5.2.1 - Photosynthesis Flashcards
What is photosynthesis ?
The process by which energy, in the form of light, is used to build organic molecules, such as glucose
What is the equation for photosynthesis ?
6CO2 + 6H2O -> C6H12O6 + 6O2
What are autotrophic organisms ?
Organisms that produce their own energy via photosynthesis
What are heterotrophic organisms ?
Organisms that obtain complex organic molecules by eating other organisms
What do both autotrophic and heterotrophic organisms do ?
They break down organic molecules during the process of respiration to release the energy they need to drive metabolic processes
Where does photosynthesis take place ?
It takes place in the chloroplasts
What is the structure of the chloroplasts ?
- Membranes form flattened sacs called thylakoids
- Thylakoids are stacked to form grana
- The grana are joined by lamellae
What is an adaptation of the granum towards photosynthesis ?
They provide a large surface area to maximise the absorption of light essential in the first step of photosynthesis
What pigment absorbs light in the chloroplasts ?
Chlorophyll
Where is chlorophyll found ?
In the chloroplasts
It is embedded within the thylakoid membranes
What is the fluid in the chloroplast called ?
Stroma
What is the stroma the site of ?
The site of many chemical reactions resulting in the formation of complex organic molecules
What gives pigments their colours ?
- The wavelength of light that they absorb
- It is specific to that pigment
What is the primary pigment in photosynthesis ?
Chlorophyll
What light does chlorophyll absorb ?
- Red and blue light
- Reflects green light
What is the primary pigment that absorbs light ?
Chlorophyll a
What are some of the other pigments that also absorb light ?
- Chlorophyll b
- Xanthophylls
- Carotenoids
What do the different combinations of pigments cause ?
They cause leaves having different shades of colours
What pigments are embedded in the thylakoid membrane of the chloroplast ?
- Chlorophyll b
- Xanthophylls
- Carotenoids
What do all these pigments form ?
A light harvesting system called an antennae complex
What is the role of the antennae complex ?
To absorb light energy of different wavelengths and transfer this energy to the reaction centre
What is the reaction centre ?
This is where the reactions that are involved in photosynthesis take place
What pigment is present in the reaction centre ?
Chlorophyll a
What are the antennae complex and the reaction centre collectively known as ?
The photosystem
What are the two stages of photosynthesis ?
- Light dependent stage
- Light independent stage
What is the light dependent stage ?
- Energy from sunlight is absorbed and used to form ATP
- Hydrogen from water is used to reduce NADP to NADPH
What is the light independent stage ?
- Hydrogen from NADPH and CO2 is used to build organic molecules
- ATP supplies the required energy
What subsections are there to the light dependent stage ?
- Non-cylic photophosphorylation
- Photolysis
- Cyclic photophosphorylation
What photosystems are involved in non cyclic photophosphorylation ?
- Photosystem 1
- Photosystem 2
What wavelength does the reaction centre of PS1 absorb light at ?
700 nm
What wavelength does the reaction centre of PS2 absorb light at ?
680 nm
What does the absorption of light do at the reaction centres ?
It excites electrons at the reaction centres of the photosystems
What happens to the excited electrons from PS2 ?
They are released from the reaction centre of PS2 and are passed to an electron transport chain.
What happens after the electrons from the reaction centre of PS2 are lost ?
They are replaced, from the electrons given off by photolysis
During what process in photosynthesis is ATP produced ?
Chemiosmosis
How is ATP produced in PS1 (after PS2) ?
- The excited electrons are released from the reaction centre and passed on to another electron transport chain.
- By chemiosmosis
What happens after the electrons from the reaction centre of PS1 are lost ?
They are replaced by electrons that have just travelled along the first electron transport chain after being released from PS2
What happens to the electrons that leave the electron transport chain of PS1 ?
They are accepted, along with a hydrogen ion, by NADP, forming NADPH
What does NADPH provide ?
It provides the hydrogen in the production of organic molecules in the light independent stage
What are these two electron transport chains called ?
The Z scheme
What does photolysis involve ?
Water molecules are split into hydrogen ions, electrons and oxygen molecules using energy from the sun
Where do the electrons released by photolysis go ?
