Surface Area: Volume

SA:V ratio in unicellular organisms

- Small size means there is a large SA:V ratio and so it is adequate for many materials to move into and out of the cells by diffusion and active transport. - But once the organism's size reaches a limit, the cell must divide in order to create two smaller organisms with larger SA:V ratios

Mitochondria and SA: V

A mitochondria is able to maximize its surface area: volume ratio because in a mitochondria there are cristae that increase the surface area but not the volume. Therefore, the SA:V ratio is able to be increased since the numerator (SA) while the denominator (V) stays the same.

SA:V importance

Cells need to be small because they rely on diffusion for getting substances into and out of their cells

As a cell grows, there is more membrane for substances to diffuse which result in the center of the cell not receiving the substances it needs

Diffusion is less efficient, cell processes slow down and the cell stops growing

SA: V ratio in multicellular organisms

Each plant and animal cell has a large SA:V ratio by having features such as gas exchange organs and circulatory systems to speed up and help the movement of materials in and out of the organism

Small cells are more efficient at diffusion as they have a HIGH SA:V ratio

Small cells are more efficient at diffusion as they have a HIGH SA:V ratio

What to do to a cell if cell reaches a limit

The cell will need to divide into two smaller cells which result in a larger SA:V ratio and so the materials can diffuse more easily

Agar lab

Think about the pink agar lab where the 4 cm cube had a larger white space because there was more membrane to get through and shows that the large cell takes long and is less efficient at diffusing materials into the center of the cell

small object

large SA:V ratio (large numerator/small denominator)

Volume

refers to the amount of space inside the object (the space inside the cell) units: cm3

surface area

refers to the outside area of an object (the area around the outside of a cell) units: cm2

large object

small SA:V ratio

As the cube size increases or the cell gets bigger....

the surface area to volume ratio decreases

Mathematical reasoning as to why SA:V ratio decreases when a cell increases in size

when a cell grows, the volume increases at a rate greater than the surface so therefore SA:V decreases (small numerator/large denominator)