B1.3 - Cell Transport

• How does diffusion work?

  Particles in a liquid and a gas move continuously. Because of this movement, particles will spread themselves evenly throughout a liquid or a gas. The particles even themselves out by moving from a high to a low concentration. This is called diffusion. Diffusion is a spreading out and mixing process. It is important to remember that the particles: will move in both directions, but there will be a net movement from high to low concentration and will end up evenly spread throughout the liquid or gas, but will continue to move
• Name some substances that are transported by diffusion

  Some of the substances transported in and out of cells by diffusion are oxygen and carbon dioxide in gas exchange, and of the waste product urea from cells into the blood plasma for excretion in the kidney
• Factors affecting rate of diffusion

  The concentration gradient - The greater the difference in concentration, the quicker the rate of diffusion.

  The temperature - The higher the temperature, the more kinetic energy the particles will have, so they will move and mix more quickly.

  The surface area of the cell membrane separating the different regions - The greater the surface area, the faster the rate of diffusion.
• Surface area to volume ratio in single celled organisms

  A single-celled organism has a relatively large surface area to volume ratio to allow sufficient transport of molecules into and out of the cell
• How are the lungs adapted for exchanging materials?

  Alveoli give them a high surface area. The epithelia of alveoli in the respiratory system and the walls of blood capillaries are only one cell thick so there's a short distance for diffusion to travel.
• How are leaves and roots adapted for exchanging materials?

  Leaves - The flattened shape of structures such as leaves give them a large surface area and a shorter distance required for diffusion. They also have stomata to maintain the concentration gradient and allow gases to move into and out of the leaves.

  Roots - These roots are very long and have root hair cells (long microscopic hairs that grow on the surface of roots of plants). This increases their surface area for them to absorb water and nutrients.
• How is the small intestine adapted for exchanging materials?

  The small intestine contains villi. These villi even have microvilli. This increases the surface area for transport. Moreover, it has a very rich blood supply to increase the rate of diffusion and a short diffusion path.
• How are fish gills adapted for exchanging materials?

  The gills are a part of the fish that act as an exchange surface between blood and the water. They have an operculum, a flap, that keeps the water flowing over the gills. This maintains the concentration gradient. Within the fish, blood keeps flowing, thus maintaining the concentration gradient.
• Factors that increase the efficiency of an exchange surface

  Are:

  1. Large surface area
  2. Thin membrane, so shorter distance for diffusion pathway
  3. (Animals only) Efficient blood supply to maintain the concentration gradient.
  4. (Animals only) Ventilation must be constant for gas exchange
• What is osmosis?

  Osmosis is the net diffusion of water from an area of. high concentration of water (dilute solution) to an area of lower concentration of water (concentrated solution) across a partially permeable membrane.
• What is active transport?

  Active transport moves substances from a more dilute solution to a more concentrated solution (against a concentration gradient). This requires energy from respiration. In plants active transport is required for the uptake of mineral ions into root hair cells. In animals, Active transport helps sugar to be transferred from a low concentration in the gut to a higher concentration in the blood. This allows the glucose from the gut to be absorbed. This glucose is used for respiration.