Metabolism: Topic 8.3 Photosynthesis Flashcards
What do light dependent reactions use?
The light dependent reactions use photosynthetic pigments (organised into photosystems) to convert light energy into chemical energy (specifically ATP and NADPH)
Where does light dependent reaction occur
The light dependent reactions occur within specialised membrane discs within the chloroplast called thylakoids
Describe the first step in light dependent reactions
Step 1: Excitation of Photosystems by Light Energy
Photosystems are groups of photosynthetic pigments (including chlorophyll) embedded within the thylakoid membrane
Photosystems are classed according to their maximal absorption wavelengths (PS I = 700 nm ; PS II = 680 nm)
When a photosystem absorbs light energy, delocalised electrons within the pigments become energised or ‘excited’
These excited electrons are transferred to carrier molecules within the thylakoid membrane
Describe the second step of light dependent reaction
Excited electrons from Photosystem II (P680) are transferred to an electron transport chain within the thylakoid membrane
As the electrons are passed through the chain they lose energy, which is used to translocate H+ ions into the thylakoid
This build up of protons within the thylakoid creates an electrochemical gradient, or proton motive force
The H+ ions return to the stroma (along the proton gradient) via the transmembrane enzyme ATP synthase (chemiosmosis)
ATP synthase uses the passage of H+ ions to catalyse the synthesis of ATP (from ADP + Pi)
This process is called photophosphorylation – as light provided the initial energy source for ATP production
The newly de-energised electrons from Photosystem II are taken up by Photosystem I
Describe the third step of light dependent reactions
Step 3: Reduction of NADP+ and the Photolysis of Water
Excited electrons from Photosystem I may be transferred to a carrier molecule and used to reduce NADP+
This forms NADPH – which is needed (in conjunction with ATP) for the light independent reactions
The electrons lost from Photosystem I are replaced by de-energised electrons from Photosystem II
The electrons lost from Photosystem II are replaced by electrons released from water via photolysis
Water is split by light energy into H+ ions (used in chemiosmosis) and oxygen (released as a by-product)
What is photophosphorylation
The production of ATP by the light dependent reactions is called photophosphorylation, as it uses light as an energy source
Describe cyclic photo-phosphorylation
Cyclic photophosphorylation involves the use of only one photosystem (PS I) and does not involve the reduction of NADP+
When light is absorbed by Photosystem I, the excited electron may enter into an electron transport chain to produce ATP
Following this, the de-energised electron returns to the photosystem, restoring its electron supply (hence: cyclic)
As the electron returns to the photosystem, NADP+ is not reduced and water is not needed to replenish the electron supply
Describe non-cyclic photo-phosphorylation
Non-cyclic photophosphorylation involves two photosystems (PS I and PS II) and does involve the reduction of NADP+
When light is absorbed by Photosystem II, the excited electrons enter into an electron transport chain to produce ATP
Concurrently, photoactivation of Photosystem I results in the release of electrons which reduce NADP+ (forms NADPH)
The photolysis of water releases electrons which replace those lost by Photosystem II (PS I electrons replaced by PS II)
What is light independent reactions and where it occurs
The light independent reactions use the chemical energy derived from light dependent reactions to form organic molecules
The light independent reactions occur in the fluid-filled space of the chloroplast called the stroma
What are the light independent reaction collective known as and what are the steps
The light independent reactions are collectively known as the Calvin cycle and involve three main steps:
Carboxylation of ribulose bisphosphate
Reduction of glycerate-3-phosphate
Regeneration of ribulose bisphosphate
What is the first step of light independent reaction
Step 1: Carbon Fixation
The Calvin cycle begins with a 5C compound called ribulose bisphosphate (or RuBP)
An enzyme, RuBP carboxylase (or Rubisco), catalyses the attachment of a CO2 molecule to RuBP
The resulting 6C compound is unstable, and breaks down into two 3C compounds – called glycerate-3-phosphate (GP)
A single cycle involves three molecules of RuBP combining with three molecules of CO2 to make six molecules of GP
What is the second step of light independent reaction
Step 2: Reduction of Glycerate-3-Phosphate
Glycerate-3-phosphate (GP) is converted into triose phosphate (TP) using NADPH and ATP
Reduction by NADPH transfers hydrogen atoms to the compound, while the hydrolysis of ATP provides energy
Each GP requires one NADPH and one ATP to form a triose phosphate – so a single cycle requires six of each molecule
What is the third step of light independent reaction
Step 3: Regeneration of RuBP
Of the six molecules of TP produced per cycle, one TP molecule may be used to form half a sugar molecule
Hence two cycles are required to produce a single glucose monomer, and more to produce polysaccharides like starch
The remaining five TP molecules are recombined to regenerate stocks of RuBP (5 × 3C = 3 × 5C)
The regeneration of RuBP requires energy derived from the hydrolysis of ATP
The structure of the chloroplast is adapted to the function it performs:
Thylakoids – flattened discs have a small internal volume to maximise hydrogen gradient upon proton accumulation
Grana – thylakoids are arranged into stacks to increase SA:Vol ratio of the thylakoid membrane
Photosystems – pigments organised into photosystems in thylakoid membrane to maximise light absorption
Stroma – central cavity that contains appropriate enzymes and a suitable pH for the Calvin cycle to occur
Lamellae – connects and separates thylakoid stacks (grana), maximising photosynthetic efficiency
Typically, chloroplast diagrams should display the following features:
Usually round in appearance with a double membrane exterior
Flattened discs (thylakoids) arranged into stacks (grana), connected by lamellae
Internal lumen of thylakoids is very small (allows for a more rapid generation of a proton motive force)
Ribosomes and chloroplast DNA are usually not visible at standard resolutions and magnifications
Starch granules may be visible and will appear as dark spots within the chloroplast