They replace the electrons lost from the reaction centre of PS2
How does the oxygen evolving complex that forms in PS2 act ?
It is an enzyme that catalyses the breakdown of water
What are water molecules split into in photolysis ?
- Hydrogen ions
- Electrons
- Oxygen molecules
How are the water molecules split in photolysis ?
Using energy from the sun
What is oxygen gas released as in photolysis ?
A by product
Where are the hydrogen ions released into in photolysis ?
- Into the lumen of the thylakoids
- They increase the proton concentration across the membrane
What do the hydrogen ions drive the formation of ?
As they move back through the membrane down a concentration and electrochemical gradient, more ATP is formed
What happens when the hydrogen ions are returned to the stroma ?
They combine with NADP and an electron from PS1 to form NADPH
Where is NADPH used ?
Light independent reactions of photosynthesis
What is cyclic photophosphorylation ?
The electrons leaving the electron chain in PS1 can return to PS1, instead of forming NADP, leading to cyclic phosphorylation
What does cyclic photophosphorylation allow to happen ?
PS1 can still lead to the production of ATP without any electrons being supplied from PS2
Is NADPH produced in cyclic photophosphorylation ?
No
Where does the light independent stage take place ?
Takes place in the stroma of chloroplasts and uses carbon dioxide as a raw material
What reactants are required for the light independent stage ?
- ATP
- Reduced NADP
What is the calvin cycle ?
Series of reactions that takes place in the light independent stage to form organic molecules
Where does carbon dioxide enter for the calvin cycle ?
- It enters the intracellular spaces within the spongy mesophyll of leaves by diffusion from the atmosphere through stomata
- Diffuses into the stroma of chloroplasts
What does carbon dioxide react with when it enters the calvin cycle ?
It reacts with a 5 carbon molecule called Ribulose bisphosphate (RuBP)
What happens to the carbon when it reacts with RuBP ?
- The carbon becomes fixed
This comes under carbon fixation
What is formed when CO2 and RuBP react ?
An unstable 6 carbon compound
What enzyme catalyses the reaction between CO2 and RuBP ?
Ribulose biphosphate carboxylase (RuBisCO)
What is RuBisCO ?
- It is the key enzyme in photosynthesis
- It is a very inefficient enzyme as it is competitively inhibited by oxygen
How much RuBisCO is required for photosynthesis to take place ?
A lot of it is needed due to the fact that it is competitively inhibited by oxygen
What happens to the unstable 6 carbon compound that is formed in the calvin cycle ?
- It is broken down
- It forms two 3 carbon Glycerate 3-phosphate (GP) molecules
What is each GP molecule converted to ?
Converted to another three carbon molecule called triose phosphate (TP)
How is TP formed ?
Using a hydrogen atom from NADPH and energy supplied by ATP
What is triose phosphate ?
It is a carbohydrate, a three carbon sugar
What happens to the majority of triose phosphate ?
The majority of it is recycled to regenerate RuBP so that the calvin cycle can continue
What can triose phosphate lead to the formation of ?
- Other carbohydrates
- Lipids
- Proteins
- Nucleic acids
What are the three steps the calvin cycle can be summarised as ?
- Fixation
- Reduction
- Regeneration
What is the fixation stage of the calvin cycle ?
Carbon dioxide is fixed in the first step
What is the reduction stage of the calvin cycle ?
GP is reduced to TP by the addition of hydrogen from NADPH
What is the regeneration stage of the calvin cycle ?
RuBP is regenerated from the recycled TP
How many carbon dioxide molecules need to enter the calvin cycle for one glucose molecule to be produced ?
- 6 carbon dioxide molecules have to enter
- This leads to 6 turns of the calvin cycle
How many TP molecules are produced in the process of 1 glucose being produced ?
- 12 TP molecules
- 2 of which will be removed to make the glucose molecule
How many TP molecules are recylced and how many RuBP molecules are regenerated ?
- 10 TP molecules are recycled
- 6 RuBP molecules are regenerated
How many carbons are involved in the production of 1 glucose molecule ?
10 x three-carbon TP = 30 carbons shuffled, this gives 6 x five carbon RuBP = 30 carbons
What supplies the energy for the regeneration of RuBP ?
ATP
