Chapter 7Chapter 7

Where It Starts - PhotosynthesisWhere It Starts - Photosynthesis

Photosynthesis - IntroPhotosynthesis - Intro

Photosynthesis – makes food (sugar and Photosynthesis – makes food (sugar and other compounds) by other compounds) by

using sunlight as an energy source using sunlight as an energy source

carbon dioxide as the carbon sourcecarbon dioxide as the carbon source

Releasing water and oxygenReleasing water and oxygen

Plants are Plants are autotrophsautotrophs, or self-nourishing , or self-nourishing organisms; they get Carbon and energy from the organisms; they get Carbon and energy from the environment and make their own food.environment and make their own food.

The first autotrophs filled Earth’s atmosphere The first autotrophs filled Earth’s atmosphere with oxygen, creating an ozone (Owith oxygen, creating an ozone (O33) layer) layer

The ozone layer became a shield against deadly The ozone layer became a shield against deadly UV rays from the sun, allowing life to move out UV rays from the sun, allowing life to move out of the ocean and diversify.of the ocean and diversify.

7.07.0 Sunlight and SurvivalSunlight and Survival

Other types of organismsOther types of organisms

HeterotrophsHeterotrophs – Cannot obtain energy and C – Cannot obtain energy and C from the environment. They feed on autotrophs, from the environment. They feed on autotrophs, one another, and organic wastes.one another, and organic wastes.

Plants are Plants are photoautotrophsphotoautotrophs. They make sugars . They make sugars and other compounds using sunlight as the and other compounds using sunlight as the energy source and COenergy source and CO2 2 and release great and release great

amounts of oxygen.amounts of oxygen. Plants around the world produce 220 billions Plants around the world produce 220 billions

tons of sugar each yeartons of sugar each year

ProkaryotesProkaryotes The first prokaryotes were The first prokaryotes were

chemoautotrophs, they did not have the chemoautotrophs, they did not have the enzymes needed. (So they extracted enzymes needed. (So they extracted energy and carbon from methane and energy and carbon from methane and hydrogen sulfide, there was little free hydrogen sulfide, there was little free oxygen.)oxygen.)

Some prokaryotes evolved to neutralize Some prokaryotes evolved to neutralize toxic oxygen radicals and flourished. toxic oxygen radicals and flourished. Those that did not perished.Those that did not perished.

7.1 7.1 Sunlight as an Energy SourceSunlight as an Energy Source

Plants (Photoautotrophs) utilize the energy Plants (Photoautotrophs) utilize the energy from the sun, in the form of photons , from the sun, in the form of photons , particles of wavelengths of visible light.particles of wavelengths of visible light.

Organisms use only a small range of Organisms use only a small range of wavelengths for photosynthesis wavelengths for photosynthesis

(380 – 750nm)(380 – 750nm) Light energy is packed as photons.Light energy is packed as photons.

Electromagnetic Spectrum Electromagnetic Spectrum

Shortest Shortest Gamma rays*Gamma rays*

wavelength wavelength X-rays*X-rays*

UV radiation*UV radiation*

Visible lightVisible light

Infrared radiationInfrared radiation

MicrowavesMicrowaves

LongestLongest Radio wavesRadio waves

wavelengthwavelength * alters or breaks * alters or breaks bonds bonds in DNA and in DNA and Proteins. Proteins. Threat to all Threat to all organisms.organisms.

PhotonsPhotons

Packets of light energyPackets of light energy Each type of photon has fixed amount of Each type of photon has fixed amount of

energyenergy

Photons having most energy travel as Photons having most energy travel as shortest wavelength (blue-violet light)shortest wavelength (blue-violet light)

Photons having least energy travel in Photons having least energy travel in longer wavelengths. (red Light, pg. 108)longer wavelengths. (red Light, pg. 108)

Visible Light Visible Light

Wavelengths humans perceive as different Wavelengths humans perceive as different colorscolors

Violet (380 nm) to red (750 nm) Violet (380 nm) to red (750 nm) Longer wavelengths, lower energyLonger wavelengths, lower energy

Figure 7-2Page 108

PigmentsPigments

Pigments are a class of molecules that absorb Pigments are a class of molecules that absorb photons in certain wavelengths only.photons in certain wavelengths only.

