http://botit.botany.wisc.edu

PhotosynthesisLecture 7

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Much of the text material in the lecture notes is from our textbook, “Essential Biology with Physiology” by Neil A. Campbell, Jane B.

Reece, and Eric J. Simon (2004 and 2008). I don’t claim authorship. Other sources were sometimes used, and are noted.

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Outline

• Our Sun• Energy production from biomass• Chloroplasts• The nature of light• Photosynthesis• Water-saving plant adaptations• Global warming• Ozone effects• The oxygen revolution• Words and terms to know• Possible test items

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Vincent Van Gogh, “Olive Trees and Yellow Sun”http:/www.vin.bg.yu

The Sun, trees, and grass are part of a fascinating story…

Morning

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Our Sun

• Our Sun accounts for 99.8 percent of the total mass of the solar system.• The Sun is a third-generation star composed of hydrogen, helium, and

small quantities of other elements formed in earlier stars.• Energy is generated through the nuclear fusion of hydrogen into helium.• The Sun formed about five billion years ago—it has enough hydrogen fuel

for another five billion years.

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Solar Image

http://students.utsi.edu

Sunlight enables life on earth through photosynthesis. It also drives the Earth’s climate and weather patterns.

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A Few Statistics

• Earth is about 93 million miles from the Sun depending upon where we are in the elliptical orbit.

• Sunlight reaches us in 500 seconds.• The light from the next nearest star, Alpha Centauri, takes four years at

the speed of light, 186,000 miles per second.

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Mt. Wilson Solar Observatory

Daily records of solar activity have been kept for over 100 years.

Check-out the skycam at http://www.astro.ucla.edu/~obs/towercam.htm

http

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.sta

rimag

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Vegetation

Aspen grove and fernshttp://www.nbil.gov

An aspen grove has a common root system.

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Photosynthetic Organisms

• Plants and other photosynthetic organisms produce about 1.5 x 1011 tons of organic material each year by converting sunlight to chemical energy.

• All food consumed by humans can be traced to the process of photo-synthesis.

• Plants also produce other raw materials we use including wood, cotton, and paper.

• Coal and hydrocarbon fuels that power our automobiles, airplanes, and trains can be traced to early plant life.

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Energy from Biomass

• Energy plantations provide a source of renewable energy—tree species include willow, sycamore, eucalyptus, and black locust.

• The trees are cut every few years and used in power plants to produce electricity.

• Wood burned for energy has few of the sulfur impurities found in coal that produces acid rain in the northeastern United States and Canada.

• Tree plantations provide habitat for wildlife, reduce erosion, renew soil, and help farmers diversify.

• The rapidly growing young trees also remove CO2 from the atmosphere.

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Biomass Energy Cycle

http://www.repp.org

Energy produced from biomass currently accounts for about four percent of all energy consumed today in the United States.

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All forms of biomass ultimately derive their energy content from sunlight.

http://www.eia.doe.gov

Forms of Biomass

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Photosynthesis

• All green parts of plants can perform photosynthesis.• The green color of plants is from the chlorophyll pigments contained in

chloroplasts.• Chlorophyll absorbs the light energy that chloroplasts use for producing

chemical energy.

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Chloroplasts

• Since leaves typically have the most chloroplasts, they are the major sites of photosynthesis.

• Chloroplasts are concentrated in the cells of the mesophyll, the tissue in the interior of the leaf.

• Carbon dioxide enters and oxygen exits by way of stomata found on the undersurface of leaves.

• Roots provide the water, which travels to the leaves via veins, needed for photosynthesis.

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Internal Structure

http://micro.magnet.fsu.edu

Photosynthesis occurs in chloroplasts.

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Internal Structure

• Chloroplasts, like the mitochondria, have a double-membrane envelope. • The inner membrane encloses a compartment containing stroma, a thick

fluid.• Thylakoids are suspended in the stroma, and the chlorophyll molecules

are contained in their membranes.• The thylakoids are organized in elaborate stacked systems known as

grana.

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Microscopic View

The stroma, grana, and individual thylakoids are visible.

