Membranes in Biological Systems Biology is never far from the headlines. You may not know it, but biology affects our daily lives in ways that would bore you! Have you ever wondered what a cell membrane or cell is made of? The cells are surrounded by thin membranes; it is these cell membranes that separate the inside from the outside, life from non-life. A cell or plasma membrane similar to those of today's cells is believed to have defined the boundary of the first cell nearly 4 billion years ago. Since then, cells have evolved such that plasma and intracellular membranes now perform many functions: · Barrier: Membranes hold the contents of the cell together, allowing nutrients to pass but retaining many harmful substances. · Signaling: the membrane transmits information about the cell's surroundings inward and in the opposite direction. · Factory site: Membranes provide places where enzymes can be arranged like an assembly line. · Energy Conversion: Membranes allow light and chemical energy to be converted into more usable forms. · Division of the cell: In most cells, membranes separate different parts of the cell that perform different functions. · Recognition: Different cell membranes have different surfaces and interact differently with other different cells. This allows cells to “recognize” each other and act accordingly; human cells cooperate with each other while they can attack foreign cells, including harmful bacteria. Cell membranes are approximately 5 nm thick while cells range from approximately 5 µm to several meters for a giraffe's nerve cell. Red blood cells (erythrocytes) are the most abundant and simplest cells in the human body. An obvious question is how red blood cells maintain their biconcave shape. It turns out that to understand its shape you need to understand the properties of the cell membrane. It took almost 100 years to discover the structure of the cell membrane as it is understood today. In 1972 the "fluid mosaic model" was proposed and is now accepted as the basis for our understanding of the cell.