Diffusion (Cellular Transport)

• The cell membrane is where molecules enter and leave the cell.
• Diffusion:

  Is the spreading out of particles resulting in a net (overall) movement from an area of higher concentration to an area of lower concentration.
• The three molecules which move in and out of cells by diffusion are oxygen, carbon dioxide, urea.
• Cells need oxygen for respiration which is carried out by mitochondria. Cells are surrounded by a high concentration of oxygen. Oxygen is transported in the bloodstream from the lungs. Like this...

  
• The oxygen molecules move into the cell by diffusion from an area of higher concentration to an area of lower concentration. Like this...

  
• The oxygen is used to generate energy in respiration and this produces the waste gas carbon dioxide. We have a higher concentration of carbon dioxide inside the cell than outside. Like this...

  
• The carbon dioxide moves out of the cell by diffusion. Like this...

  
• Urea is a waste product produced inside cells. It diffuses out of the cells into the blood plasma and is excreted by the kidneys. Like this...

  
• The rate of diffusion is affected by three main factors.

  1. The difference in concentrations.
  2. The higher the temperature, the greater the rate of diffusion.
  3. The surface area of the membrane.
• The difference in concentrations is called the concentration gradient. The greater the concentration gradient, the faster diffusion takes place. So in this example diffusion will be fast. Like this...

  
• In this example, the concentration gradient is much smaller so that means that diffusion will be slower. Like this...

  
• The higher the temperature, the greater the rate of diffusion. That is because the particles have more kinetic energy and are moving faster.
• The cell membrane of the cell on the left has a much larger surface area than the cell on the right. That means that the rate of diffusion will be greater for the cell on the left. Like this...

  
• The larger the surface area of the cell membrane, the greater the rate of diffusion.
• Transport across cell membranes:

  • In order for any organism to function properly, it needs to move substances in and out of cells.
  • Exchange of substances occurs across the cell membrane.
  • There are three transport processes that living organisms use for exchange: diffusion, osmosis and active transport.
• The definition of diffusion means, that the

  • Particles in a solution or a gas are always moving about randomly.
  • If there happen to be more particles in one area/region, then there tends to be an overall (net) movement of particles from this region of higher concentration to a region where there are fewer particles (an area of lower concentration).
• Diffusion of perfume particles through a room is one of the simplest examples of diffusion:

  
• Diffusion is an entirely passive process; the movement of particles in a fluid is dependent on how much kinetic energy they have.

  • The higher the temperature, the more kinetic energy particles have and therefore the faster they can move by diffusion.
• The cell membrane acts as a barrier which substances need to move across to get into or out of a cell:

  
• Examples of Diffusion in Living Organisms Table: