Photosynthesis Notes

Photosynthetic Reaction

• Plants are autotrophs, meaning they produce their own food using light, water, and carbon dioxide.

• They are called producers in food chains.

• Photosynthesis is an endothermic reaction where energy is transferred from the environment to chloroplasts by light.

• Leaves are the primary site of photosynthesis, containing mesophyll cells packed with chloroplasts and chlorophyll to maximize light energy absorption.

• Sugars from photosynthesis are used for plant substances and respiration for energy release.

• Word Equation:

  • Carbon Dioxide + Water → Glucose + Oxygen

• Balanced Symbol Equation:

  • $6CO<em>2 + 6H</em>2O \rightarrow C<em>6H</em>{12}O<em>6 + 6O</em>2$

• Plants obtain reactants from the environment

Rate of Photosynthesis

• Factors that affect photosynthesis include light, water, carbon dioxide, temperature and chlorophyll.

• Water is not a limiting factor because the quantity needed for photosynthesis is relatively small compared to what is transpired.

Temperature

• Temperature affects the kinetic energy of particles, influencing the speed of carbon dioxide and water movement in the plant.

• Lower temperature: results to less kinetic energy, leading to fewer successful collisions.

• Increasing temperature: increases kinetic energy, increasing the likelihood of collisions between enzymes and reactants, resulting in product formation.

• High temperature: enzymes denature (active site changes shape), reducing the rate of photosynthesis.

Light

• Light intensity influences the amount of energy available for photosynthesis.

• More light: faster rate of photosynthesis, until another factor becomes limiting.

Carbon Dioxide Concentration

• Carbon dioxide is a raw material needed for photosynthesis.

• More carbon dioxide: faster reaction rate, until another factor becomes limiting.

Chlorophyll

• The quantity of chlorophyll affects rate of photosynthesis; the more chloroplasts, the higher the rate.

• The amount of chlorophyll can be affected by:

  • Diseases (e.g., tobacco mosaic virus).

  • Lack of nutrients (e.g., magnesium).

  • Loss of leaves (fewer leaves mean fewer chloroplasts).

Interactions of Limiting Factors

• Multiple limiting factors can affect the rate of photosynthesis.

• Graphs may illustrate the interaction of two or three factors.

• The rate of photosynthesis increases with increasing light intensity, temperature, and carbon dioxide.

• Initially, light intensity limits the rate.

• As light intensity increases, temperature becomes a factor.

• Eventually, light intensity is no longer a limiting factor.

• The inverse square law shows the relationship between light intensity and distance:

  • Light intensity and distance are inversely proportional.

  • If the distance doubles, light intensity is four times less.

  • $Light Intensity = \frac{1}{d^2}$

Growing in a Greenhouse

• Commercial horticulturists control limiting factors in greenhouses to maximize photosynthesis while maintaining profit.

• Controlled factors:

  • Heat

  • Light

  • Water

  • Carbon dioxide

  • Nutrients

• Farmers avoid wasting money by not increasing a factor beyond the point where it becomes ineffective.

• Worked Example:

  • Calculate the light intensity when the distance of the plant is 30cm from the lamp

  • Answer:

    • $Lightintensity = \frac{1}{d^2}$

    • $Lightintensity = \frac{1}{30^2}$

    • $Lightintensity = 0.001 au$

Required Practical: Photosynthesis Rate

• Aim: Investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed.

• Measure the volume of oxygen produced by the pondweed as the light intensity changes as the light source is moved.

• Measure and calculate rates of photosynthesis extract and interpret graphs of photosynthesis rate involving one limiting factor.

• Method to Measure Rate:

  • Measure oxygen released from aquatic plants.

  • Place pondweed in a beaker of water.

  • Set up a light a set distance from plant

  • Record the number of bubbles observed in three minutes.

  • Repeat steps for different distances.

• Improvements:

  • Use a gas syringe to collect the volume of gas produced.

  • Repeat the experiment at least twice for each distance and calculate the mean number of bubbles.

  • Use of a glass tank between lamp and plant to prevent heating of the plant, or using an LED bulb that releases very little heat energy.

• Variables:

  • light

  • carbon dioxide

  • temperature

• Results:

  • Graph independent variable against the number of bubbles produced per minute.

  • Graph distance from the lamp against number of bubbles per minute.

Uses of Glucose from Photosynthesis

• The glucose produced in photosynthesis may be:

  • Used for respiration (both aerobic and anaerobic).

  • Converted into insoluble starch for storage in the stems, leaves, and roots.

  • Used to produce fat or oil for storage (especially in seeds).

  • Used to produce cellulose, which strengthens the cell wall.

  • Combined with nitrate ions absorbed from the soil to produce amino acids for protein synthesis.

• Testing for starch Presence:

  • Iodine solution

  • Qualitative reagent

  • Converted to starch so this test can be used to show the e ect of light on photosynthesis

• Steps for testing for starch in a leaf:

  • Leaf in boiling water - reason: break down cell walls.

  • Leaf placed in ethanol - reason: to remove chlorophyll from leaf.

  • Leaf washed in water - reason: to soften leaf.

  • Leaf placed on white tile and covered in iodine solution.

  • Observe color change - reason: iodine will change colours in presence of starch areas that contained chlorophyll will turn blue/black as starch will be present.