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Gaseous exchange: Class 10 Biology
Gaseous exchange in plants through stomata and lenticels, the human respiratory system, the mechanism of breathing, exchange in the alveoli, transport of gases and common respiratory disorders.
Key points
Gaseous exchange is the exchange of oxygen and carbon dioxide between a living organism and its environment. It is different from respiration. Respiration is the chemical process in cells that releases energy from food, and gaseous exchange supplies the oxygen for it and removes the carbon dioxide that it produces.
Gaseous exchange takes place by diffusion across a surface that is thin, moist and large. In a plant, oxygen and carbon dioxide diffuse through stomata in the leaves, through lenticels in the woody stems and through the surface of the young roots.
Stomata. Each stoma is a small pore in the epidermis of a leaf, with two guard cells around it. When the guard cells take in water and become turgid, the stoma opens. When they lose water, the stoma closes. In the day a plant uses carbon dioxide in photosynthesis and releases oxygen, and at the same time it respires. At night only respiration goes on, so oxygen is taken in and carbon dioxide is given out.
Lenticels are small spongy patches in the bark of woody stems. They allow gases to pass into and out of the stem tissues.
The human respiratory system in the order in which air passes:
- Nose and nasal cavity: air is filtered by hairs, warmed and moistened.
- Pharynx and larynx (the voice box).
- Trachea (windpipe): a tube held open by C-shaped rings of cartilage and lined with ciliated cells and mucus that trap dust and germs.
- Bronchi, one to each lung, which divide into bronchioles.
- Alveoli: tiny air sacs at the ends of the bronchioles, where gaseous exchange takes place.
The lungs lie in the thorax (chest cavity), protected by the ribs. They are covered by two pleural membranes, with a fluid between them that reduces friction. The diaphragm is a sheet of muscle below the lungs that separates the thorax from the abdomen.
Breathing is the movement of air into and out of the lungs:
- Inspiration (breathing in): the diaphragm contracts and flattens, and the external intercostal muscles contract and move the ribs up and out. The volume of the thorax increases, the air pressure inside falls below the outside pressure, and air rushes in.
- Expiration (breathing out): the diaphragm relaxes and rises into a dome, and the ribs move down and in. The volume of the thorax decreases, the pressure rises above the outside pressure and air is pushed out.
The alveoli are well suited to gaseous exchange. There are millions of them, so the surface area is very large. Their walls are one cell thick, they are moist, and each is surrounded by a network of capillaries. Oxygen dissolves in the moisture and diffuses into the blood, where it combines with haemoglobin in the red blood cells to form oxyhaemoglobin. Carbon dioxide diffuses from the blood into the alveoli and is breathed out. Most carbon dioxide is carried in the plasma as hydrogencarbonate ions, with smaller amounts dissolved in the plasma and carried by haemoglobin.
Common respiratory disorders:
- Asthma: the bronchioles narrow because of an allergy or other triggers, and breathing becomes difficult with wheezing.
- Bronchitis: inflammation of the bronchi, often caused by infection or smoking, with coughing and extra mucus.
- Emphysema: the walls of the alveoli break down, so the surface area for gaseous exchange is reduced and breathing is difficult.
- Pneumonia: an infection of the alveoli, which fill with fluid.
- Lung cancer: uncontrolled growth of cells in the lungs. Smoking is the main cause.
- Tuberculosis (TB): a bacterial infection of the lungs that spreads in droplets from coughs.
Cigarette smoke contains tar, which damages the lining of the airways and can cause cancer, nicotine, which is addictive, and carbon monoxide, which combines with haemoglobin and reduces the oxygen that the blood can carry.
Study tip: In a breathing question, give the muscles, then the movement of the ribs and diaphragm, then the change in volume and then the change in pressure. Marks are given for each link in the chain.
Definitions and flashcards
Say the meaning first, then open the card to check.
Gaseous exchange
The exchange of oxygen and carbon dioxide between an organism and its environment.
Stoma
A pore in the epidermis of a leaf that is opened and closed by two guard cells.
Lenticel
A spongy patch in the bark of a woody stem that allows gases to pass.
Trachea
The windpipe, a tube kept open by C-shaped rings of cartilage.
Alveolus
A tiny air sac in the lung where oxygen and carbon dioxide are exchanged.
Diaphragm
A sheet of muscle that separates the thorax from the abdomen and helps in breathing.
Intercostal muscles
The muscles between the ribs that move the rib cage.
Haemoglobin
The red pigment in red blood cells that carries oxygen.
Oxyhaemoglobin
The compound formed when oxygen combines with haemoglobin.
Emphysema
A disease in which the walls of the alveoli are damaged, which reduces the surface for gaseous exchange.
Short questions with model answers
These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.
Differentiate between gaseous exchange and respiration. (2 marks)
Gaseous exchange is the diffusion of oxygen and carbon dioxide between the organism and its environment. Respiration is the chemical process in the cells in which food is broken down to release energy.
How do guard cells control the opening of a stoma? (2 marks)
When the guard cells take in water they become turgid and bend apart, and the stoma opens. When they lose water they become flaccid and the stoma closes.
Why does the trachea have rings of cartilage? (2 marks)
The C-shaped rings keep the trachea open, so air can pass at all times, and they stop it from collapsing when the pressure inside falls during inspiration.
Name three features of the alveoli that make them good for gaseous exchange. (3 marks)
Their walls are only one cell thick, so the gases have a short distance to diffuse. They are moist, so the gases dissolve. They are surrounded by a network of capillaries that carry the gases away. There are millions of them, so the surface area is large.
Describe how the lungs are filled with air during inspiration. (3 marks)
The diaphragm contracts and flattens and the external intercostal muscles contract and raise the ribs up and out. The volume of the thorax increases, so the pressure in the lungs falls below the atmospheric pressure, and air flows in.
Explain how carbon monoxide in cigarette smoke affects the body. (3 marks)
Carbon monoxide combines with haemoglobin more strongly than oxygen does. Less haemoglobin is then available to carry oxygen, so the blood carries less oxygen to the tissues, and the person feels tired and short of breath.
Describe the path of air from the nose to the site of gaseous exchange and state what happens to the air on the way. (5 marks)
Air enters the nose, where hairs filter dust and the moist lining warms and moistens it. It passes through the pharynx and larynx into the trachea, whose ciliated cells and mucus trap dust and germs and move them upward. The trachea divides into two bronchi, one to each lung, which divide into smaller bronchioles. The bronchioles end in alveoli, where oxygen diffuses into the blood and carbon dioxide diffuses out.
Common mistakes
- Writing that the diaphragm moves up during inspiration. It contracts and moves down (flattens).
- Saying that air is pulled in by the lungs. The lungs have no muscle, and the air moves in because of the pressure difference when the thorax gets bigger.
- Confusing gaseous exchange with respiration.
- Writing that oxygen and carbon dioxide are carried only by haemoglobin. Most carbon dioxide is carried as hydrogencarbonate ions in the plasma.