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Pearson BiologyChapter 8 Class Notes

PhotosynthesisPhotosynthesis

Chemical Energy and ATPChemical Energy and ATP……Why is ATP useful to cells?

• Energy is the Ability to do Work.• Cells use Adenosine Triphosphate (ATP) to Store

and Release Energy for short periods of time.• ATP consists of :

• Energy is Stored between its Phosphate Groups. • Energy is Released by Breaking the Bonds

between the 2nd and 3rd Phosphate Groups (This releases a pair of electrons)

ATPATPATPATPStoring and Storing and Releasing Releasing EnergyEnergy• Adenosine Diphosphate (ADP)

• Has two phosphate groups • Contains some energy, but not as

much as ATP.• Like a Rechargeable Battery• ATP can Release the Energy by• ATP can Release the Energy by

breaking the bonds between the 2nd and 3rd Phosphate Groups.

Using Biochemical EnergyUsing Biochemical Energy• ATP is not good for storing

energy over the long term.• Glucose carries 90 times the

energy it takes to put a single Phosphate Group on to ATP

• Cells stay more efficient byCells stay more efficient by keeping only a small supply of ATP on hand.

• Cells can regenerate ATP from ADP as needed by using the energy in foods like glucose.

Cells Use the Biochemical Energy in ATP:Cells Use the Biochemical Energy in ATP:• Keep Homeostasis• Grow and develop • Movement of Cilia and Flagella• Move materials around (Active Transport)• Responses to chemical signals and environmental changes

• Build new molecules….. Synthesis of proteins

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HeterotrophsHeterotrophs and and AutotrophsAutotrophs…What happens during the process of Photosynthesis?

• Autotrophs Convert the Energy of Sunlight into Chemical Energy …. ….. and Store it in the Bonds of Carbohydrates

Think About It• How would you design a system to capture the energy

of sunlight and convert it into a useful form?

• Photosynthetic organisms capture energy from sunlight with Pigments.

Photosynthesis Reaction Photosynthesis Reaction Formula:Formula:

6 CO2 + 6 H2O + C6H12O6 + 6 O2

CO2 = Carbon DioxideH2O = WaterC6H12O6 = GlucoseO2 = Oxygen Gas

SUNLIGHTSUNLIGHT

COCO22 + + HH22OO + + CC66HH1212OO66 + + OO22

ChlorophyllChlorophyll

Sun’s SpectrumSun’s Spectrum• Energy from the sun travels to Earth by Wavelengths• Some are Visible… Some are Not• Objects Reflect, Transmit, or Absorb these Wavelengths

• When we pass white light through a PPRRIISSMM… it separates into its component colors

• This array of colors that are visible to our eyes is called the Visible SpectrumVisible Spectrum...It Ranges from VIOLETVIOLET to REDRED(nanometers = nm) 400 nm 700 nm

“White Light”“White Light”

400 nm 700 nmVIOLETVIOLET REDRED

PigmentsPigments = Compounds that absorb light waves• The light wave that a pigment does not absorb is

reflected. This is the color that we see.• Major Plant Pigments = Chlorophyll a

Chlorophyll b• Light is absorbed in blue-violet and red regions of the

i ibl t b t t i th ivisible spectrum, but not in the green region• This is why leaves are Green

Absorbed Reflected AbsorbedAbsorbed Reflected Absorbed

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Pigments Pigments (Continued)(Continued)• Plants also contain Red and Orange Pigments

(like Carotene) that absorb light in other regions of the spectrum.

FYI - The Green of the Chlorophyll overwhelms the other pigments… as temperatures drop … chlorophyll molecules break down…. the red and orange pigments may be seen…

Chloroplasts• Chloroplasts contain Interconnected Saclike

Photosynthetic Membranes called Thylakoids, arranged in stacks known as Grana, surrounded by Stroma

• Pigments are located in the Thylakoid Membrane

Chloroplast AnatomyChloroplast Anatomy

= Organelles found in plant and algae cells• Surrounded by a pair of Outer and Inner

membranes Outer Membrane

Inner Membrane

Thylakoids= System of membranes inside the inner membrane

• Appear as flattened stacksGrana (singular = Granum)

= Connected stacks of Thylakoids

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Stroma = Solution surrounding the Grana

Energy Collection• Compounds that absorbs light … absorb energy.

….Chlorophyll easily absorbs visible light… ….This light energy is transferred to electrons… These high-energy electrons make photosynthesis work.

Electron Carrier Compound Electron Carrier Compound accepts a Pair of High-Energy Electrons from Chlorophyll and Transfers them (along with most of their energy) to anotherthem (along with most of their energy) to another Molecule

High-Energy ElectronsFYI - High-energy Electron are like hot potatoes… If you

wanted to move a potato from one place to another, you would use an oven mitt—a carrier—to transport it.

• Plants use Electron Carriers to Transport High-energy Electrons from Chlorophyll to other molecules.

NADPNADP++ (Nicotinamide Adenine Dinucleotide Phosphate) = a carrier molecule that accepts and holds two high-= a carrier molecule that accepts and holds two high-

energy electrons, along with a hydrogen ion (H+). …… it is converted into NADPH.

