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S E C TI O N 3 : TH E P R O C E S S O F P H O TO S Y N TH E S I S

Chapter 8: Photosynthesis

Electron Transport Chain in Photosynthesis

� A photosystem: is a cluster of pigment molecules.

� Two photosystems are known.¡ Photosystem II¡ Photosystem I

Light Reactions� Light Reactions – called the light reactions because

they require light (Light Dependent Reactions)÷the light reactions take place inside the

thylakoid÷the light reactions require water to occur and

give off O2

÷the main purpose of the light reactions is to create ATP and NADPH

Steps of Electron Transport chain(Light Reaction)

1. Light strikes Photosystem II and the energy is passed along to pigments.

2. Electrons in the pigments get “excited” and are passed down an electron transport chain (ETC).

¡ What happens to replace all those electrons?÷ Water (H2O) from the roots is split into O2, H+, and e-

¢ Water is what oxygen, the byproduct, is made from during photosynthesis

÷ The electrons go into PhotosystemII.

÷ The oxygen is given off as waste; major atmospheric byproduct

Steps of ETC (Light Reaction)

3. The electron transport chain provides energy to turn ADP into ATP (chemical energy).

¡ The energy in the electrons is now all used up.

÷ We need more light to re-power electrons!

÷ We are about to head into Photosystem I.

Steps of ETC (Light Reaction)

4. Additional light strikes photosystem I.

¡ This “re-excites” electrons again.

¡ The electrons are sent down another electron transport chain.

¡ This ETC provides energy to turn NADP+ to NADPH (another type of chemical energy).

5. The ATP from photosystemII and NADPH from photosystem I are sent to the Calvin Cycle.

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Photosystems

� Why does Photosystem II occur before Photosystem I???

¡ The photosystems are named for the dates of discovery, not for which one goes first or second.

Photosystems

� Where do the electrons that enter PhotosystemII come from?

¡ They come from chemical reactions with splitting water.

Photosystems

� What happens if there’s no water to replace the electrons?

¡ The photosystems will shut down, photosynthesis will stop, and the plant will die.

Light Reaction

� http://my.hrw.com/sh2/sh07_10/student/flash/visual_concepts/60097.htm

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Dark Reactions

� The “dark reactions” (Light Independent Reactions).¡ Also known as the Calvin Cycle¡ This is because light is not needed for energy.

¡ The energy for this cycle comes from ATP and NADPH from the photosystems (light reactions).

¡ These reactions occur in the stroma (fluid in chloroplasts outside of the thylakoids).

Dark Reactions� Light Independent Reaction (Dark Reactions) ¡Steps of the Calvin cycle

1. CO2 starts the cycle by attaching itself to a molecule called rubisco

2. ATP and NADPH (made in the light reactions) are used to make glucose (or sugar)

� http://my.hrw.com/sh2/sh07_10/student/flash/visual_concepts/60100.htm

Photosynthesis Products

6CO2 + 6H2O + light energy = C6H12O6 (carbohydrate) + 6O2

carbon dioxide + water + light in,

sugar + oxygen out

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Alternative Pathways

� The Calvin Cycle is the most common way for plants to fix carbon (make food).

� Plants that do this are called C3 plants.

Alternative Pathways

� Some plants use a similar process, called the C4 pathway.

� This alternative process is a method for plants in hot environments to conserve water.¡ Stomata are small pores in leaves where plants exchange O2 and CO2.

¡ These plants leave their stomata partially closed, so they have less CO2 but also lose less water.

oThey have a special enzyme to deal with the lower amount of CO2.

oExamples are corn, sugar cane, and crabgrass.

Alternative Pathways

� Some plants use a different process called the CAM Pathway.

� This process is another method for plants to conserve water.

¡ Stomata are open at night and closed during the day (the opposite of most plants).

¡ Because of this, these plants grow slowly.

¡ Examples are cactuses and pineapples.

Rate of Photosynthesis

� Environmental factors can speed up or slow down the rate of photosynthesis.¡ Light

÷ Increases in light speed up photosynthesis.

÷ This only occurs up to a point.

÷ Only so many electrons can be excited by light.

÷ Once they’re all busy, increased light will have no effect

Rate of Photosynthesis

¡ CO2

÷ Increases in CO2 speed up photosynthesis.

÷ This only occurs up to a point.

÷ Only so much carbon can be fixed.

÷ Once carbon cycles are maxed, increases in CO2 will have no effect.

Rate of Photosynthesis

¡ Temperature

÷ Increases in temperature speed up photosynthesis (up to a point).

÷ If temperatures get too hot, the stomata close and less CO2 is available.

÷ High temperatures can destroy needed organic molecules and shut down photosynthesis.

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Identify the role of each substance in photosynthesis

Substance FunctionLightCarbon DioxideWaterATP & NADPHPigments

Identify the role of each substance in photosynthesis

Substance FunctionLight Excite/Energize ElectronsCarbon DioxideWaterATP & NADPHPigments

Identify the role of each substance in photosynthesis

Substance FunctionLight Excite/Energize ElectronsCarbon Dioxide Used to produce sugar=glucoseWaterATP & NADPHPigments

Identify the role of each substance in photosynthesis

Substance FunctionLight Excite/Energize ElectronsCarbon Dioxide Used to produce sugar=glucoseWater Split to make O2ATP & NADPHPigments

Identify the role of each substance in photosynthesis

Substance FunctionLight Excite/Energize ElectronsCarbon Dioxide Used to produce sugar=glucoseWater Split to make O2ATP & NADPH Store chemical energy & used in the

Calvin CyclePigments

Identify the role of each substance in photosynthesis

Substance FunctionLight Excite/Energize ElectronsCarbon Dioxide Used to produce sugar=glucoseWater Split to make O2ATP & NADPH Store chemical energy & used in the

Calvin CyclePigments Absorb light

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� Summarize the three steps in the electron transport chain that produces ATP.¡ First, water is split by an enzyme and chlorophyll molecules

take the electrons from the hydrogen leaving hydrogen ions. ¡ Second, using energy from excited electrons, a protein in the

thylakoid membrane pumps hydrogen ions into the thylakoid, creating a concentration gradient across the membrane.

¡ Finally, energy from the diffusion of hydrogen ions through ATP synthase is used to make ATP from ADP.

� Summarize the two steps in the electron transport chain producing NADPH.¡ Excited electrons are replaced by de-energized electrons from

a chlorophyll molecule. ¡ The excited electrons combine with hydrogen ions and an

electron acceptor called NADP+ to form NADPH, a substance that stores energy.

What are the reactants of the light reactions?

� Sunlight� water

What are the products of the light reactions?

� ATP� NADPH� O2 = Oxygen

What is the reactant of the dark reactions?

� CO2

� ATP� NADPH

What is the product of the dark reactions?

� Glucose

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� Describe the 4 steps of the Calvin Cycle¡ Carbon dioxide from the air enters the chloroplast; an enzyme

adds it to a five-carbon sugar to make a six-carbon sugar. ¡ Three of these are produced Each of these splits into two three-

carbon sugars. ¡ To these are added phosphate groups from ATP and electrons

from NADPH. ¡ One of the three-carbon sugar molecules leaves the Calvin

cycle and is used to make glucose or other energy-storing organic compounds.

¡ The other three-carbon sugar molecules are used to regenerate the five-carbon starting compound and continue the Calvin cycle.