Plasma membrane and transport

• Plasma membrane

  Essential for the interactions which occur between the cell and its environment

  
• Plasma membrane

  Regulates the movement of molecules in and out of the cell- selectively permeable

  
• Plasma membrane

  Helps to stick cells together in tissues and allows communication between adjacent cells

  
• Components of membranes

  • Lipids
  • Carbohydrates
  • Proteins

  
• Phospholipid

  Modified triglyceride with a glycerol backbone, 2 fatty acid chains, and a phosphate group
• Phospholipid

  • Amphipathic - has a hydrophobic tail region and a hydrophilic head region
• Arrangement of phospholipids in the plasma membrane

  1. Phosphate heads face the cytoplasm and external environment

  2. Hydrophobic tails face the inside of the membrane

  
• Other molecules besides phospholipids are found in the plasma membrane, arranged randomly between the phospholipids
• Glycolipid

  Modified triglyceride with a carbohydrate chain instead of a fatty acid chain
• Glycolipids

  • Cell surface markers important in tissue recognition
  • Receptors for certain chemicals like hormones
  • Attach cells to one another to form tissues

  
• Cholesterol

  • Structural component in animal plasma membranes
  • Increases flexibility and stability of membranes
  • Influences the permeability of the membrane

  
• Types of membrane proteins

  • Channel proteins
  • Carrier proteins
  • Cell recognition proteins (glycoproteins)
  • Cell adhesion proteins
  • Receptor proteins
  • Enzyme proteins
  • Structural proteins
• Channel proteins

  • Create a tunnel that acts as a passageway for molecules through the membrane
  • Allow a particular ion to cross the plasma membrane freely
• Carrier proteins

  • Bind to particular molecules and transport them across membranes
• Cell recognition proteins (glycoproteins)

  • Receptors important in 'self' recognition
  • Function as receptors for binding chemicals such as hormones and triggering reactions inside cells
• Cell adhesion proteins

  • Attach cells to each other
• Receptor proteins

  • Have a specific shape which allows a specific molecule (a signalling molecule) to bind to them, triggering reactions inside the cell
• Enzyme proteins

  • Catalyse specific reactions on the interior surface of the plasma membrane
• Structural proteins

  • Maintain cell shape
• Tight junctions

  Connect plasma membranes of adjacent cells, preventing molecules from leaking between the cells

  
• Desmosomes

  Attach the cytoskeleton of a cell to the cytoskeletons of other cells, like rivets

  
• Gap junctions

  Composed of connexon proteins that create a channel allowing flow of substances and electric current between cells
• Glycoproteins and glycolipids

  Act as cell surface markers to identify the cell and tissue

  
• Fluid mosaic model

  Membrane is fluid due to weak interactions between phospholipids, and proteins are scattered like a mosaic
• Fluidity of membrane

  Increases with greater concentration of unsaturated fatty acids in phospholipids

  
• Effect of cholesterol on fluidity

  • At high temperatures, stabilises membrane and decreases fluidity
  • At low temperatures, inserts between phospholipids and increases fluidity
• General functions of the plasma membrane

  • Separates cellular contents from external surroundings
  • Controls exchange of substances between cell and environment
  • Acts as a receptor site for external stimuli
  • Separates reactions within organelles from cytoplasm
  • Proteins sometimes act as enzymes

  
• Reasons for transport across membranes

  • To obtain nutrients
  • To excrete waste substances
  • To secrete useful substances
  • To generate ionic gradients for nervous and muscular activity
  • To maintain suitable pH and ionic concentration for enzyme activity

  
• Types of transport across membranes

  • Macrotransfer (endocytosis and exocytosis)
  • Microtransfer (passive transport like diffusion and osmosis, and active transport)

  
• Diffusion

  Movement of molecules or ions from a region of high concentration to a region of low concentration, down a concentration gradient
• Factors affecting rate of diffusion

  • Concentration gradient
  • Molecular size
  • Temperature
• Passive transport

  Movement of molecules down a concentration gradient without energy expenditure
• Active transport


  Movement of molecules against a concentration gradient with energy expenditure (ATP)

  
• Simple Diffusion


  1. Diffusion is the movement of molecules or ions from a region of their high concentration to a region of their low concentration

  2. Each molecule possesses kinetic energy

  3. Random motion causes diffusion

  4. Each type of molecule moves down its own diffusion gradient independently of other molecules
• Diffusion occurs in both living and non-living systems
• Factors affecting rate of diffusion

  • Steepness of the diffusion gradient
  • Surface area of the membrane
  • Distance across which diffusion takes place
• Fick's Law


  Rate of diffusion is directly proportional to concentration gradient and surface area, and inversely proportional to distance

  
• Diffusion constant


  Factor which takes into account the permeability of the membrane
• Adaptations of cells and tissues for increasing efficiency of diffusion

  • Maintain steep concentration gradient
  • Maintain large surface area
  • Assume thin lining, decreasing distance
• Other factors affecting rate of diffusion

  • Size and mass of diffusing particle
  • Temperature
  • Electrical charge distribution