Exchange Surfaces (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Surface Area : Volume Ratio

• The surface area to volume ratio of an organism affects how easily substances can be exchanged between it, and its environment

• Most bacteria are single-celled organisms. Bacteria have a relatively large surface area in comparison to their volume

  • This means that the distance between the cell membrane at a bacterial cell's surface, and the centre of the cell, is relatively low

  • Substances do not have to travel very far to get where they are needed, so transport by diffusion, osmosis or active transport alone is sufficient for the cell to meet its needs

Unicellular organisms can exchange materials directly with their environment

• Larger, multicellular organisms with smaller surface area: volume ratios need exchange surfaces and transport systems to ensure their cells obtain the materials they need

Calculating surface area to volume ratios

• You should be able to calculate and compare surface area to volume ratios

• We can model the effect of how increasing size affects surface: volume ratio using simple cubes

As the size of an organism increases, surface area: volume ratio decreases

Organisation

Multicellular organisms have many levels of organisation

• Cells are the basic building blocks of all living organisms

• Unicellular organisms are made from one cell, whereas multicellular organisms are made up of collections of cells

• In complex multicellular organisms, cells are specialised to carry out particular functions. These specialised cells form tissues, which form organs in organ systems

• In humans, the digestive system (provides the body with nutrients) and the respiratory system (provides the body with oxygen and removes carbon dioxide) are examples of organ systems that provide dissolved materials that need to be moved quickly around the body in the blood by the circulatory system

Effectiveness of Exchange

• Large multicellular organisms, such as humans, have a small surface area to volume ratio compared to single-celled organisms

  • This means the distance from the surface of the organism to cells deep inside the body is relatively large

  • As a result, diffusion, osmosis and active transport alone are not fast enough to meet the needs of the organism

• Instead, multicellular organisms rely on specialised exchange surfaces and transport systems

  • this allows materials such as oxygen, carbon dioxide, nutrients and waste products to be exchanged efficiently

  • Examples include the alveoli in the lungs, villi in the small intestine, and root hair cells in plants.

Features of an Effective Exchange Surface

• The effectiveness of an exchange surface is increased by:

  • A large surface area: This allows more particles to diffuse across the surface at the same time, increasing the rate of exchange.

  • A thin membrane: Exchange surfaces are only one cell thick, creating a short diffusion distance, which speeds up diffusion.

  • An efficient blood supply (in animals): Blood removes substances that have diffused across the surface and brings new substances to it. This maintains a steep concentration gradient, increasing the rate of diffusion.

  • Ventilation (for gaseous exchange in animals): Breathing continually replaces the air in the lungs, keeping oxygen levels high and carbon dioxide levels low. This also helps maintain a steep concentration gradient for gas exchange.

Many cells which are adapted for diffusion have increased surface area in some way – eg root hair cells in plants and cells lining the ileum in animals