CH14 Nutrition in Plants

• All life on earth ultimately depends on plants for nutrition
• Green plants are distinct from other life forms in their ability to capture light energy from the sun to produce organic nutrients from inorganic raw materials, in doing so, convert light energy into chemical potential energy that can be utilised by cells
• Photosynthesis


  Carbon dioxide + water → glucose + oxygen

  
• Light-dependent stage


  1. Chlorophyll absorbs light energy

  2. Light energy is used for the photolysis of water to form oxygen gas, hydrogen ions, and ATP

  3. Light energy is hence converted into chemical potential energy (in the form of ATP)
• Light-independent stage


  Chemical potential energy (in the form of ATP) is used for the reduction of carbon dioxide with hydrogen ions to form glucose
• Increasing light intensity


  Increases the rate of photosynthesis
• Increasing carbon dioxide concentration

  Increases the rate of photosynthesis

  
• Increasing temperature


  Increases the rate of photosynthesis

  
• Dicotyledonous leaf structure


  • Cuticle: waxy, waterproof layer to prevent excessive water loss
  • Upper/lower epidermis: single layer of transparent cells with no chloroplasts, thicker cell walls to allow light to pass through and provide mechanical protection
  • Mesophyll layer: thin lining of moisture surrounding cell wall to allow gas diffusion
  • Palisade mesophyll: elongated cylindrical cells closely packed to maximise light absorption
  • Spongy mesophyll: irregularly shaped cells loosely packed to create air spaces for gas diffusion
  • Vascular bundle: xylem transports water and mineral salts, phloem transports photosynthetates
  • Guard cells: contain chloroplasts, able to change shape to control stomata opening
• Transpiration


  The loss of water vapour from the aerial parts of the plant, especially from the stomata of the leaves
• Translocation


  The transport of photosynthetates, in the form of sucrose and amino acids, from the leaves to other parts of the plant via the phloem
• Xylem


  • No protoplasm and no cross walls, hence hollow to reduce resistance and faster transport
  • Cell walls strengthened with lignin to provide mechanical support and prevent collapse under negative pressure
• Phloem


  • Sieve tube elements have reduced cytoplasm and perforated cross walls to reduce resistance and faster translocation
  • Companion cells have numerous mitochondria to provide energy for active transport of photosynthetates into sieve tube elements
• Root pressure


  Endodermis pumps mineral salts into xylem, lowering water potential, causing water to move from soil into xylem by osmosis, generating high hydrostatic pressure to push water up stem
• Capillarity


  Adhesion between water and xylem cell walls, and cohesion between water molecules, pulls water up xylem as a continuous column
• Transpiration and transpiration pull

  1. Water moves from xylem into mesophyll cells by osmosis

  2. Evaporation of water from mesophyll cell surfaces into intercellular air spaces

  3. Diffusion of water vapour out of stomata

  4. Continuous flow of water generates a suction force (transpiration pull) that draws water and minerals up xylem from roots to leaves
• Excessive water loss from cells

  Causes cells to become flaccid, guard cells become flaccid so stomata close, mesophyll cells become flaccid so leaves curl up
• Increased air movement


  Increases rate of transpiration
• Increased temperature


  Increases rate of transpiration
• Increased humidity


  Decreases rate of transpiration
• Increased light intensity


  Increases rate of transpiration