Plant Nutrition

• Photosynthesis


  The process by which the plant makes organic substances (carbohydrates) from inorganic substances (carbon dioxide and water) using light energy, where light energy is converted into chemical energy stored in glucose
• How photosynthesis takes place

  1. Carbon dioxide diffuses into leaf through stomata, dissolves in water surrounding mesophyll cells, then diffuses through cell wall and membrane

  2. Water absorbed from soil by root hair cells, transported up xylem to mesophyll cells

  3. Light energy absorbed by chlorophyll in chloroplasts, used to help CO2 react with water producing glucose where light energy is converted into chemical energy stored in glucose
• Oxygen leaves the leaf by diffusing out through stomata down a concentration gradient, as oxygen concentration is higher inside the leaf (a product of photosynthesis) than outside
• Importance of glucose


  • Respiration
  • Starch storage
  • Sucrose to be translocated
  • Cellulose in cell wall
• Carbon dioxide is used in greenhouses to allow the plant to carry out photosynthesis at maximum rate to produce more glucose for more growth by storing starch
• Why glucose is stored as starch

  Starch is a large insoluble molecule so doesn't affect the water potential of cell, can be used as a source of glucose/hydrolysed into glucose to be used in respiration when photosynthesis is limited
• Why glucose is translocated as sucrose

  Glucose is highly reactive, yet sucrose as a disaccharide is less reactive but still soluble
• Upper epidermis


  • Transparent to allow passage of light
  • Protective layer that prevents entry of pathogens
  • Produces waxy cuticle to reduce water evaporation
• Palisade mesophyll


  • Packed with large number of chloroplasts for maximum absorption of light energy
  • Cells arranged close together with tiny air spaces to absorb more light
  • Chloroplasts arranged broad side on to maximize light absorption
• Spongy mesophyll


  • Loosely arranged with large air spaces to allow diffusion of gases
  • Surrounded by layer of water on cell walls that evaporates into air spaces to allow transpiration and cooling
• Stomata


  • Allow diffusion of carbon dioxide into leaf for photosynthesis, oxygen into leaf for respiration, and water vapour out of leaf by transpiration
• Guard cells


  • Control opening and closing of stomata
• Plant is rich in water, high light intensity

  Stomata open to allow diffusion of CO2 into leaf for photosynthesis
• Plant in shortage of water, high temperature

  Stomata close to reduce water vapour loss by transpiration
• Factors affecting rate of photosynthesis (limiting factors)

  • Carbon dioxide concentration
  • Light intensity
  • Temperature
• Limiting factor


  An environmental factor present in short supply that limits the rate of a reaction
• Temperature increase


  Rate of photosynthesis increases, then decreases
• As temperature increases till the optimum, the rate of photosynthesis increases due to increased kinetic energy and enzyme activity. Above the optimum, the rate decreases as enzymes start to denature.
• How to increase growth rate of plants in a greenhouse

  • Increase light intensity with artificial light
  • Maintain optimum temperature with ventilation
  • Enrich carbon dioxide with ventilation, composting, or CO2 cylinders
  • Maintain optimal humidity with humidifier and ventilation
  • Provide sufficient minerals with fertilizers or animal manure
• Advantages of growing crops in greenhouses include: maintaining optimal conditions for photosynthesis and growth by controlling limiting factors like light, temperature, carbon dioxide, water, and nutrients; and controlling pests and weeds
• Sodium hydrogen carbonate is a source of carbon dioxide, and can be used as an indicator - when respiration exceeds photosynthesis the solution turns yellow/dark, when photosynthesis exceeds respiration the solution turns red/purple
• Investigation of effect of different factors on rate of photosynthesis

  1. Independent variable: light intensity, temperature, or carbon dioxide concentration

  2. Controlled variables: temperature using thermostatic water bath and heat shield, carbon dioxide concentration using HCO3-, plant species/age/size

  3. Dependent variable: measure volume of oxygen gas produced in unit time using a gas syringe
• The scientists kept the temperature of the leaves at 20°C while recording results to maintain a constant temperature and avoid overheating from the lamp, using a tank of water as a heat shield