Topic H Photosynthesis lecture notes (2015)

NANYANG JUNIOR COLLEGE H2 Biology Photosynthesis J1/2015 1 TOPIC H: PHOTOSYNTHESIS Learning Outcomes Candidates should be able to: Core Topic 6 – Cellular Physiology and Biochemistry (a) With reference to the chloroplast structure, explain the light dependent reactions of photosynthesis (no biochemical details are needed but will include the outline of cyclic and non -cyclic light dependent reactions, and the transfer of energy for the subsequent manufacturing o f carbohydrates from carbon dioxide). (b) Outline the three phases of the Calvin cycle: (i) CO 2 uptake (ii) carbon reduction and (iii) ribulose bisphosphate (RuBP) regeneration and indicate the roles of ATP and NADP in the process. (c) Discuss limiting factors in photosynthesis and carry out investigations on the effects of limiting factors, such as light intensity, CO2 concentration and temperature, on the rate of photosynthesis. Content Outline 1. Autotrophs: Producers for the Earth 2. Structure of the Chloroplast 3. Photosynthetic Pigments (a) Chlorophylls (b) Carotenoids 4. Absorption and Action Spectra 5. Light-dependent Reactions (a) Photosynthetic Units (b) Excitation of Primary Pigments by Light (Absorption of Light Energy) (c) Non-cyclic Photophosphorylation (d) Cyclic Photophosphorylation 6. Light-independent Reactions (Calvin Cycle) (a) CO2 Uptake (CO2 Fixation) (b) Carbon Reduction (c) Regeneration of RuBP 7. Limiting Factors in Photosynthesis (a) Light (b) Temperature (c) Carbon Dioxide

NANYANG JUNIOR COLLEGE H2 Biology Photosynthesis J1/2015 2 1. Autotrophs: Producers for the Earth Photosynthesis nourishes almost all life on earth directly or indirectly. Autotrophs are organisms that sustain themselves without feeding on other organisms. Autotrophs can produce their own organic (carbon -containing) compounds from carbon dioxide (an inorganic source of carbon) and other inorganic compounds obtained from the environment using the energy harnessed from the environment. As plants use light as a source of energy to synthesise these organic compounds, they are specifically known as photoautotrophs. Heterotrophs feed on other organisms as they are unable to make their own organic compounds, unlike autotrophs. Autotrophs undergo photosynthesis where light energy is used to synthesize carbohydrate s from carbon dioxide and water. Oxygen is given out in the process as a by-product. The product formed is a sugar that is soon converted to starch for storage. There are two main stages in photosynthesis, namely the light-dependent and light-independent reactions / Calvin cycle.

NANYANG JUNIOR COLLEGE H2 Biology Photosynthesis J1/2015 3 2. Structure of the Chloroplast The two major steps for photosynthesis take place in specific regions within the chloroplast: 1) Light-dependent reactions – Thylakoids of the grana 2) Light-independent reactions (the Calvin cycle) - Stroma The thylakoids possess the following properties which allow the light-dependent reactions of photosynthesis to be carried out: 1) Large surface area of thylakoid membrane  Allows photosystems and electron carriers to be embedded.  Allows stalked particles containing ATP synthase to be embedded. 2) Thylakoid membrane is impermeable to protons  Allows electrochemical proton gradient to be set up between the thylakoid space and stroma. The stroma is a dense fluid which contains the enzymes and dissolved substrates required for the light-independent reactions of photosynthesis, also known as the Calvin cycle. Excess carbohydrate from photosynthesis is stored as starch grains in the chloroplasts. Lipid droplets, which are associated with the breakdown of membranes a nd lipids, also accumulate within the chloroplasts.

NANYANG JUNIOR COLLEGE H2 Biology Photosynthesis J1/2015 4 3. Photosynthetic Pigments There are 2 types of photosynthetic pigments which are the chlorophylls and the carotenoids. These pigments are found in photosystems (PS) which are embedded on the thylakoid membranes. The role of the pigments is to absorb light energy and convert it into chemical energy. (a) Chlorophylls There are 2 main types of chlorophyll – chlorophyll a and chlorophyll b. They absorb mainly red and blue-violet light , reflecting green light and therefore giving plants their characteristic green colour. Their chemical structure consists of:  a head made up of a porphyrin ring with a magnesium ion in the centre, and  a long hydrocarbon tail which is joined to its head by an ester linkage. Different chlorophylls have different side -chains on the head and th ese modify their absorption spectra. Structure of Chlorophyll a (b) Carotenoids There are 2 main types of carotenoids – carotenes and xanthophylls. They are yellow, orange, red or brown pigments that absorb strongly in the blue -violet light spectrum . They are usually masked by the green chlorophylls. Carotenoids protect chlorophylls from excess light and oxidation by oxygen produced during photosynthesis.

NANYANG JUNIOR COLLEGE H2 Biology Photosynthesis J1/2015 5 4. Absorption and Action Spectra a) Absorption spectrum  A graph showing the relative absorbance of different wavelengths of light by a pigment. b) Action spectrum  A graph showing the effectiveness of different wavelengths of light in stimulating photosynthesis.  It is a record of the amount of photosynthesis occurring at each wavelength of light. The close similarity/correlation between the absorption spectrum and action spectrum indicates that the photosynthetic pigments are responsible for absorption of light in photosynthesis. Action Spectrum: Rate of photosynthesis at different wavelengths of light Absorption Spectrum: Amount of light absorbed by different pigments at different wavelengths of light

NANYANG JUNIOR COLLEGE H2 Biology Photosynthesis J1/2015 6 5. Light-dependent Reactions Light-dependent reactions occur in the grana. The objective of these reactions is to provide ATP and reduced nicotinamide adenine dinucleotide phosphate ( reduced NADP / NADPH + H +) for the light - independent reactions (Calvin cycle). As the energy for the synthesis of ATP comes from light, it is called photophosphorylation. There are 2 types of photophosphorylation – cyclic and non-cyclic. Light-dependent reactions (photophosphorylation) Cyclic photophosphorylation Non-cyclic photophosphorylation Light-dependent reactions provide the ATP and reduced NADP required to drive the Calvin cycle.

NANYANG JUNIOR COLLEGE H2 Biology Photosynthesis J1/2015 7 (a) Photosynthetic Units There are 2 types of photosynthetic units: photosystem I (PSI) and photosystem II (PSII). Each photosystem comprises: i. Light-harvesting complexes - Made up of accessory pigments (chlorophylls and carotenoids). - Accessory pigments funnel the energy absorbed from light to the reaction centre in either PSI or PSII. - The light-harvesting complexes surround the reaction centre. ii. A reaction centre - A protein complex that includes two special chlorophyll a molecules (primary pigments) and a molecule called the primary electron acceptor. - Two different forms of special chlorophyll a, P680 and P700 (P stands for primary pigment of chlorophyll a while 680 and 700 are their absorpt ion peaks' wavelength in nm). - The two special chlorophyll a molecules each emit one electron and the two electrons are accepted by the primary electron acceptor. (b) Excitation of Primary Pigments by Light (Absorption of Light Energy) Primary pigments (i.e. special chlorophyll a in the reaction centres) are molecules that absorb energy from accessory pigments. When the energy levels of the electrons found in the primary pigments are boosted, they reach an excited s