Surface area to volume ratio Flashcards
The relationship between the size of an organism or structure and its surface area to volume ratio
Small organisms have a surface area that is large enough, compared to their volume, to allow efficient exchange across their body surface
Larger organisms have increased volume at a faster rate than their surface area - simple diffusion of substances across the outer surface can only meet the needs of relatively inactive larger organisms
Even if the outer surface could supply enough of a substance, it would still take too long for it to reach the middle of the organism if diffusion was the only method of transport
Organisms have evolved one or more of the following features for more efficient exchange:
A flattened shape so that no cell is ever far from the surface
Specialised exchange surfaces with large areas to increase the surface area to volume ratio
Changes to body shape and the development of systems in larger organisms as adaptations that facilitate exchange as this ratio reduces
They have a large variety of specialised cells, tissues, organs and systems
Eg. gas exchange system, circulatory system, urinary system, xylem and phloem
This is why larger organisms usually have exchange surfaces and transport systems; as diffusion, osmosis and active transport cannot happen sufficiently to meet a larger organism’s needs otherwise
Transport systems in animals include:
The blood and circulatory system - carries the necessary substances around the body
Transport systems in plants include:
The xylem - moves water and mineral ions from roots to shoots
The phloem - moves sugars and amino acids to where they are needed in the plant
Multicellular organisms have surfaces and organ systems that maximise the exchange of materials by increasing the efficiency of exchange in a number of ways:
Having a large surface area to increase the rate of transport
A barrier that is as thin as possible to separate two regions, to provide as short a diffusion path as possible for substances to move across
In addition, animals have:
A large network of blood vessels throughout the body:
To reduce the distance of exchange of materials between cells and the bloodstream
To move substances towards or away from exchange surfaces to maintain concentration gradients
Gas exchange surfaces that are well ventilated to maintain concentration gradients
Fick’s Law
Rate of diffusion ∝ (surface area x concentration gradient) ÷ diffusion distance
“∝” means proportional to
According to the law, if the surface area or concentration gradient doubles, or the diffusion distance halves, then the rate of diffusion will double
Fick’s Law governs the evolution of transport systems so that they maximise the rate of diffusion
