Plant Organ System (AQA GCSE Biology: Combined Science)

Plant organs

• The structure of root hair cells, xylem and phloem are adapted to their functions
• The roots, stem and leaves are all plant organs that 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 vessels – transport water and minerals (pronounced: zi-lem) from the roots to the stem and leaves
  • It is composed of hollow tubes strengthened by lignin adapted for the transport of water in the transpiration stream

• Phloem 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 (pronounced: flow-em)
• These vessels are arranged throughout the root, stem and leaves in groups called vascular bundles

Vascular tissue in a dicotyledonous plant

• 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 which increase the surface area of the cells significantly; this increases the rate of the absorption of water by osmosis and mineral ions by active transport
  • They grow between soil particles and absorb water and minerals from the soil
  • Water enters the root hair cells by osmosis
  • This happens because soil water has a higher water potential than the cytoplasm of the root hair cell

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

• 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