4 Lecture 4 EN3 Photosynthesis

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    Photosynthesis

    lecture 4 EN 3Mamta Awasthi

    CEEE, NIT Hamirpur

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    LEARNING OBJECTIVES Understand that ENERGY can be transformed from one form to another. Know that energy exist in two forms; free energy - available for doing work or as

    heat - a form unavailable for doing work. Explain why photosynthesis is so important to energy and material flow for life onearth.Autotrophs (Photo and chemo) and Heterotrophs Know why plants tend to be green in appearance. Raw Material for photosynthesis Describe the organization of the chloroplast (Site of Photosynthesis)

    Understand the terms (Thylakoids, Grana, Stroma, Pigments, Chlorophyll a, b,Carotenoids) Reaction Centre, Accessory Pigments Understand that photosynthesis is a two fold process composed of the light-

    dependent reactions (i.e., light reactions) and the light independent reactions(i.e. Calvin Cycle or Dark Reactions).

    Photosystem I, Photosystem II Photolysis of water Electron Transport Chain Photophosphorylation Cyclic Photophosphorylation Non-cyclic photophosphorylation

    Tell where the light reactions and the CO 2 fixation reactions occur in thechloroplast.

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    Energy can be transformed from oneform to another

    FREE ENERGY(available for work)

    vs.HEAT

    (not available for work)

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    Electromagnetic Spectrum and Visible Light

    Gammarays X-rays UV

    Infrared &Microwaves Radio waves

    Visible light

    Wavelength (nm)

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    Different wavelengths of visible light are seen by the

    human eye as different colors.

    WHY AREPLANTS GREEN?

    Gammarays

    X-rays UV Infrared Micro-waves

    Radiowaves

    Visible light

    Wavelength (nm)

    Light is a form of electromagnetic radiation. Visible light is acombination of many wavelengths that we can see as different colors(of the rainbow) in the range of 380 - 750 nm. Each wavelength isassociated with a specific photon, or particle of energy. In general,shorter wavelengths have more energy.

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    Why are plants green?

    Transmitted light

    Light Waves

    The light absorbing photosynthetic pigments do notabsorb all wavelengths of light equally. Some lightenergy cannot be absorbed (and is reflectedinstead) and some is transmitted, or passed throughthe chloroplasts.

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    Chloroplasts absorb light energyand convert it to chemicalenergy

    Light Reflected

    light

    Absorbedlight

    Transmittedlight

    Chloroplast

    THE COLOR OF LIGHT SEEN IS THE COLOR NOTABSORBED

    The absorption of different wavelengths,or absorption spectrum, of light by thephotosynthetic pigments can bedemonstrated in the laboratory usingspectrophotometers . The light wavesmost absorbed and most useful to

    photosynthesis are reds and blues.One can also measure rates of photosynthesis in different wavelengthsto generate an action spectrum. This isdone by growing plants in light boxesthat have just one wavelength of light.

    Not surprisingly, green light is absorbed poorly.

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    Relevance of Photosynthesis: THE FOOD WEB

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    Not all autotrophs are photosynthetic; a tinyproportion of our organic fuel is manufactured bychemosynthetic organisms (or chemoautotrophs).

    Chemosynthetic autotrophs use energy fromchemical reactions involving inorganic atoms and

    molecules, such as S, Fe, H and N, to make organiccompounds.

    Chemosynthesis is restricted to a very few groups of

    bacteria, mostly the Archaea. However,chemosynthesis sustains some deep sea-bedecosystems that surround sulfur vents.

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    Organisms that obtain energy and carbon from theirphysical surroundings are autotrophs

    The vast majority of autotrophs are photoautotrophsthat manufacture their organic molecules by theprocess of photosynthesis.

    Organisms that obtain their organic fuel moleculespre-formed from the environment are heterotrophs.Animals, fungi, many protists and many bacteria areheterotrophs.

    Most photosynthetic organisms are plants or protiststhat contain chlorophyll. Many prokaryotes are alsophotosynthetic. The Cyanobacteria have chlorophyllpigments; some Bacteria, such as the purple sulfurbacteria, have different light-capturing pigments, andslightly different photosynthetic mechanisms.

