Gas Exchange in Plants (Edexcel IGCSE Biology (Modular)) : Revision Note

Diffusion in Gas Exchange

• Diffusion is the movement of molecules from a region of its higher concentration to a region of its lower concentration

• Molecules move down a concentration gradient, as a result of their random movement

Diffusion across the cell membrane

Diffusion in organisms

• Diffusion drives the process of gas exchange 

• Single-celled organisms, such as amoeba, can exchange gases sufficiently by simple diffusion through the cell membrane

• Multicellular organisms (such as plants and animals), however, have exchange surfaces and organ systems that maximise the exchange of materials

  • Gills are the gas exchange organs in fish, lungs in humans and leaves or roots in plants

• These organs increase the efficiency of exchange in several ways:

• Having a large surface area to increase the rate of transport

• A short diffusion distance for substances to move across. This short distance is created because the barrier that separates two regions is as thin as possible

• In addition, animals have:

  • Gas exchange surfaces that are well-ventilated to maintain steep concentration gradients

Gas exchange in single-celled organisms (such as amoeba) occurs through diffusion

Gas Exchange: Photosynthesis & Respiration

• The processes of respiration and photosynthesis both rely on the exchange of oxygen and carbon dioxide

Gas exchange during respiration

• All living cells respire

• The process requires the uptake of oxygen and the release of carbon dioxide

• Oxygen diffuses down the concentration gradient from a high concentration (outside the leaf) to a low concentration (inside the leaf)

  • The cells use oxygen in respiration so the concentration is always low inside the respiring cells

• The carbon dioxide diffuses down the concentration gradient from a high concentration (inside the leaf) to a low concentration (outside the leaf)

In respiration, carbon dioxide diffuses out of the leaf and oxygen diffuses into the leaf

Gas exchange during photosynthesis

• Plant cells photosynthesize when there is enough sunlight

• The process requires the uptake of carbon dioxide and the release of oxygen

• The carbon dioxide diffuses down the concentration gradient from a region of high concentration (outside the leaf) to a region of low concentration (inside the leaf)

  • The cells use carbon dioxide in photosynthesis so the concentration is always low inside the photosynthesizing cells

• The oxygen diffuses down the concentration gradient from a high concentration (inside the leaf) to a low concentration (outside the leaf)

In photosynthesis, carbon dioxide diffuses into the leaf and oxygen diffuses out of the leaf