Transport across a membrane

• Diffusion is the movement of particles from an area of high concentration to low concentration until equilibrium is reached.
• Active transport requires energy input (ATP) and moves substances against their concentration gradients.
• Osmosis is the net movement of water molecules through a partially permeable membrane from an area of higher water potential to lower water potential.
• The rate of diffusion depends on temperature, surface area, concentration gradient, and distance traveled.
• The concentration gradient is the difference between the concentrations on either side of the cell.
• Cell membranes are described as differentially permeable; meaning that they could be semipermebale or selectively permeable (allows certain ions/molecules through but restrict the movement of others)
• Passive processes do not use up energy, whereas active processes uses the cell's energy in the form of ATP.

  There are three basic processes:

  • Simple Diffusion
  • Carrier Mediated Transport
  • Vesicular Transport
• Simple Diffusion?


  the movement of particles from a high concentration to a low concentration until at equilibrium as a result of straight and random movement (collisions)
• Carrier Mediated Transport?

  Special proteins which bind to one particular molecule and assists it in moving across a membrane. (Active or Passive)

  These proteins are channel proteins, which form protein channels and carrier protiens, which allow carrier-mediated transport.
• The higher the concentration gradient, the greater the rate of diffusion
• Osmosis is a speical case of diffusion...

  the diffusion of a solvent (often water) through a differentially permeable membrane to balance the concentration of another substance.
• Often movement of solvent creates two different fluid levels on either side of the membrane creating osmotic pressure
• To diffuse across a cell membrane, water-soluble molecules must pass throught protein channels in the membrane, allowing fascilitated diffusion.

  These protein cahnnels are very small in diameter but water and ions can easily get through; it is just larger molecules that are too big to get through.
• Carrier mediated transport is the movement of molecules across a membrane by a carrier protein. The substance can then be released on the opposite side to where is entered.
• Important Characteristics of Carrier mediated transport:

  • Carriers can become saturated
  • Carrier proteins are specific
  • Carrier activity can be regulated by substances like hormones
• 2 types of carrier-mediated transport:

  1. Fascilitated diffusion - substances are transported through a protein channel following the concentration gradient. The molecule attaches to the binding site of a specific carrier protein. PASSIVE
  2. Active transport - requires ATP (energy) to go against the concentration gradient. Via carrier proteins.
• Vesicular Transport is an active process where materials are moved by membran-bound sacs. Requires energy to form the vesicles.
• Endocytosis: movement of a substance from teh outside of a cell to the inside by the folding of the cell membrane around the substance, forming a vesicle

  • Phagocytosis - solid particles (specialised cells only)
  • Pinocytosis - liquid particles
• Exocytosis: movement of a substance from the inside of the cell to the outside by the migration of the vesicle (may be waste or a substance needed elsewhere)
• Why are cells so small?

  Efficiency; the surface area-to-volume ratio limits the size of individual cells
• What is extracellular fluid?

  Fluid found outside teh cells; includes tissue fluid and blood plasma
• Factos that could affect the exchange of materials across a membrane insludes volume to surface area ratio, concentration gradients, and the physical/chemical nature of the materials being exchanged that affect the efficieny of movement across the membrane.