2.10 Cell Membrane

• Cell Membrane

  • Lipid-based structure that separates aqueous interior of a cell from the aqueous environment surrounding the cell.
  • Up to 80 % of plasma membrane is lipid material.
  • The membranes are lipid bilayers made up of phospholipids.
• Lipid bilayer

  • Nonpolar tails of phospholipids are in the middle and polar heads are on the surface.
  • 6 - 9 nanometers thick
  • Contains at least 1 unsaturated fatty acid.
• Cholesterol

  • Are also components of plasma membranes
  • Helps regulate membrane fluidity – the fused - ring system does not allow rotation of fatty acid tails in the vicinity.
  • Fits between fatty acid chains of the lipid bilayer.
• Protein

  • Responsible for moving substances such as nutrients and electrolytes across the membrane.
  • Act as receptors that bind hormones and neurotransmitters.
  • Act as markers, substances that play roles in processes by which different cells recognize each other.
• 3 types of transport:

  • Passive transport
  • Facilitated transport
  • Active transport
• Passive Transport

  • Process in which a substance moves across a cell membrane by diffusion from a region of higher concentration to a region of lower concentration.
  • No cellular energy expenditure
  • Only a few types of molecules, including O2, N2, urea, and ethanol, can cross membranes by this process.
• Facilitated Transport

  • Process in which a substance moves across a cell membrane, with the aid of a membrane protein, from a region of higher concentration to a region of lower concentration.
  • No cellular energy expenditure 
  • Specific protein carriers or transporters are involved in the process
• Active Transport

  • Process in which a substance moves across a cell membrane, with the aid of membrane proteins, against a concentration gradient.
  • Involves expenditure of cellular energy (ATP)
  • Proteins involved in active transport are called “pumps”.