Living Cells Diffusion and Osmosis

Diffusion and Osmosis  In order to stay alive cells must be able to transport water and other substances in and out.  Substances useful to cell move IN  Substances which are waste move OUT  Two vital processes for both animal and plant cells are osmosis and diffusion.

Diffusion  This is the movement of molecules of a substance through a selectively permeable membrane, from a region of high concentration to a region of low concentration along a concentration gradient, until evenly spread.  See sheets page 9.

Osmosis  This is the movement of WATER molecules through a selectively permeable membrane, from a region of high concentration to a region of low concentration along a concentration gradient, until evenly spread.

Selectively Permeable Membrane  The cell membrane contains small pores.  See sheet page 11.  The movement of molecules depends on; Pore size Molecule size HC to LC

Concentration Gradient  The difference in concentration that exists between a region of high concentration and a region of low concentration.  During diffusion and osmosis movement of molecules always occurs down a concentration gradient, from HC to LC.

Concentration Gradient

Concentration  Examples; A 10% sugar solution contains : 10% sugar and 90% water. A 30% salt concentration contains: 30% salt and 70% water.ect.

Diffusion in action  Which direction would glucose move if inside the cell has a 20% glucose solution, and outside in the beaker has a 40% glucose solution?

Osmosis in action  Which direction would water move if inside the cell has a 20% sugar solution, and outside in the beaker has a 10% sugar solution?

Relative water concentration WWhen two solutions have a different water concentration, the solution with the highest water concentration is said to be hypotonic. TThe one with the lower concentration is said to be hypertonic. SSince pure water has the highest possible water concentration, it is said to be hypotonic to all other solutions. WWhen two solutions are found to have the same water concentration, we say they are isotonic to one another. SSee sheets page 12.

Unicellular organisms  (eg. Paramecium, animal cell) – See sheets page 9. Oxygen is constantly being used up as the animal respires. This means that oxygen conc. inside the cell is lower than outside. Oxygen will enter by diffusion. As respiration occurs, oxygen is used up and carbon dioxide is produced. Carbon dioxide will leave the cell as a waste product by diffusion.

Multicellular organisms  (eg. mammals) - see sheets page 10.  In mammals diffusion plays an important role in the movement of oxygen entering the lungs, passing to the air sacs (alvioli), to the blood capillaries and then to the body cells.  Carbon dioxide moves out in the opposite direction.

Osmosis in potato cells  Activity 1.6 (See sheets page 12)  Passage answers page 13;  In your experiment with potato cells, an overall movement of water molecules occurred from a region of high water concentration (hypotonic) to a region of low water concentration (hypertonic) through the cell membrane.  The cell membrane prevents larger molecules like sugar from diffusing across it because it only contains small pores. A membrane which only lets small molecules through like water and not larger molecules is called a selectively permeable membrane. The movement of water through this selectively permeable membrane is called osmosis.