Plants

• Stem


  • Contains xylem and phloem (vascular bundles) to allow translocation of food and transport of water and mineral salts to all parts of the plant
  • Supports the plant upright
  • Spaces out the leaves so that they can receive sunlight and absorb carbon dioxide which they need for photosynthesis

  
• Roots


  • Anchor the plant in the soil
  • Prevent it from falling over or being blown over by the wind
  • Prevent it from being carried away by rainwater (eg. when growing on slopes)
  • Absorb water (by osmosis) and mineral salts (by active transport) which the plant needs for making food in the leaves

  
• Leaves


  Make food by photosynthesis

  
• Leaf veins


  • Deliver water and salts to the leaf cells and carry away the food made by them
  • Form a kind of skeleton which supports the softer tissues of the leaf blade

  
• Leaf blade (lamina)

  Broad and thin
• Epidermis


  • A single layer of cells on the upper and lower surfaces of the leaf
  • Transparent (contains no chloroplasts) to allow sunlight to pass through it and reach the palisade cells which are responsible for photosynthesis
  • Helps keep the leaf shape
  • Prevents bacteria and fungi getting in
  • Reduces evaporation from the leaf

  
• Waxy Cuticle


  Waxy layer on the epidermis to further reduce water loss
• Palisade mesophyll cells


  • Long and contain many chloroplasts
  • Tightly packed and mainly responsible for photosynthesis

  
• Spongy mesophyll cells


  Vary in shape and fit loosely together, leaving many air spaces between them (intercellular air spaces)
• Photosynthesis

  1. Carbon dioxide diffuses into the leaf through stomata

  2. Oxygen is produced and diffuses out through the stomata

  
• Stomata


  • Pores in the leaf
  • 2 guard cells surround every stoma and are responsible for opening and closing of the stoma
  • In most dicots, stomata occur only on the lower side of the leaf
  • In monocots, stomata are evenly distributed on both sides of the leaf

  
• Stomata opening and closing

  1. Open during daylight to allow CO2 diffusion and transpiration

  2. Close at night when photosynthesis wouldn't take place anyway (due to lack of sunlight)
• Guard cells


  • Have uneven thickening in their cell walls
  • Have chloroplasts and can thus perform photosynthesis and produce sugar
  • Water enters the guard cells by osmosis, changing their shape and thus opening the pore, letting in CO2 for photosynthesis

  
• Xylem


  • No top and bottom cell walls between cells to form continuous hollow tubes
  • Cells are essentially dead, without organelles or cytoplasm, to allow free passage of water
  • Outer walls are thickened with lignin, strengthening the tubes, which helps support the plant
  • Water and mineral salts travel upwards only in the xylem
  • Xylem helps support the plant upright (due to the presence of lignin)

  
• Phloem


  • Made of living cells
  • Cells are joined end-to-end and contain holes in the end cell walls (sieve plates) forming tubes which allow sugars and amino acids to flow easily through (by translocation)
  • Soluble food travels upwards and downwards in the phloem

  
• Cortex and pith


  Large cells capable of storing food, eg. starch
• Cambium


  Cells that can divide to cause growth in diameter
• Root cap


  Protects the root tip while it pushes its way through the soil
• Root hairs


  • Take up water from the soil by osmosis and absorb mineral salts by active transport (and diffusion)
  • Increase the surface area for absorption
  • Specialized epidermal cells

  
• Translocation


  Transport of substances in solution within a plant (sucrose and amino acids)

  
• Autotrophs


  Plants produce (synthesise) food by photosynthesis

  
• Factors affecting rate of photosynthesis

  • Light intensity
  • Temperature
  • Carbon dioxide

  
• Limiting factor


  The external factor which restricts the effect of the others (which stops the reaction from going faster)
• Light intensity and photosynthesis rate


  Increases rate up to a certain point, then no further increase

  
• Temperature and photosynthesis rate


  Increases rate up to an optimum, then decreases

  
• Carbon dioxide and photosynthesis rate

  Increases rate up to an optimum, then no further increase

  
• Glucose


  • Broken down to sucrose for transport
  • Converted to starch for storage
  • Used in respiration to release energy
• Oxygen


  Diffuses out of the leaves through the stomata

  
• Photosynthesis and respiration


  • Plants photosynthesize and respire, but the rate of photosynthesis is much higher than the rate of respiration
  • At dawn and dusk, the processes of respiration and photosynthesis break even (compensation point)

  
• Nitrogen is needed to make proteins, and magnesium is needed to make chlorophyll
• Deficiency of nitrogen causes poor growth and yellow leaves, and deficiency of magnesium causes yellowing between veins of leaves
• Destarching a plant


  Leaving it in darkness for 2 or 3 days
• Requirements for photosynthesis


  • Carbon dioxide
  • Water
  • Light
  • Chlorophyll
  • Suitable temperature

  
• Testing a leaf for starch

  1. Place leaf in boiling water, then boiling ethanol to remove chlorophyll

  2. Cover with dilute iodine solution - blue-black colour indicates presence of starch
• Demonstrating need for light in photosynthesis

  1. Use a destarched plant

  2. Cover part of a leaf with dark paper, leave rest exposed to light

  3. Test covered and exposed parts for starch - only exposed part tests positive

  
• Demonstrating need for carbon dioxide in photosynthesis

  1. Use destarched plants

  2. Place one plant in a container with potassium hydroxide to absorb CO2, leave other plant in open air

  3. Test leaves for starch - only leaf from open air tests positive