Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Photosynthesis Chapter 5-6 Copyright © McGraw-Hill Companies Permission required for reproduction or display

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Outline What is photosynthesis? Chloroplasts  Structure  Light and Dark Rxns. Biophysics of Light Chlorophylls and Carotenoids Photosystems NEXT WEEK: Calvin Cycle Photorespiration

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Photosynthesis-What is it? A process by which green plant cells store energy from the sun in chemical bonds Sunlight is too much energy to be used directly (like having a $100 bill and needing to use the vending machine)  ATP and NADPH are molecules that carry smaller, usable units of energy (like having change in your pocket)

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Photosynthesis has Two Main Parts Two Stages of Photosynthesis-both occur in the chloroplast.  Light Reaction - Occurs in thylakoid membranes - Capturing energy from sunlight - Using energy to make ATP and NADPH  Dark Reaction (Calvin Cycle) - Occurs in stroma - Carbon fixation

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Chloroplasts Chloroplast Structure  Internal membranes organized into sacs of thylakoids and stacked in grana. - Light Reaction takes place within the thylakoid membrane  Semiliquid fluid (stroma) surrounds thylakoid membrane - Dark Reaction (Calvin Cycle) takes place in the stroma

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Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Chloroplast Structure  Photosynthetic pigments clustered to form photosystems in membranes of thylakoids. - When light of proper wavelength strikes a pigment molecule, resulting excitation passes from one molecule to another.  Like a bucket brigade

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Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Biophysics of Light Light moves through the air as oscillating electric and magnetic fields.  Energy content of a photon is inversely proportional to the wavelength of light. - Short wavelengths contain photons of higher energy than long wavelengths.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Biophysics of Light Ultraviolet Light  Shorter wavelength and more energy than visible light. Absorption Spectra  When a photon strikes a molecule, its energy is either - lost as heat (reflected or transmitted) - or absorbed by the electrons boosting them into higher energy levels.  Each molecule has a characteristic range of photons it is capable of absorbing.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Biophysics of Light Pigments - Light-absorbing molecules.  Photosynthesis uses two pigments: - Carotenoids and Chlorophyll  Chlorophyll a (main pigment) and b (accessory pigment) preferentially absorb violet-blue and red light.  Chlorophyll b has an absorption shifted toward green wavelengths.

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Chlorophylls and Carotenoids All plants, algae, and cyanobacteria use chlorophyll a as their primary pigment.  Achieve higher overall photon capture rates with chlorophyll than with other pigments. Carotenoids absorb photons with a wide array of energies, but are not as efficient in transferring energy as chlorophyll.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Organizing Pigments Into Photosystems Light reactions take place in four stages:  Primary Photoevent  Charge Separation  Electron Transport  Chemiosmosis Light is absorbed by clusters of chlorophyll and accessory pigments collectively called a photosystem.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Organizing Pigments Into Photosystems Light is absorbed by any one of hundreds of pigment molecules in a photosystem.  Transfer excitation energy to a molecule with lower energy level than the others. - Reaction Center serves as energy sink.  Can become saturated at high light levels.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Organizing Pigments Into Photosystems Photosystem consists of two components:  Antenna Complex - Captures photons from sunlight.  Reaction Center - Pair of chlorophyll a molecules act as trap for photon energy, passing an excited electron to an acceptor. - Electrons are passed through the electron transport chain in a series of redox reactions

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Photosystem Function Bacteria Use a Single Photosystem  Electron joined with a proton to make hydrogen.  Electron is recycled to chlorophyll. - Electron transfer process leading to ATP formation is termed cyclic phosphorylation.  Major limitation is that it is only geared towards energy production, not biosynthesis.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Photosystem Function Plants Use Two Photosystems  Second Photosystem uses another arrangement of chlorophyll a to absorb more shorter wavelength, high energy photons. - Enhancement Effect

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Steps of the Light Reaction Light strikes PII  Electrons are excited, leave chlorophyll a  Water is split - H 2 O 2H+ + O 2 +2 electrons - Electrons from water replace those lost Light strikes PI (at the same time)  Electrons are excited and move to the stroma  NADPH carries electrons (with H) to the stroma (to be used in the Dark Reaction)  Electrons from PI are replaced by those from PII

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Photosystems I and II Copyright © McGraw-Hill Companies Permission required for reproduction or display

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Two Photosystems Work Together Two-stage photosystem referred to as non- cyclic phosphorylation.  High energy electrons generated by photosystem II are used to synthesize ATP  After giving up their excess energy, these electrons are passed to photosystem I  Electrons lost from PI are used to drive NADPH production.

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Review Chloroplasts Biophysics of Light Chlorophylls and Carotenoids Photosystems

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Copyright © McGraw-Hill Companies Permission required for reproduction or display