Biological Inspection of Cell Membranes
Biological Inspection of Cell Membranes
The plasma membrane is the boundary of life; this selectively permeable membrane allows the cell to maintain a unique internal environment and to control the movement of materials into and out of the cell. The phospholipid bilayer (fluid mosiac model) with specific membrane proteins accounts for he selective permeability of the membrane and passive and active transport mechanisms. Membrane phospholipids are amphipathic. Cell membranes are phospholipid bilayers with the hydrophobic hydrocarbon tails in the center and the hydrophilic heads facing the aqueous solution on both sides of the membrane. The phospholipid bilayer also called the fluid mosiac model has proteins embedded in the phospholipid bilayer with their hydrophilic regions extending out into the aqueous environment. Membranes are held together by weak hydrophobic interactions that allow the lipids and some of the proteins to drift laterally. Phospholipids with unsaturated hydrocarbon tails maintain membrane fluidity at lower temperatures. The steroid cholesterol restricts movement of phospholipids, therefore, reducing fluidity at warmer temperatures. Cholesterol also prevents the close packing of lipids and thus enhances fluidity at lower temperatures.
The plasma membrane permits regular exchange of nutrients, waste products, oxygen, and inorganic ions. Membranes are selectively permeable which means that they allow some substances to cross more easily than others. Hydrophobic molecules, such as hydrocarbons, can dissolve in and cross through a membrane. Very small polar molecules, including water, can cross a plasma membrane easily. Ions and polar molecules may move across the plasma membrane with the aid of transport proteins, which may provide a hydrophilic channel or may physically bind and transport a specific molecule. The selectively permeability of a membrane depends on both the discriminating barrier of the lipid bilayer and the specific transport proteins built into the membrane.
The structure of membranes is directly related to the transport of materials across a membrane. Diffusion is the spontaneous movement of a substance down its concentration gradient. The cell does not expend energy when substances diffuse across membranes therefore, the process is called passive transport. Osmosis is the diffusion of water across a selectively permeable membrane. Water diffuses down its own concentration gradient and water will move from a hypotonic solution across a membrane into a hypertonic solution. Specific proteins facilitate the passive transport of selected solutes. Facilitated diffusion involves the diffusion of polar molecules and ions across a membrane with the aid of...