Photosynthesis
The Process of Photosynthesis
Photosynthesis is the process by which green plants and some other organisms use light energy to synthesise carbohydrates from raw materials. This occurs primarily in the leaves, specifically within cells containing chloroplasts. The chloroplasts contain a green pigment called chlorophyll, which absorbs light energy.
The light energy absorbed by chlorophyll is transferred into chemical energy. This chemical energy is then used to convert carbon dioxide and water into glucose and oxygen. The glucose can be used immediately for respiration, converted into other substances like starch or cellulose, or stored.
Photosynthesis Word Equation
The word equation for photosynthesis is:
Carbon dioxide + Water → Glucose + Oxygen
This reaction takes place in the presence of light and chlorophyll. Light provides the energy, and chlorophyll acts as the catalyst/pigment that captures this energy.
Balanced Chemical Equation (Supplement)
The balanced chemical equation for photosynthesis is:
6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2
Note: Light energy and chlorophyll are required for the reaction but are not included in the balanced chemical equation as they are not reactants or products in the stoichiometric sense.
Uses and Storage of Carbohydrates
The glucose produced during photosynthesis is not just stored as glucose. It is used in several ways:
| Use | Explanation |
|---|---|
| Starch | Energy store: Glucose is converted into insoluble starch for storage. This prevents it from affecting the water potential of the cell and allows large amounts to be stored without interfering with cellular processes. |
| Cellulose | Build cell walls: Glucose is used to make cellulose, which provides structural support for plant cells. |
| Respiration | Provide energy: Glucose is broken down during respiration to release energy for metabolic processes (e.g., growth, active transport). |
| Sucrose | Transport: Glucose is converted into sucrose for transport around the plant via the phloem. Sucrose is soluble and easily transported. |
| Nectar | Attract insects: Some glucose is used to produce nectar, which attracts insects. This facilitates pollination, ensuring reproduction. |
Importance of Mineral Ions
Plants require mineral ions from the soil to survive and perform photosynthesis effectively. Two key ions are:
| Ion | Function |
|---|---|
| Nitrate ions (NO_3^-) | Used to make amino acids, which are the building blocks of proteins. Without nitrates, plants cannot grow or repair tissues. |
| Magnesium ions (Mg^{2+}) | Essential for making chlorophyll. Without magnesium, chlorophyll cannot be produced, leading to chlorosis (yellowing of leaves) and reduced photosynthesis. |
Describing the Need for Chlorophyll
When to use: When asked to describe an experiment investigating the need for chlorophyll.
Why examiners accept this: Examiners look for specific reference to a variegated leaf (a leaf with green and white/non-green parts) as the standard control method. The comparison between the chlorophyll-containing and non-containing areas is critical.
Correct Usage Example: 'Test a variegated leaf for starch after exposure to light. The green parts (containing chlorophyll) will turn blue-black with iodine, indicating starch production, while the white parts (lacking chlorophyll) will remain orange-brown, showing no photosynthesis occurred.'
⚠︎ Confusing Reactants and Products
Error: Students often state that carbon dioxide is a product of photosynthesis or that oxygen is a reactant.
Correct Understanding: Remember the word equation: Carbon dioxide + Water → Glucose + Oxygen. Carbon dioxide and water are the raw materials (reactants) taken in. Glucose and oxygen are the products made.
Investigating Factors Affecting Photosynthesis
The rate of photosynthesis can be measured by the amount of oxygen produced or carbon dioxide consumed. Several factors affect this rate:
| Factor | Effect on Rate |
|---|---|
| Light Intensity | As light intensity increases, the rate of photosynthesis increases until it reaches a maximum plateau. Beyond this point, another factor (e.g., CO_2 or temperature) becomes limiting. |
| Carbon Dioxide Concentration | As CO_2 concentration increases, the rate of photosynthesis increases until it reaches a maximum plateau. Again, another factor becomes limiting at high concentrations. |
| Temperature | The rate increases with temperature up to an optimum (usually around 45°C for many plants). Above the optimum, enzymes involved in photosynthesis denature, and the rate drops sharply. |
Limiting Factors: A limiting factor is any factor that prevents the rate of photosynthesis from increasing further. According to Liebig's Law of the Minimum, the rate is limited by the scarcest resource. For example, if light intensity is low, increasing CO_2 will have no effect until the light intensity is increased.
Identifying Limiting Factors from Graphs
When to use: When interpreting graphs showing the rate of photosynthesis against light intensity, CO_2 concentration, or temperature.
Why examiners accept this: Examiners want you to identify which factor is 'holding back' the process. On a graph, if the line is rising, that factor is limiting. If the line has plateaued (flattened), that factor is no longer limiting, and another factor must be.
Correct Usage Example: 'At point X on the graph, the rate of photosynthesis increases with light intensity, so light is the limiting factor. At point Y, the line has plateaued despite increasing light intensity; therefore, light is no longer limiting, and carbon dioxide concentration or temperature must be the limiting factor.'
Gas Exchange in Aquatic Plants (Hydrogencarbonate Indicator)
Hydrogencarbonate indicator is used to detect changes in carbon dioxide concentration. It changes colour based on the level of CO_2:
| $CO_2$ Level | Indicator Colour |
|---|---|
| High (e.g., >0.04%) | Yellow |
| Atmospheric (approx 0.04%) | Red (or reddish-purple) |
| Low (e.g., <0.03%) |