5.1 Photosynthesis
Light dependent reaction:
● Photoionisation:
○ Chlorophyll molecules in PSII absorb light energy
○ The energy excites electrons, moving them to a higher energy level
○ Causing them to leave the chlorophyll, which becomes ionised
● Photolysis:
○ Light energy also causes water molecules to split, 2H2O → O2 + 4H+ + 4e-
○ Electrons released from photolysis replace those lost in photoionisation
○ H+ contributes to the proton gradient in the thylakoid before later moving into
the stroma to help form NADPH
● Electron transfer chain:
○ The excited electrons are passed along a chain of electron carriers in a
series of redox reactions between PSII and PSI, losing energy at each stage
○ Each carrier is slightly lower in energy than the previous
● Photophosphorylation:
○ Energy released from electrons moving along the ETC is used to pump
protons (H+) from the stroma into the thylakoid lumen
○ Creates proton concentration gradient, where there is a higher concentration
in the thylakoid than in the stroma
○ Causes protons to facilitatively diffuse back into the stroma via ATP synthase
○ The flow of protons provides energy for photophosphorylation, ADP + Pi →
ATP (chemiosmotic theory)
● Reduction of NADP:
○ Once the electrons that have travelled along the electron transfer chain reach
PSI, light energy excites the electrons again to an even higher energy level
○ The higher energy electrons, and protons from the stroma are transferred to
NADP to form NADPH
Light independent reaction (Calvin cycle):
● Carbon fixation:
○ CO2 diffuses into the leaf through stomata and enters the stroma by simple
diffusion
○ Rubisco catalyses the reaction between ribulose bisphosphate (RuBP, 5C)
and CO2
○ Forms a 6-carbon intermediate but is unstable so splits into two molecules
of glycerate-3-phosphate (GP, 3C)
● Reduction of GP to TP:
○ Each of the two GP molecules are reduced to triose phosphate (TP) using
hydrogen from NADPH and energy from ATP hydrolysis
● Formation of sugars:
○ 1/6 TP is converted to hexose sugars
○ 5/6 TP are used to regenerate RuBP, using energy from ATP hydrolysis
● Cycle turns:
○ For every 3 turns of the cycle, 3 CO2 are fixed, producing 6 TP
○ Of those 6, 1/6 are converted into hexose sugars, meaning the cycle must
turn 6 times to form 1 hexose sugar
, ● CO2 + RuBP –(Rubisco)→ 6C intermediate → 2x GP –(reduced NADP →
NADP + H+, ATP → ADP + Pi)→ TP
5.2 Respiration
Glycolysis:
● First stage of both aerobic and anaerobic respiration, occurs in the cytoplasm and is
an anaerobic process
● Glucose is phosphorylated by the hydrolysis of ATP, forming glucose phosphate
and ADP
● Glucose phosphate is further phosphorylated using another ATP molecule to form
hexose bisphosphate
● Hexose bisphosphate then splits into 2 molecules of triose phosphate (TP)
● Each TP is oxidised to form 2 molecules of pyruvate
● During this oxidation, 4 molecules of ATP (2 from each TP) are produced by
substrate-level phosphorylation, but since 2 ATP were used earlier, there is a net
gain of 2 ATP per glucose
● 2 H atoms also get released and transferred to NAD, forming 2 NADH
Anaerobic respiration:
● Pyruvate (3C) is reduced to ethanol in plants and yeast, or lactate in animals using H
from NADH
● NADH gets oxidised as ethanol or lactate is produced, regenerating NAD
● NAD regeneration means glycolysis can continue even in anaerobic conditions so
ATP can still be produced
Link reaction:
● The pyruvate produced by glycolysis enters the mitochondrial matrix from the
cytoplasm via active transport
● Inside the mitochondrial matrix, pyruvate is decarboxylated (-CO2)
● Then oxidised (-H) to form acetate, transferring the H atom to NAD, reducing it to
NADH
● Acetate then combines with coenzyme A (CoA) to produce acetylcoenzyme A (acetyl
CoA)
● Pyruvate + NAD + CoA → acetyl CoA + COO– + NADH
Krebs cycle:
● Acetyl CoA (2C) combines with oxaloacetate (4C) to produce citrate (6C), releasing
CoA which gets reused in the link reaction
● Citrate is decarboxylated and oxidised to α-ketoglutarate (5C), which is further
decarboxylated and oxidised to succinyl CoA (4C)
● CO2 is released, and H atoms released from oxidation reduce NAD to NADH
● succinyl CoA is converted to succinate (another 4C) which releases energy for
substrate-level phosphorylation ADP + Pi → ATP
● Succinate is then oxidised to reduce FAD to FADH2, then oxidised again to reduce
NAD to NADH, regenerating oxaloacetate
