Diffusion and the Problem of Size
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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:

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 volume of the organism, so the ability to meet the requirements depends on the surface area : volume ratio. As organisms get bigger their volume and surface area both get bigger, but volume increases much more than surface area.. This can be seen with some simple calculations for different-sized organisms. In these calculations each organism is assumed to be cube-shaped to make the calculations easier. The surface area of a cube with length of side L is LxL X6 (6L²), while the volume is L³.  

 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 mm. In anything larger than this materials simply cannot diffuse fast enough to support the reactions needed for life. Very large cells like birds' eggs are mostly inert food storage with a thin layer of living cytoplasm round the outside.

Organisms also need to exchange heat with their surroundings, and here 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 mm exists? All organisms larger than 100 mm 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 30mm, and so can exchange materials quickly and independently. Humans have about 10¹⁴ cells.

Cell Differentiation    [back to top]

Multicellular organisms have another difference from unicellular ones: their cells are specialised, or differentiated to perform different functions. So the cells in a leaf are different from those in a root or stem, and the cells in a brain are different from those in skin or muscle. In a unicellular organism (like bacteria or yeast) all the cells are alike, and each performs all the functions of the organism. 

Cell differentiation leads to higher levels of organisation: A tissue is a group of similar cells performing a particular function. Simple tissues are composed of one type of cell, while compound tissues are composed of more than one type of cell. Some examples of animal tissues are: epithelium (lining tissue), connective, skeletal, nerve, muscle, blood, glandular. Some examples of plant tissues are: epithelium, meristem, epidermis, vascular, leaf, chollenchyma, sclerenchyma, parenchyma. An organ is a group of physically-linked different tissues working together as a functional unit. For example the stomach is an organ composed of epithelium, muscular, glandular and blood tissues. A system is a group of organs working together to carry out a specific complex function. Humans have seven main systems: the circulatory, digestive, nervous, respiratory, reproductive, urinary and muscular-skeletal systems.

A multicellular organism like a human starts off life as a single cell (the zygote), but after a number of cell divisions cells change and develop in different ways, eventually becoming different tissues. This process of differentiation is one of the most fascinating and least-understood areas of modern biology. For some organisms differentiation is reversible, so for example we can take a leaf cell and grow it into a complete plant with roots, stem, leaf and vascular tissue. However for humans and other mammals differentiation appears to be irreversible, so we cannot grow new humans from a few cells, or even grow a new arm.

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Last updated 18/06/2004