Biology chapter 5

• Membranes are the structures that separate the contents of cells from their environment and also separate the different areas within cells (organelles) from each other and the cytosol.
• Some organelles are divided further by internal membranes.
• The formation of separate membrane-bound areas in a cell is called compartmentalisation.
• When plant cells are placed in a solution with a lower water potential than their own, water is lost from the cells by osmosis, leading to a reduction in the volume of the cytoplasm, which eventually pulls the cell-surface membrane away from the cell wall, causing the cell to be plasmolysed.
• Compartmentalisation is vital to a cell as metabolism includes many different and often incompatible reactions.
• Containing reactions in separate parts of the cell allows the specific conditions required for cellular reactions, such as chemical gradients, to be maintained, and protects vital cell components.
• All the membranes in a cell have the same basic structure.
• The cell surface membrane which separates the cell from its external environment is known as the plasma membrane.
• Membranes are formed from a phospholipid bilayer.
• The hydrophilic phosphate heads of the phospholipids form both the inner and outer surface of a membrane, sandwiching the fatty acid tails of the phospholipids to form a hydrophobic core inside the membrane.
• Cells normally exist in aqueous environments.
• The inside of cells and organelles are also usually aqueous environments.
• Phospholipid bilayers are perfectly suited as membranes because the outer surfaces of the hydrophilic phosphate heads can interact with water.
• Membranes were seen for the first time following the invention of electron microscopy, which allowed images to be taken with higher magnification and resolution.
• Images taken in the 1950s showed the membrane as two black parallel lines - supporting an earlier theory that membranes were composed of a lipid bilayer.
• In 1972 American scientists Singer and Nicolson proposed a model, building upon an earlier lipid-bilayer model, in which proteins occupy various positions in the membrane.
• The model is known as the fluid- mosaic model because the phospholipids are free to move within the layer relative to each other (they are fluid), giving the membrane flexibility, and because the proteins embedded in the bilayer vary in shape, size, and position (in the same way as the tiles of a mosaic).
• Membrane proteins have important roles in the various functions of membranes.
• There are two types of proteins in the cell-surface membrane - intrinsic and extrinsic proteins.
• Intrinsic proteins, or integral proteins, are transmembrane proteins that are embedded through both layers of a membrane.
• Intrinsic proteins have amino acids with hydrophobic R-groups on their external surfaces, which interact with the hydrophobic core of the membrane, keeping them in place.
• Channel and carrier proteins are intrinsic proteins.
• Channel proteins provide a hydrophilic channel that allows the passive movement of polar molecules and ions down a concentration gradient through membranes.
• Carrier proteins have an important role in both passive transport and active transport into cells.
• Glycoproteins are intrinsic proteins that are embedded in the cell-surface membrane with attached carbohydrate (sugar) chains of varying lengths and shapes.
• Glycoproteins play a role in cell adhesion and as receptors for chemical signals.
• Glycolipids are similar to glycoproteins, they are lipids with attached carbohydrate (sugar) chains.
• Glycolipids are cell markers or antigens and can be recognised by the cells of the immune system as self or non-self.
• Polar molecules, such as water (H,O) with partial positive and negative charges, can diffuse through membranes, but only at a very slow rate.
• The general process of active transport is described below - in this example transport is from outside to inside a cell.
• The non-polar tails of the phospholipids are orientated away from the water, forming a bilayer with a hydrophobic core.
• Bulk transport is another form of active transport.
• On the inside of the cell ATP binds to the carrier protein and is hydrolysed into ADP and phosphate.
• The rate at which molecules or ions diffuse across membranes is affected by surface area and membrane thickness.
• Facilitated diffusion can also involve carrier proteins, which change shape when a specific molecule binds.
• In facilitated diffusion the movement of the molecules is down a concentration gradient and does not require external energy.
• The molecule or ion is released to the inside of the cell.
• The greater the concentration difference, the faster the rate of diffusion.
• The process requires energy and carrier proteins.
• Cells are described as partially permeable because membranes are permeable to non-polar molecules such as oxygen (0) but not to polar molecules like water (H,O) with partial positive and negative charges.