AP BiologyAP Biology

Ch. 10Ch. 10

PhotosynthesisPhotosynthesis

PhotosynthesisPhotosynthesis

Transforms solar energy trapped by Transforms solar energy trapped by chloroplasts into chemical bond energy stored chloroplasts into chemical bond energy stored in sugar and other organic molecules.in sugar and other organic molecules.

Directly or indirectly supplies energy to most Directly or indirectly supplies energy to most living organismsliving organisms

Uses energy-poor molecules, COUses energy-poor molecules, CO22 and H and H22O to O to make energy-rich molecules such as glucosemake energy-rich molecules such as glucose

Autotrophs (Producers)Autotrophs (Producers) Organisms get organic molecules used for Organisms get organic molecules used for

energy by one of two ways: energy by one of two ways: autotrophicautotrophic nutrition or nutrition or heterotrophicheterotrophic nutrition. nutrition.

AutotrophsAutotrophs synthesize organic molecules from synthesize organic molecules from inorganic raw materials.inorganic raw materials.

PhotoautotrophsPhotoautotrophs use light as an energy use light as an energy source to make food. Ex) plants, algae, and source to make food. Ex) plants, algae, and some prokaryotessome prokaryotes

ChemoautotrophsChemoautotrophs use the oxidation of use the oxidation of inorganic chemicals, such as sulfur or inorganic chemicals, such as sulfur or ammonia, to make organic molecules. Ex) ammonia, to make organic molecules. Ex) some bacteria living on the ocean floor near some bacteria living on the ocean floor near volcanic vents.volcanic vents.

Purple Sulfur Bacteria

Heterotrophs Heterotrophs (Consumers)(Consumers)

Nutrition is acquired from organic Nutrition is acquired from organic molecules produced by other organismsmolecules produced by other organisms

Examples are animals that eat plants Examples are animals that eat plants (herbivores), other animals (carnivores), (herbivores), other animals (carnivores), or both (omnivores).or both (omnivores).

Some heterotrophs decompose and feed Some heterotrophs decompose and feed on organic litter (detritivores)on organic litter (detritivores)

ChloroplastsChloroplasts Site of photosynthesis in plants, mainly in the Site of photosynthesis in plants, mainly in the

leavesleaves ChlorophyllChlorophyll is the green pigment in is the green pigment in

chloroplasts that gives a leaf its color, and that chloroplasts that gives a leaf its color, and that absorbs light energy used to drive absorbs light energy used to drive photosynthesis.photosynthesis.

Chloroplasts are concentrated in cells of Chloroplasts are concentrated in cells of mesophyll,mesophyll, green tissue in the leaf’s interior. green tissue in the leaf’s interior.

Carbon dioxide enters and leaves the leaf Carbon dioxide enters and leaves the leaf through through stomatastomata

Water absorbed by roots is transported to the Water absorbed by roots is transported to the leaves through leaves through vascular bundlesvascular bundles..

Chloroplasts: structureChloroplasts: structure

Divided into three functional compartments by Divided into three functional compartments by a system of membranes:a system of membranes:

1) 1) Intermembrane space-Intermembrane space- separates the two separates the two layers of the double membrane surrounding layers of the double membrane surrounding the chloroplast.the chloroplast.

2) 2) Thylakoid space-Thylakoid space- thylakoids are stacks of thylakoids are stacks of membranes within the chloroplast, contain membranes within the chloroplast, contain chlorophyll, location of light reactionschlorophyll, location of light reactions

3) 3) StromaStroma- viscous fluid outside the - viscous fluid outside the thylakoids, location of the Calvin cycle thylakoids, location of the Calvin cycle (converts CO(converts CO22 to sugar) to sugar)

Tracing atoms through photosynthesis

Overview of Photosynthesis: cooperation of light reactions and Calvin cycle

Pathways of Pathways of Photosynthesis: Light Photosynthesis: Light reactionsreactions

Convert light energy to chemical bond energy Convert light energy to chemical bond energy in ATP and NADPH (NADP+ is reduced).in ATP and NADPH (NADP+ is reduced).

