size and surface area

exchange of substances w/ the environment

- every organism, whatever its size, needs to exchange things with its environment. cells need to take in oxygen (for aerobic respiration) and nutrients. they also need to excrete waste products like carbon dioxide and urea. most organisms need to stay at roughly the same temperature, so heat needs to be exchanged too. 

surface area : volume ratio

- an organism's surface area : volume ratio affects how quickly substances are exchanged. but before going into the effects of surface area : volume ratios, you need to understand a bit more about them. smaller organisms have higher surface area : volume ratios than larger organisms.

exchange organs & mass transport systems

- an organism needs to supply every one of its cells with substances like glucose and oxygen (for respiration). it also needs to remove waste products from every cell to avoid damaging itself. different sized organisms do this in different ways. - in single-celled organisms , these substances can diffuse directly into the cell across the cell-surface membrane. the diffusion rate is quick because of the small distances the substances have to travel. 

exchange organs & mass transport systems

- in multicellular organisms , diffusion across the outer membrane is too slow, for two reasons: some cells are deep within the body - there's a big distance between them and the outside environment.  larger animals have a low surface area to volume ratio - it's difficult to exchange enough substances to supply a large volume of animal through a relatively small outer surface.  - so rather than using straightforward diffusion to absorb and excrete substances, multicellular organisms need specialised exchange organs. - they also need an efficient system to carry substances to and from their individual cells - this is mass transport. in mammals, mass transport normally refers to the circulatory system, which uses blood to carry glucose and oxygen around the body. it also carries hormones, antibodies and waste like carbon dioxide. mass transport in plants involves the transport of water and solutes in the xylem and phloem. 

heat exchange

- as well as creating waste products which need to be transported away, the metabolic activity inside cells creates heat. staying at the right temperature is difficult and it's pretty heavily influenced by your size and shape... - body size; the rate of heat loss from an organism depends on its surface area. if an organism has a large volume, its surface area is relatively small. this makes it harder for it to lose heat from its body. if an organism is small, its relative surface area is large, so heat is lost more easily. this means smaller organisms need a relatively high metabolic rate, in order to generate enough heat to stay warm.

heat exchange

- body shape ; animals (of any size) with a compact shape have a small surface area relative to their volume - minimizing heat loss from their surface. animals with a less compact shape have a larger surface area relative to their volume - this increases heat loss from their surface. - adaptions for heat exchange ; whether an animal is compact or not depends on the temperature of its environment - the animal's body shape is adapted to suit its environment. e.g. the arctic fox has small ears and a round head to reduce its surface area : volume ratio and heat loss, making it adapted for its cold environment. 

behavioral & physical adaptations

- not all organisms have a body size or shape to suit their climate - some have other adaptations to aid exchange instead...  animals with a high surface area : volume ratio tend to lose more water as it evaporates from their surface. this is a problem particularly for animals living in hot regions where water evaporates quickly. some small desert mammals have kidney structure adaptations so that they produce less urine to compensate.  to support their high metabolic rates, small mammals living in cold regions need to eat large amounts of high energy foods such as seeds and nuts.  smaller mammals may have thick layers of fur or hibernate when the weather gets really cold.  larger organisms, living in hot regions, such as elephants and hippos, find it hard to keep cool as their heat loss is relatively slow. elephants have developed large, flat ears which increase their surface area, allowing them to lose more heat. hippos spend much of their day in the water - a behavioural adaptation to help them lose heat.