Transport of Water & Mineral Ions (Edexcel GCSE Biology: Combined Science)

• The structure of root hair cells, xylem and phloem are adapted to their functions
• The roots, stem and leaves form a plant organ system for transport of substances around the plant

Functions of xylem & phloem

• Plants contain two types of transport vessel: xylem and phloem
• Xylem (pronounced: zi-lem) vessels:
  • Transport water and minerals from the roots to the stem and leaves
  • Are composed of hollow tubes strengthened by lignin adapted for the transport of water in the transpiration stream

• Phloem (pronounced: flow-em) vessels:
  • Transport food materials (mainly sucrose and amino acids) made by the plant from photosynthesising leaves to non-photosynthesising regions in the roots and stem

• Xylem and phloem vessels are arranged throughout the root, stem and leaves in groups called vascular bundles

The location of vascular tissues in the stem and roots of a plant

• Osmosis causes water to pass into the root hair cells, through the root cortex and into the xylem vessels:

Pathway of water into and across a root

• Once the water gets into the xylem, it is carried up to the leaves where it enters mesophyll cells
• The pathway is:

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

Stomata and guard cells

• Stomata (specifically the guard cells) control the diffusion of gases in and out of leaves
  • This means stomata control the entry of carbon dioxide into leaves

• Stomata open and close in a daily rhythm
  • Even when the plant is kept in constant light or constant darkness, the daily rhythm of opening and closing of the stomata continues

• Opening of stomata during the day:
  • Maintains the inward diffusion of carbon dioxide and the outward diffusion of oxygen
  • Allows the outward diffusion of water vapour in transpiration

• Closing of stomata at night when photosynthesis cannot occur:
  • Reduces the rate of transpiration
  • Conserves water

Guard cells and the stomata. When the guard cells are turgid the stoma is open. Once the guard cells lose water by osmosis and become flaccid, the stoma closes.

Transpiration

• 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

• 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