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

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

Photosynthesis Overview Energy for all life on Earth ultimately comes from photosynthesis 6CO H 2 O C 6 H 12 O 6 + 6H 2 O + 6O 2 Oxygenic photosynthesis is carried out by:  Cyanobacteria  7 groups of algae  All land plants – chloroplasts

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Chloroplast Organelle found in plants and photosynthetic organisms Major site of photosynthesis Located in the cells of mesophyll Contains the pigment chlorophyll

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Structure of Chloroplast: Internal membranes organized into sacs of thylakoids (coins) and stacked to form granum or grana (stack of coins). Semiliquid fluid (stroma) surrounds thylakoid membrane.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Functional Unit of a Chloroplast * photosystems - photosynthetic pigments clustered in membranes of thylakoids. - when light of proper wavelength strikes a pigment molecule, excitation of electrons pass from one molecule to another.

Stages of Photosynthesis A. Light-dependent reactions Require light Capture energy from sunlight Make ATP and reduce NADP + to NADPH B. Carbon fixation reactions or light- independent reactions (Dark Rxns.) Does not require light Use ATP and NADPH to synthesize organic molecules from CO 2

Discovery of Photosynthesis Jan Baptista van Helmont (1580–1644)  Demonstrated that the substance of the plant was not produced only from the soil Joseph Priestly (1733–1804)  Living vegetation adds something to the air Jan Ingenhousz (1730–1799)  Proposed plants carry out a process that uses sunlight to split carbon dioxide into carbon and oxygen (O 2 gas)

F.F. Blackman (1866– 1947)  Came to the startling conclusion that photosynthesis is in fact a multistage process, only one portion of which uses light directly  Light versus dark reactions  Enzymes involved 99  Maximum rate  Temperature limited  Excess CO 2 ; 20ºC  CO 2 limited  Light Intensity (foot-candles)  500  1000  1500  2000  2500  Increased Rate of Photosynthesis 00  Excess CO 2 ; 35ºC  Insufficient CO 2 (0.01%); 20ºC  Light limited  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

C. B. van Niel (1897–1985)  Found purple sulfur bacteria do not release O 2 but accumulate sulfur  Proposed general formula for photosynthesis - CO H 2 A + light energy → (CH 2 O) + H 2 O + 2 A  Later researchers found O 2 produced comes from water Robin Hill (1899–1991)  Demonstrated Niel was right that light energy could be harvested and used in a reduction reaction

Biophysics of Light Light is a form of energy that moves through the air as oscillating electric and magnetic fields.  Photon is a particle of light; bundle of energy that is inversely proportional to the wavelength of light. - Short wavelengths contain photons of higher energy than long wavelengths.

(Biophysics of Light) When a photon strikes a molecule its energy is either lost as heat or absorbed by electrons of the molecule  Absorption Spectrum - the range and efficiency of a photons molecule is capable of absorbing.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies (Biophysics of Light) Pigments – molecules that absorb visible light  Photosynthesis uses two pigments: - Chlorophyll - Carotenoids

Two types of Chlorophyll in green plants: 1. Chlorophyll a (main pigment) Acts directly to convert sunlight to chemical energy Absorbs violet-blue and red light 2. Chlorophyll b (accessory pigment) - Increases the proportion of the photons in sunlight that plants can harvest - Absorbs light wavelength that chlorophyll a does not absorb

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.  Achieves 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.

 porphyrin ring – Complex ring structure with alternating double and single bonds – Magnesium ion at the center of the ring  Photons excite electrons in the ring  Electrons are shuttled away from the ring Structure of chlorophyll

Three Stages of Photosynthesis:  Capture energy from sunlight.  Use energy to make ATP and NADPH.  Use ATP and NADPH to fix carbon. ATP (adenosine triphosphate) – energy is stored in bonds between phosphate groups. NADP + (nicotineamide adenine dinucleotide phosphate) – temporarily stores energized electrons; proton carrier NADP + → NADPH (oxidized)(reduced)

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

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.

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.

Photosystem Function Plants Use Two Photosystems  Photosystem I (P 700 ) – reaction center chlorophyll; absorbs light having  ג= 700nm  Photosystem II (P 680 ) uses another arrangement of chlorophyll a to absorb more shorter wavelength, high energy photons. - Enhancement Effect Working together, the two photosystems carry out a noncyclic transfer of electrons that is used to generate both ATP and NADPH

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.  Photosystem II acts first. - High energy electrons generated by photosystem II used to synthesize ATP, and then passed to photosystem I to drive NADPH production.

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

1. Photosystem I transfers electrons ultimately to NADP +, producing NADPH 2. Electrons lost from photosystem I are replaced by electrons from photosystem II 3. Photosystem II oxidizes water to replace the electrons transferred to photosystem I 4. Photosystem I and II in series produce ATP and NADPH 5. Photosystems replenished w/electrons obtained by splitting water 6. 2 photosystems connected by cytochrome/ b 6 -f complex Summary of Light Reactions

Chemiosmosis Electrochemical gradient can be used to synthesize ATP Chloroplast has ATP synthase enzymes in the thylakoid membrane  Allows protons back into stroma Stroma also contains enzymes that catalyze the reactions of carbon fixation – the Calvin cycle reactions

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Calvin Cycle Phase I – Carbon Fixation Calvin Cycle begins when CO 2 binds RuBP to form PGA. Also called C 3 photosynthesis. Carbon fixation is made possible due to the attachment of CO 2 to ribulose 1, 5- bisphosphate (RuBP); rubisco.  6C molecule is unstable and forms two three-carbon molecules of phosphoglycerate (PGA or 3PGA).

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

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Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Summary of Calvin Cycle Phase II- CO 2 reduction (1) PGA forms (2) G3P’s We need to go through 3 cycles to continue the production of carbohydrates, as a result we need 3 CO 2 to form (6) G3P’s of these:  (1 C) of the G3P’s (precursor of a sugar molecule) goes to form Carbohydrates  (5 C) of the G3P’s goes to reforming rubisco  Cycle uses (6) ATP and (6) NADPH to generate (6) G3P and the cycle uses (3) ATP to regenerate rubisco.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Photorespiration Photorespiration incorporates O 2 into ribulose1,5-bisphosphate and releases CO 2.  Under normal conditions, 20% of photosynthetically- fixed carbon is lost to photorespiration. - Loss rises as temperature increases. C 4 photosynthesis produces a four carbon compound which does not go through photorespiration.

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

Decreasing Photorespiration C 4 plants conduct photosynthesis in mesophyll cells and the Calvin Cycle in bundle sheath cells.  Creates high local levels of CO 2.  Examples: corn, sugarcane, grasses Crassulacean Acid Metabolism (CAM) plants open stomata during the night and close them during the day to minimize water loss.  Use C 4 during the night and C3 during the day.  Examples: Pineapples, cacti, succulent plants  Used to reduce photorespiration

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

Review Chloroplasts Biophysics of Light Chlorophylls and Carotenoids Photosystems Calvin Cycle Photorespiration

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