Chapter 6

Photosynthesis: Acquiring energy from the sun

An Overview of Photosynthesis

• Most of the energy used by almost all living cells ultimately comes from the sun.• Plants, algae, and some bacteria capture the sunlight

energy by a process called photosynthesis.• Only about 1% of the available energy in sunlight is

captured.

Cross-section of leaf

Cuticle

EpidermisMesophyll

Vascularbundle

Bundlesheath

StomaVacuole Nucleus

Cell wall Chloroplasts

Inner membraneOuter membrane

GranumStroma

Chloroplast

Thylakoid Mesophyll cell

An Overview of Photosynthesis

• The leaf cells of plants contain chloroplasts.• The chloroplast

contains internal membranes called thylakoids.

• The thylakoids are stacked together in columns called grana.

• The stroma is a semi liquid substance that surrounds the thylakoids.

An Overview of Photosynthesis

• The photosystem is the starting point of photosynthesis.• It is a network of pigments in the membrane of the

thylakoid.• The primary pigment of a photosystem is

chlorophyll.• The pigments act as an antenna to capture

energy from sunlight.• Individual chlorophyll pigments pass the

captured energy between them.

An Overview of Photosynthesis

• Photosynthesis takes place in three stages:

1. Capturing energy from sunlight.

2. Using the captured energy to produce ATP and NADPH.

3. Using the ATP and NADPH to make carbohydrates from CO2 in the atmosphere.

An Overview of Photosynthesis

• The process of photosynthesis is divided into two types of reactions.• Light-dependent reactions take place only in the

presence of light and produce ATP and NADPH.

• Light-independent reactions do not need light to occur and result in the formation of organic molecules.• More commonly known as the Calvin cycle.

An Overview of Photosynthesis

• The overall reaction for photosynthesis may be summarized by this simple equation:

6 CO2

carbondioxide

+ 6 H2Owater

+ Light

energy

C6H12O6

glucose

+ 6 O2

oxygen

How Plants Capture Energy from Sunlight

• Light is comprised of packets of energy called photons.• Sunlight has photons of varying energy levels.

• The possible range of energy levels is represented by an electromagnetic spectrum.

• Human eyes only perceive photons of intermediate energy levels.• This range of the spectrum is known as visible

light.

Increasing energy of photon

Increasing wave length

0.001 nm1 nm 1,000 nm

10 nm 0.01 cm1 cm

1 m 100 m

Radioactiveelements

X-raymachines

Lightbulbs

PeopleRadar

Microwaveovens

FM radio AM radio

Gamma raysX

raysUV

light Infrared Microwaves Radio waves

Visible light

400 nm 430 nm 500 nm 560 nm 600 nm 650 nm 740 nm

How Plants Capture Energy from Sunlight

• Pigments are molecules that absorb light energy.• The main pigment in plants is chlorophyll.

• Chlorophyll absorbs light at the ends of the visible spectrum, mainly blue and red light.

How Plants Capture Energy from Sunlight

• Plants also contain other pigments, called accessory pigments, that absorb light levels that chlorophyll does not.• These pigments give color to flowers, fruits,

and vegetables.• They are present in leaves too, but are

masked by chlorophyll until the fall when the chlorophyll is broken down.

1. All wave lengths except 500- to600-nanometer wave lengths (green) absorbed by leaf

4. Brain perceives “green”

2. Green reflected by leaf

3. Green absorbed by eye pigment

Why are plants green?

CarotenoidsChlorophyll bChlorophyll a

Rel

ativ

e lig

ht

abso

rpti

on

400 450 500Wavelength (nm)

550 600 650 700

Absorption spectra of chlorophylls and carotenoids

Organizing Pigments into Photosystems

• In plants, the light-dependent reactions occur within a complex of proteins and pigments called a photosystem.

Organizing Pigments into Photosystems

• Light energy is first captured by any one of the chlorophyll pigments.

• The energy is passed along to other pigments until it reaches the reaction center chlorophyll molecule.

• The reaction center then releases an excited electron, which is then transferred to an electron acceptor.

• The excited electron that is lost is then replaced by an electron donor.

Photon

Photosystem

e–

e–

Reactioncenterchlorophyll

Electrondonor

Chlorophyllmolecules

Electronacceptor

Organizing Pigments into Photosystems

• The light-dependent reactions in plants and algae use two photosystems.• Photosystem II

• Captures a photon of light and releases an excited electron to the electron transport system (ETS).

• The ETS then produces ATP.

En

erg

y o

f ele

ctr

on

s

Electron transport system Electron transport system

Photon

Photon

Photosystem II

NADP+ + H+

Photosystem I

2H2O4H+ + O2

H+ P700

P680 e–

Electron transport system

1

23

4

5

e–

e–

e–

e–

Electron transport system

Excitedreactioncenter

Excitedreactioncenter

Reactioncenter

Proton gradientformed for ATPsynthesis

Water-splittingenzyme

ATP

Reactioncenter

NADPH

Organizing Pigments into Photosystems

• Photosystem I• Absorbs another photon of light and releases an

excited electron to another ETS.• The ETS produces NADPH.• The electron from photosystem II replaces the

electron from the reaction center.

How Photosystems Convert Light to Chemical Energy

• Plants produce both ATP and NADPH by noncyclic photophosphorylation.• The excited electrons flow through both photosystems

and end up in NADPH.• High energy electrons generated by photosystem II

are used to make ATP and are then passed along to photosystem I to drive the production of NADPH.

