Cell Membrane Structure

Cell Membrane Structure

• 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.

Phospholipid Characteristics

• 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).

Fluid Mosaic Model

• 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.

Cholesterol Function

• Cholesterol helps maintain the plasma membrane's shape and fluidity.

Protein Functions

• 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 and Peripheral Membrane Proteins

• 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.

Carbohydrates

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

Membrane Transport

• 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

• 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.