Photosynthesis

• Photosystem II (PSII) is located on the outer side of the thylakoid membrane and absorbs blue/green wavelengths of light.
• Light energy absorbed by PSII causes electrons to be excited from their ground state to an excited state.
• The light-dependent reactions occur in the thylakoid membranes, while the Calvin cycle occurs in the stroma.
• The energy absorbed by PSII is used to split water molecules into oxygen, protons, and electrons.
• The excited electron is transferred to a primary acceptor molecule, which then passes it along through a series of electron carriers until it reaches NADP+.
• Electron transport chain: A series of electron carriers that transfer electrons from one carrier to another, releasing energy along the way.
• Chlorophyll molecules absorb light energy and transfer it to other pigments called accessory pigments.
• ATP synthase: An enzyme complex that uses the energy released during electron transport to synthesize ATP.
• NADP+ accepts the electron and becomes reduced to NADPH.
• Electron transfer creates a proton gradient across the thylakoid membrane as H+ are pumped into the lumen of the thylakoids.
• Accessory pigments include carotenoids and phycobilins.
• Carotenoids are yellow or orange pigments that protect chloroplasts from excess light damage.
• ATP synthase uses the concentration gradient created by the movement of H+ to synthesize ATP.
• NADPH is produced through the reduction of NADP+ using the electrons released during photosynthesis.
• Electron transport chain: A sequence of electron carriers that transfer electrons from one carrier to another, with each carrier accepting electrons at lower energy levels than the previous one.
• NADPH: Nicotinamide adenine dinucleotide phosphate, a coenzyme involved in reducing power generation during photosynthesis.
• Photosynthesis

  Plants use sunlight, water, and carbon dioxide to create oxygen and energy in the form of sugar
• Organization of Photosynthetic apparatus

  1. Within chloroplast, thylakoid membranes contain light-harvesting antenna complexes, reaction centers, and most of electron carrier proteins

  2. PS1 and PS11 spatially separated in thylakoids
• Plants have to balance CO2 uptake with water loss
• Photosynthesis and Light Recap

  1. For plants, light is life (phototropism) – energy derived from sun

  2. Photosynthesis, the only biological process that can harvest such energy – forms planets energy resources (fossil fuels)

  3. Light energy drives synthesis of carbohydrates and generation of oxygen from carbon dioxide and water

  4. Energy stored in these molecules used to power cellular process – energy source for all forms of life
• Photosynthesis recap cont.

  1. Most active photosynthetic tissue in plants is mesophyll of leaves

  2. Such cells have many chloroplasts – contain light absorbing green pigments called chlorophylls

  3. Solar energy oxidizes water, releasing oxygen and reduces carbon dioxide forming carbon compounds, primarily sugars

  4. Complex reactions that culminate in reduction of CO2 include thylakoid reactions and carbon fixation reactions

  5. Thylakoid reactions: occur in specialized internal membranes of chloroplast – called thylakoids – reactions produce high energy ATP and NADPH used for synthesis of sugars in carbon fixation reactions
• Wave and Particle Nature of Light
• Chloroplast Light reactions

  1. In chloroplast light energy is converted into chemical energy by two different functional units called photosystems

  2. Absorbed energy used to power transfer of electrons through series of compounds that act as electron donors and acceptors

  3. Majority of electrons ultimately reduce NADP+ to NADPH and oxidize H2O to O2
• Chlorophylls are arranged in packets in the thylakoid membranes called photosystems – there are two types PSII and PSI
• Light-energized chlorophyll may fluoresce, transfer energy to another molecule, or use its energy to drive chemical reactions
• All photosynthesizers contain a mixture of pigments with distinct structures and light absorbing properties
• Chlorophylls are typical pigments of photosynthetic organisms; a and b abundant in green plants
• Carotenoids are found in all photosynthetic organisms – integral constituents of thylakoid membrane; usually intimately related with protein of photosynthetic apparatus
• Energy absorbed by carotenoids transferred to chlorophyll – hence called accessory pigments
• Photosystem I (PS I) uses light energy to convert NADP+ to NADPH2. It involves the P700, chlorophyll and other pigments
• Photosystem II (PS II) is the protein complex that absorbs light energy, involving P680, chlorophyll and accessory pigments and transfer electrons from water to plastoquinone and thus works in dissociation of water molecules and produces protons (H+) and O2
• Photosystem I (PS I) has P700 as the photo center and absorbs longer wavelengths of light at 700 nm (P700)
• Photosystem II (PS II) has P680 as the photo center and absorbs shorter wavelengths of light at 680 nm (P680)
• Photocenter or reaction centre


  The center where light energy is absorbed in photosynthesis
• P680 is the photo center
• Absorbing wavelength

  The pigments in photosystem 1 absorb longer wavelengths of light which is 700 nm (P700). The pigments in photosystem 2 absorb shorter wavelengths of light which is 680 nm (P680)
• No photolysis occur
• Photolysis occurs in this system
• Pigments in Photosystem I

  • Chlorophyll A-670, chlorophyll A-680, chlorophyll A-695, chlorophyll A-700, chlorophyll B, carotenoids
• Pigments in Photosystem II

  • Chlorophyll A-660, chlorophyll A-670, chlorophyll A-680, chlorophyll A-695, chlorophyll A-700, chlorophyll B, xanthophylls, phycobilins