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Photosynthesis

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THE LIGHT REACTIONS

All organisms use energy to carry out the functions of life. Where does this energy come from? Directly or indirectly almost all of the energy in living systems comes from the sun. Energy from the sun enters living systems one plants, algae, some unicellular protists, and some Prokaryotes absorb some light and use it to make organic compounds.

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Obtaining energy

• Organisms classified according to how they get energy • Autotrophs – use energy from sun or chemical bonds inorganic compounds to make organic compounds • Most use photosynthesis

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• Heterotrophs – must get energy from food

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Energy for life processes

• Photosynthesis is a series of chemical reactions –Products from one is used as reactants in the next • Biochemical pathway - a series of chemical reactions that are linked

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Overview of photosynthesis

• Autotrophs use photosynthesis to make organic compounds from carbon dioxide and water • Oxygen and some organic compounds made are used by cells by a process called cellular respiration

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Photosynthesis has two stages

• Light reactions • Light is converted to Chemical Energy • Temporarily stored in ATP and molecule NADPH

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Calvin cycle Organic compounds formed using CO2 And energy stored in ATP and NADPH

Capturing light energy

• First stage includes light reactions • Inside chloroplast are thylakoids arranged as flattened sacs • Thylakoids layered to form stacks called grana • Solution surrounding grana is called stroma 8

Light and Pigments

• Light from sun looks white but is actually colors –ROYGBIV –Visible spectrum

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• Light is measured in wavelengths –Different colors of light have different wavelengths

• Pigment - substance that absorbs light

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Chloroplast Pigments

• Chlorophyll • Type a - absorbs more red and less blue light • Type b - absorbs more blue and less red • Both reflect green light

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• Accessory pigment - chlorophyll b ASSISTS chlorophyll a • Carotenoids - yellow, orange, brown

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Electron transport

• Chlorophylls and carotenoids grouped in clusters in thylakoid membrane • Each cluster of pigments is called a photosystem • Two types are known: photosystem I and photosystem II

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• each has reaction center containing chlorophyll a • photosystem I (PSI) is known as P-700 • photosystem II (PSII) is P-680 • based on wavelength of light each best absorbs

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• Light reactions begin when light hits the pigments • energy from light is passed from molecule to molecule until it reaches the reaction center

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Photosystem II

• 1Light hits PSII and excites electron in P680 • Electrons leave photosystem II and goes to primary electron acceptor (pea) • PEA sends electrons through electron transport chain

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Photosystem I

• At the same time that light hits system II, it hits system I and electrons leave –Go to PEA, and through ETC

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• 4. Electrons from photostyem II now replace the electrons that just left photosystem I

• 5. Electrons from photosystem I combine with NADP+, an organic molecule that accepts electrons –Also combines with H+, forming NADPH 19

Restoring photosystem II

• Electrons from photosystem II replace electrons that left photosystem I, but what replaces the electrons that left photosystem II? • 2H2O —> 4H+ + 4e- + O2

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RESTORING PHOTOSYSTEM II - PHOTOLYSIS

Chemiosmosis

• Synthesis of ATP in light reactions • Relies on concentration gradient of protons (H+) across thylakoid membrane

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• Enzyme (ATP synthase) makes ATP by adding phosphate group to ADP –Energy that drives reaction fed by movement of protons across membrane

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THE CALVIN CYCLE

This pathway produces organic compounds, using the energy stored in ATP and NADPH during the light reactions The Calvin cycle is named after Melvin Calvin (1911-1997), the American scientist who worked out the details of the pathway

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The Calvin cycle

• Happens in stroma of chloroplast • Produces organic compounds, using energy stored in ATP and NADPH made during light reactions • Incorporates CO2 into organic compounds –carbon dioxide fixation

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• Step 1. • CO2 diffuses into stroma • Enzyme combines CO2 with 5-carbon molecule called RuBP (ribulose 1,5-biphosphate)

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• Product = 6-C molecule • Immediately splits into 2 3-C molecules (3PG, 3-phosphoglycerate)

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• Step 2. • 3-PGA converted into a new 3-C compound (G3P, glyceraldehyde 3-phosphate) in 2 steps

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• Each 3PG receives phosphate from ATP –P-C-C-C-P • Then each receives proton from NADPH, and releases phosphate –P-C-C-C —> G3P

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• These reactions produce ADP, NADP+, and phosphate –Used again in light reactions