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    Almost all plants are photosynthetic autotrophs, as are

    some bacteria and protists Autotrophs generate their own organic matter throughphotosynthesis

    Sunlight energy is transformed to energy stored in the form of

    chemical bonds

    (a) Mosses, ferns, and flowering plants

    (b) Kelp

    (c) Euglena (d) Cyanobacteria

    THE BASICS OF PHOTOSYNTHESIS

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    The location and structure of chloroplasts

    LEAF CROSS SECTIONMESOPHYLL CELL

    LEAF

    Chloroplast

    Mesophyll

    CHLOROPLAST Intermembrane space

    Outermembrane

    Innermembrane

    ThylakoidcompartmentThylakoid

    Stroma

    Granum

    StromaGrana

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    Chloroplasts: Sites of Photosynthesis

    A chloroplast contains:stroma, a fluidgrana, stacks of thylakoids

    The thylakoids contain chlorophyllChlorophyll is the green pigment thatcaptures light for photosynthesis

    In most plants, photosynthesis occurs

    primarily in the leaves, in the chloroplastsOccurs in chloroplasts, organelles incertain plantsAll green plant parts have chloroplastsand carry out photosynthesis The leaves have the most chloroplasts The green color comes from chlorophyllin the chloroplasts The pigments absorb light energyLight energy is captured by the pigmentslocated on the special membranes in thechloroplast called thylakoids, which arefolded into disk-shaped stacks calledgrana. The interior compartments of thylakoidsserve as reservoirs for hydrogen ions (H+)that are needed for producing ATP.

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    Chloroplasts contain several pigments

    Chloroplast Pigments Chlorophyll a Chlorophyll b Carotenoids

    In plants, there are two forms of chlorophyll (a, which has a methyl group, andb, which has an aldehyde group) as well as important accessory pigments, thecarotenes.

    Each pigment absorbs certain wavelengths, and collects and concentrates lightenergy for the photosynthetic process. In addition the carotenes may functionto protect chlorophyll molecules from being damaged by too intense light. Thered and blue phycocyanin pigments can also absorb light and serve as

    accessory pigments in photosynthesis.

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    Chlorophyll a & b Chl a has a methylgroup

    Chl b has a carbonylgroup

    Porphyrin ringdelocalized e -

    Phytol tail

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    Different pigments absorb light differently

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    Antenna complex The photosynthetic pigmentmolecules do not work alone. Theyare arranged on the thylakoids of thechloroplast in clusters of about 300pigment molecules in a protein matrixto form a light-harvesting or antennacomplex that gathers and transfersenergy to a photochemical reactioncenter, embedded in atransmembrane protein complex.The reaction center has a specialchlorophyll a molecule along withthe primary electron acceptor (which

    accepts the electrons released fromchlorophyll a).There are two such complexes foundin the chloroplasts, calledPhotosystem I and Photosystem II.

    Each has a unique electron acceptor.

    The reaction centers of Photosystems Iand II are activated by slightly differentwavelengths of light. The reaction center

    in Photosystem I absorbs light of 700nmbest. The reaction center of PhotosystemII absorbs wavelengths of 680nm. Eachthylakoid has thousands of the twodifferent photosystems. Both are neededfor photosynthesis.

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    A Photosynthesis Road Map

    Chloroplast

    Light

    Stack of thylakoids ADP

    + P

    NADP

    Stroma

    Lightreactions

    Calvincycle

    Sugar used for

    Cellular respiration CelluloseStarchOther organic compounds

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    The Calvin cycle makessugar from carbon dioxide

    ATP generated by the lightreactions provides the energy for

    sugar synthesis The NADPH produced by the light

    reactions provides the electronsfor the reduction of carbondioxide to glucose

    Light

    Chloroplast

    Lightreactions

    Calvincycle

    NADP

    ADP+ P

    The light reactions

    convert solar energyto chemical energy

    Produce ATP & NADPH

    AN OVERVIEW OF PHOTOSYNTHESIS

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    Process of Photosynthesis Photosynthesis involves two stages, occurring in

    separate locations within chloroplasts. In the light-dependent reactions of photosynthesis,

    light energy is transformed into chemical energy in the

    form of energy transfer molecules in a series of redoxreactions that transfer electrons and hydrogen fromwater to the energy transfer molecule NADP+.

    The light-dependent reactions are known asphotophosphorylations, because they involveproducing ATP.

    They take place on the thylakoid membranes of thegrana.

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    Light-independent reactions In the Calvin cycle (sometimes called the

    light-independent reactions), the energy fromthe light reactions is used to manufacturecarbohydrate molecules, which form glucose.

    These reactions occur in the stroma of thechloroplast.

    The two "stages" of photosynthesis are linkedby the products of the light dependentreactions.

    These products are ATP and NADPH.

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    The overall chemical equation for

    photosynthesis* is:Chlorophyll

    6CO2 + 12H 2O + energy C6H12 O6 + 6H2O + 6O 2Chlorophyll

    (Carbon dioxide + water + light energy glucose + water+ oxygen)

    The process of photosynthesis directly produces 12molecules of the 3-carbon compound, glyceraldehyde-3-phosphate (G3P).

