BIOLOGY Volume 1 - A Guide to General Biology - 2004

5. CELLS

5.9. Cell Membranes

Cell membranes play a crucial role for A number of reasons. They separate The Cell's contents from the external environment, regulate the exchange of substances between the cell and its surroundings (controlling the uptake of nutrients and the removal of metabolic waste), and compartmentalize the cell into distinct regions dedicated to specific metabolic pathways, such as Photosynthesis or aerobic Respiration. Certain Chemical Reactions—most notably the light-dependent reactions of photosynthesis in METABOLISM/14.html">Chloroplasts—take place directly on the membranes themselves. Furthermore, membranes house receptor sites responsible for recognizing Hormones, Neurotransmitters, and other chemical messengers arriving from the environment or from other PARTS OF THE Organism. Familiarity with all The properties of cell membranes is essential for understanding how a cell Functions.

5.9.1. Membranes Are Selectively Permeable

Since the late 19th century, it has been known that cell membranes behave quite differently from simple semipermeable membranes, which allow only Water and other small molecules, such as gases, to pass through. Cell membranes are selectively permeable: glucose, Amino Acids, Fatty acids, glycerol, and various ions diffuse across them slowly, with the membranes themselves actively regulating this process by allowing certain substances to pass while barring others.