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Where It Starts Photosynthesis

Chapter 7 Part 1

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7.1 Sunlight as an Energy Source

Photosynthetic organisms use pigments to

capture the energy of sunlight

Photosynthesis

The synthesis of organic molecules from

inorganic molecules using the energy of light

Chlorophyll has to be present as an enzyme

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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 wavelengths are more energy filled

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Electromagnetic Spectrum

of Radiant Energy

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The Rainbow Catchers

Different wavelengths form colors of the rainbow Photosynthesis uses wavelengths of 380-750 nm

Thats the range of visible light for us

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-reflected to us)

Are other photosynthetic pigments (Chlorophyll b, etc.)

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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

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7/54Fig. 7-3a, p. 109

Beautiful fall colors they were obscured by chlorophyll before

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7.2 Exploring the Rainbow

Engelmann identified major colors of light that

drive photosynthesis (violet and red) by using a

prism to divide light into colors

Algae using these WL gave off the most oxygen

This attracted oxygen-seeking organisms in water

An absorption spectrum shows which

wavelengths a pigment absorbs best

Organisms in different environments use different

pigments

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9/54Fig. 7-4b, p. 110

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10/54Fig. 7-4c, p. 110

phycoerythrobilin

100 chlorophyll b phycocyanobilin

80

-carotenechlorophyll a

60

40

20Lightabsorption(%)

0

Wavelength (nanometers)

C Absorption spectra of a few photosynthetic pigments. Line

color indicates the characteristic color of each pigment.

400 500 600 700

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7.1-7.2 Key Concepts:

The Rainbow Catchers

The flow of energy through the biosphere startswhen chlorophylls and other photosynthetic

pigments absorb the energy of visible light

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7.3 Overview of Photosynthesis

Chloroplast

An organelle that specializes in photosynthesis in plants

and many protists

Made by plant as needed; has active chlorophyll

Stroma and thylakoid of the chloroplast

Sunlight energy is captured in inner thylakoid membrane

Stroma is a semifluid matrix surrounded by the two outermembranes of the chloroplast

Sugars are built in the stroma using energy from thylakoid

Result is C6H12O6, - glucose

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Overview of Photosynthesis

Thylakoid membrane

Folded membrane that make up thylakoids

Contains clusters of light-harvesting pigmentsthat absorb photons of different energies

Photosystems (type I and type II)

Groups of molecules that work as a unit to begin

the reactions of photosynthesis

Convert light energy into chemical energy

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Overview of Photosynthesis

Light-dependent reactions (gotta have sun)

Light energy is transferred to ATP and NADPH

Water molecules are split, releasing O2 Electrons are released from water split and they

are used to supercharge the energy storage

Light-independent reaction (sun not needed)

Energy in ATP and NADPH drives synthesis of

glucose and other carbohydrates from CO2 and

water

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Photosynthesis Equation

12H2O + 6CO2 sun / chlorophyll C6H12O6 + 6O2 + 6H2O

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Sites of Photosynthesis

in Plants

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18/54Fig. 7-5c, p. 111

sunlight O2 H2O CO2

CHLOROPLAST

light-

dependentreactions

NADPH, ATP

NADP+, ADP

light-

independentreactions

sugars

CYTOPLASM

C In chloroplasts, ATP and NADPH form in the light-dependent stage of

photosynthesis, which occurs at the thylakoid membrane. The second

stage, which produces sugars and other carbohydrates, proceeds in the

stroma.

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7.4 Light-Dependent Reactions

In the first stage of photosynthesis, light energy

drives electrons out of photosystems

The electrons may be used in a noncyclic or

cyclic pathway of ATP formation

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Capturing Energy for Photosynthesis

Photons boost electrons in pigments to higher

energy levels thus storing more sun energy

Light-harvesting complexes absorb the energy

Electrons are released from special pairs of

chlorophyll a molecules in photosystems

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The Thylakoid Membrane

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ADP + Pi

NADP+ Light-dependent reactions(noncyclic pathway)

H2O O2

NADPH

ATP

ADP + Pi Light-dependent reactions

(cyclic pathway)

ATP

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Replacing Lost Electrons

Electrons lost from photosystem II are replaced by

photolysis of water molecules, which dissociate

into hydrogen ions and oxygen

Photolysis

Process by which light energy breaks down a

molecule such as water

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Electron Flow In Noncyclic Pathway

Electrons lost from a photosystem enter an

electron transfer chain in the thylakoid

membrane

Electron transfer chains

Organized arrays of enzymes, coenzymes, and

other proteins that accept and donate electrons ina series

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Harvesting Electron Energy

Light energy is converted to chemical energy Entry of electrons from a photosystem into the

electron transfer chain is the first step in light-

dependent reactions

ATP forms in the stroma

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

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Noncyclic Pathway Of Photosynthesis

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Electron Flow In 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

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7.5 Energy Flow In Photosynthesis

Energy flow in the light-dependent reactions isan example of how organisms harvest energy

from their environment

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Photophosphorylation

Photophosphorylation A light-driven reaction that attaches a phosphate

group to a molecule

Cyclic photophosphorylation

Electrons cycle within photosystem I

Noncyclic photophosphorylation

Electrons move from water to photosystem II, to

photosystem I, to NADPH

E Fl I

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Energy Flow In

Light-Dependent Reactions

7 3 7 5 K C t

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7.3-7.5 Key Concepts:

