13.1 Photosynthesis as an energy transfer process
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13.1.1 Describe the relationship between the structure of chloroplasts, as shown in diagrams and electron micrographs, and their function.
13.1.2 Explain that energy transferred as ATP and reduced NADP from the light-dependent stage is used during the light-independent stage (Calvin cycle) of photosynthesis to produce complex organic molecules.
13.1.3 State that within a chloroplast, the thylakoids, which occur in stacks called grana, are the site of the light-dependent stage and the stroma is the site of the light-independent stage.
13.1.4 Describe the role of chloroplast pigments in light absorption in thylakoids.
13.1.5 Interpret absorption spectra of chloroplast pigments and action spectra for photosynthesis.
13.1.6 Describe and use chromatography to separate and identify chloroplast pigments.
13.1.7 State that cyclic photophosphorylation and non-cyclic photophosphorylation occur during the light-dependent stage of photosynthesis.
13.1.8 Explain that in cyclic photophosphorylation:
only photosystem I
(PSI) is involved
photoactivation of chlorophyll occurs
ATP is synthesised.
13.1.9 Explain what happens in non-cyclic photophosphorylation.
13.1.10 Explain what happens during photophosphorylation.
13.1.11 Outline what happens in the three main stages of the Calvin cycle.
13.1.12 State that Calvin cycle intermediates are used to produce other molecules, limited to GP to produce some amino acids and TP to produce carbohydrates, lipids and amino acids.
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Emphasise the importance of photosynthesis in context using a relevant video clip such as: www.nasa.gov/content/goddard/seeing-photosynthesis-from-space-nasa-scientists-use-satellites-to-measure-plant-health/. This shows how fluorescence released by chlorophyll can be detected by satellites in space in order to produce maps of world vegetation and its health. Discuss what is meant by ‘photosynthesis’ and review prior learning. (F)
Establishing a good understanding of the key terms that learners will need for this topic is important. Ask a series of questions on the anatomy of the leaf, that require one-word answers, so that learners can visualise mesophyll tissue and mesophyll cells containing chloroplasts, and the absorption of different wavelengths of light by pigments found in plants.
Show electron micrographs of chloroplasts (e.g. www.vcbio.science.ru.nl/en/fesem/applets/chloroplast/). Move around the class and ask each learner to identify a blank label on the diagram, and/or identify cells that have a large number of these organelles. (F)
Learners undertake a practical activity using paper chromatography to separate mixtures of photosynthetic pigments according to their solubility. More-soluble compounds move further along the chromatogram than less-soluble ones. Learners make measurements and calculate Rf values. They compare these with published values to make identifications. Guidance regarding this practical is at:
www.saps.org.uk/secondary/teaching-resources/181-student-sheet-10-thin-layer-chromatography-for-photosynthetic-pigments. Extend by providing learners an opportunity to analyse the absorption spectra of chloroplast pigments that they have identified, and how this compares with the action spectra for photosynthesis.
Learners carry out a practical investigation into the Hill reaction: they investigate the effect of changing the colour of light (wavelength) on the rate of the light-dependent reaction of isolated chloroplasts, using DCPIP as an indicator. Learners suggest how they could investigate the effect of a given variable on the light-independent stage of photosynthesis. Guidance regarding this practical is at: https://pbiol.rsb.org.uk/energy/photosynthesis/investigating-the-light-dependent-reaction-in-photosynthesis
Prepare a timeline showing the sites and events of each step of the light-dependent and light-independent stages of photosynthesis. During the activity, provide learners with an opportunity to seek support from more confident learners who you identify as the activity progresses. Some good overview animations, to prepare for or conclude learners’ independent work, are:
www.sumanasinc.com/webcontent/animations/content/harvestinglight.html www.johnkyrk.com/photosynthesis.html
www.johnkyrk.com/photosynthesisdark.html (I)
Display or draw a large diagram of the thylakoid membrane showing the various components of the mechanism of the light-dependent stage of photosynthesis, but which is covered by between five and ten numbered ‘jigsaw’ pieces. Ask learners to choose which pieces to remove, which gradually reveals the image, and to identify parts of the mechanism. Learners label and annotate an unlabelled version of the diagram as you summarise the role of the molecular components. This helps to break up the amount of information you provide learners with into a series of smaller explanations.
Learners compare and contrast the Krebs cycle and the Calvin cycle. They present their work in a very visual way for display. For example, encourage them to draw up a table on a piece of poster paper to compare the processes. Subsequently hold a ‘marketplace’ activity in which one member of each group stands by their poster and offers an explanation to other groups as they circulate around the room. (I)
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