8. Metabolism, cell respiration and photosynthesis

8.1 Metabolism

Nature of science:

Developments in scientific research follow improvements in computing - developments in bioinformatics, such as the interrogation of databases, have facilitated research into metabolic pathways.

Understandings:

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.

Nature of science:

Paradigm shift - the chemiosmotic theory led to a paradigm shift in the field of bioenergetics.

Understandings:

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 membrane of the cristae is coupled to proton pumping.
Oxygen is the final electron acceptor.
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.

Nature of science:

Developments in scientific research follow improvements in apparatus - sources of 14C and autoradiography enabled Calvin to elucidate the pathways of carbon fixation.

Understandings:

Light-dependent reactions take place in the thylakoid membranes and the space inside them.
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.

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.

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