OWS Revision Notes
[Topic 5] On the Wild Side
, A Level Notes OWS: Photosynthesis (LDR/LIR) Biology
Light Dependant Reaction (occurs in thylakoid membrane)
1) Particular wavelengths of light energy (photons) from the Sun are absorbed by chlorophyll in PSII; 2e- in each
chlorophyll molecule raised to a higher energy level – now in an ‘excited’ state.
2) The 2e- leave chlorophyll molecules and lose energy as they are passed from one electron carrier to the next in a
series of redox reactions; these molecules form the electron transport chain.
3) The energy lost is used in the synthesis of ATP as ADP and inorganic phosphate undergo photophosphorylation;
ADP + Pi → ATP
4) The 2e- from PSII replace those lost from the PSI chlorophyll.
5) Within the thylakoid space, an enzyme catalyses the splitting of water (photolysis) to produced oxygen gas,
hydrogen ions (protons) and electrons; H2O → ½O2 + 2H+
6) These 2e- from photolysis replace those lost from PSII, so it is no longer positively charged, and the H+
concentration within the thylakoid space is raised a little also due to photolysis.
7) Photons of light are absorbed by 2e- in PSI which are passed down the electron transport chain; these electrons
combine with the coenzyme NADP and H+ from water to form reduced NADP (or NADPH)
Products of the light dependant reaction are ATP, NADP and oxygen (waste product) [water → oxygen]
Light Independent Reaction (occurs in stroma)
1) Co2 enters the leaf through the stomata and diffuses into the stroma of chloroplasts.
2) RuBP (5C) is fixed with CO2 (1C) by an enzyme called Rubisco (ribose bisphosphate carboxylase); carbon fixation.
Rubisco is one of the most abundant enzymes in the world and has 8 active sites.
3) This forms a 6C compound which is unstable; hence, it splits into x2 GP molecules (3C).
4) ATP is hydrolysed for energy and NADPH donates H+ ions and electrons to reduce GP (glycerate-3-phosphate) into a
triose phosphate called GALP (6C) (glyceraldehyde-3-phosphate).
5) GALP donates one carbon (now 5C) which contributes towards the formation of a glucose (6C) molecule; 6 Calvin
cycles will form one glucose molecule.
6) The glucose molecule is used for various things; such as carbohydrates (sucrose), lipids and amino acids.
7) The remaining carbons from GALP (5C) is regenerated into RuBP (5C) using energy from the hydrolysis of ATP.
The final product of the light independent reaction is glucose [carbon dioxide → glucose]
ATP (adenine triphosphate) readily diffuses around the cell and provides energy for cellular processes:
→ Made in LDR from ADP and Pi
→ Releases energy in LIR when bond is broken
→ Reduction of GP to GALP (in LIR)
→ Regeneration of RuBP (in LIR)
ATPase catalyses ATP → ADP + Pi
The third phosphate group is loosely bonded, hence, easy to
remove; once removed the phosphate group becomes hydrolysed
– forming bonds with surrounding water molecules.
NADP is a coenzyme; dinucleotide (adenine base + nicotinamide base) ADP + Pi → ATP } phosphorylation
ATP → ADP + Pi } hydrolysis (using H2O)
→ NADP is reduced in LDP with protons to form NADPH
→ NADPH is oxidised (and ATP is hydrolysed) in LIR to reduce GP into GALP
→ Reduced NADP = NADPH
[Topic 5] On the Wild Side
, A Level Notes OWS: Photosynthesis (LDR/LIR) Biology
Light Dependant Reaction (occurs in thylakoid membrane)
1) Particular wavelengths of light energy (photons) from the Sun are absorbed by chlorophyll in PSII; 2e- in each
chlorophyll molecule raised to a higher energy level – now in an ‘excited’ state.
2) The 2e- leave chlorophyll molecules and lose energy as they are passed from one electron carrier to the next in a
series of redox reactions; these molecules form the electron transport chain.
3) The energy lost is used in the synthesis of ATP as ADP and inorganic phosphate undergo photophosphorylation;
ADP + Pi → ATP
4) The 2e- from PSII replace those lost from the PSI chlorophyll.
5) Within the thylakoid space, an enzyme catalyses the splitting of water (photolysis) to produced oxygen gas,
hydrogen ions (protons) and electrons; H2O → ½O2 + 2H+
6) These 2e- from photolysis replace those lost from PSII, so it is no longer positively charged, and the H+
concentration within the thylakoid space is raised a little also due to photolysis.
7) Photons of light are absorbed by 2e- in PSI which are passed down the electron transport chain; these electrons
combine with the coenzyme NADP and H+ from water to form reduced NADP (or NADPH)
Products of the light dependant reaction are ATP, NADP and oxygen (waste product) [water → oxygen]
Light Independent Reaction (occurs in stroma)
1) Co2 enters the leaf through the stomata and diffuses into the stroma of chloroplasts.
2) RuBP (5C) is fixed with CO2 (1C) by an enzyme called Rubisco (ribose bisphosphate carboxylase); carbon fixation.
Rubisco is one of the most abundant enzymes in the world and has 8 active sites.
3) This forms a 6C compound which is unstable; hence, it splits into x2 GP molecules (3C).
4) ATP is hydrolysed for energy and NADPH donates H+ ions and electrons to reduce GP (glycerate-3-phosphate) into a
triose phosphate called GALP (6C) (glyceraldehyde-3-phosphate).
5) GALP donates one carbon (now 5C) which contributes towards the formation of a glucose (6C) molecule; 6 Calvin
cycles will form one glucose molecule.
6) The glucose molecule is used for various things; such as carbohydrates (sucrose), lipids and amino acids.
7) The remaining carbons from GALP (5C) is regenerated into RuBP (5C) using energy from the hydrolysis of ATP.
The final product of the light independent reaction is glucose [carbon dioxide → glucose]
ATP (adenine triphosphate) readily diffuses around the cell and provides energy for cellular processes:
→ Made in LDR from ADP and Pi
→ Releases energy in LIR when bond is broken
→ Reduction of GP to GALP (in LIR)
→ Regeneration of RuBP (in LIR)
ATPase catalyses ATP → ADP + Pi
The third phosphate group is loosely bonded, hence, easy to
remove; once removed the phosphate group becomes hydrolysed
– forming bonds with surrounding water molecules.
NADP is a coenzyme; dinucleotide (adenine base + nicotinamide base) ADP + Pi → ATP } phosphorylation
ATP → ADP + Pi } hydrolysis (using H2O)
→ NADP is reduced in LDP with protons to form NADPH
→ NADPH is oxidised (and ATP is hydrolysed) in LIR to reduce GP into GALP
→ Reduced NADP = NADPH
