Movement of substance

• What is diffusion?

  Diffusion is the net movement of particles from a region of high concentration to a region of Low concentration down a concentration gradient.
• Factors that affect the rate of diffusion

  Steepness of the concentration gradient, temperature, surface area to volume ratio
• Steeper the concentration gradient, higher the rate of diffusion
• The higher the temperature the higher the rate of diffusion
• The greater the surface area to volume ratio the higher the rate of diffusion
• Role of diffusion in gaseous exchange 


  Gaseous exchange in the lungs occurs via diffusion. The air spaces in the lungs have a higher concentration of oxygen molecules than the blood. There is a net movement of oxygen molecules from the lungs into the blood by diffusion down the concentration gradient. The blood has a higher concentration ion of carbon dioxide molecules than the air spaces in the lungs. There is a net movement of carbon dioxide molecules from the blood into the lungs by diffusion down the concentration gradient.
• What is osmosis?

  Osmosis is the net movement of water molecules from a higher water potential to a lower water potential through a partially permeable membrane
• Dilute solution had a higher water potential than concerted solution
• Concentrated solution has a lower water potential than a dilute solution
• Effect of osmosis on a plant tissue- higher water potential

  The solution has a higher water potential than the cell sap. There is a net movement of water molecules into the cell by osmosis down the water potential gradient. The cell expands in volume and becomes turgid. The cell wall prevents the plant cell from bursting.
• Effect of osmosis on plant tissue- lower water potential

  The cell sap in vacuole has a higher water potential than the solution. There is a net movement of water molecules out of the cell by osmosis down the water potential gradient. The cytoplasm shrinks away from cell wall and the cell becomes flaccid.
• Why is turgor important in plants?

  Turgor maintains the shape of tissues in the plants. It keeps the plant firm and upright. Loss of turgidity causes the plant to wilt.
• when a plant becomes flaccid, the plant wilts
• water may be added to the soil to dilute the soil solution. This causes water molecules to enter plants cells and keep the plants firm and upright
• effect of osmosis on animal cell- higher water potential

  the solution has a higher water potential than the cytoplasm of the animal cell. There is a net movement of water molecules into the cell by osmosis down the water potential gradient. The cell expands in volume and bursts as it does not have a cell wall to prevent it from bursting.
• effects of osmosis on animal cell- lower water potential 


  The cytoplasm of animal cell has a higher water potential than the solution. There is a net movement of water molecules out of the cell by osmosis down the water potential gradient. The cell shrinks in size and becomes crenated.
• How to plan an investigation to determine the water potential in a potato tuber


  1. Prepare 6 salt solutions of varying concentration, 0 mol/dm3, 0.5 mol/dm3, 1 mol/dm3, 1.5 mol/dm3, 2 mol/dm3, 2.5 mol/dm3

  2. Measure the initial length and mass and the final length and mass of the potato strip that is immersed in each salt solution for 25 min

  3. Ensure that the following variables are kept the same: soaking time, same tuber, same mass at the start

  4. Plot the changes in length and mass against concentration and draw a trend line

  5. Repeat the experiment at least twice to obtain the average change in length and mass to improve validity and reliability of the experiment

  6. The concentration of solution where there is no change in length and mass is the same as the concentration of cell sap of potato cells in tuber

  7. This in turn allows the water potential of the cell sap to be determined