Movement of substances

• define diffusion


  Diffusion is the net movement of substances from a region of higher concentration to a region of lower concentration.
• Factors affecting diffusion:

  • SA to volume ratio
  • relative molecular mass
  • temperature
• Gaseous exchange occurs in our lungs by diffusion. Oxygen enters the air sacs in our lungs when we breathe in, the oxygen concentration is higher in the air sacs than in the blood surrounding them. Oxygen diffuses out of the air sacs into the bloodstream.
• Air sacs in our lungs have a bubble-like structure to increase SA to volume ratio.
• Water potential is the measure of the tendency of water to move from one place to another.
• As a cell becomes bigger, its surface area to volume ratio decreases. Eg, an elephant has a smaller surface area to volume ratio as compared to a mouse
• example of diffusion in plants: 


  Diffusion of oxygen entering plant and carbon dioxide leaving plant through stomata
• Diffusion in human tissues


  Body organs have a lower concentration of oxygen and digested food, and a higher concentration of excretory substances and carbon dioxide. Oxygen and digested food diffuse out from blood capillaries into organs.
• what substances cannot pass through visiting tubing?

  Starch, sugar, salt
• Define osmosis


  it is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane
• Why do living things need transport systems?

  Multi-cellular organisms need to move substances like oxygen, nutrients and waste products from one cell to another.
• Describe movement of water through root hair cells

  The cell sap of the root hair cell contains solutes that lowers its water potential. The water potential of soil is higher than that of cell sap. Water moves into root hair cells via osmosis. Water moves from cell to cell via osmosis through the partially permeable cell membranes until entering the xylem, vessels.
• In a diluted solution a plant cell becomes Turgid
• In an isotonic solution, there is no net movement of molecules hence cell does not change in shape or size.
• In a concentrated solution , an animal cell shrivels , a red blood cell crenates and a plant cell plasmolyses.