Gas Exchange — Edexcel Linear - OL Revision Notes & Topical Questions

Particles in liquids and gases are in constant motion.

The leaf is adapted both for photosynthesis and for gas exchange: it is flat and thin, giving a large surface area and short diffusion distance; the spongy mesophyll has air spaces for gases to move; and the lower epidermis contains stomata.

Pathway of carbon dioxide in the day: atmosphere → through the open stomata → air spaces around the spongy mesophyll → dissolves in the film of water on the mesophyll cell walls → into mesophyll cells → chloroplasts, where it is used in photosynthesis.

Stomata are pores between two guard cells, mainly on the lower epidermis.

Plants photosynthesise only with sufficient light, but their cells respire all the time.

At the compensation point, at dawn and dusk, photosynthesis and respiration rates are equal so there is no net gas exchange. During the day plants photosynthesise and respire, but the rate of photosynthesis is much greater, so there is a net uptake of CO₂ and release of O₂. At night only respiration occurs.

Plants need carbon dioxide for photosynthesis and oxygen for respiration, and both must diffuse in and out through the same pores, the stomata.

Exam tip

• when asked how a plant is adapted for gas exchange, give the structure and its function, such as stomata allowing carbon dioxide in, air spaces giving a short diffusion path through the leaf, and thin flat leaves giving a large surface area.
• For night versus day, state which process is happening and the net gas movement.

Four tubes contain the same volume of hydrogencarbonate indicator (orange at atmospheric CO₂, yellow with more CO₂ and purple with less).

Any change in tube D is much smaller than in tube B. Control the volume of indicator, number and size of leaves, temperature and time.

Gas exchange occurs in the respiratory system, which lies in the thorax (chest cavity). Know the structures and the order air passes through them.

Air passes

• nose or mouth, trachea, bronchi, bronchioles, alveoli.
• The alveoli have a good blood supply, which maintains a high concentration gradient by carrying away blood rich in oxygen and bringing blood low in oxygen.

Possible improvements

• use several leaves of the same size, use a thermostatically controlled water bath, and use a colorimeter instead of judging colour by eye.
• The colour should be recorded against a colour chart.
• Plan with CORMS: change the light;
  • same species and size of leaf;
  • repeat;
  • measure the colour (before and after);
  • same conditions of volume, temperature and time.

The trachea is kept open by C-shaped rings of cartilage and is lined with ciliated cells and goblet cells that make mucus; the mucus traps dust and bacteria and the cilia sweep it up to the throat to be swallowed.

Exhaled air contains less oxygen (about 16%) and more carbon dioxide (about 4%) than inhaled air (21% oxygen and 0.04% carbon dioxide), showing that gas exchange has occurred.

Then check that you can state one function for each label, as questions often ask for a function after a label.

Muscles only pull on bones, so two sets of intercostal muscles work as antagonistic pairs. Inhalation (inspiration) and exhalation (expiration) mean breathing in and out.

In forced exhalation (during exercise), the internal intercostal muscles contract to pull the ribs down and in, decreasing the thorax volume further and forcing air out more quickly to remove the extra carbon dioxide produced.

Smoking is linked to lung disease and is a risk factor in coronary heart disease.

The brain controls the rate of breathing by detecting the concentration of carbon dioxide in the blood. When it rises, for example during exercise, the breathing centre sends impulses to the intercostal muscles and diaphragm, making them contract more often and more strongly, so breathing is faster and deeper. This keeps oxygen supply matched to demand.

The main diseases linked to smoking are lung cancer, chronic bronchitis, emphysema and coronary heart disease.

Remember to use precise terms

• external intercostal muscles raise the ribs;
  • the diaphragm contracts and flattens;
  • thorax volume increases;
  • pressure decreases.
• Reverse each word for exhalation, and then add the extra point about forced exhalation during exercise, when abdominal and internal intercostal muscles help.

A final check on terms

• ventilation is the movement of air, gas exchange is diffusion across the alveolar wall, and respiration is the release of energy in cells.
• Keep the three separate in every answer.

This investigation explores how physical activity affects breathing rate. Before you start, choose how the breathing rate will be measured: counting the number of breaths in one minute, or using a spirometer or breathing belt. Select participants from a similar age group (for example children, teenagers or adults) so that age does not affect the comparison.

It is difficult to control all variables in relation to the participants being tested, for example their fitness, lifestyle or health conditions. Exercise intensity may vary between participants; some people may work harder or less hard depending on motivation or ability. Counting breaths may be inaccurate because people change their breathing when they know they are being observed.

Safety

• exclude anyone with asthma, heart conditions or injuries, and stop immediately if a participant feels dizzy or unwell.
• Participants must consent.