plant structure yr 8

• Xylem


  Mature xylem consists of elongated dead cells, arranged end to end to form continuous vessels (tubes)
• Mature xylem vessels


  • Contain no cytoplasm
  • Are impermeable to water
  • Have tough walls containing a woody material called lignin
• Phloem


  Phloem consists of living cells arranged end to end. Unlike xylem, phloem vessels contain cytoplasm, and this goes through the holes in the sieve plates from one cell to the next.
• Phloem transport


  1. Transports sucrose and amino acids up and down the plant

  2. This is called translocation

  3. Sucrose is transported from sources in the root to sinks in the leaves in spring time

  4. Sucrose is transported from sources in the leaves to sinks in the root in the summer
• Applied chemicals, such as pesticides, also move through the plant by translocation
• Energy source


  The sugar can be broken down in plant cells by the process of respiration to generate ATP. The chemical energy released by respiration can be used by the plant for cellular activities such as protein synthesis or cell division.
• Plant energy storage


  The sugar produced by photosynthesis can be converted into the sugar glucose. Thousands of glucose molecules can be linked together to form the complex carbohydrate starch. Starch is stored inside plant cells as grains.
• Plant building material


  The sugar produced by photosynthesis can be converted into the sugar glucose. Thousands of glucose molecules can be linked together to form the complex carbohydrate cellulose. Cellulose is a very tough molecule that is used to build the cell wall of plant cells.
• Limiting factor


  Anything in short supply that prevents photosynthesis occurring at its maximum rate
• If photosynthesis occurs more slowly in plant cells

  A lower quantity of sugar will be produced and the quantity of chemical energy available for cell growth will be reduced
• Important factors that can limit the rate of photosynthesis

  • Light intensity
  • Carbon dioxide concentration
  • Temperature
• vessel elements are hollow tube-like cells with perforated end walls which allow them to join together into continuous channels
• tracheids are long tubes that transport water from the roots to the leaves
• the xylem is made up of tracheids, vessel elements, and fibres