Transport in plants - F211

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Every cell of a multicellular organism needs a regular supply of water and nutrients. the many cells deep inside of a plant, far away from the supply would not receive all the water & nutrients it needs to survive via simple diffusion. the roots can absorb water from the soil fairly easily, but cannot absorb sugars. leave can produce sugars, but cannot absorb water from the air. the transport system in plants moves water and soluble minerals in the xylem, sugars travel up/down in the phloem tissue.

Xylem vessels

  • very long, tube like structures, formed from cells joined end to end.
  • no end walls, making an uninterupted tube, allowing water to pass through the middle easily.
  • dead cells, contain no cytoplasm, so the flow of water is not impeded.
  • thickened walls with lignin, helps to support the xylem, stopping them collapsing inwards.
  • pits in the walls of the xylem allow water to move sideways from one vessel to another.

Sieve tube elements 

  • theres a companion cell for every sieve tube element, they carry out the living functions for both themselves and their sieve cells. eg, provide energy for active transport of solutes

Transpiration - the loss of water vapour from the aerial parts of the plant due to evaporation, it is a consequence of gas exchange.

a plant needs to open its stomata to get carbon dioxide in, so that it can produce glucose (by photosynthesis). it also needs to let water out, theres a higher concentration of water inside the leaf, than in the air outside, so water moves out of the leaf, down its water potential gradient when the stomata is open.

A potometer is used to estimate transpiration rates. it actually measures water uptake by a plant, but its assumed that water uptake by a plant, is directly related to water loss by the leaves. you can use it to estimate how different factors effect the transpiration rate.


1. cut a shoot underwater to prevent air from entering the xylem. cut it at an angle to increase the Surface Area available for water uptake.

2. check that the apparatus is full of water, and that there are no air bubbles.

3. insert the shoot into the apparatus underwater so that no air can enter.

4. remove the potometer from the water and make sure its airtight, and watertight (vaseline)

5. dry the leaves, allow time for the shoot to acclimatise and then shut the tap.

6. keep the conditions constant throughout the experiment, eg, temperature & air humidity. 

7. Record the position of the air bubble.

8. start a stopwatch and record the distance moved by the bubble per unit of time, eg, per hour.

factors that effect transpiration - 

  • number of leaves - a plant with more leaves, has a larger surface area, over which water vapour can be lost.
  • number, size & position of stomata - if the leaves have many large stomata, water vapour is lost quicker. if the stomata are on the


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