Photosynthesis Flashcards
1
Q
What are light-dependent reactions?
A
- These reactions convert light energy into chemical energy in the form of excited electrons
- Products are: reduced NADPH + H+ and ATP
2
Q
What are the steps for a light-dependent reaction?
A
- Chlorophyll absorbs light, which releases energized electrons that are used to produce ATP (chemical energy)
- The electrons are donated to (NADP+) which is later used in light-independent reactions
- The electrons lost are replaced with water
- The water is split through photolysis —> O2 and H2
- These reactions occur in the intermembrane space of the thylakoids (chloroplast)
- The energy produced is stored in the chloroplast in the reduced form NADPH + H+
3
Q
What are light-independent reactions?
A
- These reactions use the chemical energy to synthesise organic compounds
- A.k.a the Calvin cycle
4
Q
What are the steps of light-independent reactions?
A
- ATP and electron carriers are transferred to the stroma (where the reactions take place)
- Hydrogen and electrons are combined with CO2 to form complex compounds
- The ATP is used to fix the carbon molecules together (anabolic)
- The ATP and NADPH + H+ produced in the light-dependent reactions are used in the light-independent reactions. Once they runs out, the independent reactions stop.
5
Q
What electron carrier is used in photosynthesis? What electron carrier is used in cellular respiration?
A
- Photosynthesis: The reduced form of NADP is used: NADPH + H+ as it accepts two hydrogen atoms
- Respiration: The reduced form of NAD: NADH + H+
6
Q
How is light-energy converted into chemical energy in light-dependent reactions? Step 1
A
- Photosystems are classed according to their maximum absorption wavelengths (1. PS is 700nm, 2. PS is 680nm)
- When photosystems absorb light, delocalised electrons within the pigments become excited
- The electrons are brought to carrier molecules in the thylakoid
7
Q
How is light-energy converted into chemical energy in light-dependent reactions? Step 2
A
- Electrons are brought from PS 2. to an electron transport chain, they lose energy throughout the chain
- The energy lost transports H+ into thylakoid
- The H+ build up creating an electrochemical gradient (proton gradient)
- The H+ return to the stroma by chemiosmosis
- ATP synthase use the H+ gradient to catalyse the synthesis of ATP, called photophosphorylation
- The now ‘unexcited’ electrons from PS 2. are taken to PS 1.
8
Q
How is light-energy converted into chemical energy in light-dependent reactions? Step 3
A
- Excited electrons from PS 1. reduce NADP+ forming NADPH which is later used in light independent reactions
- The cycle continues as more ‘unexcited’ electrons from PS 2. are brought to PS 1.
- Lost electrons from PS 2. are filled by electrons from photolysis (H+ and O2) used in light dependent reactions
9
Q
What is photophosphorylation?
A
- The production of ATP by light dependent reactions
- There is cyclic and non-cyclic photophosphorylation
10
Q
What are the steps of cyclic photophosphorylation?
A
- PS 1. absorbs light, as a result the excited electrons enter the electron transport chain to produce ATP
- The de-energized electrons return to PS 1. to restore the electron supply
- NADP+ is not reduced (since the lost electrons are ‘returned’)
Check book
11
Q
What are the steps of non-cyclic photophosphorylation?
A
- Light is absorbed by PS 2., excited electrons then enter the electron transport chain to produced ATP
- Simultaneously light is absorbed by PS 1., electrons are released and NADP+ is reduced to NADPH + H+
- Water is required and broken down by photolysis to replace the lost electrons in PS 2.
Check book
12
Q
What are light independent reactions and what steps are involved?
A
- They use chemical energy derived from light dependent reactions to form organic molecules (carbohydrates), takes place in stroma
- Also
Steps: - Carboxylation of ribulose bisphosphate
- Reduction of glycerate-3-phosphate
- Regeneration of ribulose bisphosphate
13
Q
What is the first step of light independent reactions?
A
- Carboxylation of ribulose bisphosphate
- The initial compound is a 5C called ribulose bisphosphate (BuBP)
- The enzyme RuBP carboxylase (a.k.a rubisco) attaches the CO2 molecule to the RuBP
- Product: 6C compound, which breaks down into two 3C compounds called glycerate-3-phosphate (GP)
- One cycle here is three molecules of RuBP combining with three CO2 molecules, result in 6 GP
14
Q
What is the second step of light independent reactions?
A
- Reduction of Glycerate-3-Phosphate (GP)
- GP is reduced into triose phosphate (TP) using NADPH and ATP
- The reduction of GP done by NADPH. NADPH loses hydrogen atoms and simultaneously ATP is hydrolysed to provide energy
- Each GP requires one NADPH and one ATP to form a triose phosphate, hence one cycle requires 6 of each molecule
15
Q
What is the third step of light independent reactions?
A
- Regeneration of RuBP
- One of the six TP molecules is used to form 1/2 a sugar molecule
- Two cycles required to make one glucose monomer
- The remaining five TP (3C) molecules are rearranged to regenerate stocks of RuBP (5C)
- This means five TP makes up three RuBP (5 x 3C = 3 x 5C)
- This regeneration requires energy from the hydrolysis of ATP
