Photosynthesis Flashcards
Light dependant reaction location
In thylakoid membrane in chloroplast
Light independent reaction location
In stroma in chloroplast
Stages of the light dependant reaction
Photoionisation and photolysis
Photophosphorylation
Production of ATP and reduced NADP
Functions of the light-dependant reaction
To add an inorganic phosphate molecule to ADP to make ATP
To split water into H+ ions and electrons - photolysis
Photoionisation
Chlorophyll (in photosystem II) absorbs light energy (photon) which excites electrons to a higher energy level, releasing them from chlorophyll molecule
Some energy from electrons released during photoionisation is conserved in the production of ATP and reduced NADP
Photolysis
Water molecules are split using light energy
This produces protons, electrons and oxygen (2H2O -> O2 + 4e- + 4H+)
The electrons replace those lost from chlorophyll
Photophosphorylation
Adding phosphate to a molecule using light
Conversion of ADP to ATP using the energy of sunlight by activation of PSII
Each new electron carrier is at a slightly lower energy level than the previous one.
The energy that is lost from the electrons is partly used to combine ADP and Pi to form ATP
ADP + Pi -> ATP
Production of ATP (chemiosmotic theory)
Electrons from photolysis pass down an electron transfer chain (electron carriers) from PSII TO PSI via redox reactions, losing energy at each step.
This energy is used to actively transport protons from stroma into thylakoid.
This creates a proton/electrochemical gradient across the thylakoid membrane
Protons move by facilitated diffusion down the electrochemical gradient into the stroma via the enzyme ATP synthase embedded in the thylakoid membrane.
Energy from this allows ADP + Pi -> ATP (photophosphorylation)
Production of reduced NADP
In PSI electrons are excited and transferred to NADP (with a proton from photolysis) to reduce NADP to form reduced NADP
Products of the light dependant reaction
ATP -> light independant reaction
Reduced NADP -> light independant reaction
Oxygen -> leaves cell as a by-product or used in respiration
The light-independant reaction (Calvin cycle)
CO2 reacts with RuBP (ribulose biphosphate), catalysed by the enzyme rubisco
Produces 2 molecules of glycerate 3-phosphate (GP)
GP reduced to triose phosphate (TP) using products from light dependant reaction: energy from the hydrolysis of ATP as well as H+ from reduced NADP.
Some TP converted into useful organic substances e.g. glucose
TP used to regenerate RuBP (using rest of ATP)
Limiting factor
A factor is limiting when it has made a more favourable value
The rate of photosynthesis increases, until photosynthesis is limited by a different factor.
Limiting factors: temperature
Rate of photosynthesis increases as temperature increases up to an optimum, where it will decrease after
This limits the light independant reaction since it is enzyme controlled (rubisco)
More Ek
More enzyme-substrate complexes (rubisco)
Above the optimum, H bonds in the tertiary structure of the enzyme will break and the active site will change shape (rubisco denatures)
Fewer enzyme substrate complexes
Limiting factors: Light intensity
Rate of photosynthesis increases as light intensity increases (then plateaus)
If light intensity was dramatically reduced, the levels of ATP and reduced NADP would fall as the light dependant reaction is limited since less photoionisation of chlorophyll and photolysis is occurring
This leads to the light independant reaction also slowing since GP cannot be reduced to TP (requires ATP and reduced NADP) and TP cannot regenerate RuBP (requires ATP)
Limiting factors: carbon dioxide
Rate of photosynthesis increases as CO2 concentration increases (then plateaus)
If carbon dioxide concentration is dramatically decreased, it will limit the light independant reaction as there will be less CO2 to combine with RuBP to form GP, less GP reduced to TP and less TP (and GP) being converted to organic substances e.g. hexose and to regenerate RuBP
