Unit 8 Gaseous exchange in humans and plants
Key unit competence: Explain the mechanism of gas exchange in humans and plants and the effects of smoking and respiratory diseases.
Key ideas
- Respiration is the release of energy from food (glucose) in cells. Breathing (ventilation) is the movement of air into and out of the lungs. Gas exchange is the diffusion of oxygen into and carbon dioxide out of the body across a respiratory surface.
- A good respiratory surface has a large surface area, is thin (short diffusion distance), is moist (gases dissolve), has a good blood supply (in animals) and is well ventilated to keep a steep concentration gradient. Examples: alveoli (mammals), gills (fish), skin (frog, earthworm), tracheae (insects), stomata and air spaces (plants).
- Inhalation: the external intercostal muscles contract, the ribs move up and out; the diaphragm contracts and flattens; the thorax volume increases, pressure falls below atmospheric, and air rushes in.
- Exhalation (at rest): the intercostal muscles relax (ribs move down and in), the diaphragm relaxes and domes upward; volume decreases, pressure rises and air is pushed out. In forced exhalation the internal intercostal muscles contract.
- In each alveolus, oxygen diffuses from the air into the blood (and joins haemoglobin in red cells), and carbon dioxide diffuses from the blood into the air. Alveoli: millions (huge surface area), walls one cell thick, moist, surrounded by capillaries.
- Inhaled air contains about 21% oxygen and 0.04% carbon dioxide; exhaled air about 16% oxygen and 4% carbon dioxide, and more water vapour, at body temperature. Limewater turns milky faster with exhaled air.
- Smoking: tar causes cancer and paralyses cilia; nicotine is addictive and raises heart rate and blood pressure; carbon monoxide combines with haemoglobin and reduces oxygen transport. Effects: lung cancer, chronic bronchitis, emphysema (alveolar walls break down, less surface area), heart disease; smoking in pregnancy lowers birth weight.
- Other respiratory diseases: tuberculosis (bacterium, spread by droplets; prevented by BCG vaccine, treated by antibiotics), pneumonia (infection fills alveoli with fluid), asthma (airways narrow, triggered by dust, smoke, pollen), influenza and COVID-19 (viruses).
- In plants, gas exchange happens by diffusion through stomata (leaves), lenticels (woody stems) and root hairs. In light, photosynthesis is faster than respiration, so plants take in CO₂ and give out O₂; in the dark only respiration happens, so they take in O₂ and give out CO₂. At the compensation point the two are equal.
- Guard cells open stomata when they take in water and become turgid (in light) and close them when flaccid (dark, water shortage).
Common misconceptions
- "Breathing and respiration are the same." No: breathing moves air; respiration is a chemical process in cells that releases energy.
- "Air is sucked in because the lungs expand themselves." No: the lungs have no muscles; the intercostal muscles and diaphragm change thorax volume and pressure.
- "Plants only respire at night." No: plants respire all the time; in light, photosynthesis hides it because it is faster.
- "Exhaled air has no oxygen." No: it still has about 16% oxygen, which is why mouth-to-mouth resuscitation works.
Worked examples
Example 1. Explain how air enters the lungs during inhalation.
Solution
The external intercostal muscles contract, pulling the ribs up and out; the diaphragm contracts and flattens. The volume of the thorax increases, so the pressure inside the lungs falls below atmospheric pressure, and air is pushed in through the trachea.
Example 2. A leaf in a closed tube with hydrogencarbonate indicator (red) turns it purple in light and yellow in the dark. Explain.
Solution
In light, photosynthesis is faster than respiration, so the leaf removes CO₂; the indicator becomes less acidic (purple). In the dark, only respiration occurs, adding CO₂; the indicator becomes more acidic (yellow).
Practice questions
Distinguish between breathing and respiration.
Show answer
Breathing is the physical movement of air into and out of the lungs; respiration is the chemical breakdown of food in cells to release energy.
Common mistake: Saying respiration happens in the lungs only.
Describe the mechanism of exhalation at rest.
Show answer
External intercostal muscles relax, ribs move down and in; diaphragm relaxes and domes upward; thorax volume decreases, pressure inside rises above atmospheric, air is forced out.
Common mistake: Saying the diaphragm contracts during exhalation.
State four features of alveoli that make gas exchange efficient.
Show answer
Very many alveoli give a large surface area; walls one cell thick; moist lining; dense capillary network; well ventilated.
Common mistake: Listing features without explaining.
State three harmful substances in tobacco smoke and the effect of each.
Show answer
Tar: causes cancer and paralyses cilia. Nicotine: addictive, raises heart rate and blood pressure. Carbon monoxide: reduces oxygen carried by haemoglobin.
Common mistake: Mixing up the effects of tar and nicotine.
Give three differences between inhaled and exhaled air.
Show answer
Exhaled air has less oxygen (16% vs 21%), more carbon dioxide (4% vs 0.04%), more water vapour, and is warmer.
Common mistake: Saying exhaled air has no oxygen.
State two ways of preventing tuberculosis.
Show answer
BCG vaccination; good ventilation of rooms; covering the mouth when coughing; early treatment of patients; avoiding overcrowding.
Common mistake: Saying TB is prevented by mosquito nets.
Explain why plants give out carbon dioxide at night but oxygen during the day.
Show answer
Plants respire all the time, producing CO₂. In daylight, photosynthesis is faster than respiration, so there is a net uptake of CO₂ and output of O₂. At night there is no photosynthesis, so only CO₂ is released.
Common mistake: Saying plants respire only at night.
Name the respiratory surface of: (a) a fish; (b) an earthworm; (c) an insect; (d) a leaf.
Show answer
(a) Gills; (b) moist skin; (c) tracheae/tracheoles; (d) mesophyll cell surfaces in air spaces (via stomata).
Common mistake: Saying fish use lungs.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Gas exchange in the lungs takes place in the: a. BCG vaccine 2. Tar in tobacco smoke causes: b. lung cancer 3. Tuberculosis can be prevented by the: c. alveoli 4. Exhaled air contains about: d. 4% carbon dioxide Show answer
1 → c (alveoli) · 2 → b (lung cancer) · 3 → a (BCG vaccine) · 4 → d (4% carbon dioxide)
- Alveoli are the air sacs.
- Tar contains carcinogens.
- BCG is given to babies in Rwanda.
- Inhaled air has 0.04%.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
saturation point · lenticels · compensation point · oxygen carried by the blood- The point where photosynthesis equals respiration is the .
- Carbon monoxide in smoke reduces .
- Woody stems exchange gases through .
Show answer
1: compensation point · 2: oxygen carried by the blood · 3: lenticels
- There is no net gas exchange.
- It binds to haemoglobin.
- Lenticels are pores in the bark.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The substance in tobacco smoke that is addictive is: a. up and outwards 2. During inhalation, the ribs move: b. respiration 3. The release of energy from food in cells is: c. become turgid 4. Guard cells open the stoma when they: d. nicotine Show answer
1 → d (nicotine) · 2 → a (up and outwards) · 3 → b (respiration) · 4 → c (become turgid)
- Nicotine also raises heart rate.
- External intercostal muscles contract.
- It is a chemical process.
- Turgid guard cells bend apart.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
stomata · bacterium · lenticels · haemoglobin- Gases enter and leave a leaf mainly through .
- Tuberculosis is caused by a .
- Oxygen is carried in red blood cells by .
Show answer
1: stomata · 2: bacterium · 3: haemoglobin
- Stomata are pores in the epidermis.
- It is treated with antibiotics.
- It forms oxyhaemoglobin.