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Suggested Teaching Sequence F

Section 2 Energetics

Respiration is the process by which energy is released in living cells through the controlled oxidative breakdown of organic food materials.

Organisms may have to synthesise these organic food materials using energy from the sun (photosynthesis) or from the oxidation of inorganic materials (chemosynthesis).

This section aims to extend students’ understanding of the concepts of energy transformation in photosynthesis and respiration. An outline of their energy conversion processes, including an insight into their interrelationship, should be discussed. But details of the metabolic pathways, names of intermediates and individual enzymes should be de-emphasised. Chemosynthesis should be stressed as a process using an alternative source of energy to light, thus forming a solitary exception to the much-accepted concept that energy needed by living organisms comes ultimately from the Sun.

This section builds on prior knowledge in Section 1: Cell structure (especially the ultrastructures of chloroplasts and mitochondria), Chemical constituents and Enzymes. It prepares students for an understanding of the role of energy in supporting physiological processes discussed in the other sections, and provides them with a foundation for the study of Energy flow and nutrient cycling (Section 4).

Learning objectives Possible learning and teaching activities Expected learning outcomes

Students should learn Students should be able to

2.1 Photosynthesis

 the importance of photosynthesis in converting light energy to chemical energy.

 Discuss what would happen to the living world if all photosynthetic organisms disappeared from the Earth.

 explain the importance of photosynthetic organisms as producers.

2.1.1 Site of photosynthesis

 the structure of dicotyledonous leaves in relation to photosynthesis.

 Ask students to collect a variety of broad leaves.

Guide them to list out the common morphological features of the leaves and relate them to

photosynthesis.

 Examine a section of a dicotyledonous leaf microscopically to study its structure in relation to photosynthesis.

 describe the adaptive features of leaves to photosynthesis.

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Learning objectives Possible learning and teaching activities Expected learning outcomes

Students should learn Students should be able to

 the structure of chloroplast as shown in electron micrographs. [Refer to Section 1.2.]

 the occurrence of different pigments in the chloroplast.

 Extract leaf pigments with extracting solvent, and separate them by paper chromatography.

 relate the structure of chloroplast to its functions in photosynthesis.

 the absorption spectra of chlorophyll pigments and the action spectrum of photosynthesis.

 Show pictures of the spectrum of white light passing through a prism and the spectrum of white light passing through a chlorophyll extract and a prism. Guide students to deduce the light absorption property of chlorophyll.

 relate the absorption spectra of chlorophyll pigments to the action spectrum of photosynthesis.

2.1.2 Photochemical reactions

 an outline of the photochemical reactions:

(1) electrons in chlorophylls are excited by light energy, without referring to photosystems I and II;

(2) energy from these excited electrons generates ATP;

 Use audiovisual materials to illustrate the photochemical reactions.

 Discuss the importance of the photochemical reactions.

 outline the main steps of photochemical reactions.

 explain the importance of photochemical reactions.

 outline the principle of photophosphorylation.

 relate biochemical pathways of photosynthesis to their sites in cells.

(3) photolysis of water provides hydrogen for the reduction of NADP (nicotinamide adenine dinucleotide phosphate) and oxygen gas is released.

 Discuss how the establishment of photosynthesis might have led to the evolution of aerobic organisms.

 Construct a flow chart to show the process of photochemical reactions.

2.1.3 Carbon fixation

 an outline of the Calvin cycle to show that:

(1) carbon dioxide is accepted by a 5-C compound to form two molecules of a 3-C compound;

 Read how Calvin used radioactive isotopes to trace the path of carbon atoms in photosynthesis.

 Construct a flow chart to show the process of carbon fixation.

 outline the main steps of carbon fixation.

 point out the dependence of this process to the photochemical reactions.

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Learning objectives Possible learning and teaching activities Expected learning outcomes

Students should learn Students should be able to

(2) reduction of the 3-C compound by reduced NADP to triose phosphate, some of which combine to yield hexose phosphate which is subsequently metabolised to sucrose and starch;

 describe the fates of triose phosphate.

