Photosynthesis Flashcards
What is the endosymbiont theory
Theory that mitochondria and chloroplasts were once aerobic bacteria that were ingested by early eukaryotic cells
Describe the light dependent stage
Photosynthetic pigments trap light energy
Found on membranes in chloroplasts
Describe the light - independent stage
Enzyme driven reactions
In stromatolite off chloroplasts
What is cyclic phosphorylation
- A photon of light hits PSI and 2e- becomes excited
- The 2e- are passed along a chain of electron carriers
- Small amounts of ATP are made by phosphorylation
- Electrons passed back to PSI
Describe chemiosmosis
The process which the movement of hydrogen ions across a biological membrane generates ATP
How is ATP made in phosphorylation
Energy released from 2e- as they move through ETC is sued to pump H+ across the thylakoid membrane from the stroma into the thylakoid
Creating an electrochemical gradient and H+ diffuse through ATP synthase into the stroma making ATP from ADP and Pi
Describe photolysis in non-cyclic photophosphorylation
An enzyme associated with PSII splits water in the presence of light
H2O —> 2H+ + 2e- + 1/2O2
What is the primary pigment that absorbs light in chloroplasts
Chlorophyll a
What are the two stages of photosynthesis
Light-dependent stage - energy from sunlight is absorbed and used to form ATP. Hydrogen from water is used to reduce coenzyme NADP to NADPH
Light-independent stage - hydrogen from NADPH and carbon dioxide is used to build organic molecule, such as glucose. ATP supplies the required energy
Describe non-cyclic phosphorylation
When the light absorbed in photosystem II excites electrons to be released from PSII and are passed to an electron transport chain. ATP is produced via chemiosmosis
The electrons lost from PSII are replaced from water molecules broken down using energy from the sun.
Excited electrons are released from the reaction centre at PSI, passed to another electron transport chain, and ATP is again produced by chemiosmosis.
The electrons lost from this reaction centre are replaced by electrons that have just travelled along the first electron transport chain being released from PSII
Electrons leaving the electron transport chain following PSI are accepted, along with a hydrogen ion, by the coenzyme NADP forming NADPH
NADPH provides the hydrogen or reducing power in the production of organic molecules, such as glucose, in the light independent stage which follows.
Describe photolysis
When water molecules are split into hydrogen ions, electrons and oxygen molecules using energy from the sun
The electrons released replace electrons lost from the reaction of PSII.
This is why water, along with light and carbon dioxide, is a raw material of photosynthesis
Summarise the photolysis reaction
H20 —> 2H+ + 2e- + 1/2 O2
Describe cyclic phosphorylation
The electrons leaving the transport chain after PSI can be returned to PSI instead of being used to form NADPH leading to cyclic phosphorylation
This means PSI can still lead to the production of ATP without any electrons being supplied from PSII. Reduced NADP is not produced when this happens
Describe the Calvin cycle
Carbon dioxide enters the intercellular spaces within the spongy mesophyll of leaves by diffusion from atmosphere through stomata .
It diffuses into cells and into the stroma of of chloroplasts where it is combined with a five-carbon molecule RuBP. The carbon dioxide is fixed - incorporated into an organic molecule
The enzyme RuBisCO catalyses the reaction and an unstable six-carbon intermediate is produced. RuBisCO is the key enzyme in photosynthesis. It is very inefficient as competitively inhibited by oxygen - so lots is required so carry out photosynthesis successfully
6-carbon intermediate immediately breaks down, forming two 3-carbon glycerate 3-phosphate molecules (GP)
Each GP molecule is converted to another 3-carbon molecule, triose phosphate (TP), using a hydrogen atom from NADPH and energy supplied by ATP, from the light dependent stage
What processes within photosynthesis are light dependent and which are light-independent
Light dependent = non-cyclic phosphorylation, photolysis and cyclic phosphorylation
Light independent = carlvin cycle, regeneration of RuBP
Summarise the Calvin cycle in 3 steps
- fixation - carbon dioxide is fixed in the first step
- reduction - GP is reduced to TP by the addition of hydrogen from NADPH using energy supplied by ATP
- regeneration - RuBP is regenerated from the recycled TP
Describe the process of the regeneration of RuBP
For one glucose molecule to be produced, six carbon dioxide molecules have to enter the Calvin cycle, resulting in six full turns of the cycle. This will result in the production of 12 TP molecules, two of which will be removed to make the glucose molecule.
This means that 10 TP molecules are recycled to regenerate 6 RuBP molecules (used in 6 turns of the cycle)
What are the limiting factors affecting photosynthesis
Light intensity - light is needed as an energy source
Carbon dioxide concentration - carbon dioxide is needed as a carbon source - increasing carbon fixation in Calvin cycle
Temperature - affects the rate of enzyme controlled reactions
describe 2 ways grana are adapted to their function
- contain electron carriers / ETC
- large SA for light absorption
