Backnotes

Transport in Plants

Xylem and Phloem

Function

The transport system in plants is mainly made of the xylem and phloem tissue. 

  • Xylem tissue transports water, mineral ions, and solutes from the roots to the leaves. 
  • Phloem tissue transports sucrose and amino acids from the leaves to the rest of the plant. 

Structure

You need to be able to recognize the xylem & phloem in three different parts of the plant: leaf, stem, and root.

Xylem – made of many hollow dead cells joined end to end with no end walls to form a long, open tube. Runs from root of plant, through stems and branch out into leaves. Contains no cytoplasm or nuclei and has walls made out of cellulose and lignin.

Phloem – made from cells joined end to end with end walls that look like sieve tube elements (STE) – contain cytoplasm but no nucleus. Every STE has a companion cell next to it which has a nucleus and other organelles for supplying the STE with their necessitated requirements.

Vascular Bundles – Group of xylem and phloem tubes. In roots, the vascular tissue is found at the centre whereas, in a shoot, they’re arranged in a ring near the outside edge.

Water Uptake in Roots

Water, mineral ions, and other nutrients are absorbed into the plants via the roots. Root hair cells are long epidermal cells that form on the epidermis, above the root cap (tip which protects the root as it grows). Root hairs increase the surface area for absorption and waters moves from the soil into the hair via osmosis (cytoplasm and cell sap inside is extremely concentrated in comparison to soil). Via osmosis, the water seeps through the cortex (from cell to cell or between cell walls and spaces) towards the xylem. Water moves up the xylem due to a combination of factors that include: transpiration pull, root pressure, and cohesion. 

NOTE: Mineral ions move into the root via active transport not osmosis.

Pathway of Water Through a Plant

Transpiration

A transpiration stream is caused when:

  • water exits the leaf due to transpiration and therefore results in negative pressure or a transpiration pull at the top of the plant
  • when water is absorbed in the roots via osmosis there’s positive pressure or root pressure at the bottom of the plant.

A column of water is drawn up the plant from low to high pressure. Water molecules ‘stick together’ due to cohesion allowing the entire column of water to be drawn. Water movement in the xylem is much like water movement in a straw. When you stuck on the top end of the straw it reduces pressure compared to the bottom of the straw. A column of liquid is therefore drawn from low to high pressure. The liquid column sticks together due to cohesion. Water enters the leaf through the vascular bundle and can move directly into palisade or spongy mesophyll cells to be used for photosynthesis. Otherwise, water evaporates into the air spaces in the spongy mesophyll layer, where they diffuse out via the stomata. We call this transpiration. 

Transpiration is the loss of water vapour from leaves via the stomata. Once water enters the leaf via the xylem vessels of the vascular bundle, it travels to the mesophyll cells to be used in photosynthesis. Some water remains on the surfaces of these mesophyll cells and evaporates into the air spaces of the spongy mesophyll layer. Water vapour diffuses out of the leaf via the stomata. 

Factors that affect transpiration rate are: 

  • Temperature
    • Higher temperatures result in faster moving molecules and therefore increases diffusion rate, which in turn, increases transpiration rate
  • Humidity
    • Higher humidity results in a lower concentration gradient and thus reduces diffusion rate, which in turn, reduces transpiration rate
  • Wind Speed
    • Higher speeds result in water evaporating more quickly, increasing transpiration rate
  • Light Intensity
    • In bright sunlight, plant is more likely to open stomata widely for supply of CO2 which means more water can evaporate from the stomata
  • Water Supply
    • If water is in short supply, roots may not be able to replenish the amount of water lost so transpiration decreases

Translocation

Translocation is the movement of sucrose and amino acids in the phloem from the source to the sink. The sap inside the phloem tube contains a lot of sugar – mainly sucrose.

  • Source is the region from where sucrose and amino acids are being taken
  • Sink is the region where sucrose and amino acids are being deposited

Some parts of the plant may act as a source and sink at different times during the plant life. E.g.: leaves are the source most of the time but can become a sink in periods of growth i.e. after autumn when plants grow their leaves again.