Chapter 7Chapter 7

PhotosynthesisPhotosynthesis

Chapter 7 2 What Is Photosynthesis?What Is Photosynthesis?

Early cells evolved the ability to perform Early cells evolved the ability to perform photosynthesis about 2 billion years agophotosynthesis about 2 billion years ago

PhotosynthesisPhotosynthesis is the ability to capture is the ability to capture sunlight energy and convert it to chemical sunlight energy and convert it to chemical energyenergy

Most forms of life on Earth depend on the Most forms of life on Earth depend on the chemical energy produced by chemical energy produced by photosynthetic organismsphotosynthetic organisms

Chapter 7 3 The Photosynthetic EquationThe Photosynthetic Equation

6CO2 carbondioxide

+ 6H2O water

+ light energy sunlight

C6H12O6

glucose (sugar)

+ 6O2

oxygen

Photosynthesis occurs in plants, algae, and Photosynthesis occurs in plants, algae, and some prokaryotessome prokaryotesPhotosynthetic organisms are Photosynthetic organisms are autotrophsautotrophs (“self- (“self-

feeders”)feeders”)Photosynthesis in plants occurs within Photosynthesis in plants occurs within chloroplastschloroplasts

Chapter 7 4 Gases, Sugar, and Water CycleGases, Sugar, and Water Cycle

Production of carbon compounds like Production of carbon compounds like glucose (photosynthesis) is linked with glucose (photosynthesis) is linked with energy extraction (in cellular respiration)energy extraction (in cellular respiration)

Water, COWater, CO22, sugar, and O, sugar, and O22 cycle between the cycle between the two processestwo processes

Photosynthesis and cellular respiration are Photosynthesis and cellular respiration are interconnectedinterconnected

Chapter 7 5

Interconnections:Interconnections:Photosynthesis & RespirationPhotosynthesis & Respiration

Chapter 7 6 Adaptations for PhotosynthesisAdaptations for Photosynthesis

LeavesLeaves

ChloroplastsChloroplasts

Chapter 7 7

Structures ofStructures ofPhotosynthesisPhotosynthesis

Chapter 7 8 LeavesLeaves

Flattened leaf shape exposes large Flattened leaf shape exposes large surface area to catch sunlightsurface area to catch sunlight

Upper and lower leaf surfaces of a leaf Upper and lower leaf surfaces of a leaf comprise the comprise the epidermisepidermis

Waxy, waterproof Waxy, waterproof cuticlecuticle on outer on outer surfaces reduces water evaporationsurfaces reduces water evaporation

Chapter 7 9 Leaf AnatomyLeaf Anatomy

Adjustable pores called Adjustable pores called stomatastomata allow for allow for entry of air with COentry of air with CO22

Inner Inner mesophyllmesophyll cell layers contain cell layers contain majority of chloroplastsmajority of chloroplasts

Vascular bundlesVascular bundles (veins) supply water (veins) supply water and minerals to the leaf while carrying and minerals to the leaf while carrying sugars away from the leafsugars away from the leaf

Chapter 7 10 Internal Leaf StructureInternal Leaf Structure

Internal leaf structure is crucial to Internal leaf structure is crucial to photosynthesis since photosynthesis photosynthesis since photosynthesis occurs primarily in the leaves of land occurs primarily in the leaves of land plans plans

Chapter 7 11

Leaf StructureLeaf Structure

MesophyllMesophyllCellsCells

MesophyllMesophyllCellsCells

Upper EpidermisUpper EpidermisUpper EpidermisUpper Epidermis

LowerLowerEpidermisEpidermis

LowerLowerEpidermisEpidermisVeinVeinVeinVein StomaStomaStomaStoma

Chapter 7 12 Anatomy of a ChloroplastAnatomy of a Chloroplast

Mesophyll cells have 40-200 Mesophyll cells have 40-200 chloroplasts eachchloroplasts each

Chloroplasts are bounded by a double Chloroplasts are bounded by a double membrane composed of the membrane composed of the innerinner and and outer membranesouter membranes

Chapter 7 13 Anatomy of a ChloroplastAnatomy of a Chloroplast

The The stroma stroma is the semi-fluid medium is the semi-fluid medium within the inner membranewithin the inner membrane

Disk-shaped sacs called Disk-shaped sacs called thylakoidsthylakoids found within the stroma in stacks found within the stroma in stacks called called granagrana

