6a. Plant Systems: Photosynthesis Flashcards
Photosynthesis
Definition + Stages
Photosynthesis is the process in which light energy absorbed by chlorophyll is converted into ATP.
The ATP is used to synthesise glucose from water and carbon dioxide.
Water and carbon dioxide are the raw materials for photosynthesis.
Oxygen is released during the process.
Occurs in two stages: light-dependent stage & light-independent stage
Light-Dependent Stage of Photosynthesis
Process
In the light-dependent stage (occurs in thylakoid):
- Light energy is absorbed by chlorophyll and then converted into ATP. (Light energy → ATP)
- Light energy is also used to split water molecules into oxygen and hydrogen ions by photolysis of water. (12H2O → 6O2 + 24H+) Oxygen is then released out of the leaf via stomata.
Light-Independent Stage of Photosynthesis
Process
In the light-independent stage (occurs in stroma):
- The hydrogen ions released during photolysis are used to reduce carbon dioxide to glucose. The ATP needed for this process comes from the light-dependent stage. (6CO2 + 24H+→C6H12O6 + 6H2O)
Light-Dependent Stage vs Light-Independent Stage of Photosynthesis
Compare
Similarities:
- Enzymes: Enzymes are needed to catalyse the reactions in both stages
Differences:
- Site of occurance: Light-dependent stage occurs in the thylakoid while light-independent stage occurs in the stroma.
- Requirements: Light energy, water, and chlorophyll are required in the light-dependent stage while carbon dioxide, chemical energy, and hydrogen ions are necessary in the light-independent stage.
- Energy: Light is absorbed by chlorophyll and converted to chemical energy in the light-dependent stage while the energy required in the light-independent stage is obtained from the light-dependent stage.
- Reactions: Photolysis of water occurs in the light-dependent stage, where light energy is used to split water molecules into oxygen and hydrogen ions, while reduction occurs in the light-independent stage, where hydrogen ions from photolysis are used to reduce carbon dioxide to form glucose.
- Products formed: The products of the light-dependent stage are hydrogen ions, oxygen, and ATP while the products of the light-independent stage are glucose and water.
What happens to the glucose that is formed during photosynthesis?
Manufactured glucose is being transported away from the leaf via the phloem to other parts of the plant for various uses, such as:
- Respiration to produce ATP for cellular activities.
- To form cellulose cell wall.
- When rate of photosynthesis is higher than the rate of respiration, excess glucose is converted into starch for storage. When photosynthesis stops, starch is converted back to glucose for usage.
- Converted to triglycerides and lipids for storage and synthesis of cell membranes.
- Converted to sucrose for transport to other parts of the plant via the phloem.
- Glucose reacts with nitrates and other mineral salts from the soil to form amino acids in leaves. Amino acids form proteins for new cellular materials.
Importance of Photosynthesis
- Photosynthesis makes chemical energy available to animals and other organisms.
- Photosynthesis removes carbon dioxide and provides oxygen.
- Energy is stored in fossil fuels through photosynthesis.
Factors Affecting the Rate of Photosynthesis
- Temperature
- Carbon dioxide concentration
- Light intensity
Water is not a limiting factor because less than 1% of water taken up is used for photosynthesis. However, it can affect the rate of photosynthesis indirectly by closing stomata, which limits the intake of carbon dioxide.
How does temperature affect the rate of photosynthesis?
As temp. increases + At optimum temp. + Beyond optimum temp.
Photosynthesis is dependent on temperature as it is catalysed by enzymes (many different types).
- When temperature increases, kinetic energy of molecules increases which increases the frequency of effective collisions between enzymes and substrates to form enzyme-substrate complexes, thus increasing the rate of photosynthesis.
- As the enzymes approach their optimum temperatures, the overall rate of photosynthesis reaches maximum.
- When the temperature increases beyond the enzymes’ optimum temperatures, the enzymes are denatured, losing their unique 3D shape and are unable to bind to their respective subtrates, hence the rate of photosynthesis decreases rapidly until it stops.
How does carbon dioxide concentration affect the rate of photosynthesis?
+ Why carbon dioxide is an important limiting factor normally
Photosynthesis is dependent on carbon dioxide concentration as carbon dioxide is a reactant for the reaction.
- When carbon dioxide concentration increases, the rate of photosynthesis increases up to a certain point. (Carbon dioxide concentration is the limiting factor)
- Beyond this point, when the concentration of carbon dioxide increases, the rate of photosynthesis remains constant.
- This is because the rate of photosynthesis is limited by another factor such as temperature or light intensity. Carbon dioxide is no longer the limiting factor.
Under normal circumstances, carbon dioxide is an important limiting factor since atmospheric carbon dioxide remains at a constant concentration of about 0.03%.
How does light intensity affect the rate of photosynthesis?
- When light intensity increases, the rate of photosynthesis increases proportionately up to a certain point. (Light intensity is the limiting factor)
- Beyond this point, when light intensity increases, the rate of photosynthesis remains constant.
- This is because the rate of photosynthesis is limited by another factor such as temperature or carbon dioxide concentration. Light intensity is no longer the limiting factor.
Compensation point
Explain
Compensation point is the point where rate of photosynthesis equals rate of respiration, resulting in no net intake or ouput of carbon dioxide or oxygen.
The glucose produced by photosynthesis exactly compensates for the glucose broken down by respiration.
- Below the compensation point, the rate of photosynthesis < the rate of respiration.
- At the compensation point, the rate of photosynthesis = the rate of respiration.
- Above the compensation point, the rate of photosynthesis > the rate of respiration.
