3.3.1 Surface area to volume ratio

Surface area to volume ratio

High surface area to volume ratio: These organisms have a large surface area relative to their volume so the diffusion of the substances is fast as there is a small diffusion distance to the centre

Low surface area to volume ratio: These organisms have a small surface area relative to their volume ratio so the diffusion of substances is slower, and so specialised exchange surfaces are needed

• reduces the area available for diffusion per unit time

Multicellular organisms require specialised surfaces

-In single-celled organisms, substances diffuse directly across the cell membrane.

-This cannot happen in multicellular organisms because:

• Cells are not in direct contact with the external environment.

• Diffusion distances between cells and their environment are large.

• Larger organisms have higher metabolic rates, so they need more oxygen and glucose.
  
  

-To solve this problem, multicellular organisms have evolved specialised exchange surfaces.

There are two linked levels of exchange:

• At an exchange surface, substances move between the external and the internal environment of the organism

• At an individual cell, substances move between tissue fluid and the inside of the cell by crossing its plasma membrane

Metabolic rate

-Metabolic rate- the amount of energy expended by an organism within a time period 

-Basal metabolic rate (BMR)- metabolic rate of an organism when at rest 

-Metabolic rate is measured using different methods and apparatus:

• Oxygen consumptions (Respirometers)

• Carbon dioxide production (Carbon dioxide probe)

• Heat production (Calorimeter)

Features of specialised exchange surfaces

-Large surface area to volume ratio

-Thin- short diffusion pathway 

-Partially permeable- osmosis 

-Movement of the environmental medium- eg. air

-Movement of the internal environmental medium, eg. blood 

Ways to increase surface area to volume ratio

-Folding in the surface of the cell membrane to increase the surface area

-Developing into long, thin, or elongated shaped cells

-Presence of large vacuoles inside the cell that pushes the other organelles to the side of the cell for easier diffusion of materials.