Cell membrane.

The cell membrane, also known as the plasma membrane, is a thin, semi-permeable barrier that surrounds the cytoplasm of a cell. It is composed of a double layer of phospholipid molecules with embedded proteins, cholesterol, and carbohydrates. The cell membrane plays a crucial role in maintaining the integrity of the cell, regulating the passage of molecules in and out of the cell, and communicating with other cells. It also helps to protect the cell from its external environment and allows for cell recognition and signaling.

Cell Membrane (Plasma Membrane)

Structure:

- Bilayer composed of phospholipids, with polar (hydrophilic) heads facing outward and nonpolar (hydrophobic) tails facing inward.
- Embedded proteins, such as channels, pumps, and receptors, that facilitate transport and signaling.
- Cholesterol and glycoproteins also present in some regions.

Functions:

- Barrier: Selectively permeable, controlling the entry and exit of substances.
- Selective transport: Facilitates the movement of molecules and ions across the membrane through ion channels, pumps, and carrier proteins.
- Cell signaling: Transmits signals received from the extracellular environment into the cell.
- Cell recognition: Glycoproteins and other surface markers allow cells to recognize and interact with each other.
- Cell shape and support: Provides structural stability and defines the shape of the cell.
- Endocytosis and exocytosis: Facilitates the uptake (endocytosis) or release (exocytosis) of substances.
- Cell adhesion: Interacts with extracellular matrix components to anchor cells together and form tissues.

Components:

Phospholipids:
- Main structural components, forming the lipid bilayer.
- Composed of a hydrophilic head (phosphate group) and two hydrophobic tails (fatty acids).

Proteins:
- Embedded in the membrane, performing various functions.
- Integral proteins: Span the entire membrane thickness.
- Peripheral proteins: Loosely associated with membrane surfaces.
- Types of proteins include:
- Ion channels
- Carrier proteins
- Pumps
- Receptors
- Glycoproteins

Cholesterol:
- Steroid molecule embedded in the membrane, regulating fluidity and stability.

Glycoproteins:
- Proteins with attached carbohydrate groups, involved in cell recognition and adhesion.

Properties:

- Fluid mosaic model: The membrane is a dynamic, fluid structure, with lipids and proteins moving laterally.
- Semipermeable: Allows some molecules to pass through, while excluding others.
- Asymmetrical distribution: Different lipids and proteins are distributed unevenly on the two sides of the membrane.

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