The color you see is the wavelength not The color you see is the wavelength not absorbed absorbed

Light-catching part of molecule often has Light-catching part of molecule often has alternating single and double bondsalternating single and double bonds

These bonds contain electrons that are capable These bonds contain electrons that are capable of being moved to higher energy levels by of being moved to higher energy levels by absorbing light absorbing light

Pigment MoleculesPigment Molecules Pigment molecules on the thylakoid membranes Pigment molecules on the thylakoid membranes

absorb photons.absorb photons. Chlorophyll a (major) pigments absorb violet and Chlorophyll a (major) pigments absorb violet and

red, but reflect green & yellow (leaves)red, but reflect green & yellow (leaves) Chlorophyll b (accessory) pigments reflects Chlorophyll b (accessory) pigments reflects

green & blue.green & blue. Carotenoid pigments absorb blue-violet and Carotenoid pigments absorb blue-violet and

blue-green but reflect yellow, orange, and redblue-green but reflect yellow, orange, and red The light-catching portion is the flattened ring The light-catching portion is the flattened ring

structure (see page 109)structure (see page 109)

Pigment Molecules Pigment Molecules

Xanthophylls reflect yellow, brown, purple, Xanthophylls reflect yellow, brown, purple, or blue light or blue light

Anthocyanins – reflect red and purple light in Anthocyanins – reflect red and purple light in fruit and flowersfruit and flowers

Phycobilins – reflect red or blue-green light Phycobilins – reflect red or blue-green light and are accessory pigments found in red and are accessory pigments found in red algae and cyanobacteriaalgae and cyanobacteria

Pigments in PhotosynthesisPigments in Photosynthesis

BacteriaBacteria Pigments in plasma membranesPigments in plasma membranes

PlantsPlants Pigments and proteins organized into Pigments and proteins organized into

photosystems that are embedded in photosystems that are embedded in thylakoid membrane systemthylakoid membrane system

7.2 Harvesting the Rainbow7.2 Harvesting the Rainbow

Wilhelm Theodor Engelmann, a botanist, Wilhelm Theodor Engelmann, a botanist, knew that plants use sunlight, water and knew that plants use sunlight, water and something in the air. something in the air.

What he wanted to know what which parts What he wanted to know what which parts of sunlight do plants favor?of sunlight do plants favor?

He designed an experiment using He designed an experiment using photosynthetic alga, photosynthetic alga, Cladophora and Cladophora and aerobic bacterial cells.aerobic bacterial cells.

Englemann’s Experiment Englemann’s Experiment

He knew that certain bacterial cells will He knew that certain bacterial cells will move toward places where oxygen move toward places where oxygen concentration is highconcentration is high

He also understood that Photosynthesis He also understood that Photosynthesis produces oxygenproduces oxygen

He identified violet and red light are best at He identified violet and red light are best at driving photosynthesis, as most of the driving photosynthesis, as most of the bacterial cells gathered at this point.bacterial cells gathered at this point.

7.3 Overview of Photosynthesis 7.3 Overview of Photosynthesis ReactionsReactions

Photosynthesis proceeds in two reaction Photosynthesis proceeds in two reaction stages. See page 111.stages. See page 111.Light-dependent reactions Light-dependent reactions -thylakoid membrane-thylakoid membraneSunlight energy is converted to chemical bond Sunlight energy is converted to chemical bond energy of ATPenergy of ATPWater molecules are split, and typically the Water molecules are split, and typically the coenzyme NADP+ accepts the released coenzyme NADP+ accepts the released hydrogen and electrons, thus becoming hydrogen and electrons, thus becoming NADPH, oxygen is releasedNADPH, oxygen is released

