Chapter 14: Photosynthesis Flashcards
What is the overall equation of photosynthesis?
6CO2 + 6H2O → C6H12O6 + 6O2
What is a light dependent reaction?
The first series of reactions which require light energy.
Where do light dependent reactions take place?
Thylakoid membranes
Name and define the two pathways of light dependent reactions.
Cyclic photophosphorylation: the production of ATP using light energy and involving only photosystem I.
Non-cyclic photophosphorylation: the production of ATP using light energy and involving photosystems I and II, producing reduced NADP.
Describe the process of cyclic photophosphorylation.
- Light energy absorbed by photosystem I and transferred to reaction centre.
- Electron in chlorophyll a molecule is excited by energy to a higher energy level.
- Electron is emitted (photoactivation)
- Electron captured by electron acceptor in thylakoid membrane.
- Electron passed along chain of electron carriers.
- Carriers undergo alternate reduction and oxidation due to this.
- Electrons lose energy as they pass along the chain.
- This energy used to move protons from stroma across the thylakoid membrane, into the thylakoid space.
- Concentration gradient for protons set.
- Protons move down con. gradient via facilitated diffusion through ATP synthase molecules, which synthesizes ATP.
Describe the process of non-cyclic photophosphorylation.
1.Light energy absorbed by photosystem II first, followed by photosystem I, and transferred to reaction centre.
- Electron is excited by energy to a higher energy level.
- Electron is emitted (photoactivation)
- Electron captured by electron acceptor in thylakoid membrane.
- Electron passed along chain of electron carriers.
- Carriers undergo alternate reduction and oxidation due to this.
- Electrons lose energy as they pass along the chain.
- This energy used to move protons from stroma across the thylakoid membrane, into the thylakoid space.
- Concentration gradient for protons set.
- Protons move down con. gradient via facilitated diffusion through ATP synthase molecules, which synthesizes ATP.
- This electron passes onto photosystem I, where the same process repeats.
- Photolysis of water takes place in photosystem II.
- Water molecule splits into proton, electron and oxygen.
What happens to the products of photolysis?
Electron: replenishes the lost electron in photosystem II.
Oxygen: waste product, diffuses out of cell OR used by mitochondria for aerobic respiration.
Proton: combines with electron and NADP in photosystem I to produce reduced NADP.
How are electrons replenished in both photosystems I and II?
Photosystem I: from photosystem II
Photosystem II: photolysis of water
What is the light independent reaction?
The final series of reactions that take place in photosynthesis, which does not need light but needs substances produced in the light dependent stages.
Why can light independent reactions not go on for long in the dark?
Because it requires ATP and reduced NADP produced in the light dependent stages, which will run out in the absence of light.
What is the light independent stage also known as?
Calvin cycle or Melvin cycle
Where does the light independent reaction take place?
Stroma
Describe the stages of light independent reaction.
- Fixation of carbon-dioxide
-Carbon-dioxide reacts with ribulose bisphosphate (RuBP) (5C). This reaction catalyzed by rubisco. Produces 2 molecules of glycerate-3-phosphate (GP) (3C) - Reduction
-2 GP molecules are turned into carbohydrate by REDUCTION to form 2 triose phosphate molecules.
-ATP and reduced NADP used for reduction.
-Some NADP is regenerated. - Regeneration of RuBP
-5/6th of triose phosphate molecules are used to regenerate RuBP.
-ATP needed for this process.
What happens to the 1/6th of triose phosphate molecules that are not used in the light dependent stage?
Condense to become hexose phosphates, that are used to produce:
1. Starch for storage
2. Sucrose for translocation around plant
3. Cellulose for making cell walls.
What are the other uses of glycerate-3-phosphate?
- Converted to glycerol and fatty acids to produce lipids for cellular membranes.
- Used to produce some amino acids.
What are photosynthetic pigments?
-Coloured substances
-Found in thylakoid membranes
-Absorb light energy of particular wavelengths
-Supplies light energy to drive light-dependent stages of photosynthesis.
What is a photosystem?
Cluster of light harvesting pigments surrounding a reaction centre.
Name the photosynthetic pigments and their colors.
Chlorophyll a (primary pigment)- yellow-green
Chlorophyll b (accessory pigment)- blue-green
Xanthophyll (accessory pigment)- yellow
Beta carotene (accessory pigment)- orange
What colors, therefore, do these pigments absorb and reflect?
Chlorophylls- absorb red and blue-violet lights, reflect green light
Carotenoids- absorb mainly blue-violet lights
reflect red light
Primary pigment vs accessory pigment
Primary pigments absorb light and act as reaction centres.
Accessory pigments surround primary pigments.
Accessory pigments may absorb different wavelengths of light.
Light energy then passed onto primary pigments.
What is an absorption spectrum?
A graph showing the absorbance of different wavelengths of light by a photosynthetic pigment.
What is an action spectrum?
A graph showing the effect of wavelengths of light on a process e.g the rate of photosynthesis.
What are the similarities and differences between absorption and action spectrum?
Both spectra have two main peaks, one in the blue-violet region and one in the red region.
Both spectra have a trough in the green-yellow region.
the action spectrum of photosynthesis is similar to the absorption spectrum of chlorophyll. This similarity shows that chlorophyll is the most important pigment in photosynthesis. However, the spectra aren’t identical because carotenoids also play a role. Carotenoids absorb strongly in the blue, which helps fill in the absorption gaps of chlorophyll.
What is a limiting factor?
The requirement for a process to take place that is in the shortest supply