Electron micrographhttp://botany.wisc.edu

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Stomata

http://www.agr.gc.ca

Stomata are found primarily on the undersurface of leaves. The word means ‘mouth’ in Greek.

http://www.eoearth.org

Microscopic views

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Sunlight

PhotosynthesisChloroplasts in plants

Cellular RespirationMitochondria in animals and plants

CO2 (carbon dioxide)+ H2O (water)

C6H12O6 (glucose) + O2 (oxygen)

ATP

Cellular Work

http://img.dailymail.co.uk

http://pws.byu.edu

http://www.soquel.org

Chemical Cycle in Ecosystems

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C6H12O6

(glucose)+ 6O2 6CO2 + 6H2O ATP

(chemical energy)

+Cellular

Respiration

http://eurekalert.org

Glucosemolecule

htt

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ATP molecule

Cellular Respiration

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C6H12O6

(glucose)+ 6O26CO2 + 6H2O

Chloroplasts

http

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savi

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Photosynthesis

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Light Reaction and Calvin Cycle

• Photosynthesis is not one process, but two closely-related processes that each have many steps.

• The processes are the light reaction and Calvin cycle (also known as the dark reaction).

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Light Reaction and Calvin Cycle

Chloroplast

Light Reaction

Calvin Cycle

ATP and NADPH

NADP+ and ADP plus phosphorous

H2O

O2

CO2

Sunlight

Glucosehttp://eurekalert.org

Oxygen (waste

product)

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Light and Dark Reactions

• The light reaction process converts sunlight to chemical energy by splitting the covalent bonds in H2O.

• The chemical energy is stored in ATP and NADPH molecules—NADPH is related to NADH in cellular respiration.

• The Calvin cycle uses the energy-rich molecules from the light reaction to produce glucose from CO2.

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Other Photosynthetic Organisms

Algae

http

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edia

.org

Phytoplankton

http://www.dtplankton.com

Cyanobacteria

http

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The Nature of Light

• Light is a type of radiation, or electromagnetic energy, consisting of particles and waves.

• Photons are particles, and waves are measured by their wavelength.• A photon behaves as a discrete package of energy called a quantum.• Photons have greater energy in the shorter wavelengths of visible light

(blue, purple, and violet) than they do in the longer wavelengths (red and orange).

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Electromagnetic Spectrum

Increasing energy

Wavelength10-5 nm 10-3 nm 1 nm 103 nm 106 nm 1 m 103 m

Gamma rays X-rays Ultra-

violet Infrared Microwaves Radio waves

Violet Indigo Blue Green Yellow Orange Red

380 nm 750 nm

Visible light

Electromagneticspectrum

‘ROY G BIV’

R—RedO—OrangeY—YellowG—GreenB—BlueI—IndigoV—Violet

The light we see is a small slice of the electromagnetic spectrum.

Sunlight consists of 45 percent visible light, 46 percent infrared, and 9 percent

ultraviolet.

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Rainbow

http://www.missouriskies.org

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The Nature of Color

• The light shining on a pigmented material absorbs all wavelengths except those reflected from the object.

• The reflected light gives the object its color.• Plant leaves appear green because the pigments contained in chlorophyll

absorb the blue-violet and red-orange wavelengths.• Color is the brain’s processing and interpretation of wavelengths of light.

http

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tuw

ien.

ac.a

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Other Animals

• Some animals can sense somewhat different wavelengths than humans.• Honeybees sense ultraviolet light, which is used in communicating the

path to nectar-bearing flowers to other hive members.• Pit vipers, including rattlesnakes, use infrared radiation to find prey in the

night.

http

://jo

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Chloroplast Pigments

• The pigment in chlorophyll a absorbs blue-violet and red wavelengths.• Chlorophyll a participates in the light reaction process.• Chlorophyll b is sensitive to other wavelengths, and conveys energy to

chlorophyll a.

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Carotenoids

• Carotenoids in chloroplasts contain yellow-orange pigments that absorb blue-green light—their energy is also conveyed to chlorophyll a.

