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Chapter 6

Where It Starts –

Photosynthesis

6.1 Biofuels

• Coal, petroleum, and natural gas are fossil fuels – the

remains of ancient forests, a limited resource

• Biofuels such as oils, gases, or alcohols are made from

organic matter that is not fossilized – a renewable resource

• In the United States, biofuels are produced mainly from food

crops such as corn, soybeans, and sugarcane

• Researchers are looking for ways to use non-food-plant

matter such as switchgrass and agricultural wastes

Biofuels Research

Autotrophs and Heterotrophs

• Autotrophs harvest energy directly from the environment,

and obtain carbon from inorganic molecules

• Plants and most other autotrophs make their own food by

photosynthesis, a process which uses the energy of sunlight

to assemble carbohydrates from carbon dioxide and water

• Animals and other heterotrophs get energy and carbon by

breaking down organic molecules assembled by other

organisms

6.2 Sunlight as an Energy Source

• Energy flow through nearly all ecosystems on Earth begins

when photosynthesizers intercept energy from the sun

• Photosynthetic organisms use pigments to capture the energy

of sunlight and convert it to chemical energy – the energy

stored in chemical bonds

Properties of Light

• Visible light is part of an electromagnetic spectrum of energy

radiating from the sun

• Travels in waves

• Organized into photons

• Wavelength

• The distance between the crests of two successive waves

of light (nm)

• Shorter wavelength have greater energy

Electromagnetic Spectrum

of Radiant Energy

shortest wavelengths

(highest energy) range of most radiation

reaching Earth’s surface

range of heat

escaping from

Earth’s surface

longest wavelengths

(lowest energy)

visible light

gamma

rays x-rays

ultraviolet

radiation

near-infrared

radiation

radiation

infrared microwaves radio waves

400 nm 500 nm 600 nm 700 nm

Wavelength and Energy

Pigments: The Rainbow Catchers

• Different wavelengths form colors of the rainbow

• Photosynthesis uses wavelengths of 380-750 nm

• Pigment

• An organic molecule that selectively absorbs light of

specific wavelengths

• Chlorophyll a

• The most common photosynthetic pigment

• Absorbs violet and red light (appears green)

Photosynthetic Pigments

• Collectively, chlorophyll and accessory pigments absorb most

wavelengths of visible light

• Certain electrons in pigment molecules absorb photons of

light energy, boosting electrons to a higher energy level

• Energy is captured and used for photosynthesis

Some Photosynthetic Pigments

Take-Home Message:

How do photosynthesizers absorb light?

• Energy radiating from the sun travels through space in waves

and is organized as packets called photons

• The spectrum of radiant energy from the sun includes visible

light; humans perceive different wavelengths of visible light as

different colors; the shorter the wavelength, the greater the

energy

• Pigments absorb light at specific wavelengths; photosynthetic

species use pigments such as chlorophyll a to harvest the

energy of light for photosynthesis

6.3 Exploring the Rainbow

• Photosynthetic pigments work together to harvest light of

different wavelengths

• Engelmann identified colors of light that drive photosynthesis

(violet and red) by using a prism to divide light into colors –

algae using these wavelengths gave off the most oxygen

Photosynthesis and

Wavelengths of Light

bacteria

alga

400 nm 500 nm 600 nm 700 nm

Wavelength

ANIMATED FIGURE: T. Englemann's

experiment

videos_animations/Englemann.html

Absorption Spectra

• Most photosynthetic organisms use a combination of

pigments to drive photosynthesis

• An absorption spectrum shows which wavelengths each

pigment absorbs best

• Organisms in different environments use different pigments

Absorption Spectra

chlorophyll b

phycoerythrobilin

β-carotene chlorophyll a

400 nm 500 nm 600 nm 700 nm

Wavelength

phycocyanobilin

Lig

ht

ab

so

rpti

on

Take-Home Message: Why do cells use more

than one photosynthetic pigment?

