Plant Nutrition - Limiting Factors

Why Plants Make Sucrose and Starch?

  • Sucrose:
    • Glucose is highly reactive and can participate in unwanted reactions.
    • Sucrose is formed by combining glucose and fructose, resulting in a stable sugar.
    • Sucrose is soluble but doesn't engage in unwanted reactions.
    • Once sucrose reaches its destination, it's converted into starch or glucose.
  • Starch:
    • Highly stable and unreactive.
    • Suitable for long-term storage.
    • Composed of numerous glucose molecule chains.
    • Breaks down into glucose for respiration.

Limiting Factors

  • A limiting factor is a resource or environmental condition present in limited supply, which restricts chemical reactions.
  • Photosynthesis relies on several favorable conditions:
    • Temperature:
      • Increases the kinetic energy of reactants and enzymes, facilitating product formation.
    • Light Intensity:
      • Higher light intensity enables more chloroplasts to perform photosynthesis.
    • Carbon Dioxide Concentration:
      • Increased \text{CO}_2 concentration provides more reactants, leading to more glucose production.

Temperature

  • Photosynthesis is regulated by enzymes, which are sensitive to temperature variations.
  • As temperature rises, the reaction rate increases due to higher kinetic energy of reactants, resulting in more frequent collisions with enzymes.
  • Beyond a certain temperature threshold, the rate of photosynthesis declines as vital enzymes start to denature.
    • Denaturation impairs the active sites' ability to bind with substrates, reducing product formation.

Carbon Dioxide

  • Increasing carbon dioxide concentration boosts the reaction rate, leading to greater production of organic molecules.
    • More \text{CO}_2 = More Glucose
  • At a specific \text{CO}_2 concentration, the photosynthetic rate plateaus or becomes constant.
    • This occurs when enzymes/chloroplasts responsible for carbon fixation are saturated or have reached their maximum potential.
  • \text{CO}_2 can be a limiting factor when it is not sufficiently available.
  • \text{CO}_2 is not a limiting factor when the system is saturated.

Light Intensity

  • Higher light intensity generally leads to a greater rate of photosynthesis.
  • However, beyond a certain point, increasing light intensity no longer enhances the rate of photosynthesis.
    • This occurs when all available chlorophyll molecules are saturated with light or have reached their maximum potential.
  • At this point, another factor becomes the limiting factor, not light intensity.
  • Light is a limiting factor when it is not sufficiently available.
  • Light is not a limiting factor when the system is saturated.

Respiration and Photosynthesis

  • Plants constantly respire, consuming oxygen and releasing carbon dioxide due to aerobic respiration.
  • During daylight, plants also photosynthesize, absorbing carbon dioxide and releasing oxygen.
  • At night, plants only respire, taking in oxygen and releasing carbon dioxide since photosynthesis does not occur without light.

Investigations

  • During the day, especially under bright sunlight, plants photosynthesize at a faster rate than they respire.
    • This results in a net intake of carbon dioxide and a net output of oxygen.
  • The effect of light on net gas exchange in an aquatic plant can be investigated using a pH indicator like hydrogen carbonate indicator.
  • Carbon dioxide is an acidic gas when dissolved in water, making this possible.
  • Hydrogen carbonate indicator indicates the concentration of carbon dioxide in solution.
  • The indicator's color changes based on the carbon dioxide concentration.

Aquatic Plant Investigations

  • Commonly used plants: Elodea or Cabomba (types of pondweed).
  • As photosynthesis takes place, oxygen gas is released.
  • The released oxygen appears as bubbles emerging from the cut end of the pondweed.
  • The rate of photosynthesis can be determined by counting the number of bubbles produced per minute.
  • A higher number of bubbles per minute indicates a faster rate of photosynthesis.
  • Alternatively, a gas syringe can be used to measure the amount of \text{O}_2 produced.

Effect of Limiting Factors on Photosynthesis

  • Experimental Setup:
    • Aquatic plant in water with sodium hydrogencarbonate ($\text{NaHCO}3) to provide \text{CO}2$$.
    • Inverted boiling tube to collect oxygen bubbles produced during photosynthesis (Dependent Variable).
    • Lamp as a light source.
    • Thermometer to monitor temperature.
    • Glass tank filled with water to maintain constant conditions.
    • Hot plate to change temperature (Independent Variable).
  • Controlled Variables:
    • Distance of lamp.
    • Concentration of sodium hydrogencarbonate solution.
    • Aquatic plant type.
    • Temperature.
  • The number of oxygen bubbles produced indicates the rate of photosynthesis.
  • By altering the independent variable (e.g., temperature or light intensity) and measuring the oxygen production, the effect of that factor on photosynthesis can be determined.