Limits to Cell Size

A cell will only reach a certain size before it divides

This is because the rate at which substances enter, pass and leave the cell must be optimised

The surface area and volume of a cell will dictate the rate at which this occurs.

Substances that need to get in the cell:

• Oxygen

• Nutrients

• Water

• Energy

Substances that need to get out of the cell:

• Carbon dioxide

• Waste products

• Energy lost

The cell membrane acts as the surface area of a cell

It is the place for exchange of heat and materials

Determines the rate at which materials enter or leave the cell

The cytoplasm and cell interior act as the volume of a cell

It is the place for the transport of materials and metabolic processes

Determine the rate at which materials are used or produced

If more surface area of a cell is available (because the cell is larger), substances like nutrients diffuse in more rapidly

However, substances are also lost faster

The larger the cell the larger its volume

The rate at which substances are transported around within the cell will become slower

Surface area and volume do not increase proportionally

The volume of the cell dictates the rate of metabolism (including transport of substances, waste production, consumption of resources and production of heat)

• If big, more effort and energy is required for all metabolic actions and processes

• If small, substances are used and processed quickly and efficiently

The surface area of the cell dictates how fast substances can enter and exit

• If big, the cell may lose heat too rapidly - and a high metabolism is needed to compensate for the loss of energy

• If small, required substances and heat cannot enter/exit the cell through the cell membrane quickly enough

The SA:V ratio decreases as the size of a cube increases. This means that the bigger a cell grows the less SA they have available for the exchange of materials in relation to their volume and internal processes.

A single cell is therefor limited in its growth

The solution to the problem of a decreases or low SA:V ratio - the cell must divide and/or organisms are becoming multicellular