Biology: Respiration, Digestion and Gas Exchange
Respiration, digestion and gas exchange, in 13+ Biology, covers how cells release energy from food, how the digestive system breaks food down using enzymes so it can be absorbed, and how the lungs are adapted to exchange gases efficiently with the blood.
Method
- Learn the word equation for aerobic respiration: glucose + oxygen produces carbon dioxide + water (releasing energy), and know that it happens continuously in every living cell, mostly in the mitochondria.
- Learn that anaerobic respiration happens when the body cannot supply enough oxygen (for example during vigorous exercise); in humans it converts glucose into lactic acid instead of carbon dioxide and water, and releases much less energy than aerobic respiration.
- Learn the three main digestive enzymes, the food group each one digests, and the simple product formed: amylase digests starch into sugars, protease digests protein into amino acids, and lipase digests fats into fatty acids and glycerol.
- Know the standard food test practical: iodine solution tests for starch (turns from orange-brown to blue-black), Benedict's solution tests for reducing sugar (turns from blue to green, yellow, orange or brick-red when heated in a water bath), and biuret solution tests for protein (turns from blue to purple/lilac); test tubes must be rinsed out between tests, or a trace of one reagent can contaminate the next test and give a false result.
- Learn that bile, made in the liver and stored in the gall bladder, is not an enzyme but emulsifies fats (breaks large fat drops into many smaller droplets), which increases the surface area available for lipase to act on.
- Learn that digested food is absorbed into the blood through the wall of the small intestine, which is adapted for this with millions of finger-like villi that give it a large surface area, a good blood supply to maintain a concentration gradient, and a wall only one cell thick.
- Learn the pathway air takes into the lungs, in order: trachea, bronchi, bronchioles, alveoli; learn that alveoli are adapted for efficient gas exchange with a large surface area, thin (one-cell-thick) walls, a moist lining, and a rich network of capillaries.
- Learn how breathing in (inhalation) works: the intercostal muscles and the diaphragm contract, the ribcage moves up and out and the diaphragm flattens, which increases the volume of the chest cavity and lowers the pressure inside it below atmospheric pressure, so air is drawn in.
Worked example
A pupil carries out food tests on an unknown food sample. Adding iodine solution turns the sample blue-black. Adding Benedict's solution and heating produces no colour change (it stays blue). Identify which nutrient(s) the food sample contains, and explain your reasoning.
- Recall that iodine solution tests for starch, and a colour change from orange-brown to blue-black is a positive result.
- The sample turned blue-black with iodine, so the food contains starch.
- Recall that Benedict's solution tests for reducing sugar, and a positive result is a colour change from blue to green, yellow, orange or brick-red on heating.
- The Benedict's test produced no colour change (it stayed blue), which is a negative result, so the food does not contain a significant amount of reducing sugar.
- Final answer: the food sample contains starch (a positive iodine test) but does not contain reducing sugar (a negative Benedict's test).
Practice questions
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Q1Complete the word equation for aerobic respiration: glucose + oxygen produces ___ + ___.Show answer
Answer: Carbon dioxide + water
Q2Name the substance that builds up in muscles during anaerobic respiration and causes fatigue.Show answer
Answer: Lactic acid
Q3Name the enzyme that digests starch, and state the product it is broken down into.Show answer
Answer: Amylase; sugar(s) (maltose/glucose)
Q4A food sample turns from blue to brick-red when Benedict's solution is added and the mixture is heated. State which nutrient this positive result shows is present.Show answer
Answer: Reducing sugar
Q5State one way bile helps with the digestion of fats.Show answer
Answer: It emulsifies fats, breaking them into smaller droplets, which increases the surface area available for lipase to work on
Q6Put these structures in the order air passes through them when breathing in, starting from the windpipe: alveoli, bronchi, trachea, bronchioles.Show answer
Answer: Trachea, bronchi, bronchioles, alveoli
Q7State one way the alveoli are adapted for efficient gas exchange.Show answer
Answer: Any one of: a large surface area, walls that are one cell thick (a short diffusion distance), a moist lining (gases dissolve before diffusing), or a rich blood supply (capillary network)
Exam-style questions
Written in the style of a 13+ Common Entrance exam paper, with a full mark scheme.
A sprinter runs a 100 m race in 11 seconds. During the race, her leg muscles cannot get enough oxygen from her blood to meet the demand for energy. (a) Name the type of respiration that happens in her muscles when oxygen supply cannot meet demand. (b) Write a word equation for this type of respiration in a human, starting from glucose. (c) State one difference between this type of respiration and aerobic respiration, other than the type of waste product formed.
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The small intestine is well adapted for the absorption of digested food. (a) Name the finger-like structures that increase the surface area of the small intestine wall. (b) State two other features of the small intestine wall that make it well adapted for absorption, other than having a large surface area. (c) Explain why food must be digested into small, soluble molecules before it can be absorbed into the blood.
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A student breathes in deeply. (a) State what happens to the diaphragm during inhalation (breathing in). (b) State what happens to the volume of the chest cavity during inhalation, and explain how this causes air to move into the lungs. (c) Name the muscles, found between the ribs, that contract to help move the ribcage upward and outward during inhalation.
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Free printable worksheet
Want more practice on paper? Download the biology: respiration, digestion and gas exchange worksheet pack - 20 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 5 of 13+ Science Workbook, the whole course as one free printable PDF.
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