Cell Membrane

• Cell membrane functions:

  • Encloses the contents of the cell
  • Controls the movement of substances in and out of the cell
  • Involved in cell recognition and communication with other cells
• Cell membrane is selectively permeable, allowing some molecules to pass across and not others
• Fluid Mosaic Model:

  • Proposed by Jonathan Singer and Garth Nicholson in 1972
  • Widely accepted as the basic model of all biological membranes
• Cell membrane structure consists of:

  • Phospholipid bilayer (two layers)
  • Embedded proteins, carbohydrates, and cholesterol
• Phospholipid Bilayer:

  • Hydrophilic head composed of a phosphate group
  • Hydrophobic tails composed of long chain fatty acids
  • Hydrophilic heads in contact with extracellular and intracellular fluids, while hydrophobic tails meet in the middle
• Proteins:

  • Integral proteins are permanent and extend through the membrane
  • Peripheral proteins are temporary and either bind to integral proteins or partly penetrate one surface of the membrane
  • Some proteins are fixed, while others can move freely
  • Functions of proteins include transport channels and receptor binding sites for substances like hormones
• Carbohydrates:

  • Attached to surface proteins forming glycoproteins and to lipids forming glycolipids
  • Important in cell-cell interactions, recognition, adhesion between cells, and acting as marker molecules for self and non-self cell recognition
• Cholesterol:

  • Embedded within the membrane between phospholipid molecules
  • Provides stability to the cell membrane without affecting its fluidity
• Phospholipids form two layers in cell membranes because:

  • The head is a phosphate molecule attracted to water (hydrophilic)
  • The tails are made of fatty acids that repel water (hydrophobic)
  • The cell membrane is exposed to water mixed with electrolytes and other materials
  • Phospholipids assemble into two layers due to hydrophilic and hydrophobic properties, with the heads facing the aqueous environment and the tails hiding between the layers
• The cell membrane is described as 'fluid' because the lipid bilayer is flexible and constantly shifting in shape, and 'mosaic' because the embedded proteins vary in shapes, sizes, and patterns, resembling a mosaic
• Carbohydrates provide energy for the cell to survive, assist in cell membrane function, aid in cell recognition, and offer cell protection
• Cholesterol in the cell membrane maintains stability, grabs other molecules, and helps keep the membrane at a cold temperature
• Proteins in the cell membrane form channels allowing materials to cross, some are fixed while others move freely, giving a mosaic effect
• The cell membrane needs to be selectively permeable for a cell to function to maintain stable internal conditions different from the environment, regulate osmotic pressure, pH, and ion concentration, and enable homeostasis