Chapter 7 14

Chloroplast StructureChloroplast Structure

Inner & OuterInner & OuterMembranesMembranes

ThylakoidsThylakoids

GranumGranum

Stroma Stroma (space)(space)

Chapter 7 15 The Energy in Visible LightThe Energy in Visible Light

The sun radiates The sun radiates electromagnetic energyelectromagnetic energyVisible light is radiation falling between 400-Visible light is radiation falling between 400-750 nanometers of wavelength750 nanometers of wavelength

Packets of energy called Packets of energy called photons photons with with different energy levelsdifferent energy levels• Short-wavelength photons are very Short-wavelength photons are very

energeticenergetic• Longer-wavelength photons have lower Longer-wavelength photons have lower

energiesenergies

Chapter 7 16

Light, ChloroplastLight, ChloroplastPigments, & PhotosynthesisPigments, & Photosynthesis

Gamma RaysGamma Rays X-RaysX-Rays UVUV InfraredInfrared Micro-Micro-waveswaves

RadioRadioWavesWaves

Visible LightVisible Light

400400 450450 500500 550550 600600 650650 700700 750750Wavelength (nanometers, nm)Wavelength (nanometers, nm)

Chapter 7 17 Light Captured by PigmentsLight Captured by Pigments

Action of light-capturing pigmentsAction of light-capturing pigments• AbsorptionAbsorption of certain wavelengths of certain wavelengths

(light is “trapped”)(light is “trapped”)• ReflectionReflection of certain wavelengths of certain wavelengths

(light bounces back)(light bounces back)• TransmissionTransmission of certain wavelengths of certain wavelengths

(light passes through)(light passes through)

Chapter 7 18 Light Captured by PigmentsLight Captured by Pigments

Absorbed light drives biological Absorbed light drives biological processes when it is converted to processes when it is converted to chemical energychemical energy

Common pigments found in Common pigments found in chloroplasts include:chloroplasts include:• ChlorophyllChlorophyll aa andand bb• Accessory pigments such as Accessory pigments such as

carotenoidscarotenoids

Chapter 7 19

Light Captured by PigmentsLight Captured by Pigments

Chlorophyll a and bChlorophyll a and b absorb violet, blue, absorb violet, blue, and red light but reflect green light and red light but reflect green light (hence they appear green)(hence they appear green)

CarotenoidsCarotenoids absorb blue and green light absorb blue and green light but reflect yellow, orange, or red (hence but reflect yellow, orange, or red (hence they appear yellow-orange)they appear yellow-orange)

Pigment absorbs visible lightPigment absorbs visible light

Chapter 7 20

00

2020

4040

6060

8080

100100

% L

ight

Abs

orpt

ion

% L

ight

Abs

orpt

ion

400400 450450 500500 550550 600600 650650 700700 750750Wavelength (nanometers)Wavelength (nanometers)

400400 450450 500500 550550 600600 650650 700700 750750

Absorption SpectraAbsorption Spectra

Visible LightVisible Light

ChlorophyllChlorophyll

CarotenoidsCarotenoids

PhycocyaninPhycocyanin

CollectivelyCollectively

Chapter 7 8Light, ChloroplastLight, ChloroplastPigments & PhotosynthesisPigments & Photosynthesis

Gamma RaysGamma Rays XX-- RaysRays UVUV InfraredInfrared MicroMicro--waveswaves

RadioRadioWavesWaves

Visible LightVisible Light

400400 400400 400400 400400 400400 400400 400400 400400Wavelength (nanometers)Wavelength (nanometers)

Chapter 7 21

GlucoseGlucose

OO22

Overview of PhotosynthesisOverview of Photosynthesis

Light-DependentLight-DependentReactionsReactions

(in thylakoids)(in thylakoids)

Light-IndependentLight-IndependentReactionsReactions

(in stroma)(in stroma)

DepletedDepletedCarriersCarriers

(ADP, NADP(ADP, NADP++))

DepletedDepletedCarriersCarriers

(ADP, NADP(ADP, NADP++))

EnergizedEnergizedCarriersCarriers

(ATP, NADPH)(ATP, NADPH)

HH22OO

COCO22

Chapter 7 22

The PhotosystemsThe Photosystems

Chapter 7 23 Light-Dependent ReactionsLight-Dependent Reactions

Steps of the light reactions:Steps of the light reactions:Accessory pigments in Photosystems Accessory pigments in Photosystems

absorb light and pass energy to absorb light and pass energy to reaction centersreaction centers containing containing chlorophyllchlorophyll

