Photosynthesis Flashcards
Describe the light dependent reaction (5)
- chlorophyll absorbs light energy via photoionisation
- excites electron / electrons removed from chlorophyll
- electrons move along electron transport chain releasing energy
- energy released used to form proton gradient
- H+ ions move through ATP synthase;
- providing energy to join ADP and Pi to form ATP
- photolysis of water produces 2 protons, 2 electrons and 1/2 oxygen
- NADP reduced by electrons
Describe photoionisation (of chlorophyll)
- oxidation of chlorophyll (loses electrons) when hit by light
Describe photolysis (of water)
- 2H2O —-> 4H+ + 4e- + O2
- splitting water to produce protons and e-
Describe photophosphorylation (of ADP)
- making ATP from (ADP + Pi) using energy from light
In plants, ATP is produced in the light dependant reaction. Suggest why this is not a plants only source of ATP (4)
- plants dont photosynthesise in the dark
- not all the parts of plants photosynthesise
- plants require more ATP than is produced in the light dependant reaction
- ATP used in active transport
Describe the effect of introducing a herbicide on the electron transport chain (4)
- reduced transfer of protons across thylakoid membrane
- so less ATP is produced
- so less reduced NADP produced
- so light indpendent reaction slows/stop
OR - less reduction of GP to triose phosphate
What role does light play in this process?
- photoionisation of chlorophyll (excitation of electrons to a higher energy level)
- photolysis of water
How is ATP produced?
- photophosphorylation
- energy released used to change tertiary structure of proton pumps, used to form electrochemical gradient
- H+ ions diffuse through ATP synthase
- providing energy to join ADP and Pi to form ATP
How is reduced NADP produced?
- The H+ and associated electron are used to reduce a hydrogen carrier NADP to form NADPH catalysed by a hydrogenase enzyme
Explain the role of water in the light dependent reactions
- the electrons from photolysis replace those - lost by the chlorophyll
- the H+ and an associated electron are used to reduce NADP to form NADPH, evolved O2 released as waste product (used for aerobic respiration)
Describe the light independent reaction (6)
- carbon dioxide combines with ribulose phosphate
- produces two glycerate 3-phosphate (GP)
- GP reduces to triose phosphate (TP)
- using reduced NADP
- using energy from ATP
- triose phosphate converted to glucose
What is Fixation?
- RuBP acts as a CO2 acceptor
- reaction is catalysed by rubsico
- CO2 reacts with RuBP to form two molecules of GP
What is reduction?
- GP is reduced to TP
- requires reduced NADP from LDR which provides the reducing power & ATP
What is regeneration?
- Most triose phosphate (TP) is used to regenerate RuBP
The concentrations of carbon dioxide in the air at different heights above ground in a forest changes over a period of 24 hours.
Use your knowledge of photosynthesis to describe these changes and explain why they occur (5)
- high concentration of carbon dioxide linked with darkness
- no photosynthesis in the dark
- in the dark plants respire
- in light, plants use more carbon dioxide than they produce
- decrease in carbon dioxide concentration with height
- at ground level less photosynthesising tissue
