DIFFUSION

• River currents are comparable to diffusion process that occurs in cells.

  • Materials move through the cell membrane from higher concentration region to lower one.
• The difference in concentration makes up a current effect through the cell which we call as diffusion.

  • Due to this effect, materials are moved freely - that is going with the flow in or out of the cell.
• Diffusion - is the movement of molecules from an area of high concentration of the molecules to an area with a lower concentration

  • Type of passive transport
• Concentration gradient - the difference in the concentration of the molecules in the two areas.

  • Diffusion will continue until this gradient has been eliminated
• Since diffusion moves materials from an area of higher concentration to the lower, it is described as moving solutes "down the concentration gradients"
• The result of diffusion is an equal concentration, or equilibrium of molecules on both sides of the membrane.
• Diffusion is a process that does not require any energy input hence, it is a passive transport.

  • A concentration gradient itself is a form of stored (potential) energy, and this energy is used up as the concentrations equalize.
• Molecules can move through the cell's cytosol by diffusion and some molecules also diffuse across the plasma membrane.
• Each individual substance in a solution or space has its own concentration gradient, independent of the concentration gradients of other materials, and will diffuse according to that gradient.
• Other factors being equal, a stronger concentration gradient (larger concentration difference between regions) results in faster diffusion.
• In a single cell, there can be different rates and directions of diffusion for different molecules.

  • EX: Oxygen might move into the cell by diffusion, while at the same time, carbon dioxide might move out in obedience to its own concentration gradient.
• FACTORS THAT AFFECT DIFFUSION

  • Extent of the concentration gradient
  • Mass of the molecules diffusing
  • Temperature
  • Solvent density
  • Solubility
  • Surface area and thickness of the plasma membrane
  • Distance travelled
• Molecules move constantly in a random manner at a rate that depends on their mass, their environment, and the amount of thermal energy they possess, which in turn is a function of temperature.

  • This movement accounts for the diffusion of molecules through whatever medium in which they are localized.
• A substance will tend to move into any space available to it until it is evenly distributed throughout it.
• After a substance has diffused completely through a space removing its concentration gradient, molecules will still move around in the space, but there will be no net movement of the number of molecules from one area to another.
• The lack of a concentration gradient in which there is no net movement of a substance is known as DYNAMIC EQUILIBRIUM.

  • While diffusion will go forward in the presence of a concentration gradient of a substance.
• FACTOR THAT AFFECT THE RATE OF DIFFUSION

  Extent of the concentration gradient

  • The greater the difference in concentration, the more rapid the diffusion.
  • The closer the distribution of the material gets to equilibrium, the slower the rate of diffusion becomes.
• FACTOR THAT AFFECT THE RATE OF DIFFUSION

  Mass of the molecules diffusing

  • Heavier molecules move more slowly, therefore, they diffuse more slowly
  • The reverse is true for lighter molecules
• FACTOR THAT AFFECT THE RATE OF DIFFUSION

  Temperature

  • Higher temperatures increase the energy and therefore the movement of the molecules, increasing the rate of diffusion.
  • Lower temperatures decrease the energy of the molecules, thus decreasing the rate of diffusion.
• FACTOR THAT AFFECT THE RATE OF DIFFUSION

  Solvent density

  • As it increases, the rate of diffusion decreases.
  • If the medium is less dense, diffusion increases.
• As the density of the solvent increases, the rate of diffusion decreases.

  • The molecules slow down because they have a more difficult time getting through the denser medium.
• If the medium is less dense, diffusion increases.

  • Because cells primarily use diffusion to move materials within the cytoplasm, any increase in the cytoplasm's density will inhibit the movement of the materials.
• EXAMPLE, WHEN THE RATE OF DIFFUSION DECREASES

  A person experiencing dehydration - as the body's cells lose water, the rate of diffusion decreases in the cytoplasm and the cell's function deteriorate.

  • Neurons tend to be very sensitive to this effect.
• Dehydration frequently leads to unconsciousness and possibly coma because of the decrease in diffusion rate within the cells.
• FACTOR THAT AFFECT THE RATE OF DIFFUSION

  Solubility

  • Nonpolar or lipid-soluble materials pass through plasma membranes more easily than polar materials, allowing a faster rate of diffusion.
• FACTOR THAT AFFECT THE RATE OF DIFFUSION

  Surface area and thickness of the plasma membrane

  • Increased surface area increases the rate of diffusion, whereas thicker membrane reduces it.
• FACTOR THAT AFFECT THE RATE OF DIFFUSION

  Distance travelled

  • The greater the distance that a substance must travel, the slower the rate of diffusion.
• GREATER DISTANCE = SLOWER RATE OF DIFFUSION

  • This places an upper limitation on cell size. A large spherical cell will die because nutrients or waste cannot reach or leave the center of the cell.
  • Therefore, cells must either be small in size, as in the case of many prokaryotes, or be flattened, as with many single-celled eukaryotes.