Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Photosynthesis Light Reaction (AP) Chapter 10

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 (making carbohydrates)

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Chloroplasts Chloroplast Structure  Internal membranes organized into stacks of thylakoid membranes (stacks called 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 Leaf Organization Copyright © McGraw-Hill Companies Permission required for reproduction or display

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, a pair of electrons becomes excited. They are passed from one molecule to another giving up energy as this happens…Like a bucket brigade!

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.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies 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 (AP) 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 (AP) 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 (AP) Organizing Pigments Into Photosystems Photosystem consists of two components:  Antenna Complex - Captures photons from sunlight.  Reaction Center - Pair of chlorophyll a molecules act as a 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

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(AP) 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 (AP) 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 Light Reaction Steps: Light strikes PII  Electrons are excited, leave chlorophyll a  Water is split - 2H 2 O 4H + + O 2 +4 electrons - Electrons from water replace those lost (PII) 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 (AP) 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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Yes or No Can sunlight be used directly by the plant?

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Why can’t sunlight be used directly? A) It is not able to be absorbed B) It is too much energy C) It is too little energy D) Too much energy is lost

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies True or False Photosynthesis occurs in one step. T/F?

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