Flashcards in Photosynthesis (Chapter 13) Deck (102)
What are photo autotrophs?
Green plants, photosynthetic prokaryotes and both single-called and many-called protoctists
They all depend on light energy to make glucose and ATP
What are chemoautotrophs?
Nitrifying bacteria which obtain energy from oxidising ammonia to nitrite (NO2-) or nitrite to nitrate (NO3-)
What is photosynthesis?
The fixation (trapping) of CO2 and its reduction to carbohydrate, using hydrogen from water
Where does photosynthesis take place?
Inside the chloroplasts
What is the overall equation of photosynthesis in green plants?
6CO2 + 6H2O --> C6H12O6 + 6O2
What is necessary for photosynthesis to take place?
Light energy and chlorophyll
What are the two reactions involved in photosynthesis?
Light dependent reaction
Light independent reaction
What is necessary for the light dependent reaction to take place?
1) suitable pigments that absorb certain wavelengths of light
2) light energy for photolysis of water into hydrogen + oxygen
What is oxygen in the photolysis of water?
A waste product
How is light energy indirectly needed in the light independent reaction?
To provide chemical energy, in the form of ATP, for the reduction of CO2 to carbohydrate
What are the two types of photosynthetic pigments involved in photosynthesis?
1) primary pigments
2) accessory pigments
How are photosynthetic pigments arranged?
What are photosystems?
Light harvesting clusters of pigments
How many types of photosystem are there?
2 (I + II)
Describe the structure of a photosystem
- several hundred accessory pigment molecules surround one primary pigment molecule
- each photosystem is arranged as a funnel-like structure
What happens to light energy in a photosystem?
It is absorbed by accessory pigments and passed onto the primary pigment
What are primary pigments?
Reaction centres that emit electrons
What are the two reactions that the light dependent reaction includes?
1) the splitting of H2O by photolysis to give H+
2) the synthesis of ATP in photophosphorylation
What happens in the light dependent reaction?
1) the H+ ions combine with NADP to make reduced NADP
2) ATP and reduced NADP are passed from the LDR to the LIR
- photophosphorylation of ADP to ATP can be cyclic or non-cyclic, depending on the pattern of electron flow in one or both types of photosystem
Which photosystem(s) does cyclic photophosphorylation involve?
Photosystem I (P1)
What happens during cyclic photophosphorylation?
1) light is absorbed by P1 and passed to the primary pigment (a chlorophyll molecule)
2) an electron in the chlorophyll molecule is excited to a higher energy level and emitted (photo activation)
3) the excited electron is captured by an electron acceptor and passed back to a chlorophyll molecule via a chain of electron acceptors
4) enough energy is released during this process to synthesise ATP from ADP + Pi by the process of chemiosmosis
5) the ATP is then passed onto the LIRs
Which photosystem(s) does non-cyclic photophosphorylation involve?
Both photosystems (P1 and P2) in the 'Z-scheme' of electron flow
What happens during non-cyclic photophosphorylation?
1) light is absorbed by P1 and P2 and excited electrons are emitted from the primary pigments of both reaction centres
2) these electrons are absorbed by electron acceptors and pass along chains of electron carriers, leaving the photosystems positively charged
3) the primary pigment of P1 absorbs the electrons from P2 and the primary pigment of P2 receives replacement electrons from the photolysis of water
4) ATP is synthesised as the electrons lose energy while passing along the carrier chain
Describe how the photolysis of water works
1) P2 contains a water-splitting enzyme that catalyses the breakdown of water: H2O --> 2H+ + 2e- + ½O2
2) the H+ ions combine with electrons from P1 and NADP to give reduced NADP: 2H+ + 2e- + NADP --> reduced NADP
3) reduced NADP passes to the LIRs and is used in the synthesis of carbohydrate
How can the photolysis of water be demonstrated?
By the Hill reaction
Describe the Hill reaction
1) Robert Hill showed that isolated chloroplasts had 'reducing power' and liberated O2 from H2O in the presence of an oxidising agent (electron acceptor) e.g. Fe3+ or DCPIP to replace NADP
2) the 'reducing power' was demonstrated as the redox agent changed colour on reduction e.g DCPIP goes from blue to colourless when reduced
3) oxidised DCPIP -- (H2O --> O2) --> reduced DCPIP
What is another name of the LIRs?
The Calvin cycle
What kind of process is he fixation of CO2
Light independent process
What happens during the Calvin cycle?
1) carbon fixation - CO2 combines with ribulose biphosphate, RuBP, (5C sugar) forming 2 molecules of glycerate-3-phosphate, GP, (3C sugar) in the presence of an enzyme called rubisco
2) GP, in the presence of ATP and reduced NADP from the LDR, is reduced to triose phosphate, TP, (3C sugar)
3) Most (5/6) of the TP is used to regenerate RuBP but 1/6 is used to produce other molecules e.g. hexose phosphates to starch/sucrose/cellulose OR glycerol+fatty acids/acetyl CoA/amino acids