    Two of these molecules are metabolized to glucoseduring a "complete" photosynthesis. The remaining 10 molecules of G3P are recycled in the

    photosynthetic process.

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    Chlorophyll

    molecule

    2

    (b) fluorescence of isolated chlorophyll in solution

    Excitation of chlorophyll ina chloroplast

    Loss of energy due to heat causesthe photons of light to be lessenergetic.

    Less energy translates into longer

    wavelength.Energy = (Plancks constant) x

    (velocity of light)/(wavelength of light)

    Transition toward the red end of thevisible spectrum.

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    When pigment molecules absorb energy from light,electrons in the molecules are excited and moved to ahigher energy orbital in an excited state.

    The energy level of a photon absorbed and the rise inenergy level to the higher energy orbital must match, whichis why only certain wavelengths can be absorbed by certainpigments.

    This rise in energy is temporary. Electrons fall back to theirground state if the energy is not transferred elsewhere.

    When the electrons fall back to their ground state energy isgiven off as heat, or sometimes as light (fluorescence).Chlorophyll a does both,

    fluorescing as red light.

    El t T t S t M l l

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    Electron Transport System Molecules(Energy Transfer Molecules)

    many chemical reactions of metabolism are coupledoxidation-reductions that utilize a chain of electrontransport molecules.

    These electron carriers specialize in oxidizing and reducingat specific energy levels to minimize energy loss in energytransfers. Both photosynthesis and cellular respiration relyon such molecules. In the process of photosynthesis, theelectron transport carriers are embedded in the thylakoidmembranes.

    Some of the molecules which specialize in these energytransfers in the thylakoids are:

    NADP+, plastoquinone, two cytochromes, plastocyanin and ferredoxin

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    Water-splittingphotosystem

    NADPH-producingphotosystem

    ATPmill

    Two types of photosystemscooperate in the lightreactions

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    The O 2 liberated by photosynthesis is made fromthe oxygen in water (H + and e -)

    Plants produce O 2 gas by splitting H 2O

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    Photosynthesis & Electron Transport

    Cyclic Photophosphorylation

    PS IGenerates H+ gradient (used for ATP synthesis) only.

    Non-cyclicPhotophosphorylation Both PS I and PS II Generates H+ gradient (used for ATP synthesis) and

    NADPH.

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    Two connected photosystems collect photons of light and transfer the energy to chlorophyllelectrons

    The excited electrons are passed from theprimary electron acceptor to electron transportchains

    Their energy ends up in ATP and NADPH

    In the light reactions, electron transportchains generate ATP, NADPH, & O 2

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    Cyclic Photophosphorylation Process for ATP generation associated with some

    Photosynthetic Bacteria Reaction Center => 700 nm

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    Primaryelectron acceptor

    Primaryelectron acceptor

    Photons

    PHOTOSYSTEM I

    PHOTOSYSTEM II

    Energy forsynthesis of

    by chemiosmosis

    Noncyclic Photophosphorylation Photosystem II regains electrons by splitting water,

    leaving O 2 gas as a by-product

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    The electron transport chains are arranged withthe photosystems in the thylakoid membranesand pump H + through that membrane

    The flow of H + back through the membrane isharnessed by ATP synthase to make ATP

    In the stroma, the H + ions combine with NADP + to

    form NADPH

    Chemiosmosis powers ATP synthesis in thelight reactions

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    The production of ATP by chemiosmosis inphotosynthesis

    Thylakoidcompartment(high H +)

    Thylakoidmembrane

    Stroma(low H +)

    Light

    Antennamolecules

    Light

    ELECTRON TRANSPORTCHAIN

    PHOTOSYSTEM II PHOTOSYSTEM I ATP SYNTHASE

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    Review: Photosynthesis uses light energy tomake food molecules

    Light

    Chloroplast

    Photosystem IIElectron transport

    chainsPhotosystem I

    CALVINCYCLE Stroma

    LIGHT REACTIONS CALVIN CYCLE

    Cellularrespiration

    Cellulose

    Starch

    Other organiccompounds

    A summary of the chemicalprocesses of photosynthesis

    http://www.flickr.com/photos/ghonet/27421952/
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    It's not that easybein' green but

    it is essential forlife on earth!

    http://www.flickr.com/photos/ghonet/27421952/http://www.flickr.com/photos/ghonet/27421952/http://www.flickr.com/photos/ghonet/27421952/http://www.flickr.com/photos/ghonet/27421952/http://www.flickr.com/photos/ghonet/27421952/http://www.flickr.com/photos/ghonet/27421952/