• A combination of pigments allows a photosynthetic organism

to most efficiently capture the particular range of light

wavelengths that reaches the habitat in which it evolved

6.4 Overview of Photosynthesis

• In plants and other photosynthetic eukaryotes, photosynthesis

occurs in chloroplasts

• Photosynthesis occurs in two stages

Two Stages of Photosynthesis

• Light-dependent reactions (noncyclic pathway)

• First stage of photosynthesis

• Light energy is transferred to ATP and NADPH

• Water molecules are split, releasing O2

• Light-independent reactions

• Second stage of photosynthesis

• Energy in ATP and NADPH drives synthesis of glucose

and other carbohydrates from CO2 and water

Summary: Photosynthesis

6CO2 + 6H2O → light energy → C6H12O6 + 6O2

The Chloroplast

• Chloroplast

• An organelle that specializes in photosynthesis in plants

and many protists

• Thylakoid membrane

• Folded membrane that make up thylakoids

• Contains clusters of light-harvesting pigments that absorb

photons of different energies and convert light energy into

chemical energy (first stage of photosynthesis)

The Chloroplast

• Stroma

• A semifluid matrix surrounded by the two outer

membranes of the chloroplast

• Sugars are built in the stroma (second stage of

photosynthesis)

Figure 6-5b p105

two outer membranes

of chloroplast

stroma

part of thylakoid

membrane system:

thylakoid

compartment,

cutaway view

INTERACTION: Structure of a chloroplast

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videos_animations/chloroplast_dnd.html

Take-Home Message: Where do the

reactions of photosynthesis take place?

• In the first stage of photosynthesis, light energy drives the

formation of ATP and NADPH, and oxygen is released; in

eukaryotic cells, these light-dependent reactions occur at the

thylakoid membrane of chloroplasts

• The second stage of photosynthesis, the light-independent

reactions, occur in the stroma of chloroplasts; ATP and

NADPH drive the synthesis of carbohydrates

3D ANIMATION: Photosynthesis Bio

Experience 3D

videos_animations/Photosynthesis/Photosynthesis.html

6.5 Light-Dependent Reactions

• Light-dependent reactions convert light energy to the energy

of chemical bonds

• Photons boost electrons in pigments to higher energy levels

• Light-harvesting complexes absorb the energy

• Electrons are released from special pairs of chlorophyll a

molecules in photosystems

• Electrons may be used in noncyclic or cyclic pathways of ATP

formation

Figure 6-7 p106

The Thylakoid Membrane

photosystem light-harvesting complex

The Noncyclic Pathway

• Photosystems (type II and type I) contain “special pairs” of

chlorophyll a molecules that eject electrons

• Electrons lost from photosystem II are replaced by photolysis

of water molecules – the process by which light energy

breaks down a water molecule into hydrogen and oxygen

• Electrons lost from a photosystem enter an electron transfer

chain (ETC) in the thylakoid membrane


• In the ETC, electron energy is used to build up a H+ gradient

across the membrane

• H+ flows through ATP synthase, which attaches a phosphate

group to ADP

• ATP is formed in the stroma by chemiosmosis, or electron

transfer phosphorylation


• Electrons from the first electron transfer chain (from

photosystem II) are accepted by photosystem I

• Electrons ejected from photosystem I enter a different

electron transfer chain in which the coenzyme NADP+ accepts

the electrons and H+, forming NADPH

• ATP and NADPH are the energy products of light-dependent

reactions in the noncyclic pathway

Noncyclic Pathway of Photosynthesis

to second stage

of reactions light energy electron transfer chain light energy

ATP

synthase

photosystem II photosystem I

thylakoid

compartment

stroma

H+

ADP + Pi

The Cyclic Pathway

• When NADPH accumulates in the stroma, the noncyclic

pathway stalls

• A cyclic pathway runs in type I photosystems to make ATP;

electrons are cycled back to photosystem I and NADPH does

not form

Take-Home Message: What happens during the

light-dependent reactions of photosynthesis?