• The carotenoids in carrots and other vegetables can absorb excessive light energy to prevent damage to chlorophyll.

http://www.worldcommunitycookbook.org

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Autumn Color

Fuji-Hakone-Izu National Park, Japanhttp://www.tropicalislands.de

Color changes are partly due to decreases in green chlorophyll, allowing the red-orange hues of the carotenoids to show through

the epidermis of the leaves.

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Photosystem

• The pigments in thylakoids are organized into complex photosystems.• Each photosystem has several hundred elements of chlorophyll a,

chlorophyll b, and carotenoids.• The photosystem acts as a light gathering antenna—when a photon

strikes a pigment molecule, an electron in the molecule gains energy.

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Photosystem

http://fig.cox.miami.edu

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Harvesting of Light Energy

• The electron jumps from element to element in the photosystem until it arrives at the reaction center of the photosystem.

• The reaction center of chlorophyll a, is located adjacent to a primary electron acceptor.

• This acceptor traps the electron, and passes it to molecules in the thylakoid membrane to produce ATP from ADP, and NADPH from NADP+.

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• The Calvin cycle produces sugar molecules in the stroma of the chloro-plasts.

• As with the Krebs cycle, materials are regenerated with each turn of the cycle.

• Inputs are CO2 from the atmosphere, and ATP and NADPH produced by the light reaction process.

Calvin Cycle

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• The output of the Calvin cycle is the sugar molecule, glyceraldehyde 3-phosphate, or G3P.

• Plant cells use the energy-rich molecule as material to produce glucose and other organic molecules, including starches.

G3P Molecule

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Biochemistry of the Calvin Cycle

http

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ox.m

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i.edu

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Afternoon

http://users.tk.fi/jsreunan

Sunflowers offer a reminder of the importance of the Sun in the formation and sustainment of life.

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Varieties of Sunflowers

http://gmushrooms.com

Many varieties of sunflowers exist.

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Water-Saving Plant Adaptations

• Like animals, plants have adapted to their unique environments to help assure their survival.

• Different biological processes are employed, but with the same purpose of retaining water.

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Wheatfield

Eastern Montanahttp://dnrc.mt.gov

Wheat is a C3 plant.

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C3 Plants

• Some plants are able to thrive in dry or arid conditions due to adapta-tions of the photosynthesis process.

• Water-saving plants that use CO2 directly from the atmosphere are called C3 plants.

• C3 plants found in agriculture include wheat, soybeans, oats, and rice.

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C3 Plants

• C3 plants close their stomata on hot, dry days to reduce water loss to the atmosphere.

• The stomata closing prevents CO2 from entering the leaf, which slows or stops the Calvin cycle.

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Cornfield

Kansashttp://www.oznet.ksu.edu

Corn is a C4 plant.

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C4 Plants

• C4 plants can retain water in dry conditions without the photosynthesis process shutting-down.

• In hot, dry weather the stomata are closed most of the time although photosynthesis continues.

• C4 plants found in agriculture include corn, sorghum, and sugarcane.• These plants evolved in hot regions where frequent dry spells occur.

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C4 Plants

• C4 plants have an enzyme that incorporates an additional carbon from CO2 to form a four-carbon molecule in the Calvin cycle.

• This enzyme has an intense affinity (attraction) for CO2, to mine carbon dioxide from the air spaces of leaves even when the stomata are closed.

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Jade

Crassula Argenteahttp://www.botany.wisc.edu

Jade is a CAM plant.

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CAM Plants

• CAM plants are adapted to very dry environments, such as the deserts in many parts of the world.

• The stomata open at night to admit CO2 to minimize water loss. • CAM plants include cactus, succulents (named for their fleshy leaves),

and pineapple.

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CAM Plants

• CO2 molecules are incorporated into a four-carbon compound as in C4 plants.

• During the day, the four-carbon compound releases stored CO2 for use by the Calvin cycle when the stomata are closed.

• CAM is the acronym for crassulacean acid metabolism, named for the crassulaceae (jade plant) family in which this water-saving adaptation was first discovered.

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Global Warming

• Our planet’s atmosphere is getting warmer, and it is expected to get even warmer during this century.