Occurs in the thylakoid membranes of Occurs in the thylakoid membranes of chloroplastschloroplasts

Oxygen given off as by-product from the Oxygen given off as by-product from the splitting of water. H+ passed on to the Calvin splitting of water. H+ passed on to the Calvin cycle, joining COcycle, joining CO2 2 to from sugar.to from sugar.

ATP is generated by photophosphorylation.ATP is generated by photophosphorylation.

Properties of LightProperties of Light Sunlight is Sunlight is electromagneticelectromagnetic energy, energy,

having wavelike and particle-like having wavelike and particle-like properties.properties.

The The visiblevisible range range of light is the radiation of light is the radiation that drives photosynthesis.that drives photosynthesis.

BlueBlue and and redred, the two wavelengths most , the two wavelengths most effectively absorbed by chlorophyll, are effectively absorbed by chlorophyll, are the colors most useful as energy for the colors most useful as energy for photosynthesis.photosynthesis.

Photosynthetic Pigments:Photosynthetic Pigments:Light receptorsLight receptors

Light may be reflected, transmitted, or Light may be reflected, transmitted, or absorbed when is contacts matter.absorbed when is contacts matter.

Pigments are substances that absorb Pigments are substances that absorb different wavelengths of light: different wavelengths of light: wavelengths absorbed disappear, while wavelengths absorbed disappear, while the color seen is reflected.the color seen is reflected.

For example, leaves look green because For example, leaves look green because chlorophyll absorbs red and blue light, chlorophyll absorbs red and blue light, but transmits and reflects green light.but transmits and reflects green light.

Absorption vs. Action spectrumAbsorption vs. Action spectrum

Each pigment has a characteristic Each pigment has a characteristic absorption absorption spectrumspectrum, or pattern of wavelengths that it , or pattern of wavelengths that it absorbs. It is expressed as a graph of absorbs. It is expressed as a graph of absorption vs. wavelength.absorption vs. wavelength.

Absorption spectrums for a pigment are Absorption spectrums for a pigment are determined using a spectrophotometer.determined using a spectrophotometer.

Action spectrumsAction spectrums profile the relative profile the relative effectiveness of different wavelengths of visible effectiveness of different wavelengths of visible light for photosynthesis. Graph is wavelength light for photosynthesis. Graph is wavelength vs. rate of photosynthesis.vs. rate of photosynthesis.

Pigments of PhotosynthesisPigments of Photosynthesis Chlorophyll a-Chlorophyll a- participates directly in the participates directly in the

light reactions, accessory pigments can light reactions, accessory pigments can absorb and transfer energy to chlorophyll absorb and transfer energy to chlorophyll aa

Accessory pigments- include chlorophyll b (yellow-green pigment), and carotenoids (yellow and orange pigments)

Accessory pigments expand the range of wavelengths of light available for photosynthesis

Photoexcitation of ChlorophyllPhotoexcitation of Chlorophyll

When pigments absorb photons, the When pigments absorb photons, the absorbed photons boost one of the absorbed photons boost one of the pigment molecule’s electrons in its lowest pigment molecule’s electrons in its lowest energy state (energy state (ground stateground state) to an orbital ) to an orbital of higher potential energy (of higher potential energy (excited stateexcited state))

The excited state is unstable, so excited The excited state is unstable, so excited electrons fall back to the ground state, electrons fall back to the ground state, releasing heat or by fluorescing.releasing heat or by fluorescing.

Photoexcitation of chlorophyll, Photoexcitation of chlorophyll, cont.cont.

Pigment molecules in the thylakoid Pigment molecules in the thylakoid membrane do not fluoresce because the membrane do not fluoresce because the primary electron acceptor molecules trap primary electron acceptor molecules trap excited state electrons that have excited state electrons that have absorbed photons.absorbed photons.

