Photosynthesis

1. Overview

Photosynthesis is the fundamental process by which plants function as "producers," converting light energy from the sun into chemical energy stored in food. This process is the foundation of almost all food chains on Earth and is responsible for maintaining atmospheric oxygen levels.

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Key Definitions

• Photosynthesis: The process by which plants synthesise carbohydrates from raw materials (carbon dioxide and water) using energy from light.
• Chlorophyll: A green pigment found in chloroplasts that absorbs light energy for use in photosynthesis.
• Limiting Factor: Something present in the environment in such short supply that it restricts life processes (e.g., in photosynthesis, it limits the rate of the reaction).
• Stomata: Small pores on the underside of a leaf that allow gases (CO2, O2, water vapour) to diffuse in and out.

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Core Content

The Process of Photosynthesis

Plants take in carbon dioxide from the air and water from the soil to create glucose. This reaction requires energy, which is captured by chlorophyll.

• Role of Chlorophyll: Chlorophyll transfers energy from light into energy in chemicals; this chemical energy is then used to synthesise carbohydrates.
• Word Equation: carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll)

Use and Storage of Carbohydrates

The glucose produced in photosynthesis has five primary fates:

1. Starch: Converted into starch for storage in leaves or storage organs (tubers). Starch is insoluble, so it does not affect the water potential of the cell.
2. Cellulose: Used to build cell walls, providing structural support to the plant cell.
3. Glucose for Respiration: Broken down to release energy for cellular processes.
4. Sucrose: Glucose is converted to sucrose to be transported efficiently through the phloem to other parts of the plant.
5. Nectar: Used to attract insects for pollination.

Important Mineral Ions

Plants require specific minerals from the soil for healthy growth:

• Nitrate Ions: Needed for making amino acids, which are then used to build proteins for growth.
  • Deficiency: Stunted growth and yellow older leaves.
• Magnesium Ions: Needed for making chlorophyll.
  • Deficiency: Chlorosis (leaves turn yellow because they cannot produce enough pigment).

Investigating Photosynthesis

To test for photosynthesis, we usually test a leaf for the presence of starch using iodine solution (turns from orange/brown to blue-black).

• Testing for Chlorophyll: Use a variegated leaf (green and white). Only the green parts (containing chlorophyll) will turn blue-black with iodine.
• Testing for Light: Cover part of a leaf with aluminum foil. Only the exposed parts will produce starch.
• Testing for Carbon Dioxide: Place a plant in a sealed jar with soda lime (which absorbs CO2). The leaf will test negative for starch.
• 📊A diagram showing the "Starch Test" steps: 1. Boiling leaf in water (kills it), 2. Boiling in ethanol (removes chlorophyll), 3. Dipping in warm water (softens it), 4. Adding iodine solution.

Factors Affecting the Rate

1. Light Intensity: As light increases, the rate increases until it reaches a plateau.
2. CO2 Concentration: As CO2 increases, the rate increases until it reaches a plateau.
3. Temperature: The rate increases as temperature rises until the optimum is reached. Beyond this, the enzymes denature and the rate drops sharply.

Gas Exchange Investigation

We can use hydrogencarbonate indicator to see the effect of light on gas exchange in aquatic plants (like Elodea):

• In Light: Photosynthesis > Respiration. CO2 is taken in. Indicator turns Purple (low CO2).
• In Dark: Respiration only. CO2 is released. Indicator turns Yellow (high CO2).
• At Equilibrium: Photosynthesis = Respiration. Indicator remains Red/Orange.

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Extended Content (Extended Only)

The Balanced Chemical Equation

The chemical reaction for photosynthesis is: $$6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$$ (In the presence of light and chlorophyll)

Explaining Limiting Factors

A limiting factor is the factor that is "furthest from its optimum," thereby restricting the rate of photosynthesis.

• Light Intensity: On a dark, cloudy day, light is the limiting factor. Increasing light will increase the rate.
• CO2 Concentration: On a sunny day in a field, CO2 is often the limiting factor (atmospheric CO2 is only ~0.04%).
• Temperature: In winter, temperature is the limiting factor because the kinetic energy of molecules is low, slowing down the enzymes responsible for the reaction.

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Key Equations

1. Word Equation: Carbon Dioxide + Water $\rightarrow$ Glucose + Oxygen
2. Balanced Chemical Equation: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$
   • $CO_2$: Carbon Dioxide
   • $H_2O$: Water
   • $C_6H_{12}O_6$: Glucose
   • $O_2$: Oxygen

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Common Mistakes to Avoid

• ❌ Wrong: Thinking plants only respire at night.
  • ✅ Right: Plants respire all the time (day and night). They only photosynthesise when light is available.
• ❌ Wrong: Stating that chlorophyll "creates" energy.
  • ✅ Right: Chlorophyll transfers or converts light energy into chemical energy. (Law of Conservation of Energy).
• ❌ Wrong: Saying "Magnesium is used for growth."
  • ✅ Right: Magnesium is used for chlorophyll; Nitrates are used for amino acids/proteins which lead to growth.

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Exam Tips

• Command Words: "State" is the most common command word (21x in past papers). Be prepared to simply provide the equations or the names of the minerals.
• "Explain" vs "Describe": If asked to describe a graph, say what you see (e.g., "as light increases, the rate increases"). If asked to explain, say why (e.g., "because light provides the energy for the reaction").
• Real-World Contexts: Questions often involve greenhouses. Remember that glasshouses are used to control limiting factors (adding heaters for temp, or burning paraffin for CO2).
• Typical Values: Be comfortable reading graphs where light intensity or CO2 concentration is measured in arbitrary units (a.u.) or percentages.