• Global warming is likely to lead to substantial changes in the climate and biosphere.

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Some of the Consequences

• Severe weather patterns• Rising sea levels• Higher sea temperatures—important in hurricane generation• Desertification• Reduced agricultural output and food production in many regions• Species extinction• Insect spread—such as mosquitoes • Dislocation of human populations• Unforeseen and other undesirable consequences

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Top of the World

http://upload.wikimedia.org

Extent of the Arctic icepack in winter, 1978 through 2004 (shown in light blue).

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Summertime

http://www.nytimes.com

The USS Queenfish at the North Pole on August 6, 1970In the near future, the Arctic Ocean could be ice-free during the

summer.

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At Risk

Polar bears near the North Pole observed from the submarine, USS Honolulu.

http://upload.wikimedia.org

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Open Ocean

http://www.greenparty.ca

Will the polar bear survive other than in zoos?

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Carbon Dioxide and Methane

• The increasing amount of CO2 in the atmosphere is strongly implicated in global warming.

• Carbon dioxide is a by-product of using hydrocarbon (fossil) fuels.• Methane (MH4) is also implicated. • The causes of global warming is no longer a matter of serious scientific

dispute.

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Sources

• On the average, every person in the United States is responsible for releasing substantial amounts of CO2 into the atmosphere each year.

• The amount is much higher than other countries except China, which recently overtook the U.S.

• Europe averages about one-half as much CO2 release per person as the United States.

• MH4 is a by-product of grazing animals.

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Incident and Reflected Light

Eastern Sierra Nevadahttp://doe-mbi.ucla.edu

http

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peds

pace

.com

Incident light

Reflected light

Blue surface

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Refracted Light

Water glass and strawshttp://cache.eb.com

http://www.btinternet.com

Incident light

Refracted light

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Greenhouse

http://www.acmecompany.com

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Greenhouse Effect

• Global warming results from the greenhouse effect. • Sunlight has the full spectrum of visible light, and infrared and ultraviolet

wavelengths.• Some of the sunlight is reflected from the glass panes of a greenhouse.• Much of the light will be transmitted (refracted) through the panes, and

the IR will warm the interior.

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Greenhouse Effect (continued)

• Some of the light will be reflected from the interior of the greenhouse and retransmitted through the glass into the atmosphere.

• Still, some of this light will be reflected from the interior of the glass panes back into the greenhouse.

• The IR component will lead to even more heating of the greenhouse interior.

• The effect is similar to global warming, but with glass panes rather than CO2, methane, and other gases trapping heat from IR radiation.

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On a Global Scale

http://www.astro.virginia.edu

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Beneficial Role of Forests

• Since plants use CO2 in photosynthesis, they could help ameliorate the problem to some degree.

• Many vast forests, however, are being cut or burned to provide material for wood products and land for agricultural.

• Rainforests are being clear-cut in the Amazon River basin and coastal areas of British Columbia.

• With global warming, we are facing a very serious crisis to our biosphere, unlike any in the past.

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Deforestation in a Tropical Rainforest

Satellite image of the Amazon River Basinhttp://www.mongabay.com

Clear-cutting of land for farming.

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Deforestation in a Temperate Rainforest

British Columbia, Canadahttp://www.midwestearthbuilders.com

Clear-cutting of spruce trees for wood products.

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Ozone

• Ozone is a component of pollution formed as a by-product of the burning of fossil fuels.

• Ozone can cause mild-to-serious respiratory problems.• The ozone from ground-level pollution does not enter the upper atmos-

phere.

http://www.abc.net.au

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Our Atmosphere

http://www.fas.org

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Ozone Formation

• Ozone in the upper atmosphere screens-out high-energy, ultraviolet (UV) light rays from the Sun, which can be harmful to living cells.

• Oxygen is an allotrope—it exists in atomic form (O), as oxygen gas (O2), and as ozone (O3).

• Intense UV light in the upper atmosphere breaks the covalent bond in O2 to form O that bonds with O2 to form O3.

• Although ozone molecules have a short life, the reaction occurs frequently enough to form a protective layer.

