YIJC [H2] CI3.1 Photosynthesis (N)(S)

2025 JC2 BIOLOGY LECTURE NOTES 1 CORE IDEA 3: ENERGY AND EQUILIBRIUM TOPIC 3.1 PHOTOSYNTHESIS Learning Outcomes: (a) Identify components of chloroplasts in drawings, photomicrograph and electron micrographs. (b) Explain the absorption and action spectra of photosynthetic pigments. (c) With reference to the chloroplast structure, describe and explain how light energy is harnessed and converted into chemical energy during the light-dependent reactions of photosynthesis. (d) Outline the three phases of Calvin cycle in C3 plants: (i) CO 2 fixation, (ii) PGA reduction and (iii) ribulose bisphosphate (RuBP) regeneration, indicating the roles of rubisco, ATP and reduced NADP in these processes that ultimately allow synthesis of sugars. (e) Discuss limiting factors in photosynthesis and carry out investigations on the effect of limiting factors such as temperature, light intensity and carbon dioxide concentration on the rate of photosynthesis. (l) Outline chemiosmosis in photosynthesis. (names of complexes in the ETC are not required.) References: Campbell and Reece. Biology. (11th edition). Chapter 11: Photosynthetic Processes, pg.259 – 283 (or corresponding chapter in other editions) Note: This textbook is available in our library. You may wish to borrow it to supplement your reading when necessary. 1) Introduction • The Chloroplast • Photosynthetic Pigments • Absorption and Action Spectra • Stages of Photosynthesis 2) Light-Dependent Stage • Non-cyclic Photophosphorylation • Cyclic Photophosphorylation • Chemiosmosis 3) Light-Independent Stage 4) Factors Affecting Rate of Photosynthesis • Light Intensity • Carbon Dioxide Concentration • Temperature H2

2025 JC2 BIOLOGY LECTURE NOTES 2 1 INTRODUCTION • Most plants synthesise their own organic molecules from carbon dioxide and other inorganic raw materials (such as water and minerals) obtained from the environment. • They use light as a source of energy to synthesise these organic compounds. This light is absorbed by photosynthetic pigments present in the chloroplasts of leaf cells or green organs of the plant. • Photosynthesis is the conversion of light energy to chemical energy that is stored in sugars or other organic compounds • The process of photosynthesis can be summarised by the following equation: light 6 CO2 + 6 H2O → → → C6H12O6 + 6 O2 chlorophyll Fig. 1.1: Plants as autotrophs chlorophyll absorbs mostly violet-blue and red wavelengths of light light energy water water is absorbed through the roots and carried through the stem to all parts of the plant. oxygen excess sugar synthesised in the leaf cells are stored. carbon dioxide

2025 JC2 BIOLOGY LECTURE NOTES 3 1.1 THE CHLOROPLAST Learning Outcome (a): Identify components of chloroplasts in drawings, photomicrograph and electron micrographs. • Chloroplasts, found in plants and algae, are the sites of photosynthesis . They convert light energy to chemical energy by absorbing light and using it to drive the synthesis of organic compounds (such as sugars) from carbon dioxide and water. • Chloroplast are 3-10 μm in diameter, and are visible under the light microscope. • The chloroplast is a double membrane-bound organelle, with an outer and inner chloroplast membrane. Fig. 1.1.1: Structure of chloroplast Part of chloroplast Structure Function Thylakoid Chloroplast contains an internal membrane system known as the thylakoid system Thaylakoids are flat, interconnected sacs bound by a membrane known as thylakoid membrane. Thylakoid membrane is embedded with photosynthetic pigments (e.g. chlorophyll), proteins such as ATP synthase and electron transport chain. Within the thylakoid is the thylakoid space / lumen. Thylakoids will stack up to form a granum (plural: grana) , which are connected to each other by intergranal lamellae. Site of Light -dependent Stage Thylakoid membrane: 1. Affords a large surface area to accommodate the electron transport chain and to absorb light energy. 2. Impermeable to charged particles such as of H⁺ ions, enabling the generation of a proton gradient. 3. Contains ATP synthase, which phosphorylates ADP to ATP by chemiosmosis. Stroma The fluid outside the thylakoid system is called stroma and it contains starch grains, enzymes, circular double-stranded DNA and ribosomes. Site of Light -independent Stage/ Calvin Cycle

2025 JC2 BIOLOGY LECTURE NOTES 4 1.2 PHOTOSYNTHETIC PIGMENTS • Photosynthetic pigments absorb light for the plant’s use. Different pigments absorb different wavelengths of light. Non-absorbed wavelengths are reflected. o E.g. Blue and red light are absorbed by chlorophyll, causing green light to be reflected . This gives leaves their signature green colour. o E.g. Blue and green light are absorbed by carotenoids, causing yellow, orange, red light to be reflected. Hence leaves appear to be such colours during autumn as chlorophyll levels decrease. Fig. 1.2.1: Absorption of light by pigments in the chloroplast • Chloroplasts contain the following photosynthetic pigments: 1) Chlorophyll – Chlorophyll (e.g. chlorophyll a and b) absorbs light maximally at the blue-violet (400 – 500nm) and red regions (600 – 700nm) of the visible light spectrum. Green light is reflected to give chlorophyll its characteristic colour. (refer to 1.4 Absorbance and Action Spectra) – Chlorophyll functions to absorb light energy for photoexcitation of electrons during photoactivation. 2) Carotenoids – Carotenoids [e.g. carotene (yellow / orange) and xanthophylls (yellow) ] act as accessory pigments to broaden the spectrum of light absorbed for photosynthesis. – It also serves in photoprotection, preventing damage by absorbing excess light energy.

2025 JC2 BIOLOGY LECTURE NOTES 5 Fig. 1.2.2: Structure of chlorophyll • These photosynthetic pigments are found in photosystems which are embedded on the thylakoid membrane. Photosystems are protein complexes containing: - Reaction Centre: o contains special chlorophyll a molecules (e.g. P680 or P700) with a primary electron acceptor. - Light Harvesting Complex: o made up of hundreds of accessory pigment molecules (e.g. chl a, chl b and carotenoids). o Light Harvesting Complex surrounds the reaction centre and channels energy absorbed from light to it.

2025 JC2 BIOLOGY LECTURE NOTES 6 Fig. 1.2.3: A photosystem embedded in the thylakoid membrane • There are two photosystems present in chloroplasts – photosystem I (PS I) absorbs light optimally at 700 nm, while photosystem II (PS II) absorbs light optimally at 680 nm.

2025 JC2 BIOLOGY LECTURE NOTES 7 CHECKPOINT 1.2 1 Name the components A, B, C, D and E in the diagram below. A B C D E 2 Where is B found in the photosystem? A Reaction Centre B Primary Electron Donor C Primary Electron Acceptor D Light Harvesting Complex 3 Which options below is the primary electron donor? A Carotenoids B Accessory pigments C Chlorophyll D Special chlorophyll a 4 What is the purpose of having a primary electron donor? A Photoprotection B Photoexcitation during photoactivation C Donate an electron to primary electron acceptor D Convert light energy to chemical energy *there can be more than 1 right answer for this question

2025 JC2 BIOLOGY LECTURE NOTES 8 1.3 ABSORPTION & ACTION SPECTRA Learning Outcome (b): Explain the absorption and action spectra of photosynthetic pigments. • An absorption spectrum plots the amount of light absorbed at each wavelength by a particular pigment. • An action spectrum plots the rate of photosynthesis at each wavel