PHOTOSYNTHESIS: ACQUIRING
ENERGY FROM THE SUN
C H A P T E R 6

• 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
Epidermis
Mesophyll
Vascular bundle
Stoma
Bundle sheath
Vacuole Nucleus
Inner membrane
Outer membrane
Granum
Stroma
Thylakoid
Chloroplast
Cell wall
Chloroplasts
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 CO
2 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 CO
2
+ 6 H
2
O carbon dioxide water
+ Light energy
C
6
H
12
O
6 glucose
+ 6 O
2 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
0.001 nm
Increasing wave length
1 nm 1,000 nm
10 nm 0.01 cm
1 cm
1 m 100 m
Radioactive elements
X-ray machines
Light bulbs
People
Radar
Microwave ovens
FM radio
AM radio
Gamma rays
X rays
UV 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

ABSORPTION SPECTRA OF
CHLOROPHYLLS AND CAROTENOIDS
CarotenoidsChlorophyll bChlorophyll a
400 450 500 550 600 650 700
Wavelength (nm)

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.
Photon
Electron
Reaction acceptor center chlorophyll e
–
• The energy is passed along to other pigments until it reaches the reaction center chlorophyll molecule.
• The reaction center then
Chlorophyll molecules e
– releases an excited electron, which is then transferred to
Photosystem an electron acceptor.
• The excited electron that is lost is then replaced by an electron donor.
Electron donor

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.

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.
Excited reaction center e
–
Excited reaction center e
–
4
ATP e
– NADP + + H +
5
NADPH
P
700
Reaction center
Photon e
–
Photon
Reaction center
P
680
2
1
3
H +
Water-splitting enzyme e – 2H
2
O
4H + + O
2
Proton gradient formed for ATP synthesis
Photosystem II Electron transport system Photosystem I Electron transport system

• 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
Calvin cycle
ATP NADP
Light-dependent reactions
Thylakoid space
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Stroma
Photon
Thylakoid membrane
Antenna complex
Photon
H +
H + + NADP +
NADP e
– e – e – e –
2H
2
O Proton gradient
H +
Water-splitting enzyme
O
2
4 H +
H +
H +
H +
H +
Thylakoid space
Photosystem II Electron transport system Photosystem I Electron transport system

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
Calvin cycle
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Stroma
Photon
H +
ADP
H +
ATP
ATP NADPH
Light-dependent reactions
Thylakoid space e – e –
2H
2
O
Thylakoid space
O
2
4 H +
Photosystem II
H +
H +
Electron transport system
H +
H +
H +
ATP synthase http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio13.swf::Photosynthetic%20Electron%20Transport%20and%20ATP%20Synthesis

• 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.

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

• The Calvin cycle reactions occur in 3 stages
• 1. Carbon fixation
• Carbon from CO
2 molecule, RuBP.
• 2. Making sugars in the air is attached to an organic
• The carbons are shuffled about through a series of reactions to make sugars.
• 3. Reforming RuBP
• The remaining molecules are used to reform RuBP.

1
3
CO
2
3
RuBP
(Starting material)
P
6
3-phosphoglycerate
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
2 3
P
6
3-phosphoglycerate
6 ATP
6 NADPH
3
RuBP
(Starting material)
6
P
Glyceraldehyde
3-phosphate
1
P
Glyceraldehyde
3-phosphate
3 ATP
P
5
Glyceraldehyde
3-phosphate
The Calvin cycle begins when a carbon atom from a CO
2 molecule is added to a five-carbon molecule (the starting material). The resulting six-carbon molecule is unstable and immediately splits into three-carbon molecules.
(Three “ turns ” of the cycle are indicated here with three molecules of CO
2 entering the cycle.)
Glucose
Then, through a series of reactions, energy from ATP and hydrogens from
NADPH (the products of the lightdependent reactions) are added to the three-carbon molecules. The nowreduced three-carbon molecules either combine to make glucose or are used to make other molecules.
Most of the reduced three-carbon molecules are used to regenerate the five-carbon starting material, thus completing the cycle.
http://highered.mcgraw-hill.com/sites/0070960526/student_view0/chapter5/animation_quiz_1.htmla

• The Calvin cycle must “turn” 6 times in order to form a new glucose molecule.
• Only one carbon is added from CO
2 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.

Stroma of chloroplast
3
CO
2 Calvin cycle
AT
P
NADPH
Light-dependent reactions
Thylakoid space
3 ADP
3
RuBP
• Rubisco is thought to be the most abundant protein on earth
3 ATP
2 P i
Reforming
RuBP
P
5
Glyceraldehyde 3-phosphate
Rubisco
P
6
3-phosphoglycerate
6 ATP
Carbon fixation
6 ADP
P P
6
1,3-bisphosphoglycerate
6 NADPH
Making sugars
6 NADP
P
6 P i
6
Glyceraldehyde 3-phosphate
P
Glyceraldehyde
3-phosphate
1
Glucose and other sugars

PHOTORESPIRATION: PUTTING THE
BRAKES ON PHOTOSYNTHESIS
• Many plants have trouble carrying out C
3 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.
• O
2 levels build up inside the leaves while the concentrations of CO
2 fall.
• The enzyme rubisco fixes oxygen instead of carbon.
• This process is called photorespiration and shortcircuits the Calvin cycle and photosynthesis.

PLANT RESPONSE IN HOT, ARID WEATHER
Leaf epidermis
Under hot, arid conditions, leaves lose water by evaporation through openings in the leaves called stomata.
Heat
H
2
O H
2
O
Stoma
The stomata close to conserve water but as a result, O
2 builds up inside the leaves, and CO
2 cannot enter the leaves. This leads to photorespiration.
O
2
CO
2
O
2
CO
2

CARBON FIXATION IN C
4
PLANTS
CO
2
• Some plants have adapted to hot climates by performing C
4 photosynthesis .
• These C
4 plants include sugarcane, corn, and many grasses.
• They fix carbon using different types of cells and reactions than C
3 plants and do not run out of CO
2 even in hot weather.
• CO
2 becomes trapped in cells called bundle-sheath cells .
Phosphoenolpyruvate (PEP)
Oxaloacetate
PP i
+
AMP
P i
+
Mesophyll cell
ATP
Pyruvate Malate
Pyruvate Malate
CO
2
Calvin cycle
Glucose
Bundlesheath cell

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 C
4 and the C
3 pathway at night pathway during the day.

COMPARING CARBON FIXATION IN C
4
AND CAM PLANTS
CO
2
CO
2
Mesophyll cell
Bundlesheath cell
C
4 pathway
CO
2
Calvin cycle
C
4 pathway
Night
CO Mesophyll
2 cell
Calvin cycle
Day
Glucose
C
4 plants
Glucose
CAM plants