The Functions of the Plasma Membrane

Glycocalyx functions

1. Lubrication and protection - glycoproteins and glycolipids form a protective layer for membrane 2. Anchoring and locomotion 3. Specificity in binding - ability to alter properties of the cell's surface and behavior. 4. Recognition - recognize genetic normalality/abnormality of cells

Peripheral proteins

Bound to the inner or outer surface of the membrane and easily separated from it.

Carbohydrates in plasma membrane

Components of large, complex molecules extending beyond the outer surface of the membrane, forming a layer of glycocalyx. Accounts for 3% of membrane's weight.

Channel

A central pore that forms a passageway completely across the plasma membrane, permitting the movement of water, and small solutes across the membrane. Ions do not dissolve in lipids and cannot pass through the membrane any other way. Many channels are highly specific, permitting only one type of ion through.

Selectively permeable

A level of permeability between impermeability and freely permeable.

Freely permeable

Membrane through which any substance can pass without difficulty.

Impermeability

Membrane through which nothing can pass.

Anchoring proteins

Proteins that attach the plasma membrane to another structure and stabilize it.

Carrier proteins

Proteins that bind solutions and transport them across the plasma membrane, sometimes this movement requires ATP as an energy source.

Lipids in plasma membrane

Form most of the surface area of the membrane, accounting for 42% of its weight.

Materials that cannot easily pass through plasma membrane

Ions and water-soluble compounds that are not lipid soluble. These must pass through a membrane channel.

Functions of plasma membrane

1. Physical isolation - separates inside of cell from the surrounding extracellular fluid. 2. Regulation of exchange with the environment - controls the entry of ions and nutrients, elimination of wastes, and release of secretions. 3. Sensitivity to the environment - first part of cell to be affected by changes in the pH of the extracellular fluid. Different receptors allow cell to recognize and respond to specific molecules.

Plasma membrane

AKA cell membrane, the outermost boundary of a cell. Extremely thin (6-10 nm). Contains lipids, proteins, and carbohydrates.

Recognition proteins (identifiers)

AKA glycoproteins. Cells of the immune system recognize other cells as normal or abnormal based on the presence of this characteristic.

Leak channels

AKA passive channels. Channels that remain open and allow the passage of ions across the plasma membrane.

Integral proteins

AKA transmembrane proteins. Cannot be removed without damaging or destroying the membrane. These proteins span the width of the membrane two or three times and greatly outnumber peripheral proteins.

Ligands

Anything from a small ion to a large hormone that will bind to the appropriate receptor protein, triggering changes in the cell's activity.

Enzymes in cells

Could be integral or peripheral proteins. They catalyze reactions in the extracellular fluid or in the the cytosol.

Phospholipid bilayer

Phospholipid molecules in the cell membrane form two layers, laying with their hydrophilic heads along the membrane surface and hydrophobic tails inside.

Permeability

Property of the plasma membrane that determines which substances can enter or leave the plasma membrane.

Receptor proteins

Proteins that are sensitive to the presence of ligands.

Concentration gradient

The difference between high and low concentrations-- a potential energy gradient. Diffusion tends to eliminate this gradient.

Simple diffusion

The diffusion of alcohol, fatty, acids, and steroids through the phospholipid bilayer.

Channel-mediated diffusion

The diffusion of ions and water-soluble compounds through tiny, specifically sized channels. Whether or not an ion can cross a specific channel is dependent on the size and charge of the ion, the size of the hydration sphere, and interactions between the ion and channel walls.

Diffusion

The distribution process and movement of a substance from an area of higher concentration to an area of lower concentration.

Materials that can easily pass through plasma membrane

alcohol, fatty acids, steroids