Photosynthesis Reaction Photosynthesis Reaction Formula:Formula:

6 6 COCO22 + + 6 6 HH22O + CO + C66HH1212OO66 + + 6 6 OO2 2

COCO22 = Carbon Dioxide= Carbon DioxideHH22OO = Water= WaterCC66HH1212OO66 = Glucose= GlucoseOO22 = Oxygen Gas= Oxygen Gas

Photosynthesis involves two sets of reactions:Photosynthesis involves two sets of reactions:Stage 1 = LightStage 1 = Light--Dependent Dependent { { directly involve sunlight }…because they require the direct involvement of light

and light-absorbing pigments.• Use energy from Sunlight to Convert ADP and NADP+

into the energy carriers ATP and NADPH • Water is required as a source of electrons andWater is required as a source of electrons and

hydrogen ions. Oxygen is released as a byproduct.• Reactions occur within the Thylakoid Membranes

Stage 1 of Photosynthesis Stage 1 of Photosynthesis = Light-Dependent Reactions • Occur within the Thylakoid Membrane

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LightLight--Dependent Reactions: Generating ATP and NADPHDependent Reactions: Generating ATP and NADPHPhotosystemsPhotosystems { II and I }• Clusters of chlorophyll and proteins within the

thylakoid that absorb sunlight and generate high-energy electrons that are then passed to a series of electron carriers embedded in the thylakoid membrane.

• Photosystem II comes before Photosystem I

Photosystem IIPhotosystem II•• Light energy is absorbed Light energy is absorbed

by electrons in pigments in by electrons in pigments in photosystemphotosystem II…II…

•• These HighThese High--energy energy electrons are passed to the electrons are passed to the Electron Transport ChainETC = a series of electron ETC = a series of electron carriers that move the carriers that move the highhigh--energy electrons energy electrons during ATPduring ATP--generating generating reactions.reactions.

Photosystem II Photosystem II (Continued)(Continued)

• New electrons come from the breakdown of Water to replace the ones that left chlorophyll and travelled into the ETC

• Enzymes of the innerEnzymes of the inner surface of the Thylakoid break up each water into 2 electrons, 2 H+ ions, and 1 oxygen atom

2H2O 4e- + 4H+ + O2

Photosystem II Photosystem II (Continued(Continued))

• The 2 electrons replaced the high-energy electrons that were lost to the electron transport chain.

• Oxygen gas is released into the air… This reaction is the source of most of the oxygen in Earth’s atmosphere.

• The H+ ions are released into the space inside the Thylakoid

Electron Transport ChainElectron Transport Chain• Energy from electrons

travelling through the ETC is used by the proteins in the electron transport chain to pump additional H+ ions from th St i t ththe Stroma into the Thylakoid space.

Electron Transport Chain Electron Transport Chain (Continued)(Continued)• At the end of the electron

transport chain, the electrons pass to photosystem I.

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Photosystem IPhotosystem I• Because energy was used

to pump H+ ions across the thylakoid membrane … electrons do not contain as much energy when they reach the end

f th ETC t thof the ETC at the beginning of Photosystem I.

• Pigments in the Photosystem I use energy from light to re-energize these electrons.

Photosystem I Photosystem I (Continued)(Continued)

• At the end of a short second ETC… the NADP+ (in the stroma) picks up the high-energy electrons and H+ ions …. And becomes NADPH.

Hydrogen Ion Movement and ATP FormationHydrogen Ion Movement and ATP Formation• H+ ions accumulate in the thylakoid space from:

the splitting of water …. and …. from being pumped in from the stroma.

Hydrogen Ion Movement and ATP Formation Hydrogen Ion Movement and ATP Formation (continued)(continued)

• The buildup of H+ ions makes the stroma negatively charged relative to the space within the thylakoids.

• This ‘gradient’….. the difference in both charge and H+ ion concentration across the membrane …. provides the energy to make ATP.

• The thylakoid membrane contains a protein enzyme called ATP Synthase ATP Synthase that allows H+ ions to pass through it

Hydrogen Ion Movement and ATP Formation Hydrogen Ion Movement and ATP Formation (continued)(continued)

• Powered by the gradient…. H+ ions pass through the ATP Synthase ATP Synthase and force it to rotate.

• As it rotates…. ATP Synthase binds ADP and a phosphate group to produce ATP

ADP + P group ATP

Hydrogen Ion Movement and ATP Formation Hydrogen Ion Movement and ATP Formation (continued)(continued)

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= ChemiosmosisChemiosmosis• Enables light-dependent electron transport to

produce not only NADPH (at the end of the electron transport chain), but ATP as well.

Hydrogen Ion Movement and ATP Formation Hydrogen Ion Movement and ATP Formation (continued)(continued) Summary of LightSummary of Light--Dependent ReactionsDependent Reactions• The light-dependent reactions produce oxygen gas

and convert ADP and NADP+ into the energy carriers ATP and NADPH.

During Stage 2 = LightDuring Stage 2 = Light--Independent Independent • ATP and NADPH provide the energy needed to build

high-energy sugars from low-energy carbon dioxide.high energy sugars from low energy carbon dioxide.