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• Step 3. • Most of G3P converted back to RuBP –Requires phosphate from another ATP

• Some G3P molecules NOT converted –Leave Calvin cycle and used by plant to make other organic compounds

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Balance sheet for photosynthesis

• How much ATP and NADPH are required to make one molecule of 3GP from CO2? • Each turn of cycle fixes ONE CO2

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• Since G3P is 3-C, it takes 3 turns of cycle to make 1 • 3 turns gives 1 G3P and 5 3-C compounds which rearrange to form RuBP again

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• Each turn = 2 ATP and 2 NADPH used in step 2, 1 more ATP in step 3

• SO….3 turns of Calvin cycle (1 3GP) = 9 ATP, 6 NADPH

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Alternative pathways

• Calvin cycle most common form of carbon fixation –C3 plants (produce 3-C compounds) • Other plants fix carbon a different way, then send to Calvin cycle

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–Generally found in plants that evolved in hot, dry climates • Rapidly lose water through stomata - holes in leaves • Controls exchange between oxygen and carbon dioxide

–Stomata close to conserve water loss • CO2 decreases, O2 increases inside the plant • Inhibits Calvin cycle

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The C4 pathway

• Allows plants to fix CO2 into 4-C molecules • During hottest part of day, stomata are partially closed –Enzyme can fix CO2 into 4-C compounds even when CO2 is low and O2 is high –Compounds then transported to Calvin cycle

–Corn, sugar cane, crabgrass36

The CAM pathway

• Open stomata at night and close during the day • At night, CAM plants take in CO2 and fix into different organic compounds • During the day, CO2 released from compounds and enter Calvin cycle

–Cactuses, pineapple37

Summary of Photosynthesis

• Happens in two stages in chloroplasts • Light reactions – energy absorbed from sunlight and converted into chemical energy temporarily stored in ATP and NADPH • Calvin cycle – CO2 and chemical energy in ATP and NADPH used to form organic compounds

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• Photosynthesis is an ongoing cycle • Products of light reactions are used in Calvin cycle • Some products of Calvin cycle used in light reactions

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• Other products used to make organic compounds (amino acids, lipids, carbohydrates) • Extra carbs can be stored as starch in chloroplasts and roots and fruits • Provide chemical energy that autotrophs and heterotrophs depend on

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• This figure shows how light reactions and Calvin cycle work as a continuous cycle • This happens in every one of the thousands of chloroplasts in a plant

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• Remember water is split during light reactions • Yields electrons, protons (H+) and oxygen as byproduct • Simplest equation for photosynthesis

• (CH2O) represent general formula for carb42

• Light reactions are light dependent reactions • Depend on presence of sunlight • Calvin cycle is light independent (dark) reactions • Does not depend on light • Often happens during the day when the light reactions are producing materials for the Calvin cycle

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Rate of photosynthesis

• THREE THINGS IN THE PLANT'S ENVIRONMENT AFFECT THE RATE OF PHOTOSYNTHESIS: • 1. LIGHT INTENSITY • 2. CO2 LEVELS • 3. TEMPERATURE

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Light Intensity

• Rate of photosynthesis increases as light intensity increases • Higher light excites more electrons in photosystems • At some point all available electrons are excited • Maximum rate

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CO2 Levels

• Increasing levels stimulate photosynthesis until maximum is reached

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Temperature

• Increasing temp accelerates chemical reactions • Rate peaks at certain temp • Many enzymes that catalyze reactions become ineffective • Stomata start to close to limit water loss

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ADAPTATIONS OF PLANTS FOR PHOTOSYNTHESIS

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Classification of plant types

• Plants have adapted efficient transport systems • Also have adaptations to conserve water

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• Many habitats found in nature with respect to water supply • Xeric, mesic and hydric habitats

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xerophytes

• Plants adapted to living in very dry habitats • Found in desert • Have specialized tissue for storing water

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• Waxy coating on leaves and stems helps reduce water loss • Slower growth rate to conserve energy

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mesophytes

• Plants growing in moderate water habitats • Most common plant type

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• Have well-developed root system and vascular tissue • Most crop plants, grasses, broad-leaf and trees in temperate climates are mesophytes

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hydrophytes

• Plants that grow wholly or partially submerged in water • Aquatic plants

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• Less need to conserve water • Have reduced cuticle and fewer stomata

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