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

  4. Oxygen diffuses out of leaf through stomata down concentration gradient
• Increase in carbon dioxide concentration/ temperature / light intensity causes an increase in crop yield
• Carbon dioxide


  A limiting factor for photosynthesis, it is a reactant needed for photosynthesis
• Light


  A limiting factor as light is a source of energy trapped by chlorophyll, light energy is converted into chemical energy stored in glucose
• Temperature


  A limiting factor, photosynthesis is a process catalysed by enzymes, where increase in temperature till optimum, increase kinetic energy of molecules so increase number of collisions and number of ESCs (enzyme substrate complexes), and at lower temperature we have lower enzymatic activity
• Carbon dioxide is used in glass houses
• Why the plant stores glucose as starch

  • Starch is a store of energy
  • Starch is large insoluble molecule so doesn't affect the water potential of cell (glucose small and highly reactive and affects the water potential inside the cells)
  • To be used in presence of limiting factors such as no leaves and the plant can't carry photosynthesis
• Why glucose is translocated as sucrose

  Glucose is highly reactive, yet sucrose as a disaccharide is less reactive but still soluble
• Leaf structure: Adaptations


  • Upper epidermis: Transparent to allow passage of light, Protective layer that prevent entry of pathogens, Produce waxy cuticle to reduce water evaporation
  • Palisade mesophyll: Packed with large number of chloroplast, Cells arranged close together with tiny air spaces, Chloroplast 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 the air spaces
  • Stomata: Allow diffusion of carbon dioxide into leaf, oxygen into leaf, water vapour out of leaf
  • Guard cells: Control opening and closing of stomata
• Plant is rich in water, high light intensity

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

  Guard cells become flaccid, stomata close to reduce the water vapour loss by transpiration
• Limiting factor


  An environmental factor present in short supply that limits rate of reaction (for plant growth: light intensity, minerals concentration, temperature and carbon dioxide concentration)
• At point F (leveled off) in graph 3, the temperature is the limiting factor
• In graph 4, the limiting factor is light intensity
• In the blue part of graph 2, the limiting factor is carbon dioxide concentration
• How temperature affects rate of photosynthesis

  1. As temperature increase till the optimum, the rate of photosynthesis increase (due to increased kinetic energy and enzyme activity)

  2. At peak optimum temperature

  3. Increase in temperature above optimum is accompanied by a decrease in rate of photosynthesis (due to enzyme denaturation and stomata closure)
• Hypothesis: As temperature increase, the rate of photosynthesis increase then decrease
• How to increase the growth rate of plants in a glass house

  • Increase light intensity using artificial light source
  • Maintain optimum temperature using ventilation
  • Increase carbon dioxide concentration using ventilation, burning organic matter, or CO2 cylinders
  • Maintain optimum humidity using humidifier and ventilation
  • Provide sufficient minerals using artificial fertilisers or animal manure
• Advantages of growing crops in green / glass houses
• Sodium hydrogen carbonate


  Source of carbon dioxide, indicator for CO2 concentration - acid lowers pH, increase in CO2 concentration decreases pH, photosynthesis greater than respiration increases pH
• Investigation of effect of different factors on rate of photosynthesis

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

  2. Controlled variables: temperature using thermostatic water bath and heat shield, CO2 concentration using HCO3-, plant species/age/size

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