• In light-dependent reactions, chlorophylls and other pigments

in thylakoid membrane transfer light energy to photosystems

• Photosystems eject electrons that enter electron transfer

chains in the membrane; electron flow through ETCs sets up

hydrogen ion gradients that drive ATP formation

• In the noncyclic pathway, oxygen is released and electrons

end up in NADPH

• A cyclic pathway involving only photosystem I allows the cell

to continue making ATP when the noncyclic pathway is not

running; NADPH does not form; O2 is not released

3D ANIMATION: Photophosphorylation

videos_animations/Photophosphorylation/photophosphorylation.html

ANIMATED FIGURE: Noncyclic pathway of

electron flow




videos_animations/noncyclic_v3.html

6.6 Energy Flow in Photosynthesis

• Energy flow in the light-dependent reactions is an example of

how organisms harvest energy from their environment

Photophosphorylation

• Photophosphorylation is a light-driven reaction that

attaches a phosphate group to a molecule

• In noncyclic photophosphorylation, electrons move from water

to photosystem II, to photosystem I, to NADPH

• In cyclic photophosphorylation, electrons cycle within

photosystem I

light energy

Excited P680

en

erg

y

P680

(photosystem II)

P700

(photosystem I)

Stepped Art

Energy flow in the noncyclic reactions of photosynthesis

light energy

Excited P700

Figure 6-9a p108

Energy flow in the cyclic reactions of

photosynthesis

P700

(photosystem I) light energy

Excited P700

en

erg

y

Stepped Art

Figure 6-9b p108

Take-Home Message: How does energy flow

during the reactions of photosynthesis?

• Light provides energy inputs that keep electrons flowing

through electron transfer chains

• Energy lost by electrons as they flow through the chains sets

up a hydrogen ion gradient that drives the synthesis of ATP

alone, or ATP and NADPH

6.7 Light-Independent Reactions

• The cyclic, light-independent reactions of the Calvin-Benson

cycle are the “synthesis” part of photosynthesis

• Calvin-Benson cycle

• Enzyme-mediated reactions that build sugars in the

stroma of chloroplasts

Carbon Fixation

• Carbon fixation

• Extraction of carbon atoms from inorganic sources

(atmosphere) and incorporating them into an organic

molecule

• Builds glucose from CO2

• Uses bond energy of molecules formed in light-dependent

reactions (ATP, NADPH)

The Calvin-Benson Cycle

• The enzyme rubisco attaches CO2 to RuBP

• Forms two 3-carbon PGA molecules

• PGAL is formed

• PGAs receive a phosphate group from ATP, and hydrogen

and electrons from NADPH

• Two PGAL combine to form a 6-carbon sugar

• Rubisco is regenerated

1

2

4

Stepped Art

Calvin–

Benson

Cycle

glucose 3 other molecules

Figure 6-10 p109

ANIMATED FIGURE: Photosynthesis

overview




videos_animations/photosynthesis_summary_v2.html

Take-Home Message: What happens in light-

independent reactions of photosynthesis?

• Light-independent reactions of photosynthesis run on the

bond energy of ATP and energy of electrons donated by

NADPH; both formed in the light-dependent reactions

• Collectively called the Calvin–Benson cycle, these carbon-

fixing reaction use hydrogen (from NADPH), and carbon and

oxygen (from CO2) to build sugars

6.8 Adaptations:

Different Carbon-Fixing Pathways

• Environments differ, and so do details of photosynthesis:

• C3 plants

• C4 plants

• CAM plants

Stomata

• Stomata

• Small openings through the waxy cuticle covering

epidermal surfaces of leaves and green stems

• Allow CO2 in and O2 out

• Close on dry days to minimize water loss

C3 Plants

• C3 plants

• Plants that use only the Calvin–Benson cycle to fix carbon

• Forms 3-carbon PGA in mesophyll cells

• Used by most plants, but inefficient in dry weather when

stomata are closed

• Example: barley

Photorespiration

• When stomata are closed, CO2 needed for light-independent

reactions can’t enter, O2 produced by light-dependent

reactions can’t leave

• Photorespiration

• At high O2 levels, rubisco attaches to oxygen instead of

carbon

• CO2 is produced rather than fixed

A C3 Plant: Barley

palisade

mesophyll cell

spongy

mesophyll cell

Figure 6-11b p110

mesophyll cell

CO2 O2

glycolate RuBP

Calvin–

PGA Cycle

Benson

ATP

NADPH

B On dry days, stomata close and oxygen accumulates

inside leaves. The excess causes rubisco to attach oxygen

instead of carbon to RuBP. This is photorespiration, and it

makes sugar production inefficient in C3 plants. sugars

C4 Plants

• C4 plants

• Plants that have an additional set of reactions for sugar

production on dry days when stomata are closed;