IsolatedIsolated chlorophyll fluoresces in the red chlorophyll fluoresces in the red part of the spectrum and dissipates heat.part of the spectrum and dissipates heat.

PhotosystemsPhotosystems Chlorophyll in the chloroplast is organized with Chlorophyll in the chloroplast is organized with

proteins into proteins into photosystemsphotosystems.. A photosystem has a light-gathering “antenna A photosystem has a light-gathering “antenna

complex” consisting of chlorophyll complex” consisting of chlorophyll aa, , chlorophyll chlorophyll bb, and carotenoid molecules., and carotenoid molecules.

When light energy hits the chloroplast, energy When light energy hits the chloroplast, energy is transferred to chlorophyll a in the is transferred to chlorophyll a in the reaction reaction center.center.

The solar-powered transfer of electrons The solar-powered transfer of electrons from chlorophyll a to the primary electron from chlorophyll a to the primary electron acceptor is the first step of the light acceptor is the first step of the light reactions. It is a redox reaction.reactions. It is a redox reaction.

Photosystems I and IIPhotosystems I and II The thylakoid membrane has two types The thylakoid membrane has two types

of photosystems that cooperate in the of photosystems that cooperate in the reactions of photosynthesis.reactions of photosynthesis.

Photosystem IIPhotosystem II occurs first, and has occurs first, and has chlorophyll P680 (absorbs at 680 nm chlorophyll P680 (absorbs at 680 nm wavelengths of light, red).wavelengths of light, red).

Photosystem IPhotosystem I has chlorophyll P700, has chlorophyll P700, which absorbs best at 700 nm (far red which absorbs best at 700 nm (far red part of the spectrum)part of the spectrum)

Noncyclic electron flowNoncyclic electron flow

Light drives the synthesis of NADPH and Light drives the synthesis of NADPH and ATP by energizing the two photosystems ATP by energizing the two photosystems in the thylakoid membranes of in the thylakoid membranes of chloroplasts.chloroplasts.

During photosynthesis, there are two During photosynthesis, there are two possible routes for electron flow: cyclic possible routes for electron flow: cyclic and non-cyclic electron flow (non-cyclic is and non-cyclic electron flow (non-cyclic is most common)most common)

Noncyclic electron flow, cont.Noncyclic electron flow, cont.

Photosystem II absorbs light, excites Photosystem II absorbs light, excites e-e- to a higher level, chlorophyll becomes to a higher level, chlorophyll becomes oxidizedoxidized

Water is split; oxygen is releasedWater is split; oxygen is released Each photoexcited electron passes from Each photoexcited electron passes from

photosystem II to photosystem I by an photosystem II to photosystem I by an electron transport chain. Electron electron transport chain. Electron carriers are plastoquinone (Pq) and a carriers are plastoquinone (Pq) and a cytochrome complex.cytochrome complex.

As electrons move down the chain, ATP As electrons move down the chain, ATP is formed by is formed by noncyclic noncyclic photophosphorylationphotophosphorylation..

Primary electron acceptor of Primary electron acceptor of Photosystem I passes photoexcited Photosystem I passes photoexcited electrons to a second electron transport electrons to a second electron transport chain, producing NADPH.chain, producing NADPH.

ATP and NADPH will be used in the ATP and NADPH will be used in the Calvin cycle to make sugar.Calvin cycle to make sugar.

Cyclic electron flowCyclic electron flow Under certain conditions, photoexcited Under certain conditions, photoexcited

electrons take an alternate path called cyclic electrons take an alternate path called cyclic electron flow.electron flow.

Cyclic electron flow uses Photosystem I, but Cyclic electron flow uses Photosystem I, but not Photosystem II.not Photosystem II.

ATP is produced by ATP is produced by cyclic cyclic photophosphorylationphotophosphorylation, but no oxygen or , but no oxygen or NADPH.NADPH.

Noncyclic electron flow produces ATP and Noncyclic electron flow produces ATP and NADPH in equal amounts, but the Calvin cyclic NADPH in equal amounts, but the Calvin cyclic requires more ATP.requires more ATP.

