Plant Structure

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Xylem Vessels

  • function = to transport water and mineral ions up the plant, and provide support
  • very long, tube-like structures fromed from dead cells joined end to end (the tubes are found together in bundles)
  • the cells are longer than they are wide - the have a hollow lumen (doesn't contain any cytoplasm)
  • they don't have end walls, making an uninterrupted tube which allows water and mineral ions the pass up the middle easily
  • walls are thickened with a woody substance called lignin which helps support the plant
  • water and mineral ions move into and out of the vessels through pits in the walls where there's no lignin
  • found throughout the plant, but particularly around the centre of the stem where they group together with other vessels to form vascular bundles
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Sclerenchyma Fibres

  • function = to provide support
  • made of bundles of dead cells that run vertically up the stem (like xylem vessels)
  • the cells are longer than they are wide
  • have a hollow lumen 
  • have end walls
  • cell walls are thickened with lignin
  • they have more cellulose than other plant cells
  • usually associated with the vascular bundles
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Starch

  • cells get energy from glucose (plants store excess glucose as starch - when they need more glucose for energy they break down starch to release the glucose
  • starch is a mixture of two polysaccharides of alpha-glucose:
    • amylose = a long unbranched chain of alpha-glucose. the angles of the glycosidic bonds give it a coiled structure - this makes it compact and good for storage as you can fit mor into a small space
    • amylopectin = a long branched chain of alpha-glucose. its side branches allow the enzymes that break down the molecule to reach the glycosidic bonds easily, meaning glucose can be released quickly
  • starch is insoluble in water, so it doesn't cause water to enter cells by osmosis (which would make them swell) this makes it good for storage
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Cellulose

  • is made of long, unbranched chains of beta-glucose, joined by glycosidic bonds
  • the glycosidic bonds are straight, so the cellulose chains are straight
  • between 50 and 80 cellulose chains are linked together by a large number of hydrogen bonds to from strong threads called microfibrils
  • the microfibrils mean cellulose provides structural support for cells (strengthening plant cell walls)
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Plant Fibres

  • made up of long tubes of plant cells
  • are strong, making them useful for making things e.g. ropes or fabrics like hemp
  • strong because of:
    • the arrangement of microfibrils = the cell wall contains cellulose microfibrils in a net-like arrangement
    • secondary thickening = when some structural plant cells (sclerenchyma) have finished growing, they produce a secondary cell wall between the normal cell wall and the cell membrane which is thicker and contains more lignin than the normal cell wall - this is called secondary thickening
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Core Practical: Tensile Strength of Plant Fibres

Tensile strength = mamximum load it can take before it breaks

  • attach the fibre to a clamp stand and hang a weight from the other end
  • keep adding weights, one at a time, until the fibre breaks
  • record the mass needed to break the fibre (the hgher the mass, the higher the tensile strength)
  • repeat the experiment with different samples of the same fibre
  • the fibres being tested should be the same length
  • all other variables (like temperature and humidity) should be kept constant
  • safety measures - goggles to protect eyes, leave weith drop are clear so they won't drop on feet
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