compensates for inefficiency of rubisco

• Forms 4-carbon oxaloacetate in mesophyll cells, then

bundle-sheath cells make sugar

• Examples: Corn, switchgrass, bamboo

A C4 Plant: Millet

mesophyll cell

bundle-sheath cell

Figure 6-12b p110

B C4 plants.

Oxygen also builds

up inside leaves

when stomata

close during

photosynthesis.

An additional

pathway in these

plants keeps the

CO2 concentration

high enough in

bundle-sheath

cells to prevent

photorespiration.

mesophyll cell CO2 from inside plant

oxaloacetate

bundle-sheath cell

Calvin–

Benson

Cycle PGA

RuBP

sugars

C4

Cycle

CO2

CAM Plants

• CAM plants (Crassulacean Acid Metabolism)

• Plants with an alternative carbon-fixing pathway that

allows them to conserve water in climates where days are

hot

• Forms 4-carbon oxaloacetate at night, which is later

broken down to CO2 for sugar production

• Example: succulents, cactuses

A CAM Plant: Jade Plant

Figure 6-13a p111

mesophyll cell CO2 from outside plant

oxaloacetate C4

Cycle

night

day CO2

RuBP

PGA

sugars

Calvin–

Benson

Cycle

ANIMATED FIGURE: Carbon-fixing

adaptations




videos_animations/carbon_fixing.html

Take-Home Message:

How do carbon-fixing reactions vary?

• When stomata are closed, oxygen builds up inside leaves of

C3 plants; rubisco then can attach oxygen (instead of carbon

dioxide) to RuBP; photorespiration reduces the efficiency of

sugar production, so it can limit the plant’s growth

• Plants adapted to dry conditions limit photorespiration by

fixing carbon twice: C4 plants separate the two sets of

reactions in space; CAM plants separate them in time

Biofuels Revisited

• The first cells on Earth were chemoautotrophs that extracted

energy and carbon from inorganic molecules in the

environment, such as hydrogen sulfide and methane

• The evolution of photosynthesis dramatically and permanently

changed Earth’s atmosphere

• Photoautotrophs use photosynthesis to make food from CO2

and water, releasing O2 into the atmosphere

Earth’s Early Atmosphere

Earth With an Oxygen Atmosphere

Effects of Atmospheric Oxygen

• Selection pressure on evolution of life

• Oxygen radicals

• Development of ATP-forming reactions

• Aerobic respiration

• Formation of ozone (O3) layer

• Protection from UV radiation

The Atmospheric Carbon Cycle

• Photosynthesis removes carbon dioxide from the atmosphere,

and locks carbon atoms in organic compounds

• Aerobic organisms break down organic compounds for

energy, and release CO2 into the atmosphere

• Since photosynthesis evolved, these two processes have

constituted a more or less balanced cycle of the biosphere

• Today, Earth’s atmosphere is out of balance – the level of

CO2 is increasing, mainly as a result of human activity

Fossil Fuels

• When we burn fossil fuels, carbon that has been locked for

hundreds of millions of years is released back into the

atmosphere, mainly as carbon dioxide

• Today, we release about 28 billion tons of carbon dioxide into

the atmosphere each year, more than ten times the amount

we released in the year 1900

• Increased atmospheric CO2 contributes to global warming

and disrupts natural biological systems

Fossil Fuel Emissions

Renewable Energy Sources

• Biofuels are a renewable source of energy

• The carbon in plant matter comes from atmospheric CO2,

fixed by photosynthesis

• Making biofuel production economically feasible is a high

priority for today’s energy researchers

• Research Homework: Research different forms of biofuels.

Which form seems the most reasonable choice: Biomass?

Biogas?

• Look into biofuel use in Brazil. Do you think the United States

should use their program as a template?

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