2 Nutrition in flowering plants

• Xylem vessels


  • Transports water and mineral salts from the roots to the other parts of the plant
  • Provides mechanical support to the plant

  
• Phloem


  Transports sucrose and amino acids from the leaves to the other parts of the plant

  
• Xylem vessels


  • Hollow with no cytoplasm or cross walls
  • Allows water to move continuously within the xylem vessel
  • Inner walls of xylem is strengthened with a hard substance known as lignin
• Phloem


  • Consists of sieve tubes and companion cells
  • Sieve tube consists of living cells known as sieve tube elements
  • Sieve plates separate the sieve tube elements
  • Companion cells have numerous mitochondria to release energy to help sieve tube elements transport sucrose and amino acids by active transport
• Translocation is the transport of manufactured food substances in plants e.g. sucrose and amino acids
• How do we know that phloem transports "food"?

  1. Ringing experiment

  2. Using aphids in translocation studies

  
• Transpiration


  Loss of water vapour from the aerial parts of a plant, mainly through the stomata of the leaves

  
• Importance of transpiration


  • The loss of water vapour creates a pull known as transpiration pull, which draws water and mineral salts upwards from the roots
  • Evaporation of water from the surface of leaves cools the plant
  • Water that is drawn up from the roots are used for a number of purposes like photosynthesis and maintaining turgor pressure in plants

  
• Pathway of water in a plant (transpiration)

  1. Water molecules move from soil into root hair cells by osmosis

  2. Water molecules continue to move by osmosis, down a water potential gradient, to the root cells and to the xylem in the roots

  3. From the xylem in the roots to the xylem in the stem and finally to the xylem in the leaf, water molecules move upwards by transpiration pull and capillary action

  4. Water molecules move from xylem in the leaf into the mesophyll cells, and finally to the thin film of water

  5. Water molecules evaporate to form water vapour which occupies the intercellular air spaces

  6. Water vapour diffuses out of the leaf into the external environment through the stomata, down a diffusion gradient

  
• Light intensity


  The higher the light intensity, the bigger the size of the stomata, the higher the rate of transpiration
• Wind intensity


  The higher the wind intensity, the more removal of water vapour outside the leaves, creating a steep concentration gradient of water vapour between air spaces in the leaves and the external environment, hence, rate of transpiration is higher
• Humidity


  The higher the humidity, the gentler the concentration gradient of water vapour between the air spaces and the external environment, hence, transpiration rate is lower
• Temperature


  The higher the temperature, the higher the rate of evaporation of water on the thin film of moisture in the air spaces, increasing the concentration of water vapour in the air space and establishing a steep concentration gradient of water vapour between air space and external environment, hence, rate of transpiration is higher
• Wilting


  Excessive transpiration where the rate of transpiration is higher than the rate of water absorption, causing plant cells to lose water and become flaccid, decreasing turgor pressure in the plant
• Wilting might not be a bad thing

  • When the leaves fold downwards, exposure to light decreases, guard cells become flaccid, reducing excessive loss of water and rate of transpiration decreases
  • When the stomata closes, the rate of diffusion of carbon dioxide into the leaves decreases, so the rate of photosynthesis decreases
  • When the leaves fold downwards, the exposure to light decreases, and therefore rate of photosynthesis decreases

  
• Transpiration experiments


  1. Potometer is used to measure the rate of transpiration by measuring how much/fast the plant absorbs water

  2. Assumption: rate of transpiration = rate of absorption

  
• Where is transpiration occurring on a leaf?