transport in plants

• Water travels to the leaves and some is used for photosynthesis, but most evaporates and diffuses out of the leaves via ores on the underside of the leaf called stomata. This loss of water vapour from the stomata of the leaves is called transpiration
• Water is important for plants as ;

  • it is needed to dissolve chemicals in cells and in order to carry out the reactions that make up life
  • needed in photosynthesis so plants can make their own food
  • water carries minerals for healthy growth
  • support - water keeps the cells turgid and stops them becoming flaccid and wilting
• Stem transports glucose from the leaves to the roots and shoots, and water from the roots to the leaves
• Absorption in the roots

  • soil water in more dilute than the root hair cell sap. Water moves into root hair cell by osmosis
  • Root hair cell sap is diluted by the water and so becomes more dilute than the next cell. Water moves into the next cell by osmosis down the water conc. gradient
  • process is repeated and water moves further along the root by osmosis till it reaches the xylem
• How does water get up the stem - equipment needed

  • 250ml conical flask
  • Scalpel
  • Hand lens
  • Stop clock
  • Ruler
  • 100cm³ coloured water
  • White tile
  • Celery stalk
• How does water get up the plant - method

  1. Cut off 5mm from bottom of celery stick
  2. Place this in conical flask containing the coloured water and start the stop clock
  3. Leave for 5 minutes
  4. Remove celery and blot the end dry
  5. Measure the length the coloured water has risen - use the scalpel to help expose the vessels carrying the coloured water
• Phloem carries sucrose away from the photosynthetic parts of the plant (leaves) up and down to other parts of the plants where it is needed for respiration or stored
• Diffusion of water vapour through the stomata, causes more water to be pulled up by transpiration. The water molecules stick together and this causes water to move up the xylem as a column. This ensures there's a continuous flow of water from roots to leaves
• The transpiration stream has several functions. These include:

  • transporting mineral ions
  • providing water to keep cells turgid by filling vacuoles in order to support the plant and stop it wilting
  • providing water to leaf cells for photosynthesis
  • keeping the leaves cool by evaporation
• Investigating rate of transpiration with a potometer

  A) resevoir of water

  B) leafy shoot

  C) beaker of water

  D) air bubble

  E) capillary tube

  F) graduated scale

  
• A summary of water uptake, water transport and transpiration:

  
• Potometer - method (1)

  1. Immerse potometer in bowl of water and make sure apparatus is full of water with on bubbles
  2. Put cut end of leafy shoot in water,taking care to keep leaves above surface
  3. Diagonally cut last cm from stem underwater
  4. With potometer and stem still underwater, gently push stem into the bung and make sure it is tight
  5. Remove assembled apparatus from water and apply vaseline to all joints as a water resistant seal
  6. Gently dab leaves with paper towel to remove excess water
• Potometer - method (2)

  7. Clamp potometer in an upright position with the capillary tube in the beaker of water

  8. Remove capillary tube from beaker to allow an air bubble to form and then return it to the beaker

  9. When the air bubble reaches the start of scale, begin timing

  10.After a set amount of time, record distance travelled by bubble

  11. open the resevoir to set the bubble back to zero. Close resevoir and repeat steps 9-10 twice more
• A potometer measures rate of transpiration indirectly. Directly, it measures uptake of water
• 3 weather conditions that speed up rate of transpiration is

  • sunny - more K.E so they can diffuse faster and more likely to evaporate
  • windy - The wind blows away water vapour as soon as it diffuses out of the leaf. This maintains a strong concentration gradient between the inside and outside of the leaf, which increases the rate at which water diffuses out.
  • dry - if humidity outside leaf is low , the steeper the conc. gradient
• Light affects where transpiration happens or not, but does not affect its rate. Explain why?

  Stomata closed in dark, so transpiration rate drops to almost zero. One light and stomata is open so increasing light intensity has no effect
• How are plants well adapted for photosynthesis?

  • Plants have broad leaves to trap sunlight
  • Leaves are thin to provide a short diffusion path for oxygen and carbon dioxide
  • Long roots are good for absorbing more water for photosynthesis
  • Stomata allow gases into and out of the leaf
  • Guard cells to control the opening and closing of the stomata under different environmental conditions
  • Lots of chloroplast to absorb sunlight for photosynthesis
• Transpiration Stream

  1. water enter root hair cells by osmosis
  2. water moves from cell to cell in the root by osmosis
  3. Water moves into xylem by osmosis
  4. water molecules stick together and this causes water to be pulled up by the xylem as an column
  5. water moves from cell to cell in the leaf by osmosis
  6. water evaporates from some of the leaf cells, causing more water to be pulled up by the xylem
  7. water diffused from air spaces into the spongy layer out of stomata into the air
• The role of water in photosynthesis is to provide the Hydrogen found in glucose

  Oxygen released as a by product
• Transpiration ;

  water evaporates from the mesophyll layer and diffused out of the stomata

  water molecules are drawn up by xylem vessels to replace water that has been lost

  This causes more water molecules to be absorbed from the soil into root hair cells
• Root hair cells

  • long hairs that extend from the cell body, increasing surface area for absorption
  • lots of mitochondria which produces energy for the active transport of mineral ions
  • thin so shorter diffusion distance