Cell Membranes and Transport

Cell Membranes

Cell membranes, also known as plasma membranes, are the semi-permeable barriers that enclose cells.
They are composed of a lipid bilayer with embedded proteins.
The lipid bilayer is made up of phospholipids, which have a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) tail. This allows the membrane to regulate the movement of substances.
Some proteins within the membrane act as receptors for specific molecules or signal transduction pathways, while others function as enzymes or transport proteins.
The fluid mosaic model describes the structure of the cell membrane. It suggests the membrane is flexible (fluid) and consists of various proteins embedded in the lipid bilayer (mosaic).
Cholesterol molecules within the membrane regulate fluidity and maintain membrane stability.

Passive transport involves the movement of substances across the cell membrane without the input of energy. This commonly occurs through diffusion or facilitated diffusion.
Diffusion is the random movement of particles from an area of high concentration to an area of low concentration. Small, non-polar molecules can diffuse directly across the lipid bilayer.
Facilitated diffusion enables the passive transport of larger or polar molecules, like glucose or amino acids, via specific carrier proteins or through protein channels.
The rate of diffusion can be influenced by factors such as the temperature, gradient concentration, and the size of the particles.
Osmosis is a special case of diffusion where water molecules move across a semi-permeable membrane from an area of lower solute concentration to an area of higher solute concentration.
Active transport moving substances against their concentration gradient, thus requires energy in the form of ATP. This is often achieved by carrier proteins known as ‘pumps’.
Endocytosis and exocytosis are forms of bulk transport, where large particles are moved in or out of cells. Endocytosis is the ingestion of material, while exocytosis is the expulsion.

The primary function of the cell membrane is to protect the cell from its surroundings, while allowing certain materials to pass in and out of it.
It is involved in maintaining the balance of substances (homeostasis) within the cell.
The cell membrane is also responsible for cell recognition and adhesion, important for immune responses and cell-to-cell communication.