8.3 Photosynthesis Flashcards
What is photosynthesis?
Photosynthesis is the process by which cells synthesise organic molecules (e.g. glucose) from inorganic molecules (CO2 and H2O) in the presence of sunlight
What does photosynthesis require?
This process requires a photosynthetic pigment (chlorophyll) and can only occur in certain organisms (plants, some bacteria)
Where does photosynthesis occur within plants?
In plants, photosynthesis occurs within a specialised organelle called the chloroplast
What are the two (v. general) steps of photosynthesis?
light dependent and light independent reactions
What is the general role of the light dependent reactions?
The light dependent reactions convert light energy from the Sun into chemical energy (ATP)
What is the general role of the light independent reactions?
The light independent reactions use the chemical energy to synthesise organic compounds (e.g. carbohydrates)
- What is the role of the light dependent reactions?
The light dependent reactions use photosynthetic pigments (organised into photosystems) to convert light energy into chemical energy (specifically ATP and NADPH)
Where do the light dependent reactions take place?
These reactions occur within specialised membrane discs within the chloroplast called thylakoids
What are the 3 main steps of the light dependent reactions?
Excitation of photosystems by light energy
Production of ATP via an electron transport chain
Reduction of NADP+ and the photolysis of water
- What are photosystems?
light d
Photosystems are groups of photosynthetic pigments (including chlorophyll) embedded within the thylakoid membrane
2.What are photosystems classed according to?
LD
Photosystems are classed according to their maximal absorption wavelengths (PS I = 700 nm ; PS II = 680 nm)
- What happens when a photosystem absorbs light energy?
LD
When a photosystem absorbs light energy (chlorophyll within it), delocalised electrons within the pigments become energised or ‘excited’
- What is done with the excited electrons?
LD
These excited electrons are transferred to carrier molecules within the thylakoid membrane
- Where are the excited electrons from photosystem II transferred to?
LD
Excited electrons from Photosystem II (P680) are transferred to an electron transport chain within the thylakoid membrane
- WHat happens as the electrons pass through the chain?
LD
As the electrons are passed through the chain they lose energy, which is used to translocate H+ ions into the thylakoid
- What does the buildup of protons cause?
LD
This build up of protons within the thylakoid creates an electrochemical gradient, or proton motive force
- How do the H+ ions return to the stroma?
LD
The H+ ions return to the stroma (along the proton gradient) via the transmembrane enzyme ATP synthase (chemiosmosis)
- What does ATP Synthase do?
LD
ATP synthase uses the passage of H+ ions to catalyse the synthesis of ATP (from ADP + Pi)
- What is ATP synthesis in the light dependent reaction known as?
This process is called photophosphorylation – as light provided the initial energy source for ATP production
- Where are the de-energised electrons moved to?
LD
The newly de-energised electrons from Photosystem II are taken up by Photosystem I
- Once the electrons are at photosystem I, what are they used for?
LD
Excited electrons from Photosystem I may be transferred to a carrier molecule and used to reduce NADP+
- What does the reduction of NADP+ created?
LD
This forms NADPH – which is needed (in conjunction with ATP) for the light independent reactions
- What replaces the lost electrons from photosystem I?
LD
The electrons lost from Photosystem I are replaced by de-energised electrons from Photosystem II
- What replaces the lost electrons from photosystem II?
LD
The electrons lost from Photosystem II are replaced by electrons released from water via photolysis
- What is water split into and how?LD
Water is split by light energy into H+ ions (used in chemiosmosis) and oxygen (released as a by-product)
- Where are the products of the light dependent reactions used?
The products of the light dependent reactions (ATP and NADPH) are used in the light independent reactions
Why is it called photophosphorylation?
Photophosphorylation may be either a cyclic process or a non-cyclic process
- What does cyclic phosphorylation involve?
Cyclic photophosphorylation involves the use of only one photosystem (PS I) and does not involve the reduction of NADP+
- What happens when light is absorbed by photosystem 1?
Cyclic P
When light is absorbed by Photosystem I, the excited electron may enter into an electron transport chain to produce ATP
- WHat happens to the de-energised electrons?
Cyclic P
Following this, the de-energised electron returns to the photosystem, restoring its electron supply (hence: cyclic)
- What are the 2 differences of cyclic phosphorylation?
As the electron returns to the photosystem, NADP+ is not reduced and water is not needed to replenish the electron supply
- How many photosystems does non-cyclic phosphorylation involve?
Non-cyclic photophosphorylation involves two photosystems (PS I and PS II) and does involve the reduction of NADP+
- What happens when light is absorbed?
NCP
When light is absorbed by Photosystem II, the excited electrons enter into an electron transport chain to produce ATP
- What does photoactivation of photosystem I do?
NCP
Concurrently, photoactivation of Photosystem I results in the release of electrons which reduce NADP+ (forms NADPH)
- What reaction is in NCP and not in cyclic P?
The photolysis of water releases electrons which replace those lost by Photosystem II (PS I electrons replaced by PS II)
When is cyclic phosophorylation used?
Cyclic photophosphorylation can be used to produce a steady supply of ATP in the presence of sunlight
Can ATP be stored?
However, ATP is a highly reactive molecule and hence cannot be readily stored within the cell
What does NCP produce?
Non-cyclic photophosphorylation produces NADPH in addition to ATP (this requires the presence of water)
What is needed to synthesise organic molecules in light independent?
Both NADPH and ATP are required to produce organic molecules via the light independent reactions
What is the main advantage of NCP?
Hence, only non-cyclic photophosphorylation allows for the synthesis of organic molecules and long term energy storage
What is the purpose of the light independent reactions?
The light independent reactions use the chemical energy derived from light dependent reactions to form organic molecules
Where do the light independent reactions occur?
The light independent reactions occur in the fluid-filled space of the chloroplast called the stroma
What are the light independent reactions collectively known as?
The light independent reactions are collectively known as the Calvin cycle
What are the 3 main steps of Calvin cycle?
Carboxylation of ribulose bisphosphate
Reduction of glycerate-3-phosphate
Regeneration of ribulose bisphosphate
- What compound does Calvin begin with?
The Calvin cycle begins with a 5C compound called ribulose bisphosphate (or RuBP)
- WHat happens to ribulose biphosphate?
calvin
An enzyme, RuBP carboxylase (or Rubisco), catalyses the attachment of a CO2 molecule to RuBP
- What happens to the carboxylated RuBP?
calvin
The resulting 6C compound is unstable, and breaks down into two 3C compounds – called glycerate-3-phosphate (GP)