Respiratory systems and gas exchange: principles and simple organisms

Learning objectives

By the end of this lesson, you should be able to:

• distinguish breathing, gas exchange and cellular respiration
• explain why organisms exchange oxygen and carbon dioxide
• describe gas exchange in plants and simple aquatic organisms
• identify the features of an efficient exchange surface
• explain how diffusion gradients are maintained

Three related processes

Breathing, or ventilation, is the movement of air or water over a respiratory surface. Gas exchange is the diffusion of oxygen and carbon dioxide between an organism and its environment. Cellular respiration is the chemical process in cells that releases energy from food molecules. These terms are related but are not interchangeable.

In aerobic respiration, glucose reacts with oxygen to release energy, producing carbon dioxide and water:

glucose + oxygen → carbon dioxide + water + energy

Oxygen is needed by many cells for aerobic respiration. Carbon dioxide is produced and must be removed because excessive accumulation affects cell chemistry.

Gas exchange in plants

Plants exchange gases mainly through stomata in leaves and young green stems, and through lenticels in some woody stems. A stoma is a pore surrounded by guard cells. When guard cells become turgid, the pore usually opens; when they lose water and become flaccid, it usually closes.

During daylight, photosynthesis may use carbon dioxide and release oxygen. Respiration continues both day and night, using oxygen and releasing carbon dioxide. The net direction of gas exchange depends on the balance between photosynthesis and respiration and on environmental conditions. At night, when photosynthesis stops, respiration generally causes net oxygen uptake and carbon dioxide release.

Air spaces in the spongy mesophyll allow gases to diffuse between stomata and photosynthesising cells. The moist surfaces of mesophyll cells allow gases to dissolve before diffusing across cell membranes.

Gas exchange in simple aquatic organisms

Amoeba and other unicellular aquatic organisms exchange gases directly across the cell membrane. Dissolved oxygen diffuses into the cell and carbon dioxide diffuses out. Their small size provides a large surface-area-to-volume ratio and short diffusion distances. They do not require specialised lungs or gills.

Efficient exchange surfaces

An efficient gas-exchange surface has a large surface area, a thin barrier, a moist surface and a mechanism for maintaining a steep concentration gradient. A large area provides more space for diffusion. A thin barrier shortens the diffusion distance. Moisture allows gases to dissolve. Ventilation and blood flow, where present, continually refresh the medium and carry gases away from or towards the surface.

Diffusion occurs down a concentration gradient. If the concentration difference becomes small, net diffusion slows. Therefore, ventilation and circulation are important in larger animals because they maintain differences in oxygen and carbon dioxide concentrations between the respiratory surface and the blood or tissues.

WASSCE examination focus

Do not confuse breathing with respiration. Breathing moves air; respiration is a chemical reaction in cells. When asked for adaptations of an exchange surface, link each feature to its effect on diffusion. For plants, remember that stomata permit exchange but their opening also causes water loss through transpiration.

Quick check

1. Distinguish breathing from cellular respiration.
2. Through which structures do most leaves exchange gases?
3. Why is a moist exchange surface useful?
4. Why does gas exchange continue in a plant at night?

Answers: 1. Breathing moves air or water over a surface; cellular respiration releases energy through chemical reactions in cells. 2. Stomata. 3. Oxygen and carbon dioxide dissolve in moisture before crossing cell membranes. 4. Respiration continues even though photosynthesis stops in darkness.