Unit 2 AQA Biology - Size and Surface Area

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Size and surface area
All organisms need to exchange materials with their surroundings. Things such as:
Respiratory gases ­ oxygen, carbon dioxide
Nutrients ­ glucose, fatty acids, amino acids, vitamins, minerals
Excretory products ­ urea, carbon dioxide
This exchange can take place in two ways:
Passively ­ no energy is required, by diffusion and osmosis
Actively ­ energy is required, by active transport
The size and surface area problem:
The amount needing to be exchanged varies on according to the animal's volume and
amount of living tissues it has.
However, the quantity of a particular material that an organism is able to exchange is
proportional to its surface area. As organisms get larger, the surface area: volume ratio
decreases. For larger organisms, this effect is a problem. It makes it increasingly difficult to
exchange both materials and heat with the environment fast enough to keep conditions
inside the organism constant.
Surface area, heat and metabolic rate:
The control of body temperature is known as thermoregulation. Mammals and birds can
maintain a constant core body temperature using their physiology and their behaviour. They
are said to be endothermic because their body heat is generated inside the body.
All other animals are, to a large extent, at the mercy of the environment. They can gain or
lose heat by their behaviour. However, their body temperature generally reflects that of their
surroundings. These animals are said to be ectothermic because their body heat comes
mainly from outside their body.
Mammals and birds produce heat as a by-product of metabolism, and thermoregulate by
controlling its loss to the environment. The amount of heat an organism can make depends
on two things.
Its volume, in other words, the mass of living tissue it has
Its metabolic rate, the number and speed of chemical reactions going on in its
Metabolic rate can be measured by the amount of oxygen used or by the amount of heat

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A large animal such as an elephant has a lot of heat-producing tissue and a relatively small
surface area through which to lose it. Such an animal needs strategies to lose heat. These
can include:
Physical adaptions ­ i.e. large ears with a good blood supply near to the surface, to
lose heat from the blood
Behavioural adaptions ­ increasing evaporation of water from the body surface by
mud and water bathing
Physiological adaptions ­ having a relatively low metabolic rate.…read more


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