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Chlorinated Fluorocarbons

• Chlorinated fluorocarbons (CFCs) include the gases, freon and halon.• CFCs were used in refrigeration systems, fire extinguishers, and as

propellant in spray paint, hairspray, and aerosol medicines, among other products.

• CFCs enter the upper atmosphere and break-down ozone molecules, diminishing or destroying the protective layer.

http://www.arcaneminiatures.co.uk

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Antarctic Ozone Hole

Computer-generated image of low ozone concentrations over Antarctica as observed from the NASA Aura satellite.

http://www.nasa.gov

Blue = low ozone levels

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Ozone Hole

• An ozone hole is evident over the Antarctic landmass and ocean. • The hole means the concentration of ozone is relatively low compared

to other regions of our planet.• The problem is widespread, just more so in the Antarctic.• The ozone hole has expanded over the past 30 years—the hole also

changes in size with the seasons, being larger in the austral (southern) winter.

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Expansion Over Time

A series of illustrations show the expansion of the ozone hole over the Antarctic continent and ocean from October 1970 to

October 1998.

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Data from the European Space Agency Nimbus satellitehttp://upload.wikimedia.org

Ozone levels are highest at the blue-end of the color spectrum in the diagram.

Altitude Differences

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Effects on Life

• Prolonged exposure to UV radiation can produce cataracts of the eyes that can lead to visual impairment including blindness.

• UV can also damage the DNA in the cell nuclei and cause skin cancer, or melanoma.

• Some cities at higher latitudes in the southern hemisphere advise their populations to wear sunglasses and cover-up their skin during the day.

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Sunglasses Needed

A public sign posted along streets in Melbourne, Victoria, Australia—the city has about 3.7 million residents.

http://www.health.vic.au

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Fashion Trend?

http://www.wholeearthprovisions.com

Sun-protective clothing

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CFC Phase-out

• CFCs are being phased-out of production and replaced with non-destructive compounds because of their danger to the biosphere.

• International cooperation on solving this issue has been substantial.• Although progress has been made, many years will pass before the

CFCs in the upper-atmosphere are no longer a threat to the ozone layer.

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Anaerobic Life

Bacteria methanospirillumhttp://www.nrc-cnrc.gc.ca

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The Oxygen Revolution

• The oxygen we breathe is the by-product of the light reaction process in photosynthesis.

• The first organisms that could produce O2, through photosynthesis, were the prokaryotes known as cynanobacteria.

• These microorganisms evolved between 3.4 and 2.5 billion years ago.• As their numbers grew, O2 gradually accumulated in the atmosphere.

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The Oxygen Revolution

• Many prokaryotic species died-off since oxygen is corrosive to organic molecules.

• Some species survived in anaerobic conditions including deep in the soil where their descendants live today.

• Other organisms adapted to the new environment and put O2 to use in extracting energy from food, including today’s animal and plants species.

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Evening

http://www.unsanity.org

The first part of the lecture is almost over.

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Nighttime

http://www.saratogaskies.com

About 100 million (108) stars similar to our Sun exist in our galaxy, the Milky Way of about 10 billion (1010) stars.

Does photosynthesis occur elsewhere?

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Words and Terms to Know

• Biomass• Calvin cycle• CAM plant• Carotenoid• CFC• Chlorophyll• Chloroplast• C3 plant• C4 plant• Deforestation• Electromagnetic spectrum• Global warming• Greenhouse effect• G3P

•Light reaction•Mesophyll•Ozone•Ozone hole•Photon•Photosynthesis•Photosystem•Primary electron acceptor•Reaction center•Stomata•Stroma•Thylakoid•UV wavelengths•Visible light

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Possible Test Items

1. Describe five features of chloroplasts and their functions in the photo-synthesis process.

2. Describe how the light reaction produces oxygen, and how the Calvin cycle produces sugars.

3. Describe three water-saving adaptations of plants and two plants that use each process.

4. Describe mechanisms that produce the greenhouse effect and global warming.

5. Describe how ozone forms in the upper atmosphere, and the protective function it serves.

6. Describe how scientists think oxygen first formed on Earth, and how the earliest organisms survived and adapted.