Photosynthesis – Second StagePhotosynthesis – Second Stage

Light Independent reactions Light Independent reactions - stroma- stroma Runs on energy delivered by ATP.Runs on energy delivered by ATP. This energy drives the synthesis of glucose This energy drives the synthesis of glucose

and other carbohydrates.and other carbohydrates. The building blocks are the hydrogen atoms The building blocks are the hydrogen atoms

and electron from NADPH, as well as carbon and electron from NADPH, as well as carbon and oxygen atoms stripped from carbon and oxygen atoms stripped from carbon dioxide and water.dioxide and water.

CO2H2O

SUNLIGHT

Fig. 7-6c, p.111

O2

light-dependant reactions

light-independant

reactions

sugars

CHLOROPLAST

NADPH, ATP

NADP+, ADP

Two stages of Photosynthesis Two stages of Photosynthesis

Occurs in the thylakoid membrane of the Occurs in the thylakoid membrane of the chloroplastchloroplast

Hundreds of Pigments absorb light energy, give Hundreds of Pigments absorb light energy, give up eup e--, which is transferred to a photosystem. , which is transferred to a photosystem.

Two molecules of chlorophyll a are at the Two molecules of chlorophyll a are at the center of a photosystem.center of a photosystem.

Chloroplasts have two kinds of photosystems, Chloroplasts have two kinds of photosystems, type I and type II. (Focus on type II)type I and type II. (Focus on type II)

7.4 Light-Dependent Reactions7.4 Light-Dependent Reactions

Light Dependent continuedLight Dependent continued

The freed eThe freed e-- enter an electron transfer enter an electron transfer chain, an orderly array of enzymes and co- chain, an orderly array of enzymes and co- enzymes. enzymes.

This is the first step in the conversion of This is the first step in the conversion of light to chemical energy.light to chemical energy.

Water molecules split, ATP and NADH Water molecules split, ATP and NADH form, and oxygen is released.form, and oxygen is released.

Light Dependent continuedLight Dependent continued

Pigments (PS II)that gave up electrons get Pigments (PS II)that gave up electrons get replacement electrons (from water molecules –replacement electrons (from water molecules –through the process of photolysis)through the process of photolysis)

HH+ + moves from the stroma into the thylakoid moves from the stroma into the thylakoid compartment. compartment.

The eThe e-- now enter photosystem 1 now enter photosystem 1 This is also known as the noncyclic pathway of This is also known as the noncyclic pathway of

ATP formation.ATP formation.

NADPH

NADP + + H+

thylakoidcompartment

thylakoidmembrane

stroma

ATPADP + Pi

H+

H+

H+H+

H+

H+

H+

H+ H+

H+

H+

PHOTOSYSTEM I P 700

sunlightPHOTOSYSTEM II P 680

LIGHT-HARVESTING

COMPLEX

Fig. 7-8, p.113

H+

e- e-e- e-e- e-

H+

e-

O2

H2O

cross-section through a disk-shaped fold in the

thylakoid membrane

Noncyclic pathway of ATPNoncyclic pathway of ATP

This pathway of photosynthesis This pathway of photosynthesis photon energy forces electrons out of photon energy forces electrons out of photosystem II to an electron transfer photosystem II to an electron transfer chain, which sets up Hchain, which sets up H++ gradient that gradient that drive ATP formation, and ultimately drive ATP formation, and ultimately end up in NADPH.end up in NADPH.

The electrons are not cycled back into The electrons are not cycled back into photosystem II.photosystem II.

Cyclic Pathway of ATP Cyclic Pathway of ATP

Photosystem I may run independently so Photosystem I may run independently so that cells can continue to make ATP.that cells can continue to make ATP.

It is a cyclic because the electrons that It is a cyclic because the electrons that leave photosystem I get cycled back to it.leave photosystem I get cycled back to it.

