Multicellular Organisms

Many organisms including man are multicellular (multi = many, multicellular = made up of many cells) organisms.

The more complex multicellular organisms cope with the problem of being vulnerable to changes in the environment by surrounding their individual cells with fluid that is separated from the environment by a barrier such as the skin.

This fluid is now the immediate environment of the cells of such multicellular organisms and is called the extracellular fluid. The extracellular fluid acts in the same way as the pond water does for the amoeba. The extracellular fluid is sometimes known as the internal environment because it is within the organism.

In complex multicellular organisms, only the surface cells that are in contact with the external environment are able to exchange substances with it. Cells within the organism however are too far away from the environment for direct exchange. This is the reason multicellular organisms have developed transport systems such as the circulatory system. The blood brings gases and nutrients from the external environment to the cells and removes waste products from them. Unless changes in the external environment alter the internal environment, there will be little effect on the cells themselves. So if the organism maintains the internal environment constant, it protects the cells from external changes. The cellular enzymes can function efficiently at all times no matter what is happening in the external environment. By maintaining the internal environment constant, multicellular organisms are less vulnerable to changes in the external environment. Thus the cellular enzymes can function efficiently at all times.