The photosynthetic electron transport system

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Thylakoidspace

2H2O

4 H+

H+ + NADP+

O2

e–

H+

Light-dependentreactions

Calvincycle

ATP

Photon

Thylakoidmembrane

Photosystem II Electron transport system Photosystem I Electron transport system

Water-splittingenzyme

Stroma

NADPPhotonAntenna

complex

Protongradient

Thylakoidspace

H+

H+

H+

H+

H+

e–

e–e–

NADP

How Photosystems Convert Light to Chemical Energy

• As a result of the proton pump of the ETS, a large concentration of protons builds up in the thylakoid space.• The thylakoid membrane is impermeable to protons.• Protons can only re-enter the stroma by traveling

through a protein channel called ATP synthase.• As the protons follow their concentration gradient

and pass through ATP synthase channels, ADP is phosphorylated into ATP.

• This process is called chemiosmosis.

Chemiosmosis in a chloroplast

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H+

2H2O

4 H+O2

Light-dependentreactions

ATP

Calvincycle

NADPH

Thylakoid space

Thylakoidspace

Stroma

PhotonADP ATP

Photosystem II Electron transport system ATP synthase

e– e–

H+

H+

H+

H+

H+

H+

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Building New Molecules

• Cells use the products of the light-dependent reactions to build organic molecules.• ATP is needed to drive endergonic reactions.• NADPH is needed to provide reducing power in the

form of hydrogens.

Building New Molecules

• The synthesis of new molecules employs the light-independent, or Calvin cycle, reactions.• These reactions are also known as C3

photosynthesis.

Building New Molecules

• The Calvin cycle reactions occur in 3 stages

• 1. Carbon fixation • Carbon from CO2 in the air is attached to an organic

molecule, RuBP.• 2. Making sugars

• The carbons are shuffled about through a series of reactions to make sugars.

• 3. Reforming RuBP• The remaining molecules are used to reform RuBP.

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PP

The Calvin cycle begins when a carbonatom from a CO2 molecule is added toa five-carbon molecule (the startingmaterial). The resulting six-carbonmolecule is unstable and immediatelysplits into three-carbon molecules.(Three “turns” of the cycle are indicatedhere with three molecules of CO2

entering the cycle.)

Then, through a series of reactions,energy from ATP and hydrogens fromNADPH (the products of the light-dependent reactions) are added to thethree-carbon molecules. The now-reduced three-carbon molecules eithercombine to make glucose or are used tomake other molecules.

Most of the reduced three-carbonmolecules are used to regenerate thefive-carbon starting material, thuscompleting the cycle.

63-phosphoglycerate

6

6

Glucose

3-phosphoglycerate

1

5

3 3

3

CO2

321

P P

P

3

6

6

RuBP(Starting material)

ATP

ATP

Glyceraldehyde3-phosphate

Glyceraldehyde3-phosphate

Glyceraldehyde3-phosphate

RuBP(Starting material)

NADPH

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Building New Molecules

• The Calvin cycle must “turn” 6 times in order to form a new glucose molecule.• Only one carbon is added from CO2 per turn.

• The Calvin cycle also recycles reactants needed for the light-dependent reactions.• It returns ADP so that it is available for chemiosmosis

in photosystem II.• It returns NADP+ back to the ETS of photosystem I.

Reactions of the Calvin cycle

• Rubisco is thought to be the most abundant protein on earth

3

Rubisco

Carbon fixation

3-phosphoglycerate6

3

6

1

5

6

CO2

6 ADP

1,3-bisphosphoglycerate

6 NADP

Glyceraldehyde 3-phosphate

Glyceraldehyde3-phosphate

3 ADP

RuBP

Stroma of chloroplast

Glyceraldehyde 3-phosphate

2 Pi

6

Making sugars

Pi

P

PP

P

P

P

Glucose andother sugars

3 ATP

6 ATP

ReformingRuBP

Thylakoid space

Light-dependentreactions

ATP

Calvincycle

NADPH

6 NADPH

Photorespiration: Putting the Brakes on Photosynthesis

• Many plants have trouble carrying out C3 photosynthesis when it is hot.• Plants close openings in their leaves, called stomata

(singular, stoma), in order to prevent water loss.• The closed stoma also prevent gas exchange.

• O2 levels build up inside the leaves while the concentrations of CO2 fall.

• The enzyme rubisco fixes oxygen instead of carbon.• This process is called photorespiration and short-

circuits the Calvin cycle and photosynthesis.

Plant response in hot, arid weather

H2O

O2

Under hot, aridconditions, leaveslose water byevaporation throughopenings in theleaves called stomata.

The stomata closeto conserve waterbut as a result, O2

builds up inside theleaves, and CO2

cannot enter theleaves. This leadsto photorespiration.

H2O

O2

CO2 CO2

Leafepidermis Heat

Stoma

Carbon fixation in C4 plants

• Some plants have adapted to hot climates by performing C4 photosynthesis.• These C4 plants include sugarcane,

corn, and many grasses.• They fix carbon using different

types of cells and reactions than C3 plants and do not run out of CO2 even in hot weather.

• CO2 becomes trapped in cells called bundle-sheath cells.

CO2

CO2

Phosphoenol-pyruvate (PEP)

Oxalo-acetate

Mesophyllcell

Pyruvate Malate

Pyruvate Malate

ATP

PPi +AMP

Pi +

Bundle-sheath cell

Calvincycle

Glucose

Photorespiration: Putting the Brakes on Photosynthesis

• Another strategy to avoid a reduction in photosynthesis in hot weather occurs in many succulent (water-storing) plants, such as cacti and pineapples.• These plants undergo crassulacean acid

metabolism (CAM).

• Photosynthesis occurs via the C4 pathway at night and the C3 pathway during the day.

Comparing carbon fixation in C4 and CAM plants

C4 plants

Glucose

CO2

CAM plants

Night

Day

CO2CO2

Glucose

CO2

C4

pathwayC4

pathway

Calvincycle

Calvincycle

Mesophyllcell

Mesophyllcell

Bundle-sheath

cell