Topic 3: Exchange: Surface area to volume ratio

What is meant by the external environment of an organism?

The external environment is the immediate surroundings of a cell or tissue with which it exchanges material

Organisms needs to exchange materials like oxygen, glucose, excretory products like urea, and heat with their environment

This occurs across plasma membranes

High SA:Volume

These organisms have a large surface area relative to their volume so the diffusion of substances is fast. Generally smaller organisms have a higher surface area:volume ratio. Taller and thinner shapes

Low SA:Volume

Organisms have a smaller surface area relative to their volume so diffusion of substances is lower, large organisms have a lower SA:Volume ratio. Round and bigger.

Unicellular organisms don’t need exchange services as their cell service membrane is their exchange surface

Substances can diffuse directly across the membrane

Substances cannot diffuse directly across the membrane 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

What have multicellular organisms evolved to solve the issue of diffusion?

Specialised exchange surfaces

Surface area

The total area of the organism that is exposed to the external environment

Volume

Total internal volume of the organism (total amount of space inside the organism)

Key features of specialised exchange surfaces

• Large surface area – provides a large larger area across which substances can be exchanged

• Thin walls – these minimise the diffusion distance

• An extensive blood supply and/or ventilation – this maintains steep concentration gradient

• Being surrounded by selectively permeable plasma membrane – controls which substances are exchanged

Small organisms

• E.g. amoeba

• Have a very large surface area in comparison to their volume

• This means that there is a big surface area for exchange of substances, but also there is a smaller distance from outside to middle of the organism

• As a result, very small organisms can simply exchange substances across their surface

Large organisms

• Smaller surface area to volume ratio

• Larger distance from middle to outside

• Higher metabolic rate too, which demands efficient transport of waste out of cells and reactants into the cell

• As a result, they have adaptations to help make exchange across surfaces more efficient

Small objects

Rate of heat loss is fast and rate of diffusion is fast

Large objects

Rate of heat loss is slow and rate of diffusion is slow

Adaptations to increase SA:Volume ratio

1. Villi and microvilli - absorption of digested food

2. Alveoli and bronchioles – gas exchange

3. Spiracles and tracheoles - gas exchange

4. Gill filaments and Lamellae – gas exchange

5. Thin wide leaves – gas exchange

6. Many capillaries – capillary network

Behavioural and physiological adaptations to aid exchange in kangaroo rat

• High surface area to volume ratio, lose more water, have a kidney structure that produces less urine (physiological adaptation – to do with functioning of organ systems)

Behavioural and physiological adaptations to aid exchange in cold regions

• have high metabolic rate, need to eat large amounts of high energy food such as seeds and nuts (behavioural adaptation)

• Small mammals have thick fur (structural adaptation) or hibernate (behavioural) when the weather gets really cold

Behavioural and physiological adaptations to aid exchange in large mammals

• Structural adaptation – large mammals living in hot regions – they have large flat areas which increase the surface area, allowing them to lose more heat

• Behaviour adaptation – hippo spend much of the day in water

Metabolic rate

Amount of energy expended by an organism in a given period

Basal metabolic rate (BMR)

The metabolic rate at rest. BMR lower than when organism is actively moving.

Metabolic rate can be measured by

1. Oxygen consumption

2. Carbon dioxide production

3. Heat production

All organisms need to exchange substances with the environment to survive

They need to take in oxygen and nutrients and remove carbon dioxide and urea. Temperature and water levels also need to be kept constant so heat and water also need to be exchanged.

Metabolic rate

The amount of energy expanded by an organism in a time period, usually daily

Metabolic demand

How much oxygen and nutrients and organism needs to take in daily to respire enough to maintain the metabolic rate

Larger organisms – surface area and metabolic rate

As a general rule, the greater the mass of an organism the higher that organisms metabolic rate this is because organisms of high metabolic rates require more efficient delivery of oxygen to cells as more respiration is needed

Smaller organisms – surface area and metabolic rate

Small animals have a lower metabolic rate because they have a greater SA: Vol but they lose heat more easily. this means they need more energy and a higher metabolic rate to maintain a constant internal temperature so per unit of body mass metabolic rate is actually higher in small animals compared to larger ones. Organisms will therefore evolve and develop adaptations to increase or decrease the surface area to match their metabolic demands. This means that organisms with similar volumes in different environments may need to have different surface areas.

Large surface area relative to volume

Increases the rate of exchange

Very thin

So diffusion distance is short and therefore materials cross the exchange surface rapidly

Selectively permeable

To allow selected molecules to cross

Movement of the environmental medium

To maintain a diffusion gradient

A transport system

Maintains a concentration gradient

Rate of diffusion directly proportional to (Flick’s law)

(Surface area x concentration difference)/length of diffusion path