Reaction centers receive energized Reaction centers receive energized electrons…electrons…

Chapter 7 24 Light-Dependent ReactionsLight-Dependent Reactions

Energized electrons then passed down Energized electrons then passed down a series of electron carrier molecules a series of electron carrier molecules ((Electron Transport ChainElectron Transport Chain))

Energy released from passed electrons Energy released from passed electrons used to synthesize used to synthesize ATPATP from ADP from ADP and phosphateand phosphate

Energized electrons also used to makeEnergized electrons also used to make NADPHNADPH from NADP+ + H+ from NADP+ + H+

Chapter 7 25

Thylakoid Structure & Thylakoid Structure & Light-Dependent ReactionsLight-Dependent Reactions

Chapter 7 26

Oxygen as a By-productOxygen as a By-product

Chapter 7 27

Chemiosmosis 1Chemiosmosis 1

Chapter 7 28

Chemiosmosis 2Chemiosmosis 2

Chapter 7 29 Light-Light-InIndependent Reactionsdependent Reactions

NADPH and ATP from light-dependent NADPH and ATP from light-dependent reactions used to power glucose reactions used to power glucose synthesissynthesis

Light not Light not directlydirectly necessary for light- necessary for light-independent reactions if ATP & NADPH independent reactions if ATP & NADPH availableavailable

Light-independent reactions called the Light-independent reactions called the Calvin-BensonCalvin-Benson CycleCycle or or CC33 Cycle Cycle

Chapter 7 30 The CThe C33 Cycle Cycle

6 CO6 CO22 used to synthesize 1 glucose used to synthesize 1 glucose (C(C66HH1212OO66))

Carbon dioxide is captured and linked to Carbon dioxide is captured and linked to ribulose bisphosphate (RuBP)ribulose bisphosphate (RuBP)

ATP and NADPH from light dependent ATP and NADPH from light dependent reactions used to power Creactions used to power C33 reactions reactions

Chapter 7 31 CC33 Cycle Has Three Parts Cycle Has Three Parts

1. Carbon fixation (carbon capture)1. Carbon fixation (carbon capture)• 6 Ribulose bisphosphate (RuBP) 6 Ribulose bisphosphate (RuBP)

molecules combine with 6COmolecules combine with 6CO22

• Fixation step and subsequent reactions Fixation step and subsequent reactions yield twelve 3-carbon phosphoglyceric yield twelve 3-carbon phosphoglyceric acid (PGA) molecules…acid (PGA) molecules…

Chapter 7 32 CC33 Cycle Has Three Parts Cycle Has Three Parts

2. Synthesis of Glyceraldehyde 3-2. Synthesis of Glyceraldehyde 3-Phosphate (G3P)Phosphate (G3P)

• Energy is donated by ATP and NADPHEnergy is donated by ATP and NADPH• Phosphoglyceric acid (PGA) molecules Phosphoglyceric acid (PGA) molecules

are converted into glyceraldehyde 3-are converted into glyceraldehyde 3-Phophate (G3P) molecules …Phophate (G3P) molecules …

Chapter 7 33 CC33 Cycle Has Three Parts Cycle Has Three Parts

3. Regeneration of Ribulose bis-3. Regeneration of Ribulose bis-phosphate (RuBP)phosphate (RuBP)

• 10 of 12 G3P molecules converted into 10 of 12 G3P molecules converted into 6 RuBP molecules6 RuBP molecules

• 2 of 12 G3P molecules used to 2 of 12 G3P molecules used to synthesize 1 glucosesynthesize 1 glucose

• ATP energy used for these reactionsATP energy used for these reactions

Chapter 7 34 The CThe C33 Cycle of Cycle of

Carbon Carbon FixationFixation…or……or…Dark ReactionsDark Reactions…or……or…Light-Light-Independent Independent ReactionsReactions…or……or…Calvin CycleCalvin Cycle…or……or…Calvin-Benson Calvin-Benson CycleCycle

Chapter 7 35 Relationship Between ReactionsRelationship Between Reactions

The “photo” part of photosynthesis The “photo” part of photosynthesis refers to the capture of light energy refers to the capture of light energy (light dependent reactions)(light dependent reactions)