Stage 2 = LightStage 2 = Light--Independent Independent { { No light is required } •• OcccurOcccur in the Stroma of the Chloroplast• Commonly referred to as the Calvin cycle• ATP and NADPH molecules produced in the light-

dependent reactions are used to produce high-energy sugars from carbon dioxide

The LightThe Light--Independent Reactions: Producing SugarsIndependent Reactions: Producing Sugars

g• These carbohydrate compounds can be stored for a

long time.

Carbon Dioxide ( Carbon Dioxide ( COCO22 ) Enters the Calvin Cycle) Enters the Calvin Cycle• An enzyme in the stroma of the chloroplast combines

CO2 molecules with 5-carbon compounds that are already present in the organelle… producing 3-carbon compounds that continue in the cycle.

• For every 6 CO2 molecules that enter the cycle, a total of twelve 3-carbon compounds are produced.

6 CO + 6 (5 C) 12 (3 C)6 CO2 + 6 (5-C) 12 (3-C)

• Other enzymes in the chloroplast then convert the 3-carbon compounds into higher-energy forms the rest of the cycle, using

Carbon Dioxide ( Carbon Dioxide ( COCO22 ) Enters the Calvin Cycle) Enters the Calvin Cycle

energy from ATP and high-energy electrons from NADPH.

Sugar ProductionSugar Production

• At midcycle, two of the twelve 3-carbon molecules are removed from the cycle.

• These molecules become the building blocks that the plant cell uses to produce sugars, lipids, amino acids, and other compounds.

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Sugar Production Sugar Production (continued)(continued)

• The remaining ten 3-carbon molecules are converted back into six 5-carbon molecules that combine with six new carbon dioxide molecules to begin the next cycle.

Summary of the Calvin CycleSummary of the Calvin Cycle

• The Calvin cycle uses 6 molecules of carbon dioxide to produce a single 6-carbon sugar molecule.

Summary of the Calvin Cycle Summary of the Calvin Cycle (continued)(continued)

• The energy for the reactions is supplied by compounds produced in the light-dependent reactions.

Summary of the Calvin Cycle Summary of the Calvin Cycle (continued)(continued)

• The plant uses the sugars produced by the Calvin cycle to meet its energy needs and to build macromolecules needed for growth and development.

• When other organisms eat plants, they can use the energy and raw materials stored in these compounds.

The End ResultsThe End Results• The two sets of photosynthetic reactions work

together—the light-dependent reactions trap the energy of sunlight in chemical form, and the light-independent reactions use that chemical energy to produce stable, high-energy sugars from carbon dioxide and water.

• In the process, animals, including humans, get food and an atmosphere filled with oxygen.

Factors Affecting Photosynthesis:Factors Affecting Photosynthesis:• Temperature• Light Intensity • Availability of Water

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TemperatureTemperature• The reactions of photosynthesis are made possible by

enzymes that function best between 0°C and 35°C.• Temperatures above or below this range may affect

those enzymes, slowing down the rate of photosynthesis or stopping it entirely.

LightLight• High light intensity increases the rate of

photosynthesis.• After the light intensity reaches a certain level,

however, the plant reaches its maximum rate of photosynthesis, as is seen in the graph.

WaterWater• Because water is one of the raw materials in

photosynthesis, a shortage of water can slow or even stop photosynthesis.

• Water loss can also damage plant tissues.

• Plants that live in dry conditions often have waxy coatings on their leaves to reduce water loss. They may also have biochemical adaptations that make photosynthesis more efficient under dry conditions.

Photosynthesis Under Extreme ConditionsPhotosynthesis Under Extreme Conditions

• In order to conserve water, most plants under bright, hot conditions close the small openings in their leaves that normally admit carbon dioxide.

• This causes carbon dioxide within the leaves to fall toThis causes carbon dioxide within the leaves to fall to very low levels, slowing down or even stopping photosynthesis.

• C4 and CAM plants have biochemical adaptations that minimize water loss while still allowing photosynthesis to take place in intense sunlight.

C4 PhotosynthesisC4 Photosynthesis• C4 plants have a specialized chemical pathway that

allows them to capture even very low levels of carbon dioxide and pass it to the Calvin cycle.

• The name “C4 plant” comes from the fact that the first compound formed in this pathway contains 4 carbon atomscarbon atoms.

• The C4 pathway requires extra energy in the form of ATP to function.

• C4 organisms include crop plants like corn, sugar cane, and sorghum.

CAM PlantsCAM Plants• Members of the Crassulacae family, such as cacti and

succulents, incorporate carbon dioxide into organic acids during photosynthesis in a process called Crassulacean Acid Metabolism (CAM).

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CAM Plants• CAM plants admit air into their leaves only at night,

where carbon dioxide is combined with existing molecules to produce organic acids, “trapping” the carbon within the leaves.

• During the daytime, when leaves are tightly sealed to prevent water loss, these compounds release carbon dioxide, enabling carbohydrate production.

• CAM plants include pineapple trees, many desert cacti, and “ice plants”.