L6 - Cell Transport

• Diffusion is the net movement of particles from an area of high concentration to low conentration.
• A partially permeable membrane allows only some particles to go through.
• Diffusion is cells occur when the concentration of a substance is higher in one area than another across a partially permeable membrane.
• A factor that affects the rate of diffusion is:

  The concentration gradient - If the difference in concentration is higher, then diffusion happens quicker
• A factor that affects the rate of diffusion is:

  The temperature - As the temperature increases the particles gain more energy so they move faster, speeding up the rate of diffusion.
• A factor that affects the rate of diffusion:

  The surface area of the membrane - A single-celled organism has a large surface area compared to its volume, this allows more molecules to diffuse through the membrane.
• The surface area to volume ratio can be calculated by dividing an object’s surface area by its volume
• Surface area = Area of face X Number of faces

  Volume = Length x Width x Height
• Cube A has sides that are 1 cm

  each and there are 6 sides. What is the surface area to volume ratio?

  Answer = 6:1
• As organisms get larger, the ratio of surface area to volume decreases
• Since small organisms have a large surface area to volume ratio. They can rely on diffusion to exchange everything they need.
• However, large organisms like Humans have a low surface area to volume ratio. This means that they can not rely on diffusion and instead they have specialised exchange surfaces like the lungs.
• In large organisms, the distance that molecules must diffuse through are too large for diffusion to be effective (it would be far too slow). We have transport systems such as the circulatory system which transports blood around the body.
• Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
• Water concentration is the proportions of water to solutes.

  A dilute solution has a high concentration of water.
• Hypertonic – more concentrated solution than in the cells.

  Isotonic – same concentration as the solution in the cell.

  Hypotonic – more dilute than the solution in the cells.
• Active transport moves substances from a low concentration to high concentration (against a concentration gradient). This process requires energy to occur and the energy is provided by respiration.
• Diffusion and Osmosis are passive processes, this means that they do not require energy to occur.
• Plants have a network of roots in the soil, with root hair cells on the outside which absorbs water and minerals needed for the plant.
• Active transport occurs in root hair cells, the minerals are at a higher concentration in the root hair cell than in the soil. So the minerals move into the cell against the concentration gradient.
• Root hair cells have a lot of mitochondria to release enough energy needed for Active transport.

  They also have a long structure (projection) to increase surface area.

  
• Specialised exchange surfaces are parts of an organism that exchanges substances with their environment.
• In the small Intestine:

  •Internal surface is covered with villi.

  •Villi increase the surface area.

  •Villi have a very good blood supply. This maintains the concentration gradient.

  •Membranes of the villi are very thin to allow for a short diffusion distance.
• In the lungs:

  •Lungs contain millions of tiny air sacs called alveoli.

  •Alveoli increase the surface area.

  •Alveoli have a very good blood supply. This maintains the concentration gradient.

  •Membranes of the alveoli are very thin to allow for a short diffusion distance.