiration in plant cells and in some microorganisms results in the production of ethanol and carbon dioxide.
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- Where do most of the reactions of aerobic respiration take place? A. In the nucleus B. In the mitochondria C. In the chloroplasts D. In the cell wall Answer: B
- Which products are formed by anaerobic respiration in a human muscle cell? A. Carbon dioxide and water B. Ethanol and carbon dioxide C. Lactic acid only D. Lactic acid and oxygen Answer: C
- Which products are formed by anaerobic respiration in yeast? A. Lactic acid only B. Ethanol and carbon dioxide C. Carbon dioxide and water D. Glucose and oxygen Answer: B
- Which change does NOT happen in the human body during exercise? A. Heart rate increases B. Breathing becomes deeper C. Glycogen in the muscles is converted to glucose D. The concentration of oxygen in the air breathed in increases Answer: D
- Why does anaerobic respiration transfer less energy per glucose molecule than aerobic respiration? A. It happens outside the mitochondria B. Glucose is broken down incompletely C. It uses less glucose D. It produces carbon dioxide Answer: B
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