Topic 8.3 Photosynthesis Flashcards
1
Q
Light dependent reactions
A
The light dependent reactions convert light energy from the Sun into chemical energy (ATP)
- Light is absorbed by chlorophyll, which releases energized electrons that are used to produce ATP (chemical energy)
- The electrons are donated to carrier molecules (NADP+), which is used (along with ATP) in the light independent reactions
- The electrons lost from the chlorophyll are replaced by water, which is split (photolysis) to produce oxygen and hydrogen
- The light dependent reactions occur in the inter-membrane space of membranous discs called thylakoids
2
Q
Light independent reactions
A
The light independent reactions use the chemical energy to synthesize organic compounds (e.g. carbohydrates)
- ATP and hydrogen / electrons (carried by NADPH) are transferred to the site of the light independent reactions
- The hydrogen / electrons are combined with carbon dioxide to form complex organic compounds (e.g. carbohydrates)
- The ATP provides the required energy to power these anabolic reactions and fix the carbon molecules together
- The light independent reactions occur within the fluid-filled interior of the chloroplast called the stroma
3
Q
Step 1 - Excitation of Photosystems by Light Energy
A
- 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, delocalized electrons within the pigments become energized or ‘excited’
- These excited electrons are transferred to carrier molecules within the thylakoid membrane
4
Q
Step 2 - Production of ATP via an Electron Transport Chain
A
- 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 catalyze 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-energized electrons from Photosystem II are taken up by Photosystem I
5
Q
Step 3 - Reduction of NADP+ and the Photolysis of Water
A
- 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-energized 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)
6
Q
Cyclic Photophosphorylation
A
- 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
7
Q
Non-Cyclic Photophosphorylation
A
- 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)
8
Q
Cyclic vs Non-Cyclic Photophosphorylation
A
- Cyclic photophosphorylation can be used to produce a steady supply of ATP in the presence of sunlight
- However, ATP is a highly reactive molecule and hence cannot be readily stored within the cell
- Non-cyclic photophosphorylation produces NADPH in addition to ATP (this requires the presence of water)
- Both NADPH and ATP are required to produce organic molecules via the light independent reactions
- Hence, only non-cyclic photophosphorylation allows for the synthesis of organic molecules and long term energy storage
9
Q
Step 1 - Carbon Fixation
A
- 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
10
Q
Step 2 - Reduction of Glycerate-3-Phosphate
A
- 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
11
Q
Step 3 - Regeneration of RuBP
A
- 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
12
Q
Key events of Calvin Cycle
A
The Calvin cycle outlines the events that result in the formation of organic molecules from inorganic sources
- RuBP is carboxylated by carbon dioxide to form a hexose biphosphate compound
- The hexose biphosphate compound immediately breaks down into molecules of glycerate-3-phosphate
- The GP is converted by ATP and NADPH into molecules of triose phosphate
- TP can be used to form organic molecules or can be recombined by ATP to reform stocks of RuBP
13
Q
Chloroplasts
A
are the solar energy plants of a cell - they convert light energy into chemical energy
the chemical energy may be either ATP or organic compounds
14
Q
Chloroplast structure
A
- Thylakoids - flattened discs have a small internal volume to maximize hydrogen gradient upon proton accumulation
- Grana - thylakoids are arranged into stacks to increase SA:Vol ratio of the thylakoid membrane
- Photosystems - pigments organized into photosystems in thylakoid membrane to maximize light absorption
- Stroma - central cavity that contains appropriate enzymes and a suitable pH for the Calvin cycle to occur
Lamellae 0 connects and separates thylakoid stacks (grana), maximizing photsynthetic efficiency
