B11- Photosynthesis Flashcards
Chloroplasts
organelle that is responsible for photosynthesis
The stroma
contains a flattened, fluid-filled system of membranes = thylakoids
Thylakoids
stack up to form grana (singular - granum)
Photosystems
Pigments molecules (chlorophyll) are arranged in light- harvesting clusters= photosystems:
Photoionization of chlorophyll
- Light is absorbed by chlorophyll pigments in the thylakoid membrane
- Electrons in the primary pigment is excited by this light energy, emitted and lost to the electron transport chain
Photolysis of water
The thylakoid contains a water-splitting enzyme called the oxygen-evolving complex which catalyses the breakdown (photolysis) of water by light:
H2O → 2H+ + 2e- + ½O2
Products:
- The electrons passed down the electron transport chain in the previous step are replaced by the electrons released from this process
- H+ ions - used to create & drive a proton gradient for the synthesis of ATP
- Oxygen - reforms diatomic oxygen, and is released into the atmosphere
Chemiosmosis / Chemiosmotic theory
The excited electrons are passed along a chain of electron carriers (proteins) known as the electron transport chain.
Each protein carrier is reduced (as they gain an electron), then oxidised (as the electron is lost to the next carrier)
- Energy is released as the electrons move from protein to protein, and is used to actively transport H+ ions from the stroma, into the thylakoid lumen
Chemiosmosis (continued) & ATP Synthesis
The H+ ions have now created a proton gradient (high concentration of H+ ions in the thylakoid lumen, low concentration of H+ ions in the stroma
H+ ions return to the stroma through ATP synthase enzymes, which are embedded in the thylakoid membrane
- This process provides the energy for ATP synthesis - ADP + Pi → ATP
Electrons from the final electron carrier, and newly-pumped H+ ions in the stroma combine with NADP to form reduced NADPH
2H+ + 2e- + NADP → reduced NADH
Both reduced NADPH & ATP are used in light-independent reactions
The Light-Independent Reaction / Calvin Cycle
- Rubisco catalyses the fixation of carbon dioxide
- It is combined with a molecule called ribulose bisphosphate (RuBP - a 5 carbon compound)
- The resulting 6C compound is split into two molecules of glycerate 3-phosphate (GP - a 3C compound) - GP is reduced to triose phosphate (TP) - requires NADPH & ATP from the LDR
- RuBP is regenerated from TP - requires ATP
Calvin Cycle: Regeneration of RuBP
5/6ths of the TP molecules are used to regenerate RuBP- this process requires ATP
Limiting Factors Light Intensity
- Light intensity
The rate of photosynthesis increases as light intensity increases
- More energy is supplied for the light-dependent stage
- Produces more ATP & reduced NADPH for the calvin cycle → occurs at a greater rate
Limiting Factor Carbon Dioxide
The rate of photosynthesis increases as carbon dioxide concentration increases
- More CO2 present → faster carbon fixation can happen
Limiting Factor Temperature
As temperature increases the rate of photosynthesis increases as the reaction is controlled by enzymes
