Nutrition in Plants

• External Leaf Structure and Function

  • Lamina (leaf blade) has a large, thin surface area to maximise absorption of sunlight, allowing rapid diffusion of CO2 to reach the inner cells of the leaf - Petiole positions the lamina for maximum absorption of sunlight and gaseous exchange
  • Veins allow transport of water and mineral salts to the cells in the lamina. They also transport manufactured food from the leaves to other plant parts
  • Regularly patterned around the stem - leaves are not blocking one another from sunlight, and each leaf receives optimum amount of light
• Internal Leaf Structure and Function (Pt. 1)

  • Cuticle: Waxy layer above epidermis which prevents excessive water loss. It's transparent to allow sunlight to penetrate to the mesophyll
  • Upper epidermis: Single layer of closely packed cells and don't contain chloroplast
  • Palisade mesophyll: A few layers of closely packed cells which are long, cylindrical and contain most number of chloroplasts for maximum light absorption - arranged longitudinally
• Internal Leaf Structure and Function (Pt. 2)

  • Spongy mesophyll: Loosely packed, irregularly shaped cells containing chloroplast and numerous large intercellular air spaces - rapid gaseous diffusion inside the leaf
  • Vascular bundle: Contains xylem (inner) and phloem (outer), to transport water and food materials within the plant
  • Lower epidermis: Single layer of closely packed cells not containing chloroplast. Stomata is found here
• Guard cells

  • Regulates transpiration rate and rate of gaseous and water vapour diffusion by opening and closing the stomata
  • Contains chlorophyll not present in epidermal cells
  • Manufactures glucose by photosynthesis
• Overall equation for photosynthesis

  • Word: Carbon dioxide + water + sunlight -> glucose + oxygen + water
  • Chemical: 6CO2 + 6H2O + sunlight -> C6H12O6 + 6O2 + 6H2O
• Photosynthesis

  • The process where light energy is absorbed by chlorophyll and converted into chemical energy
  • Requires inorganic molecules like carbon dioxide and water to synthesis organic molecules like glucose
• Light-dependent stage

  • Absorption of light energy by chlorophyll
  • Conversion of light energy to chemical energy
  • Photolysis of water
  • Light energy -chlorophyll-> chemical energy
  • 12H2O -photolysis of water-> 6O2 + 24H
• Light-independent stage

  • Hydrogen atoms produced are used to reduce CO2 to form glucose by enzyme reactions
  • 6CO2 -enzyme-controlled reactions-> C6H12O6 + 6H2O
• Entry of CO2 into the leaf

  • Rapidly used during photosynthesis, so CO2 concentration inside the leaf < atmospheric air
  • CO2 diffuses into leaf via the stomata down the concentration gradient
  • CO2 dissolves into the film of water surrounding the mesophyll cells and diffuses into cells
  • Structure of the lamina helps to maximise CO2 intake
• Stomata during the day:

  • Water from adjacent epidermal cells enters the guard cells
  • Guard cells swell and become turgid. This causes the guard cells to curve and pull the stoma open
• Stomata during the night:

  • Potassium ions diffuse out of the guard cells
  • Water potential in the guard cells increases, leading to exit of water by osmosis
  • Guard cells become flaccid and stoma closes
• Glucose in leaves

  • Used immediately by plant cells for cellular respiration or to form cellulose cell walls
  • Excess glucose is temporarily stored as starch in the leaves
  • Converted into sucrose which is transported to storage organs via the phloem
  • Reacts with nitrates and mineral salts to form amino acids which are combined to form proteins for synthesis of new protoplasm in the leaf. Excess amino acids are transported away for synthesis of new protoplasm or for storage as proteins
  • Used to form fats for storage, cellular respiration or synthesis of new protoplasm
• Limiting factor: A factor that directly affects or limits a process if its quantity or concentration is altered is called a limiting factor

  • Light intensity
  • Humidity
  • Carbon dioxide
  • Temperature
  • Availability of water
• Light intensity on the rate of photosynthesis

  • Increasing the light intensity will increase the rate of photosynthesis up to a certain point. After which, light intensity is no longer the limiting factor
  • When the light intensity first increase, it affects the light dependent stage of photosynthesis. Hence, when the light intensity increases, water molecules split faster in the chloroplasts
  • Other factors like CO2 concentration or temperature becomes the limiting factor
• Temperature on the rate of photosynthesis

  • At low temperatures, the rate of photosynthesis is slow as enzymes are less active
  • Rate of photosynthesis is at the maximum when the enzymes are at its optimum temperature, increasing the frequency of effective collisions
  • At a very high temperature, the rate of photosynthesis slows down as enzymes begin to denature
• CO2 on the rate of photosynthesis

  • From 0 to a certain point (Y), the rate of photosynthesis increases as the concentration of CO2 increases
  • As the concentration of CO2 increases, the plant can absorb more CO2 as more CO2 will diffuse into the leaves due to a steeper concentration gradient between the intercellular air spaces and the surrounding air
  • After a certain point, CO2 is no longer the limiting factor. Temperature or light intensity can now be the limiting factor