Osmosis and Plant Transport

• Cell membranes allow some substances to pass through so they are selectively permeable
• diffusion is the movement of particles from an area of high concentration to an area of low concentration down a concentration gradient
• osmosis in plant cells

  when water enters the vacuole increases in size and the cell membrane pushes against the cell wall which increases the pressure in the cell (turgid)
• turgor pressure prevents too much water from entering and the cell bursting

  it also gives plant support
• in a turgid cell the water flows from a higher concentration of water molecules outside the cell to a lower concentration in the cell sap
• when cells don’t receive enough water they can’t remain turgid and wilting occurs (flaccid)
• if a cell loses too much water it is called plasmolysis
• in a plasmolysed cell the water flows from a higher concentration of water molecules in the cell sap to a lower concentration outside the cell
• osmosis is the diffusion of water from a dilute solution to a more concentrated solution through a selectively permeable membrane
• osmosis in animal cells

  if a red blood cell is placed in pure water it will enter the cell by osmosis. Water will move down the gradient and so much water enters the blood cell that it would swell and eventually burst
• concentration of blood is carefully controlled so bursting does not happen
• plants have cellulose cell wall which prevents too much water entering and cell bursting but red blood cells do not have a cell wall so they can burst
• transpiration is the loss of water by evaporation from mesophyll cells followed by diffusion through air spaces and stomata
• continuous movement of water through the plant is importsnt for:

  1. supply of water for photosynthesis
  2. transport of minerals
  3. providing support
  4. cooling the plant
• there is more stomata on the lower surface of a leaf compared to the upper surface
• the bubble potometer measures water uptake in a leafy shoot
• as water evaporates from leaves the shoot sucks water up through the potometer however it cannot give an absolute value for transpiration
• bubble potometer

  as leaf loses water the air bubble moves the distance moved over a time period is measured
• Bubble potometer apparatus is assembled underwater to prevent development of unwanted air bubbles
• high wind speed increase transpiration
• high temperature increases transpiration
• high humidity decreases transpiration
• to investigate temperature effect on transpiration put thermometer on different temperatures
• to investigate humidity effect on transpiration put plastic bag over plant
• to investigate wind speed effect on transpiration use a fan/hairdryer
• the weight potometer weighs how much water a plant loses over time
• weight potometer can also investigate temp. humidity and wind speed
• weight potometer loses most water from transpiration in non humid conditions
• increase in temperature=increase in rate of transpiration