The “synthesis” part of photosynthesis The “synthesis” part of photosynthesis refers to glucose synthesis (light-refers to glucose synthesis (light-inindependent reactions)dependent reactions)

Chapter 7 36 Relationship Between ReactionsRelationship Between Reactions

Light dependent reactions produce ATP Light dependent reactions produce ATP and NADPH which is used to drive and NADPH which is used to drive light-independent reactionslight-independent reactions

Depleted carriers (ADP and NADPDepleted carriers (ADP and NADP++) ) return to light-dependent reactions for return to light-dependent reactions for rechargingrecharging

Chapter 7 37

A Summary of PhotosynthesisA Summary of Photosynthesis

Light-dependent Light-dependent reactions occur reactions occur

in thylakoidsin thylakoids Light-Light-independent independent reactions (creactions (c33

cycle) occur in cycle) occur in stromastroma

Chapter 7 38 Water, COWater, CO22, and the C, and the C44 Pathway Pathway

The ideal leaf:The ideal leaf:• has large surface area to intercept sunlighthas large surface area to intercept sunlight• are porous to allow for COare porous to allow for CO22 entry from air entry from air

Problem: Substantial leaf porosity leads to Problem: Substantial leaf porosity leads to substantial water evaporation, causing substantial water evaporation, causing dehydration stress on the plantdehydration stress on the plant

Plants evolved waterproof coating and Plants evolved waterproof coating and adjustable pores (stomata) for COadjustable pores (stomata) for CO22 entry entry

Chapter 7 39 When Stomata Are ClosedWhen Stomata Are Closed

When stomata close, COWhen stomata close, CO22 levels drop and O levels drop and O22 levels riselevels rise

Carbon fixing enzyme combines OCarbon fixing enzyme combines O22 instead of instead of COCO22 with RuBP (called with RuBP (called photorespirationphotorespiration))

Photorespiration:Photorespiration:• OO22 is used up as CO is used up as CO22 is generated is generated• No useful cellular energy madeNo useful cellular energy made• No glucose producedNo glucose produced• Photorespiration is unproductive and Photorespiration is unproductive and

wastefulwasteful

Chapter 7 40 When Stomata Are ClosedWhen Stomata Are Closed

Hot, dry weather causes stomata to stay Hot, dry weather causes stomata to stay closedclosed

Oxygen levels rise as carbon dioxide levels Oxygen levels rise as carbon dioxide levels fall inside leaffall inside leaf

Photorespiration very common under such Photorespiration very common under such conditionsconditions

Plants may die from lack of glucose Plants may die from lack of glucose synthesissynthesis

Chapter 7 41 CC44 Plants Reduce Photorespiration Plants Reduce Photorespiration

““CC44 plants” have chloroplasts in bundle plants” have chloroplasts in bundle sheath cells as well as mesophyll cellssheath cells as well as mesophyll cells• Bundle sheath cells surround vascular Bundle sheath cells surround vascular

bundles deep within mesophyllbundles deep within mesophyll• CC33 plants lack bundle sheath cell plants lack bundle sheath cell

chloroplastschloroplasts

Chapter 7 42 CC33 Plants & the Pathway Plants & the Pathway

CC33 plants use the C plants use the C33 pathwaypathway

In a CIn a C33 plant, most plant, most chloroplasts are in chloroplasts are in

mesophyll cells.mesophyll cells.

Chapter 7 43 CC33 Plants & the Pathway Plants & the Pathway

Much photorespiration Much photorespiration occurs under hot, dry occurs under hot, dry

conditionsconditions

Chapter 7 44 CC44 Plants & the C Plants & the C4 4 PathwayPathway

CC44 plants use the C plants use the C44 pathwaypathway

In a CIn a C44 plant, both plant, both mesophyll & bundle-mesophyll & bundle-sheath cells contain sheath cells contain

chloroplasts.chloroplasts.

Chapter 7 45

CC44 Plants & the Plants & the CC4 4 PathwayPathway

Almost no Almost no photorespiration photorespiration occurs under hot, occurs under hot,

dry conditions.dry conditions.

COCO22 is captured is captured with a highly with a highly

specific enzyme.specific enzyme.

Much glucose Much glucose synthesis occurs.synthesis occurs.

Mesophyll cell in a CMesophyll cell in a C44 plant plant

Bundle-sheath cell in a CBundle-sheath cell in a C44 plant plant

Chapter 7Chapter 7

The endThe end