Topic 8: Metabolism, Cell respiration and Photosynthesis (14h)

8.1 Metabolism

“Developments in scientific research follow improvements in computing—developments in bioinformatics, such as the interrogation of databases, have facilitated research into metabolic pathways. (3.8)”

Theory of Knowledge

Many metabolic pathways have been described following a series of carefully controlled and repeated experiments. To what degree can looking at component parts give us knowledge of the whole?


Metabolic pathways consist of chains and cycles of enzyme-catalysed reactions
• Enzymes lower the activation energy of the chemical reactions that they catalyse
• Enzyme inhibitors can be competitive or non-competitive.
• Metabolic pathways can be controlled by end-product inhibition.

Applications and Skills

• Application: End-product inhibition of the pathway that converts threonine to isoleucine.
• Application: Use of databases to identify potential new anti-malarial drugs.
• Skill: Calculating and plotting rates of reaction from raw experimental results.
• Skill: Distinguishing different types of inhibition from graphs at specified substrate concentration.


Many enzyme inhibitors have been used in medicine. For example ethanol has been used to act as a competitive inhibitor for antifreeze poisoning.
• Fomepizole, which is an inhibitor of alcohol dehydrogenase, has also been used for antifreeze poisoning.

8.2 Cell Respiration

Paradigm shift—the chemiosmotic theory led to a paradigm shift in the field of bioenergetics. (2.3)

Theory of Knowledge

Peter Mitchell’s chemiosmotic theory encountered years of opposition before it was finally accepted. For what reasons does falsification not always result in an immediate acceptance of new theories or a paradigm shift?


Cell respiration involves the oxidation and reduction of electron carriers.
• Phosphorylation of molecules makes them less stable.
• In glycolysis, glucose is converted to pyruvate in the cytoplasm.
• Glycolysis gives a small net gain of ATP without the use of oxygen.
• In aerobic cell respiration pyruvate is decarboxylated and oxidized, and converted into acetyl compound and attached to coenzyme A to form acetyl coenzyme A in the link reaction.
• In the Krebs cycle, the oxidation of acetyl groups is coupled to the reduction
of hydrogen carriers, liberating carbon dioxide.
• Energy released by oxidation reactions is carried to the cristae of the mitochondria by reduced NAD and FAD.
• Transfer of electrons between carriers in the electron transport chain in the
• In chemiosmosis protons diffuse through ATP synthase to generate ATP.
• Oxygen is needed to bind with the free protons to maintain the hydrogen gradient, resulting in the formation of water.

• The structure of the mitochondrion is adapted to the function it performs.

Applications and Skills

 Application: Electron tomography used to produce images of active mitochondria. 

Skill: Analysis of diagrams of the pathways of aerobic respiration to deduce where decarboxylation and oxidation reactions occur. 

Skill: Annotation of a diagram of a mitochondrion to indicate the adaptations to its function.

8.3 Photosynthesis

Paradigm shift—the chemiosmotic theory led to a paradigm shift in the field of bioenergetics. (2.3)

Theory of knowledge

 The lollipop experiment used to work out the biochemical details of the Calvin cycle shows considerable creativity. To what extent is the creation of an elegant protocol similar to the creation of a work of art?


• Light-dependent reactions take place in the intermembrane space of the thylakoids.

• Light-independent reactions take place in the stroma.

• Reduced NADP and ATP are produced in the light-dependent reactions.

• Absorption of light by photosystems generates excited electrons.

• Photolysis of water generates electrons for use in the light-dependent reactions.

• Transfer of excited electrons occurs between carriers in thylakoid membranes.
• Excited electrons from Photosystem II are used to contribute to generate a proton gradient.
• ATP synthase in thylakoids generates ATP using the proton gradient.

• Excited electrons from Photosystem I are used to reduce NADP.
• In the light-independent reactions a carboxylase catalyses the carboxylation of ribulose bisphosphate.
• Glycerate 3-phosphate is reduced to triose phosphate using reduced NADP and ATP.

• Triose phosphate is used to regenerate RuBP and produce carbohydrates.
• Ribulose bisphosphate is reformed using ATP.
• The structure of the chloroplast is adapted to its function in photosynthesis.


The Global Artificial Photosynthesis (GAP) project aims to create an artificial “leaf” within the next decade. An electronic version of the leaf that creates oxygen and hydrogen from water and sunlight has already been invented
and will be developed for use in the next decade.

Applications and Skills

Application: Calvin’s experiment to elucidate the carboxylation of RuBP.

Skill: Annotation of a diagram to indicate the adaptations of a chloroplast to its function.


Aim 6: Hill´s method demonstrating electron transfer in chloroplasts by observing DCPIP reduction, immobilization of a culture of an alga such as Scenedesmus in alginate beads and measurement of the rate of photosynthesis by monitoring th

¿Necesitas ayuda?