Making ATP And NADPH

Photosynthesis proceeds through two stages in

the chloroplasts of plants and many types of

protists

In the first stage, sunlight energy is converted to

the chemical bond energy of ATP

The coenzyme NADPH forms in a pathway that

also releases oxygen

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Where It Starts Photosynthesis

Chapter 7 Part 2

7 6 Light Independent Reactions:

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7.6 Light-Independent Reactions:

The Sugar Factory

The cyclic, light-independent reactions of the

Calvin-Benson cycle are the synthesis part of

photosynthesis (where sugar is put together)

Calvin-Benson cycle (the dark side)

Enzyme-mediated reactions that build sugars in

the stroma of chloroplasts Use energy from radiant energy captured by the

chloroplast in light dependent stage

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Carbon Fixation

Carbon fixation

Extraction of carbon atoms from inorganicsources (atmosphere) and incorporating them

into an organic molecule

Builds glucose from CO2 plus energy bonds

Uses bond energy of molecules formed in light-dependent reactions (stored ATP, NADPH)

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The Calvin-Benson Cycle

Enzyme rubisco attaches C from 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 with each turn of wheel

But takes 6 wheel turns to complete sugar for

normal growing conditions, with one carbon

added to growing glucose with each turn

Inputs And Outputs Of

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Inputs And Outputs Of

Calvin-Benson Cycle

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Calvin-Benson Cycle Summary

Each turn adds 1 carbon to sugar and rebuilds 5 carbon RuBP

7 7 Adaptations:

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7.7 Adaptations:

Different Carbon-Fixing Pathways

Environments differ, and so do details ofphotosynthesis

C3 plants most plants, with wilting when water

becomes in short supply, closing stomata

C4 plants drought resistant plants that can

store carbon in two places and resist wilting

longer

CAM plants desert plants, require little waterdue to ability to fix carbon only at night and thus

keep stomata closed during day

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Stomata

Stomata (not stroma)

Small openings through the waxy cuticle covering

epidermal surfaces of leaves and green stems

especially on lower surfaces

Allow CO2 in and O2 and water out

Close on dry days to minimize water loss; this

stops carbon attainment by preventing intake ofCO2 from air and leads to wilting

There are C3, C4, and CAM plants based upon

ability to deal with drought conditions

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C3 Plants

C3 plants

Plants that use only the CalvinBenson cycle to

fix carbon

Most plants are C3

Forms 3-carbon PGA in mesophyll cells

Used by most plants, but inefficient in dry weather

when stomata are closed Examples: most flowering and vegetable plants

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Photorespiration

When stomata are closed, CO2 needed for light-

independent reactions cant enter, O2 produced

by light-dependent reactions cant leave

Photorespiration

At high O2 levels, rubisco attaches to oxygen

instead of carbon

CO2 is produced rather than fixed

So efficiency lost and more turns needed to

make sugar product

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C4 Plants

C4 plants

Plants that have an additional set of reactions forsugar 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

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C3 And C4 Plant Leaves

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CAM Plants

CAM plants (Crassulacean Acid Metabolism)

Plants with an alternative carbon-fixing pathwaythat 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

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A CAM Plant Many People Grow

Jade plant (Crassula argentea)

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C3, C4, And CAM Reaction Summary

7 6-7 7 Key Concepts:

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7.6-7.7 Key Concepts:

Making Sugar By Photosynthesis

The second stage is the synthesis part of

photosynthesis, in which sugars are assembled

from CO2

The reactions use ATP and NADPH that form in

the first stage of photosynthesis as radiant

energy is captured from sunlight

Details of the reactions vary among organisms

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7.8 Photosynthesis And Atmosphere

The evolution of photosynthesis dramatically andpermanently changed Earths atmosphere

Oxygen comes only from photosynthesis and

prior to development in plants the atmosphere

did not contain oxygen

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Food Sources And Consumers

Autotrophs

Organisms that make their own food using

energy from the environment and inorganic

carbon

Heterotrophs

Organisms that get energy and carbon fromorganic molecules assembled by other organisms

that are autotrophs or feed on autotrophs

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Two Kinds Of Autotrophs

Chemoautotrophs

Extract energy and carbon from simple molecules

in the environment (hydrogen sulfide, methane)

Used before the atmosphere contained oxygen

Photoautotrophs

Use photosynthesis to make food from CO2 and

water, releasing O2

Allowed oxygen to accumulate in the atmosphere

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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

7.8 Key Concepts:

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7.8 Key Concepts:

Evolution And Photosynthesis

The evolution of photosynthesis changed the

composition of Earths atmosphere

New pathways that detoxified the oxygen by-

product of photosynthesis evolved

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The Carbon Cycle

Photosynthesis locks CO2 from the atmosphere

in organic molecules; aerobic respiration returns

CO2

to the atmosphere

A balanced cycle of the biosphere is the result

Humans burn wood and fossil fuels for energy,

releasing locked carbon into the atmosphere

Contributes to global warming, disrupting

biological systems as it is not balanced by Nature

7.9 Key Concepts:

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7.9 Key Concepts:

Global Warming?

Photosynthesis by autotrophs removes CO2from

the atmosphere; metabolism by all organismsputs it back into atmosphere

Human activities have disrupted this balance,

and contribute to global warming by releasinggreater amounts of CO

2than what can be

balanced by natural processes