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

Diffusion in Gas Exchange

• Particles in a liquid and gas are in constant motion

• Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient

• Diffusion is important in living organisms. Substances that may move into and out of cells by diffusion include the molecules oxygen (O₂) and carbon dioxide (CO₂)

If there is a higher concentration of a substance outside of the cell compared to the inside (or vice versa), then diffusion occurs

Diffusion in organisms

• Gas exchange refers to the exchange of the gases oxygen (O₂) and carbon dioxide (CO₂) between a cell/organism and its environment

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

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

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

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

    • Having a large surface area, providing more surface area relative to volume to allow sufficient diffusion

    • 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: Photosynthesis & Respiration

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

Gas exchange during respiration

• Plant cells, like all living cells, require energy for cellular processes. Energy is released when respiration occurs. Respiration occurs in plant cells all the time - both during the day and at night

  • In the daytime, if conditions are sufficient then photosynthesis will also occur in plants cells with chloroplasts

  • During the night when there is no light no photosynthesis occurs, but respiration will still be occurring

• Respiration requires oxygen and produces carbon dioxide

• At night, when no photosynthesis is occurring:

  • Oxygen diffuses down a concentration gradient from a higher concentration outside the leaf to a lower concentration inside the leaf

    • The cells in the leaf use oxygen in respiration so the concentration will remain low inside the respiring cells

  • Carbon dioxide diffuses down a concentration gradient from a higher concentration inside the leaf to a lower concentration outside the leaf

At night, when no photosynthesis is occurring, oxygen diffuses into the leaf and carbon dioxide diffuses out of the leaf. This is an example of gas exchange.

Gas exchange during photosynthesis

• Plant cells with chloroplasts can photosynthesise when there is enough sunlight

• Photosynthesis requires carbon dioxide and releases oxygen

  • Carbon dioxide diffuses down a concentration gradient from a region of higher concentration outside the leaf to a region of lower concentration inside the leaf

    • Cells with chloroplasts use carbon dioxide in photosynthesis so the concentration of carbon dioxide inside the photosynthesising cells is lower than outside

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

    • Note - some of the oxygen released in photosynthesis will be used in respiration!

When photosynthesis occurs, carbon dioxide diffuses into the leaf and oxygen diffuses out of the leaf.