Cell Membrane Notecards

Cell Transport

is movement of materials across cell membranes.

Transport Proteins

proteins involved in the movement of ions, small molecules, or macromolecules, such as another protein, across a biological membrane.

Selectively Permeable Diagram of each of the functions of the membrane proteins

Integral polytopic proteins, also known as "transmembrane proteins," are integral membrane proteins that span across the membrane at least once. These proteins may have different transmembrane topology.[7][8] These proteins have one of two structural architectures: helix bundle proteins, which are present in all types of biological membranes; beta barrel proteins, which are found only in outer membranes of Gram-negative bacteria, lipid-rich cell walls of a few Gram-positive bacteria, and outer membranes of mitochondria and chloroplasts. Integral monotopic proteins are integral membrane proteins that are attached to only one side of the membrane and do not span the whole way across.

4 Functions of Membrane Proteins

Membrane proteins perform a variety of functions vital to the survival of organisms: Membrane receptor proteins relay signals between the cell's internal and external environments. Transport proteins move molecules and ions across the membrane. They can be categorized according to the Transporter Classification database. Membrane enzymes may have many activities, such as oxidoreductase, transferase or hydrolase. Cell adhesion molecules allow cells to identify each other and interact. For example, proteins involved in immune response.

Selectively Permeable Diagram of each of the functions of the membrane proteins

Peripheral membrane proteins are temporarily attached either to the lipid bilayer or to integral proteins by a combination of hydrophobic, electrostatic, and other non-covalent interactions. Peripheral proteins dissociate following treatment with a polar reagent, such as a solution with an elevated pH or high salt concentrations. Integral and peripheral proteins may be post-translationally modified, with added fatty acid or prenyl chains, or GPI (glycosylphosphatidylinositol), which may be anchored in the lipid bilayer.

Fluid Mosaic Model

A model that describes the structure of cell membranes. In this model, a flexible layer made of lipid molecules is interspersed with large protein molecules that act as channels through which other molecules enter and leave the cell.

Selectively Permeable Diagram of each of the functions of the membrane proteins

Main articles: Integral membrane protein and Transmembrane protein Integral membrane proteins are permanently attached to the membrane. Such proteins can be separated from the biological membranes only using detergents, nonpolar solvents, or sometimes denaturing agents. They can be classified according to their relationship with the bilayer:

Selectively Permeable Diagram of each of the functions of the membrane proteins

Polypeptide toxins[edit] Main article: Pore-forming toxin Polypeptide toxins and many antibacterial peptides, such as colicins or hemolysins, and certain proteins involved in apoptosis, are sometimes considered a separate category. These proteins are water-soluble but can aggregate and associate irreversibly with the lipid bilayer and become reversibly or irreversibly membrane-associated.

Phospholipid bilayer

The phospholipid bilayer consists of two layers of phospholipids, with a hydrophobic, or water-hating, interior and a hydrophilic, or water-loving, exterior.

Phospholipid

consist of a glycerol molecule, two fatty acids, and a phosphate group that is modified by an alcohol. The phosphate group is the negatively-charged polar head, which is hydrophilic. The fatty acid chains are the uncharged, non polar tails, which are hydrophobic.