Cyclic electron flow makes up the difference.Cyclic electron flow makes up the difference.

Chemiosmosis: Chloroplasts vs. Chemiosmosis: Chloroplasts vs. Mitochondria (similarities)Mitochondria (similarities)

Chloroplasts and mitochondria both Chloroplasts and mitochondria both generate ATP by chemiosmosis.generate ATP by chemiosmosis.

Both have an electron transport chain Both have an electron transport chain assembled in a membraneassembled in a membrane

Both have an ATP synthase complex Both have an ATP synthase complex built into the same membrane that built into the same membrane that couples the diffusion of H+ down their couples the diffusion of H+ down their gradient to the phosphorylation of ADP.gradient to the phosphorylation of ADP.

The ATP synthase complexes of both The ATP synthase complexes of both organelles are similar.organelles are similar.

Chemiosmosis: Chloroplast vs. Chemiosmosis: Chloroplast vs. Mitochondria (differences)Mitochondria (differences)

Mitochondria transfer chemical energy from food Mitochondria transfer chemical energy from food molecules to ATP, while chloroplasts transform light molecules to ATP, while chloroplasts transform light energy into chemical energy.energy into chemical energy.

Spatial organization of chemiosmosis differs in Spatial organization of chemiosmosis differs in mitochondria and chloroplasts:mitochondria and chloroplasts:

The inner membrane of the mitochondrion pumps The inner membrane of the mitochondrion pumps protons from the mitochondrial matrix out to the protons from the mitochondrial matrix out to the intermembrane space, which is a reservoir of protons intermembrane space, which is a reservoir of protons that power ATP synthase.that power ATP synthase.

The thylakoid membrane of the chloroplast pumps The thylakoid membrane of the chloroplast pumps protons from the stroma into the thylakoid space. ATP protons from the stroma into the thylakoid space. ATP is produced as protons diffuse from the thylakoid is produced as protons diffuse from the thylakoid compartment back to the stroma through ATP compartment back to the stroma through ATP synthase. ATP forms in the stroma where it drives synthase. ATP forms in the stroma where it drives sugar synthesis during the Calvin cycle.sugar synthesis during the Calvin cycle.

Summary of Light ReactionsSummary of Light Reactions Noncyclic electron flow pushes electrons from Noncyclic electron flow pushes electrons from

water ( low P.E.) to NADPH (high P.E.)water ( low P.E.) to NADPH (high P.E.) Water is split by photosystem II on the side of Water is split by photosystem II on the side of

the membrane facing the thylakoid space.the membrane facing the thylakoid space. The diffusion of H+ from the thylakoid space to The diffusion of H+ from the thylakoid space to

the stroma (along the H+ gradient) powers ATP the stroma (along the H+ gradient) powers ATP synthase.synthase.

Light-driven reactions across the membrane Light-driven reactions across the membrane store chemical energy in NADPH and ATP, store chemical energy in NADPH and ATP, which shuttle energy to the Calvin cycle.which shuttle energy to the Calvin cycle.

Melvin Calvin

Calvin cycleCalvin cycle Carbon enters the Calvin cycle in the form of Carbon enters the Calvin cycle in the form of

COCO22 and leaves in the form of sugar. and leaves in the form of sugar. It consumes ATP as an energy source, and It consumes ATP as an energy source, and

NADPH as the reducing agent to add high NADPH as the reducing agent to add high energy electrons to form sugar.energy electrons to form sugar.

The Calvin cycle produces a 3-C sugar, The Calvin cycle produces a 3-C sugar, glyceraldehyde-3-phosphate (G3P). The cycle glyceraldehyde-3-phosphate (G3P). The cycle must take place 3 times, fixing three molecules must take place 3 times, fixing three molecules of COof CO2, 2, to make one molecule of G3P.to make one molecule of G3P.

