plants

• Levels of organization in plants

  • Cells
  • Tissues
  • Organs
  • Organ systems
• Leaf


  An organ that, along with a stem and roots, forms an organ system for the transport of substances around the plant
• Photosynthesis


  1. Requires carbon dioxide and water

  2. Water transported from soil by roots and xylem

  3. Carbon dioxide diffuses into leaf through stomata

  4. Photosynthesis occurs in palisade mesophyll layer
• Stomata


  • Tiny holes in the lower epidermis of the leaf
  • Allow carbon dioxide to diffuse into the leaf
  • Controlled by guard cells to balance carbon dioxide intake and water loss
• Xylem and phloem


  • Xylem transports water from roots to leaves
  • Phloem transports sugars produced in photosynthesis to rest of plant
• Cuticle


  • Waxy, waterproof layer on top of leaf to reduce water loss
• Guard cells are turgid (well hydrated)


  Stomata open wider to allow more carbon dioxide diffusion
• Guard cells are flaccid (dehydrated)


  Stomata close to conserve water
• Meristem tissue


  • Plant stem cells found at growing tips
  • Can differentiate into many cell types to allow plant growth
• Most stomata are on the underside of leaves to reduce water evaporation
• Translocation


  Plants transport sugars made through photosynthesis in leaves to rest of plant
• Phloem cells


  Arranged end to end to form long columns called phloem tubes to transport cell sap (water and sugar)
• Phloem tubes


  • Have small pores/gaps between adjacent cells to enable movement of cell sap
  • Can transport substances in either direction (up or down the plant)
• Plants


  Respond to their environment much more slowly than animals, rely entirely on hormones rather than a nervous system
• Transpiration


  Movement of water from roots to leaves driven by evaporation of water from leaves
• Auxins


  • Plant hormones that control growth at the ends of shoots and roots
• Transpiration stream


  Chain of water molecules pulled up the plant as water evaporates from leaves
• Auxin movement


  Produced in tips, dissolve in cell solution, diffuse backwards along shoot or root
• Auxin accumulation


  On shaded side rather than sunny side, on lower side rather than upper side
• Factors influencing rate of transpiration
• Phototropism


  Response to light, shoots grow towards light
• High light intensity


  More photosynthesis, more stomata open, higher rate of transpiration
• No photosynthesis at night

  Stomata closed, very little transpiration
• Geotropism/Gravitropism


  Response to gravity, roots grow downwards, shoots grow upwards
• Higher temperature


  Water particles have more energy, higher rate of transpiration
• Auxin accumulation in shoots

  Causes faster growth on shaded side, leading to curving towards light
• High airflow


  Water molecules blown away quickly, higher concentration gradient, higher rate of transpiration
• Auxin accumulation in roots

  Inhibits growth on lower side, causing upper side to grow faster and root to curve downwards
• High humidity


  Lower concentration gradient, lower rate of transpiration
• Roots are normally underground so don't detect light, but if exposed will curve downwards
• Xylem tubes are made of dead xylem cells with no ends, strengthened with lignin, to transport water and mineral ions from roots to leaves
• Auxin


  A type of growth hormone that stimulates plants to grow
• Using auxin commercially


  1. Add auxin to plant cells in tissue culture to stimulate growth

  2. Add auxin powder to plant cuttings to stimulate root growth

  3. Add high levels of auxin to disrupt plant growth and kill weeds
• Gibberellin


  A plant hormone used to control dormancy, induce flowering, and grow larger fruit
• Using gibberellin commercially


  1. Expose seeds to gibberellin to induce germination

  2. Apply gibberellin to induce flowering or produce more/larger flowers

  3. Apply gibberellin to grow larger fruits, especially in seedless varieties
• Ethylene


  A plant hormone used to stimulate fruit ripening
• Using ethylene commercially


  Pick fruit before fully ripe, then expose to ethylene to quickly ripen it for sale
• Most plants naturally produce their own ethylene, which is often blocked during transport to prevent premature ripening
• Ethylene mechanism


  Stimulates an enzyme that causes the fruit to ripen