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Gas Exchange Edexcel IGCSE Biology 4BI1 notes: Describe gas exchange in leaves and how it changes between day and night.In daylight photosynthesis is faster than respiration: net O2 out, CO2 in.At night only respiration occursnet O2 in, CO2 out.Air pathwaynose/mouth → trachea → b…
Gas exchange in plants
Leaf adaptations
Night vs day
Respiratory system
Ventilation
Alveoli adaptations
Part 1 of 8Key ideas
1
In daylight photosynthesis is faster than respiration: net O2 out, CO2 in.
2
At night only respiration occurs
At night only respiration occurs net O2 in, CO2 out.
The trachea is kept open by C-shaped rings of cartilage.
5
Alveolar walls and capillary walls are each one cell thick, giving a short diffusion distance.
6
Ventilation and blood flow maintain steep concentration gradients.
7
Inhaled air ~21% O2, 0.04% CO2; exhaled air ~16% O2, ~4% CO2.
Part 2 of 8Learning objectives
1
Describe gas exchange in leaves and how it changes between day and night.
2
Describe an experiment using hydrogencarbonate indicator to investigate light and gas exchange.
3
Describe the structure of the thorax and the mechanism of ventilation.
4
Explain how alveoli are adapted, and the effects of smoking.
Part 3 of 8Essential knowledge
1
Gas exchange in plants
Gas exchange in plants leaves take in carbon dioxide for photosynthesis and oxygen for respiration, and release the other gas. Gases diffuse through stomata into air spaces in the spongy mesophyll and then into cells.
2
Leaf adaptations
Leaf adaptations leaves are thin and flat (large surface area, short distance); many stomata on the lower surface; air spaces in the spongy mesophyll; guard cells open stomata in the light and close them in the dark to reduce water loss.
3
Night vs day
Night vs day in bright light the rate of photosynthesis exceeds respiration, so the leaf takes in CO2 and gives out O2. In the dark there is only respiration, so the leaf takes in O2 and releases CO2. At dawn and dusk the two rates can balance (compensation point).
4
Practical
Practical put leaves in sealed boiling tubes containing hydrogencarbonate indicator, some in light, some wrapped in gauze (dim) and some in foil (dark), with a control tube with no leaf. After a few hours, purple shows CO2 has been removed, yellow shows CO2 has built up.
5
Respiratory system
Respiratory system air enters through nose and mouth, down the trachea (supported by cartilage rings, lined with ciliated cells and mucus-producing goblet cells), into two bronchi, many bronchioles,
Structures of the human gas exchange system
and finally alveoli.
The lungs lie in the thorax, protected by the ribs and surrounded by pleural membranes, with the diaphragm below.
6
Ventilation
Ventilation external intercostal muscles and the diaphragm contract during inhalation, increasing thorax volume and lowering pressure so air flows in. During normal exhalation they relax, volume decreases, pressure rises and air flows out. Internal intercostals contract during forced exhalation.
Exhalation: diaphragm relaxation, rib movement and pressure changesInhalation: diaphragm contraction and increase in thoracic volume
7
Alveoli adaptations
Alveoli adaptations millions of alveoli give a huge surface area; walls are one cell thick; they are surrounded by a dense capillary network; a moist lining lets gases dissolve; ventilation and blood flow keep concentration gradients steep.
Diffusion at the alveolar surface
8
Smoking
Smoking tar contains carcinogens and damages cilia, leading to mucus build-up, infections, chronic bronchitis and lung cancer.
Emphysema destroys alveolar walls, reducing surface area so sufferers are breathless.
Carbon monoxide reduces oxygen transport;
nicotine is addictive and, with CO, increases the risk of coronary heart disease.
9
How guard cells open stomata
In light with plenty of water, water enters guard cells by osmosis; they become turgid, bend apart and open the pore. When water is short or light is low, guard cells lose water, become flaccid and close the stoma, reducing water loss.
10
Practical — exercise and breathing
Count breaths per minute at rest, then immediately after a fixed exercise (e.g. 1 minute of step-ups at a set pace); count for 15 s and multiply by 4. Keep exercise type, duration and conditions the same. Breathing rate and depth rise to take in more O2 and remove more CO2.
11
How emphysema develops
Smoke damage and repeated infections bring phagocytes into the lungs, which release an enzyme that digests elastic fibres in the alveolar walls. The alveoli lose elasticity and burst, leaving fewer, larger air spaces with less surface area.
Part 4 of 8Process & sequence
Intercostal muscles contract, moving the ribs up and out.
The diaphragm contracts and flattens.
Volume of the thorax increases, so pressure inside falls below atmospheric pressure.
Air is drawn in through trachea, bronchi and bronchioles to the alveoli.
Oxygen diffuses into blood; carbon dioxide diffuses into alveoli.
To exhale, muscles relax, ribs move down and in, diaphragm domes up, volume falls and pressure rises, pushing air out.
Breathing inwrite these steps in this order; each step is a marking point.
Intercostal muscles contract
Ribs move up and out
Diaphragm contracts and flattens
Thorax volume increases
Pressure falls below atmospheric
Air flows into lungs
Part 5 of 8Worked examples
1Purple
A leaf in hydrogencarbonate indicator was left in bright light for 3 hours and the indicator turned purple. Explain this result.
In bright light the rate of photosynthesis was greater than the rate of respiration, so the leaf took in more CO2 than it released.
The CO2 concentration in the tube fell, so the indicator changed from red to purple.
2Explain why a person with emphysema becomes breathless when exercising.
The alveolar walls are broken down, forming fewer, larger air spaces, so the surface area for gas exchange is reduced.
Less oxygen diffuses into the blood, so respiring muscles receive less oxygen and the person breathes faster to compensate.
Part 6 of 8Summary
Leaves exchange gases by diffusion through stomata and air spaces.
Net gas exchange in plants depends on light intensity.
Breathing in: intercostals and diaphragm contract, volume up, pressure down.
Alveoli have large surface area, thin walls and a rich blood supply.
Smoking causes lung cancer, bronchitis, emphysema and heart disease.
Part 7 of 8Keep going
Gas Exchange Edexcel IGCSE Biology 4BI1: what to do next
These Gas Exchange Edexcel IGCSE Biology 4BI1 notes covered the key ideas, worked examples and exam technique for this topic. Check your Gas Exchange Edexcel IGCSE Biology 4BI1 understanding with the practice questions below, then reread any Gas Exchange Edexcel IGCSE Biology 4BI1 point you were unsure about before moving on.
Gas Exchange Edexcel IGCSE Biology 4BI1: revision checklist
Revise the main Gas Exchange Edexcel IGCSE Biology 4BI1 definitions until you can write them from memory.
Practise Gas Exchange Edexcel IGCSE Biology 4BI1 exam questions under timed conditions.
Link each Gas Exchange Edexcel IGCSE Biology 4BI1 idea to a diagram or a worked example.