Calvin cycle: Phase ICalvin cycle: Phase ICarbon fixationCarbon fixation

Each CO2 is incorporated by attaching to Each CO2 is incorporated by attaching to a 5-C sugar, ribulose bisphosphate a 5-C sugar, ribulose bisphosphate (RuBP). Rubisco is the enzyme that (RuBP). Rubisco is the enzyme that catalyzes this step.catalyzes this step.

A 6-C intermediate forms that A 6-C intermediate forms that immediately splits into two molecules of immediately splits into two molecules of 3-phosphoglycerate.3-phosphoglycerate.

Calvin cycle: Phase 2Calvin cycle: Phase 2ReductionReduction

Endergonic reduction is a 2-step process that Endergonic reduction is a 2-step process that couples ATP hydrolysis with the reduction of 3-couples ATP hydrolysis with the reduction of 3-phosphoglycerate to glyceraldehyde phosphate phosphoglycerate to glyceraldehyde phosphate (G3P).(G3P).

For every three CO2 molecules that enter the For every three CO2 molecules that enter the Calvin cycle, six G3P molecules are produced, Calvin cycle, six G3P molecules are produced, only one of which can be counted as net gain.only one of which can be counted as net gain.

The other five G3P molecules are recycled to The other five G3P molecules are recycled to regenerate molecules of RuBP.regenerate molecules of RuBP.

Calvin cycle: Phase IIICalvin cycle: Phase IIIRegeneration of CORegeneration of CO22 acceptor (RuBP) acceptor (RuBP)

A complex series of reactions rearranges A complex series of reactions rearranges the carbon skeletons of five G3P the carbon skeletons of five G3P molecules into three RuBP molecules.molecules into three RuBP molecules.

3 ATP molecules are used, RuBP is now 3 ATP molecules are used, RuBP is now prepared to receive COprepared to receive CO22 again, and the again, and the cycle continues.cycle continues.

Energy totalsEnergy totals To make one G3P molecule, the Calvin cycle To make one G3P molecule, the Calvin cycle

consumes 9 ATPs and 6 molecules of NADPH.consumes 9 ATPs and 6 molecules of NADPH. G3P is the starting material for the synthesis of G3P is the starting material for the synthesis of

glucose and other carbohydrates. glucose and other carbohydrates. The Calvin cycle uses 18 ATPs and 12 The Calvin cycle uses 18 ATPs and 12

molecules of NADPH to produce one molecule molecules of NADPH to produce one molecule of glucose.of glucose.

The light reactions and Calvin cycle are The light reactions and Calvin cycle are bothboth required to make sugar from COrequired to make sugar from CO22..

Carbon fixation: Alternative Carbon fixation: Alternative methods in hot, dry methods in hot, dry climatesclimates

In most plants, the fixation of carbon via In most plants, the fixation of carbon via rubisco results in a 3-carbon compound. rubisco results in a 3-carbon compound. These are called These are called C3 plantsC3 plants. Examples are . Examples are rice, wheat, and soybeans.rice, wheat, and soybeans.

On hot, dry days, these plants close their On hot, dry days, these plants close their stomata to conserve water. CO2 coming into stomata to conserve water. CO2 coming into the plant decreases. the plant decreases.

Rubisco can also bind oxygen, in place of Rubisco can also bind oxygen, in place of CO2. O2 enters the Calvin cycle. This is CO2. O2 enters the Calvin cycle. This is photorespiration.photorespiration.

PhotorespirationPhotorespiration This process is called photorespiration because This process is called photorespiration because

it occurs in light (it occurs in light (photophoto), and uses oxygen ), and uses oxygen ((respirationrespiration).).

However, photorespiration does not generate However, photorespiration does not generate ATP or make glucose.ATP or make glucose.

Photorespiration Photorespiration decreases decreases photosynthetic photosynthetic output by removing organic material from the output by removing organic material from the Calvin cycle.Calvin cycle.

