IB DP Biology: HL复习笔记1.1.3 Surface Area to Volume Ratio

• • Materials constantly being exchanged across the plasma membrane
  • The volume/mass of cytoplasm (as this is where the reactions take place)For cells to survive, metabolic reactions must be occurring, these reactions depend on:
• • There is less surface area for the absorption of nutrients and gases and secretion of waste products
  • The greater volume results in a longer diffusion distance to the cells and tissues of the organismAs organisms increase in size their SA:V ratio decreases
• Thus the rate at which substances (e.g. oxygen and heat) are exchanged across the plasma membrane is dependent on the surface area (the larger the surface area the more substances are exchanged)
• The rate at which a cell metabolises is dependent on the mass/volume of the cytoplasm (the larger the mass/volume the longer it takes for metabolic reactions to occur)

Limitations to cell size

• • The large surface area allows for maximum absorption of nutrients and gases and secretion of waste products
  • The small volume means the diffusion distance to all organelles is shortSingle-celled organisms have a high SA:V ratio which allows for the exchange of substances to occur via simple diffusion
• A consequence of the SA:V ratio is that cells cannot grow bigger indefinitely. Once the ratio becomes too small, growth must stop and the cells must divide
• To overcome this, large multicellular animals and plants have evolved adaptations to facilitate the exchange of substances between their environment
• • Eg. gas exchange system, circulatory system, lymphatic system, urinary system, xylem and phloemThey have a large variety of specialised cells, tissues, organs and systems

As the size of an organism increases, it’s surface area : volume ratio decreases. Notice for this particular shape the distance between the surface and the centre increases with size.