Cell Membrane Structure

Study Notes

The cell membrane is composed of a lipid bilayer, making it semipermeable and outlining the cell borders.
Phospholipids are a main component of the plasma membrane, consisting of a phosphate group attached to a glycerol unit with 2 fatty acid tails.

Phospholipids are amphipathic (dual-loving), with a hydrophilic head and hydrophobic tails.
The head of the phospholipid is hydrophilic (water-loving), while the fatty acid tails are hydrophobic (water-fearing).

The fluid mosaic model of the plasma membrane was first proposed by S.J. Singer and Garth L. Nicolson in 1972.
This model includes phospholipids, cholesterol, proteins, and carbohydrates as the mosaic components of the cell membrane.

Proteins make up the second major component of plasma membranes.
Proteins can serve multiple purposes:
- Receptor proteins: perform signal transduction, transmitting information to the cell by sensing the presence or absence of their cognate ligands.
- Channel proteins: allow specific substances to enter the cell, such as tiny substances and water.
- Gated channel proteins: open or close depending on external stimuli, with a binding site.
- Transport proteins (carrier proteins): move substances against the concentration gradient, using ATP, and are specific for ions and molecules.

Glycoproteins are proteins attached to carbohydrate chains, serving multiple functions such as cell-to-cell communication and initiating immune responses.
Integral membrane proteins:
- Transmembrane proteins: span the entire plasma membrane, can be biopic (spans once) or polytopic (spans multiple times).
- Integral monotopic proteins: permanently attached to the membrane from one side.
Peripheral membrane proteins:
- Most are hydrophilic and found at the surface of the membrane.
- Can attach to a portion of the bilayer or integral membrane protein.
- Can be easily removed as they are only temporarily associated with the membrane.

Found outside the cell, carbohydrates can attach to proteins, forming glycoproteins, or to lipids, forming glycolipids.

Membrane transport is fundamental to the functioning of living organisms, regulating the transport of substances across cell membranes.
Principles of membrane transport are crucial for scientific knowledge and advancements in medicine, biotechnology, and pharmacology.

Protein-free lipid bilayers are highly impermeable to ions.
Molecules diffuse across the bilayer down their concentration gradient, with rates varying depending on molecule size and solubility in oil.