Light dependent reaction:
● Photoionisation:
○ Chlorophyll molecules in PSII absorb light energy
○ The energy excites electrons, moving them to a higher energy level
○ Causing them to leave the chlorophyll, which becomes ionised
● Photolysis:
○ Light energy also causes water molecules to split, 2H2O → O2 + 4H+ + 4e-
○ Electrons released from photolysis replace those lost in photoionisation
○ H+ contributes to the proton gradient in the thylakoid before later moving into
the stroma to help form NADPH
● Electron transfer chain:
○ The excited electrons are passed along a chain of electron carriers in a
series of redox reactions between PSII and PSI, losing energy at each stage
○ Each carrier is slightly lower in energy than the previous
● Photophosphorylation:
○ Energy released from electrons moving along the ETC is used to pump
protons (H+) from the stroma into the thylakoid lumen
○ Creates proton concentration gradient, where there is a higher concentration
in the thylakoid than in the stroma
○ Causes protons to facilitatively diffuse back into the stroma via ATP synthase
○ The flow of protons provides energy for photophosphorylation, ADP + Pi →
ATP (chemiosmotic theory)
● Reduction of NADP:
○ Once the electrons that have travelled along the electron transfer chain reach
PSI, light energy excites the electrons again to an even higher energy level
○ The higher energy electrons, and protons from the stroma are transferred to
NADP to form NADPH
Light independent reaction (Calvin cycle):
● Carbon fixation:
○ CO2 diffuses into the leaf through stomata and enters the stroma by simple
diffusion
○ Rubisco catalyses the reaction between ribulose bisphosphate (RuBP, 5C)
and CO2
○ Forms a 6-carbon intermediate but is unstable so splits into two molecules
of glycerate-3-phosphate (GP, 3C)
● Reduction of GP to TP:
○ Each of the two GP molecules are reduced to triose phosphate (TP) using
hydrogen from NADPH and energy from ATP hydrolysis
● Formation of sugars:
○ 1/6 TP is converted to hexose sugars
○ 5/6 TP are used to regenerate RuBP, using energy from ATP hydrolysis
● Cycle turns:
○ For every 3 turns of the cycle, 3 CO2 are fixed, producing 6 TP
○ Of those 6, 1/6 are converted into hexose sugars, meaning the cycle must
turn 6 times to form 1 hexose sugar
, ● CO2 + RuBP –(Rubisco)→ 6C intermediate → 2x GP –(reduced NADP →
NADP + H+, ATP → ADP + Pi)→ TP
5.2 Respiration
Glycolysis:
● First stage of both aerobic and anaerobic respiration, occurs in the cytoplasm and is
an anaerobic process
● Glucose is phosphorylated by the hydrolysis of ATP, forming glucose phosphate
and ADP
● Glucose phosphate is further phosphorylated using another ATP molecule to form
hexose bisphosphate
● Hexose bisphosphate then splits into 2 molecules of triose phosphate (TP)
● Each TP is oxidised to form 2 molecules of pyruvate
● During this oxidation, 4 molecules of ATP (2 from each TP) are produced by
substrate-level phosphorylation, but since 2 ATP were used earlier, there is a net
gain of 2 ATP per glucose
● 2 H atoms also get released and transferred to NAD, forming 2 NADH
Anaerobic respiration:
● Pyruvate (3C) is reduced to ethanol in plants and yeast, or lactate in animals using H
from NADH
● NADH gets oxidised as ethanol or lactate is produced, regenerating NAD
● NAD regeneration means glycolysis can continue even in anaerobic conditions so
ATP can still be produced
Link reaction:
● The pyruvate produced by glycolysis enters the mitochondrial matrix from the
cytoplasm via active transport
● Inside the mitochondrial matrix, pyruvate is decarboxylated (-CO2)
● Then oxidised (-H) to form acetate, transferring the H atom to NAD, reducing it to
NADH
● Acetate then combines with coenzyme A (CoA) to produce acetylcoenzyme A (acetyl
CoA)
● Pyruvate + NAD + CoA → acetyl CoA + COO– + NADH
Krebs cycle:
● Acetyl CoA (2C) combines with oxaloacetate (4C) to produce citrate (6C), releasing
CoA which gets reused in the link reaction
● Citrate is decarboxylated and oxidised to α-ketoglutarate (5C), which is further
decarboxylated and oxidised to succinyl CoA (4C)
● CO2 is released, and H atoms released from oxidation reduce NAD to NADH
● succinyl CoA is converted to succinate (another 4C) which releases energy for
substrate-level phosphorylation ADP + Pi → ATP
● Succinate is then oxidised to reduce FAD to FADH2, then oxidised again to reduce
NAD to NADH, regenerating oxaloacetate
