Transport in cells

• Root hair cells


  Absorb substances
• Transpiration


  Evaporation of water from parts of a plant
• Transport of water and minerals

  Through parts of a plant
• Transport of sucrose


  To other parts of a plant
• Osmosis


  The net diffusion of water across a partially permeable membrane, from a solution with a higher water potential to one with lower water potential
• Osmosis


  • Not affected by changes in temperature, wind speed, humidity and light intensity
• Investigate the role of environmental factors in determining the rate of transpiration from a leafy shoot

  1. Observe

  2. Measure

  3. Analyse
• Partially permeable membrane


  Allows small molecules like water to pass through but not larger molecules like sucrose
• Partially permeable membrane


  • Visking tubing
• Osmosis demonstration using Visking tubing

  1. Fill Visking tubing with concentrated sucrose solution

  2. Attach to capillary tube

  3. Place in beaker of water

  4. Observe liquid level rise in capillary tube
• Water potential


  How 'free' the water molecules are to move
• Water moves from a high to low water potential during osmosis
• Cell surface membrane


  Partially permeable membrane in plant and animal cells
• Cell wall


  Tough outer structure that keeps the shape of plant cells and can resist changes in pressure
• Plant cell in dilute solution or water


  Absorbs water by osmosis, becomes turgid
• Plant cell in concentrated solution

  Loses water by osmosis, becomes flaccid and may become plasmolysed
• Animal cells do not have a strong cell wall and cannot resist changes in internal pressure from water movement
• Animal cells in water or concentrated salt solution


  • Swell up and burst or shrink with crinkly edges
• Turgor


  The state of a plant when its cells are turgid, providing support for non-woody parts
• Plant loses too much water


  Cells become flaccid, causing wilting
• Water movement through plant tissues


  From cell to cell down a water potential gradient
• Selectively permeable


  Cell surface membrane can control which chemicals can pass in and out
• Diffusion


  Net movement of molecules or ions from a high concentration to a low concentration, down a concentration gradient
• Diffusion


  When a substance is more concentrated in one place than another, the molecules move from the higher concentration to the lower concentration
• Diffusion happens because of the kinetic energy of the particles, it does not need an 'extra' source of energy from respiration
• Active transport


  A cell uses energy from respiration to take up substances, like a pump moving a liquid from one place to another
• Osmosis


  Water moves across cell membranes from a more dilute solution to a more concentrated one
• Osmosis can only happen if the membrane is permeable to water but not to some other solutes
• Alveoli (air sacs) of the lungs and villi of the small intestine


  • They have a very large surface area in proportion to their volume to allow efficient exchange of materials by diffusion
• Diffusion is a slow process, so organs that rely on diffusion need a large surface over which it can take place
• Root hairs


  Tiny projections on the root epidermis that greatly increase the surface area for water absorption
• Water uptake by roots


  1. Water enters root hair cells by osmosis due to a water potential gradient

  2. Water then moves out of the root hair cells into the root cortex

  3. A water potential gradient is set up across the root cortex

  4. Water enters the xylem and is transported to all parts of the plant
• Adaptations of plants in dry habitats


  • Thick waxy cuticle on leaves
  • Reduced leaf surface area
• Transpiration


  1. Water evaporates from the mesophyll cells into the air spaces

  2. Water vapour diffuses out through the stomatal pores

  3. Loss of water from the mesophyll cells sets up a water potential gradient

  4. The xylem supplies the leaf mesophyll tissues with water
• Transpiration stream


  The continuous flow of water up the xylem in the stem and roots
• The transpiration stream has more than one function: supplies water for photosynthesis, carries mineral ions, provides water to keep cells turgid, and allows evaporative cooling