B11 - Photosynthesis Flashcards
Where does photosynthesis occur?
In the chloroplast
What are thylakoids and what do they stack to form?
The stroma contains a flattened, fluid-filled system of membranes called thylakoids. Thylakoids stack up to form granum which are connected by membrane channels called lamellae
What are photosystems?
Pigment molecules (chlorophyll) are arranged in light-harvesting clusters called photosystems, which are funnel like structures in the thylakoid membrane and are made from accessory pigments which absorb light and transfer it to the primary pigment
What occurs in photoionisation of the light dependent reaction?
Light is absorbed by accessory pigments in the thylakoid membrane and transferred to the primary pigment. Electrons are excited by this light energy and are passed along the electron transport chain
What occurs in photolysis in the light dependent reaction?
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
The electrons are then passed down the electron transport chain
H+ ions create a proton gradient leading to the synthesis of ATP
Oxygen reforms a diatomic atom and is released into the atmosphere
What occurs in chemiosmosis in the light dependent reaction?
When electrons move along the transport chain each protein carrier is reduced, then oxidised. 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
The H+ ions have now created a proton gradient with a high concentration of H+ ions in the thylakoid lumen and a 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 NADPH
Both reduced NADPH & ATP are used in light-independent reactions
What happens in the first reaction of the Calvin cycle?
CO2 combines with ribulose bisphosphate (RuBP, a 5C compound) - this is catalysed by Rubisco
The resulting compound is unstable & splits into two molecules of glycerate 3-phosphate (GP, a 3C compound)
What happens in the second reaction of the Calvin cycle?
ATP and NADPH are used to reduce Glycerate 3-phosphate, to a phosphorylated 3C compound called triose phosphate (TP). 1/6th of TP molecules produced are used to produce useful organic molecules like hexose phosphates, sugars, lipids and amino acids
What happens in the third reaction of the Calvin cycle?
5/6ths of the Triose phosphate molecules are used to regenerate Ribulose biphosphate (RuBP) - this process requires ATP
