3.1 - Structure of Plasma Membrane

The Necessity of Movement of Substances Across a Plasma Membrane

• Living cells take in substances from the external environment to carry out living processes.
• Metabolic processes produce waste that must be disposed of from cells.
• The plasma membrane regulates the movement of substances into and out of cells.

The Fluid Mosaic Model

• Protein molecules float in the phospholipid bilayer and form an ever-changing mosaic pattern.
• The phospholipid bilayer, proteins and cholesterols are not static; they form a dynamic, flexible structure that gives the membrane its fluid characteristic.
• The fluid mosaic model proposed by S. J. Singer and G. L. Nicolson in 1972 is the model accepted by scientists.

Components of the Plasma Membrane

Phospholipid Bilayer

• Each phospholipid has:
  • a polar, hydrophilic head attracted to water;
  • a nonpolar, hydrophobic tail that repels water.
• Heads in the outer layer face the extracellular fluid; heads in the inner layer face the cytoplasm.
• Tails of the two layers face each other.

Membrane Proteins

• Protein molecules are embedded partially or fully and widely dispersed in the phospholipid bilayer.
• Channel proteins contain channels or canals.
• Carrier proteins function as carriers.

Glycoproteins and Glycolipids

• Glycoprotein: a protein with an attached carbohydrate chain.
• Glycolipid: a lipid with an attached carbohydrate chain.
• Both:
  • act as receptors for hormones such as insulin;
  • stabilise the membrane by forming hydrogen bonds with water;
  • act as antigens for cell identification.

Cholesterol

• Cholesterol molecules lie between phospholipid molecules.
• They make the phospholipid bilayer stronger, more flexible and less permeable to water-soluble substances such as ions.

Permeability of the Plasma Membrane

• A membrane is permeable to a substance when it allows the substance to move across freely; it is impermeable when the substance cannot move across.
• The plasma membrane is selectively permeable: it allows the free movement of certain substances but prevents or limits the movement of others.
• The phospholipid bilayer and proteins determine permeability to specific substances.