These electrons pass through an ETC that These electrons pass through an ETC that moves H+ into the thylakoid moves H+ into the thylakoid compartments. This drives ATP formation, compartments. This drives ATP formation, but no NADPH forms.but no NADPH forms.

7.5 Energy Flow in 7.5 Energy Flow in Photosynthesis Photosynthesis

Most pigments in photosystem are Most pigments in photosystem are harvester pigmentsharvester pigments

When excited by light energy, these When excited by light energy, these pigments transfer energy to adjacent pigments transfer energy to adjacent pigment moleculespigment molecules

Each transfer involves energy loss Each transfer involves energy loss

Photosystem Function: Photosystem Function: Reaction Center Reaction Center

Energy is reduced to a level that can be Energy is reduced to a level that can be captured by molecule of chlorophyll captured by molecule of chlorophyll aa

This molecule P700 (I) or P680 (II) is the This molecule P700 (I) or P680 (II) is the reaction center of a photosystemreaction center of a photosystem

Reaction center accepts energy and Reaction center accepts energy and donates electron to acceptor molecule donates electron to acceptor molecule

Cyclic Electron FlowCyclic Electron Flow

Electrons Electrons are donated by P700 in photosystem I to are donated by P700 in photosystem I to

acceptor moleculeacceptor molecule flow through electron transfer chain and back flow through electron transfer chain and back

to P700 to be reusedto P700 to be reused Electron flow drives ATP formationElectron flow drives ATP formation This is the process of the first anaerobic This is the process of the first anaerobic

photoautotrophs. See page 114 photoautotrophs. See page 114

Noncyclic Electron FlowNoncyclic Electron Flow

Two-step pathway for light absorption and Two-step pathway for light absorption and

electron excitationelectron excitation

Uses two photosystems: type I and Uses two photosystems: type I and

type II type II

ee__ that leave PS II are not returned to it. that leave PS II are not returned to it.

Produces ATP and NADPHProduces ATP and NADPH

Involves photolysis - splitting of waterInvolves photolysis - splitting of water

See page 114See page 114

Chemiosmotic Model Chemiosmotic Model of ATP Formationof ATP Formation

Electrical and HElectrical and H++ concentration gradients concentration gradients are created between thylakoid are created between thylakoid compartment and stromacompartment and stroma

HH++ flows down gradients into stroma flows down gradients into stroma through ATP synthesis ( ATP synthase)through ATP synthesis ( ATP synthase)

Flow of ions drives formation of ATPFlow of ions drives formation of ATP

Synthesis part of photosynthesis – Synthesis part of photosynthesis – make sugarmake sugar

Takes place in the stroma and can Takes place in the stroma and can proceed in the darkproceed in the dark

Steps – carbon fixation, Rubisco Steps – carbon fixation, Rubisco

mediatedmediated Calvin-Benson cycle – a series of Calvin-Benson cycle – a series of

enzyme mediated reactionsenzyme mediated reactions

7.6 Light-Independent 7.6 Light-Independent ReactionsReactions

Carbon FixationCarbon Fixation

A Carbon atom from COA Carbon atom from CO22 becomes becomes

attached to an organic compound. attached to an organic compound. Plants get the carbon dioxide from the air, Plants get the carbon dioxide from the air,

and algae get it from carbon dioxide and algae get it from carbon dioxide dissolved in water.dissolved in water.

Rubisco (ribulose bisphosphate Rubisco (ribulose bisphosphate carboylase/oxygenase meditates this step carboylase/oxygenase meditates this step in most plants.in most plants.

