Edexcel IGCSE Biology 复习笔记 2.8.3 Water Movement in Flowering Plants

Uptake of water into the root

• Root hair cells are adapted for the efficient uptake of water (by osmosis) and mineral ions (by active transport)
• Root hairs are single-celled extensions of epidermis cells in the root
• They grow between soil particles and absorb water and minerals from the soil
• Root hairs increase the surface area to volume ratio significantly
  • This increases the rate of the absorption of mineral ions by active transport

   

• The high proportion of dissolved minerals and sugars in the cytoplasm (of the root hair cell) give it a low water potential (less watery)
  • Water moves into the root hair cell by osmosis

   

A root hair cell

The structure of a root specifically allows it to maximise absorption of water by osmosis and mineral ions by active transport

The route of water through the plant

• Water moves, by osmosis, into the root hair cells, through the root cortex and into the xylem vessels:
• Once the water gets into the xylem, it is carried up to the leaves where it enters mesophyll cells
• So the pathway is:

root hair cell → root cortex cells → xylem → leaf mesophyll cells

Pathway of water into and across a root

• The pathway can be investigated by placing a plant (like celery) into a beaker of water that has had a stain added to it (food colouring will work well)
• After a few hours, you can see the leaves of the celery turning the same colour as the dyed water, proving that water is being taken up by the celery
• If a cross-section of the celery is cut, only certain areas of the stalk is stained the colour of the water, showing that the water is being carried in specific vessels through the stem - these are the xylem vessels

Investigating water movement in plants using a stain

• Transpiration is defined as the loss of water vapour from the parts of the plant that are above ground (leaves, stem, flowers)
• Loss of water occurs through evaporation of water at the surfaces of the spongy mesophyll cells followed by diffusion of water vapour through the stomata
• The many interconnecting air spaces between the mesophyll cells and the stomata creates a large surface area
• This means evaporation can happen rapidly when the stomata are open

Transpiration in plants

The effect of transpiration

• Water moves through the xylem vessels in a continuous transpiration stream from the roots to the leaves via the stem to replace the water that has been lost due to transpiration
• Due to cohesion, the water in the xylem creates a continuous unbroken column (each individual molecule ‘pulls’ on the one below it)
• Transpiration produces tension or ‘pulls’ on the water in the xylem vessels
• If the rate of transpiration from the leaves increases, water molecules are pulled up the xylem vessels quicker

Water uptake, transport and transpiration

The function of transpiration

• Transpiration has several functions in plants:
  • Transporting mineral ions
  • Providing water to keep cells turgid in order to support the structure of the plant
  • Providing water to leaf cells for photosynthesis
  • Keeping the leaves cool, the conversion of water (liquid) into water vapour (gas) as it leaves the cells and enters the airspace requires heat energy. The use of heat to convert water into water vapour helps to cool the plant down

   

• There are several environmental conditions which have an impact on the rate of transpiration
  • Air movement
  • Humidity
  • Temperature
  • Light intensity

   

Factor affecting the Rate of Transpiration Table

• The table above explains how these four factors affect the rate of transpiration when they are all high; the opposite effect would be observed if they were low