From Cells to Organisms WALT Explain: The relationship between the size of an organism or structure and the surface area: volume ratio, and the significance of this for the exchange of substances and of heat. Changes to body shape and the development of systems in larger organisms as adaptations to facilitate exchanges as the ratio reduces WILF To be able to Explain how surface to volume ratio differs from unicellular organisms to large multicellular organisms and how this affects exchanges of substances and heat
Diffusion and the Problem of Size • All organisms need to exchange substances such as food, waste, gases and heat with their surroundings. • These substances must diffuse between the organism and the surroundings. • The rate at which a substance can diffuse is given by Fick's law: Rate of Diffusion α Surface area × difference in concentration Thickness of membrane
The rate of exchange • The rate of exchange of substances therefore depends on the organism's surface area that is in contact with the surroundings. • The requirements for materials depends on the mass or volume of the organism, so the ability to meet the requirements depends on • which is known as the surface area : volume ratio. Surface area Thickness of membrane
As organisms get bigger their volume and surface area both get bigger, but not by the same amount. • Which has the bigger surface area to volume ratio?
As organisms get bigger their volume and surface area both get bigger, but not by the same amount. This can be seen by performing some simple calculations concerning different-sized organisms. In these calculations each organism is assumed to be cube-shaped to make the calculations easier. 8cm 2cm 8cm 2cm 2cm 8cm Surface area = 2x2 x6 = 24 Volume = 2x2x2 = 8cm SA:V = 24:8 24/8 = 3 Surface area = 8x8x6 = 384 Volume = 8x8x8 = 512cm SA:V = 384:512 384/512 = 0.75
So as organisms get bigger their surface area/volume ratio gets smaller. A bacterium is all surface with not much inside, while a whale is all insides with not much surface. This means that as organisms become bigger it becomes more difficult for them to exchange materials with their surroundings. In fact this problem sets a limit on the maximum size for a single cell of about 100 µm. In anything larger than this materials simply cannot diffuse fast enough to support the reactions needed for life.
Organisms also need to exchange heat with their surroundings Large animals have an advantage in having a small surface area/volume ratio: they lose less heat than small animals. Large mammals keep warm quite easily and don't need much insulation or heat generation. Small mammals and birds lose their heat very readily, so need a high metabolic rate in order to keep generating heat, as well as thick insulation. So large mammals can feed once every few days while small mammals must feed continuously. Human babies also loose heat more quickly than adults, which is why they need woolly hats.
So how do organisms larger than 100 µm exists? All organisms larger than 100 µm are multicellular, which means that their bodies are composed of many small cells, rather than one big cell. Each cell in a multicellular organism is no bigger than about 30µm, and so can exchange materials quickly and independently. Humans have about 1014 cells.
1 1 × 1 × 6 = 6 6 2 × 2 × 2 = 8 2 × 2 × 6 = 24 3 2 3 × 3 × 6 = 54 3 × 3 × 3 = 27 4 × 4 × 6 = 96 4 × 4 × 4 = 64 1.5 5 × 5 × 6 = 150 5 × 5 × 5 =125 1.2 6 × 6 × 6 = 216 6 × 6 × 6 = 216 1
The larger the organism, the smaller its surface area to volume ratio. • Single-celled organisms are small. Therefore the surface area is very large when compared with thevolume. This provides a large gasexchange surface.
2. For the maximum rate of diffusion, surface area and difference in concentration should be as large as possible. The thickness of the membrane should be as small as possible. In Amoeba: Surface area is large as it is a single-celled organism (look at thetable in question 1). Difference in concentration is large as oxygen is being used in respiration. Thin membrane as diffusion is only into a single cell. In the human lungs: Surface area is large as there are many alveoli and capillaries. Difference in concentration is large as oxygen is being replaced by the breathing process and removed by the blood. Thin membrane as diffusion is only through the flat squamous epithelial cells of the alveoli and the cells of the capillary walls. Low oxygen concentration at high altitudes. The folds of loose skin, through which the toad breathes, provide a larger surface area for oxygen absorption. In air, water would be lost very rapidly through the thin membrane, so the Amoeba would dehydrate.