Biology

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The Cell Surface Membrane B io F actsheet September 1997 Number 8 1 The cell surface membrane (formerly called the plasma membrane) surrounds the cytoplasm of eukaryotic cells. The membrane forms a selectively permeable barrier, controlling the substances that enter and leave the cell and therefore enables the cell to regulate its internal environment. 2. Glycolipids - which make up 5% of membrane lipids. Glycolipids occur on the external surface of the cell surface membrane and the carbohydrate portion of the glycolipid extends into the intercellular space and is called a glycocalyx. These are important in cell-to-cell recognition. 3. Cholesterol - a steroid which makes up 20% of lipids in animal membranes but is rarely found in plant cell membranes.

1 B io F actsheet September 1997 Number 1 The kidney: excretion and osmoregulation Kidneys have two main functions. 1. They are excretory organs, removing nitrogenous and other waste from the body. 2. They play an important part in maintaining a constant internal environment by helping to regulate pH, water and sodium ion concentrations in the blood and tissues. This Factsheet will focus on the role of the kidney in excretion and osmoregulation. Excretion Surplus nitrogen-containing compounds such as amino acids have to be broken down in the body because they are toxic and are then excreted as ammonia, urea or uric acid (Table 1). Table 1. Nitrogenous excretory products Renal corpuscle Proximal convoluted tubule Distal convoluted tubule Collecting duct Ascending limb