Plant Nutrition

• Plants are required to undergo respiration to release useable energy. However, they do not eat food to respire. Instead, they make their own food through the process of photosynthesis.
• Photosynthesis


  1. Chlorophyll absorbs light

  2. Enzymes convert carbon dioxide and water into glucose and oxygen
• Glucose


  Can be respired to release energy, stored as starch, used to make cellulose, converted to sucrose, or combined with other chemicals
• Factors affecting rate of photosynthesis


  • Carbon dioxide concentration
  • Light intensity
  • Temperature
• One of the three factors (carbon dioxide, light, temperature) will always be limiting the rate of photosynthesis


  The other two will be in excess
• Adding more of the limiting factor causes the rate of photosynthesis to increase, and eventually one of the other two factors becomes the limiting factor
• At very high temperature, rate of photosynthesis will decrease due to enzyme denaturation, increased water loss, and stomatal closure
• Mineral ions required for plant growth


  • Magnesium
  • Nitrate
• Conditions required for optimal plant growth


  • Magnesium ions to make chlorophyll
  • Nitrate ions to make amino acids and DNA
  • High light intensity
  • High carbon dioxide concentration
  • Warmth for enzymes
• Leaf structure


  • Stomata allow gas exchange
  • Palisade cells contain many chloroplasts
• Xylem can be distinguished from phloem due to their thicker walls
• Leaf adaptations for effective photosynthesis


  • Thin leaf with large surface area
  • Transparent upper epidermis
  • Palisade mesophyll layer close to surface with many chloroplasts
  • Spongy mesophyll layer with air spaces for gas diffusion
  • Stomatal pores open and close to allow carbon dioxide diffusion
  • Xylem vessels provide water from roots
• Leaf adaptations for effective gas exchange


  • Contain stomata controlled by guard cells
  • Spongy mesophyll layer with air spaces
  • Thin leaf with large surface area
• Demonstrating requirements of photosynthesis by testing for starch


  1. Boil leaf in water to kill it

  2. Boil in ethanol to remove chlorophyll and waxy cuticle

  3. Remove leaf from ethanol and dip in water

  4. Add iodine solution - blue-black colour indicates presence of starch
• Ethanol is highly flammable, so safety precautions should be taken when heating it
• Destarching a plant


  Place plant in dark for 24 hours to stop photosynthesis and convert starch to glucose
• Showing a plant requires light for photosynthesis


  Cover part of plant with paper
• Showing a plant requires carbon dioxide for photosynthesis


  Place plant in sealed bell jar with soda lime to absorb carbon dioxide
• Showing a plant requires chlorophyll for photosynthesis


  Test a variegated leaf with both green and white parts
• Investigating photosynthesis (commonly limiting factors)


  Change light intensity, light wavelength, carbon dioxide concentration, temperature, plant mass, plant species, plant age
• Measuring volume of oxygen produced (using a photosynthometer) is more accurate than counting bubbles
• Respiration


  Oxygen to carbon dioxide
• Photosynthesis


  Carbon dioxide to oxygen
• Compensation point


  The point when rate of respiration equals rate of photosynthesis, so there is no net gas exchange
• Investigating net gas exchange using hydrogen carbonate indicator


  1. Leaf in light turns indicator purple (low CO2)

  2. Leaf in dark turns indicator yellow (high CO2)

  3. Leaf in dim light stays orange (equal CO2)