Cell Membrane

• The Cell Membrane aka Plasma Membrane 


  •Structure found in all cells.

  •A layer surrounding the cell. It defines the cell and separates the cell from its surroundings.

  •Is semi-permeable –it controls and allows certain substances to enter and exit the cell.

  •Assists in communication, cell adhesion and binding of hormones to cell.
• The Cell Membrane

  Made up of:

  •Two layers (bi-layer) of phospholipid molecules.

  •Two types of proteins:

  •Integral proteins – always in the bilayer.

  •Peripheral proteins – can move temporarily.

  •Cholesterol (in animal cells only) & Sitosterol (plant cells) – membrane fluidity and permeability.

  •Glycoproteins – assist in body immunity (WBC).

  •Glycolipids – serve as markers for cell recognition and cell stability.
• The Cell Membrane

  • Selectively permeable membrane: allows certain molecules and ions into the cell while keeping others out.
  • Molecule moving through is dependent on size, charge (ions) and solubility in water.
• Fluid Mosaic Model


  •The name for how scientists describe what the cell membrane looks like.

  •Fluid refers to lipids and proteins continually moving around the cell membrane.

  •Mosaic refers to the pattern formed by proteins in the bilayer.
• The Cell Membrane - Phospholipids

  •The cell membrane is composed mainly of phospholipids.

  •Each phospholipid has a head and 2 tails.

  •The head of the phospholipid contains a charged phosphate and is hydrophilic (“water-loving”) and faces outwards.

  •The 2 tails are fatty acids (lipid) and are hydrophobic (“water-fearing/repel water”).

  •When phospholipids come into contact with water, they self-assemble into a double layer, shielding the hydrophobic tails from the water

  •This makes the membrane impermeable (cannot pass) to water soluble and polar molecules.
• Membrane Proteins

  •Membrane proteins carry out a variety of functions.

  •Integral proteins are permanently attached to the membrane and generally span the membrane (go from one end to the other).

  •Peripheral proteins have the ability to move around the membrane and are only attached to the surface of the membrane (temporary).
• Hormone Proteins 


  •Many things such as hormones that interact with the cell membrane require complementary protein receptors on the outside of membranes.

  •This is similar to a lock and key – only some keys open certain locks.

  •Cell communication can occur in a similar way.
• Channel proteins and pump proteins

  • These proteins allow movement across the membrane. Some require energy.
• Cholesterol 


  •Cholesterol immobilises the outer surface of the cell membrane by forming interactions with phospholipid tails and heads.

  •This reduces the fluidity of and makes the membrane more rigid, giving it more stability.

  •Same as sitosterol in plant cells.