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.