Perhaps photorespiration is an evolutionary Perhaps photorespiration is an evolutionary “relic” from a time when there was more CO“relic” from a time when there was more CO2 2 in in the atmosphere than Othe atmosphere than O22, and rubisco did not , and rubisco did not have to differentiate between them.have to differentiate between them.

C4 PlantsC4 Plants C4 plants have adapted to hot, dry climates by C4 plants have adapted to hot, dry climates by

producing a 4-carbon compound as its first producing a 4-carbon compound as its first product.product.

Sugar cane and corn are examples of C4 plants.Sugar cane and corn are examples of C4 plants. There two types of photosynthetic cells: Bundle-There two types of photosynthetic cells: Bundle-

sheath cells and mesophyll cells.sheath cells and mesophyll cells. The Calvin cycle is confined to chloroplasts of The Calvin cycle is confined to chloroplasts of

the bundle-sheath cells, but COthe bundle-sheath cells, but CO22 can be can be incorporated into organic compounds in the incorporated into organic compounds in the mesophyll cell to be used later in the Calvin mesophyll cell to be used later in the Calvin cycle.cycle.

C4 plants, cont.C4 plants, cont. CO2 is added to phosphoenolpyruvate (CO2 is added to phosphoenolpyruvate (PEPPEP) )

by the enzyme by the enzyme PEP carboxylasePEP carboxylase.. Compared to rubisco, PEP carboxylase has a Compared to rubisco, PEP carboxylase has a

higher affinity for COhigher affinity for CO2 2 and can fix it under and can fix it under conditions (hot and dry) when rubisco cannot.conditions (hot and dry) when rubisco cannot.

The four carbon products are then exported to The four carbon products are then exported to the bundle-sheath cells through the bundle-sheath cells through plasmodesmata.plasmodesmata.

C4 photosynthesis minimizes photorespiration, C4 photosynthesis minimizes photorespiration, and enhances sugar production.and enhances sugar production.

CAM plantsCAM plants A second adaptation to arid conditions in succulent A second adaptation to arid conditions in succulent

(water-storing) plants is (water-storing) plants is crassulacean acid crassulacean acid metabolismmetabolism (CAM). (CAM).

Cacti and pineapples are examples of CAM plants.Cacti and pineapples are examples of CAM plants. The stomata are closed during the day, but open at The stomata are closed during the day, but open at

night to collect COnight to collect CO22. . Carbon fixation into organic acids occurs at night. Carbon fixation into organic acids occurs at night.

These molecules are stored in the mesophyll cell.These molecules are stored in the mesophyll cell. The Calvin cycle operates during the day when the The Calvin cycle operates during the day when the

light reactions can provide ATP and NADPH to light reactions can provide ATP and NADPH to make sugar from the stored COmake sugar from the stored CO22..

Review of PhotosynthesisReview of Photosynthesis Light reactions convert light energy to the Light reactions convert light energy to the

chemical energy of ATP and NADPH.chemical energy of ATP and NADPH. Pigments and protein molecules that Pigments and protein molecules that

carry out the light reactions are found in carry out the light reactions are found in the thylakoid membranes and include two the thylakoid membranes and include two photosystems and electron transport photosystems and electron transport chains.chains.

The light reactions split water and The light reactions split water and release oxygen to the atmosphere.release oxygen to the atmosphere.

Review of photosynthesis, Review of photosynthesis, cont.cont.

The Calvin cycle takes place in the The Calvin cycle takes place in the stroma of the chloroplast, and uses ATP stroma of the chloroplast, and uses ATP and NADPH to convert COand NADPH to convert CO22 to to carbohydrates (glucose).carbohydrates (glucose).

The direct product of the Calvin cycle is The direct product of the Calvin cycle is G3P. Enzymes in the chloroplast convert G3P. Enzymes in the chloroplast convert this molecule into a diverse group of this molecule into a diverse group of other organic compounds.other organic compounds.

The Calvin cycle returns ADP and The Calvin cycle returns ADP and NADP+ to the light reactions.NADP+ to the light reactions.