Photosynthesis Flashcards
Explain the significance of chloroplast distribution at the palisade mesophyll cells compared to the spongy mesophyll cells.
A higher number of chloroplasts are found in the palisade mesophyll cells compared to the spongy mesophyll cells. This is because palisade mesophyll cells are located near the leaf surface, where more light energy can be absorbed for photosynthesis.
Define photosynthesis.
Photosynthesis is the process where green plants transform light energy from the sun into chemical energy from inorganic CO2 and water.
State the chemical equation for photosynthesis
Carbon dioxide + Water + Light energy → Glucose + Oxygen
6 CO2 + 6H2O + light energy → C6H12O6 + 6O2
Explain the REDOX reaction that occurs for the chemical equation of photosynthesis
Carbon dioxide is reduced to sugar as they gain hydrogen ions from the water. Water molecules are oxidised when they lose electrons along with the hydrogen ions, thus forming oxygen.
State the location where light-dependent and light-independent reactions take place respectively.
The light-dependent reactions take place in the photosystems of the grana while the light-independent reactions take place in the stroma.
Describe the processes of the light-dependent reactions
Light energy is absorbed by the chloroplast pigments in the photosystems of the grana. This light energy is then harnessed for the photolysis of water to split water molecules into oxygen and hydrogen atoms, yielding oxygen as the by-product. Hydrogen atoms are removed by the hydrogen acceptor, NADP to form NADPH. ATP is generated from ADP and phosphate. NADPH and ATP are then used to drive reactions in the light-independent stage to synthesize complex organic compounds.
State the 3 stages of the light-independent stage/ Calvin cycle. Describe the reactions at each stage.
- Carbon Fixation
- Carbon dioxide diffuses into the stroma and combines with RuBP in the presence of RuBisCO (an enzyme), producing 2 molecules of 3C glycerate-phosphate - Reduction
- Glycerate-phosphate is reduced to triose phosphate by NADPH and ATP. From triose phosphate, carbohydrates, lipids and amino acids can be synthesized - Regeneration of RuBP
- RuBP is regenerated using energy from ATP so that more carbon dioxide can be fixed.
Define a limiting factor.
A limiting factor is one that, when in short supply, limits
rate of reaction.
State 3 factors affecting photosynthesis and explain why these factors are limiting factors.
- Light intensity
→ Light is needed for the photolysis of water in the light-dependent reactions and excitation of electrons in the photosystems - Carbon dioxide concentration
→ Carbon dioxide is needed for carbon fixation to occur during the light-independent stage which takes place in the stroma - Temperature
→ A suitable temperature is needed for photosynthetic enzymes such as RuBisCo to function. This would affect the light-independent reactions of photosynthesis.
Explain why there is no change in the rate of photosynthesis from a certain light intensity onwards.
From a certain light intensity onwards, light-dependent reactions would already be occurring at a maximal rate and will not be affected by any further increase in light intensity. Hence other factors such as carbon dioxide concentration and temperature have become the limiting factor.
Explain the effect of carbon dioxide on the photosynthetic rate.
As the concentration of carbon dioxide increases, there would be more carbon dioxide molecules available to bind to the active site of rubisco enzymes, thus increasing the rate of photosynthesis. However, from a certain carbon dioxide concentration onwards, the rate of photosynthesis will remain constant because all the active sites of the rubisco enzymes are now fully occupied. The enzymes are saturated. Thus carbon dioxide is no longer the limiting factor and other factors such as light intensity or temperature may remain as the limiting factors.
Explain the effect of temperature on the photosynthetic rate.
As the temperature increases, there is an increase in kinetic energy of substrate molecules, thus an increase in effective collision between the enzyme and substrate molecules. This leads to an increase in the formation of enzyme-substrate complexes, hence an increase in formation of glycerate-phosphate. Thus a higher rate of photosynthesis. Beyond a certain temperature, the enzyme rubisco is denatured, thus the rate of photosynthesis will drop. From this point onwards, temperature is no longer a limiting factor and other factors such as carbon dioxide concentration and light intensity may be the prevailing limiting factors.