(3) metabolism of some of the triose phosphate to provide a continuous supply of the 5-C carbon dioxide acceptor.

 that triose phosphate can be used as a substrate to produce lipids and amino acids.

2.1.4 Factors affecting the rate of photosynthesis

 the effects of light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis.

 Ask students to predict the possible effects of various factors on the rate of photosynthesis.

Guide students to design and perform investigations to test their ideas.

 describe and explain the effects of various factors on the rate of photosynthesis.

 the concept of limiting factors, as exemplified by light intensity and carbon dioxide concentration.

 Perform experiments to study the factors affecting the rate of photosynthesis using a bubbler / syringe, J-tube or a data logger with oxygen or pressure sensors.

 explain the concept of limiting factors.

 the principle for maximising plant growth in greenhouse by the control of light, temperature and carbon dioxide concentration.

 Discuss how to increase the yield of plants through the design of a greenhouse.

 apply the concept of limiting factors in the design of a greenhouse.

2.2 Chemosynthesis

 the general nature of chemosynthesis using nitrifying bacteria as an example.

 Search for information on the importance of other types of bacteria in the maintenance of the ecosystem.

 realise the occurrence of chemosynthesis.

 point out the difference between chemosynthesis and photosynthesis.

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Learning objectives Possible learning and teaching activities Expected learning outcomes

Students should learn Students should be able to

2.3 Respiration

 the importance of respiration in converting chemical energy in food to chemical energy in ATP.

 define respiration.

 compare respiration and photosynthesis.

2.3.1 The sites of respiration

 the sites of the various biochemical pathways of respiration.

 state the sites of different stages of respiration.

 the structure of mitochondrion as shown in electron micrographs. [Refer to Section 1.2.]

 Use electron micrographs to show the structure of mitochondrion.

 relate the structure of mitochondrion to its function.

2.3.2 Glycolysis

 an outline of glycolysis to show:

(1) the phosphorylation of glucose;

(2) the break down of hexose phosphate to triose phosphate;

 Construct a flow chart to show the process of glycolysis.

 Read how scientists worked out the glycolytic pathway.

 describe the main steps of glycolysis.

 point out the significance of glycolysis.

(3) the conversion of triose phosphate to pyruvate with the production of reduced NAD and ATP.

2.3.3 Aerobic pathway

 the conversion of pyruvate to acetyl-CoA.

 an outline of the Krebs cycle to show:

(1) the combination of acetyl-CoA with a 4-C compound to form a 6-C compound;

 Construct a flow chart to show the aerobic pathway.

 Discuss the ways to measure the rate of aerobic respiration. Then conduct investigations to find the rate of aerobic respiration in plants and animals, e.g. germinating seeds and mealworms.

 describe the main steps of Krebs cycle.

 review the interrelationships between glycolysis, Krebs cycle and electron transport chain.

 state the importance of Krebs cycle.

(2) that the 6-C compound undergoes a series of reactions to regenerate the 4-C compound with the release of carbon dioxide;

(3) the production of reduced NAD and ATP.

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Learning objectives Possible learning and teaching activities Expected learning outcomes

Students should learn Students should be able to

 that lipids and proteins can be used to produce reduced NAD and ATP.

 the electron transport chain as a process of oxidative phosphorylation; the role of molecular oxygen as the final electron acceptor.

 point out the alternative substrates for respiration.

2.3.4 Anaerobic pathway

 the fate of pyruvate under anaerobic condition.

 the formation of lactic acid in muscle; the oxygen debt.

 the formation of ethanol and carbon dioxide in yeast.

 Design and perform investigations to find the rate of anaerobic respiration in yeast.

 Search for information on the brewing of beer and wine making.

 outline the biochemical pathways of alcoholic fermentation and lactic acid fermentation.

 suggest how the knowledge of anaerobic respiration can be used in everyday life.

2.3.5 Energy yield

 the comparison of the energy yield of aerobic and anaerobic respiration, without calculating the number of ATP produced.

 compare the energy yield of aerobic and anaerobic respiration.

2.3.6 Role of ATP

 the role of ATP in energy transfer.  explain the role of ATP in energy transfer.

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