• For cells to survive, metabolic reactions must be occurring, these reactions depend on:
  • Materials?constantly being?exchanged?across the?plasma membrane
  • The?volume/mass?of cytoplasm (as this is where the reactions take place)
• As organisms?increase in size?their?SA:V ratio decreases
  • 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 organism
• 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

• Single-celled organisms have a high SA:V ratio which allows for the exchange of substances to occur via simple diffusion
  • 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?short
• 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
• They have a large variety of specialised cells, tissues, organs and systems
  • Eg. gas exchange system, circulatory system, lymphatic system, urinary system, xylem and phloem

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.