Calvin-Benson CycleCalvin-Benson Cycle

This is known as the sugar factoryThis is known as the sugar factory It is a series of enzyme mediated reactions It is a series of enzyme mediated reactions

that take place in the stroma.that take place in the stroma. Reactants – carbon dioxide attaches to Reactants – carbon dioxide attaches to

rubisco, and splits into phosphoglycerate. rubisco, and splits into phosphoglycerate. ATP, and NADPHATP, and NADPH

Products – Glucose, ADP, and NADP+Products – Glucose, ADP, and NADP+ This is cyclic and RuBP is regeneratedThis is cyclic and RuBP is regenerated

ATP

6 RuBP

phosphorylated glucose

10 PGAL

1 Pi

12 PGA

Calvin-Benson cycle

Fig. 7-10b, p.115

6 ADP

ATP

12 ADP +12 Pi

6CO2

NADPH

12 NADP+

12 PGAL

4 Pi

1

12

12

Calvin- Calvin- Benson Benson

CycleCycle

Six turns to Six turns to make one make one

glucose glucose

moleculemolecule

Gases diffuse in/out of a plant through Gases diffuse in/out of a plant through stomata.stomata.

In Calvin-Benson cycle, the first stable In Calvin-Benson cycle, the first stable intermediate is a three-carbon PGAintermediate is a three-carbon PGA

Because the first intermediate has three Because the first intermediate has three carbons, the pathway is called the C3 carbons, the pathway is called the C3 pathwaypathway

7.7 The C3 Pathway7.7 The C3 Pathway

Photorespiration in C3 PlantsPhotorespiration in C3 Plants

On hot, dry days stomata closeOn hot, dry days stomata close Inside leaf Inside leaf

Oxygen levels riseOxygen levels rise Carbon dioxide levels dropCarbon dioxide levels drop

Rubisco attaches RuBP to oxygen instead Rubisco attaches RuBP to oxygen instead of carbon dioxide (photorespiration)of carbon dioxide (photorespiration)

Only one PGAL forms instead of twoOnly one PGAL forms instead of two See page 117See page 117

C4 Plants C4 Plants

Carbon dioxide is fixed twiceCarbon dioxide is fixed twice In mesophyll cells, carbon dioxide is fixed to In mesophyll cells, carbon dioxide is fixed to

form four-carbon oxaloacetate form four-carbon oxaloacetate

Oxaloacetate is transferred to bundle-sheath Oxaloacetate is transferred to bundle-sheath

cellscells

Carbon dioxide is released and fixed again in Carbon dioxide is released and fixed again in

Calvin-Benson cycle. See page 117.Calvin-Benson cycle. See page 117.

CAM PlantsCAM Plants

Crassulacean Acid MetabolismCrassulacean Acid Metabolism Carbon is fixed twice (in same cells)Carbon is fixed twice (in same cells) Never uses oxygen, no matter the concentrationNever uses oxygen, no matter the concentration Night – stomata openNight – stomata open

Carbon dioxide is fixed to form organic acidsCarbon dioxide is fixed to form organic acids DayDay

Carbon dioxide is released and fixed in Carbon dioxide is released and fixed in Calvin-Benson cycle. See page 117.Calvin-Benson cycle. See page 117.

PhotoautotrophsPhotoautotrophs

Carbon source is carbon dioxideCarbon source is carbon dioxide

Energy source is sunlightEnergy source is sunlight

HeterotrophsHeterotrophs

Get carbon and energy by eating autotrophs or one Get carbon and energy by eating autotrophs or one

anotheranother

7.8 Autotrophs and the 7.8 Autotrophs and the BiosphereBiosphere

Linked ProcessesLinked Processes

PhotosynthesisPhotosynthesis

Energy-storing Energy-storing pathway pathway

Releases oxygenReleases oxygen

Requires carbon Requires carbon dioxidedioxide

Aerobic RespirationAerobic Respiration

Energy-releasing Energy-releasing pathwaypathway

Requires oxygenRequires oxygen

Releases carbon Releases carbon dioxidedioxide

Photoautotrophs Photoautotrophs

Capture sunlight energy and use it to carry Capture sunlight energy and use it to carry out photosynthesisout photosynthesis

PlantsPlants

Some bacteriaSome bacteria

Many protistansMany protistans

See Figure 7-14, on page 120 for an overviewSee Figure 7-14, on page 120 for an overview