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Topic 3.4: Photosynthesis
Gas Exchange in Plant Leaves

  • Gases (primarily carbon dioxide and oxygen) pass in and out of plant leaves through small pores on the leaf surface called stomata (singular: stoma).

  • These stomata are regulated by guard cells that open and close them.

Structure of a Chloroplast

  • A chloroplast is an organelle found in plant cells and other eukaryotic photosynthetic organisms.

  • Its structure includes:

    • Outer and Inner Membranes: Enclose the chloroplast.

    • Stroma: The dense fluid within the inner membrane where the Calvin cycle takes place.

    • Thylakoids: Flattened, interconnected sacs suspended in the stroma. The light-dependent reactions occur here.

    • Grana (singular: granum): Stacks of thylakoids.

    • Lumen: The space inside the thylakoid sac.

Overall Equation for Photosynthesis

  • 6CO2 \ (Carbon~Dioxide) + 6H2O \ (Water) + Light~Energy \rightarrow C6H{12}O6 \ (Glucose) + 6O2 \ (Oxygen)</p></li></ul><h6id="fb2dce8f−8d85−4932−a37c−b42187691347"data−toc−id="fb2dce8f−8d85−4932−a37c−b42187691347"collapsed="false"seolevelmigrated="true">MajorPhasesofPhotosynthesis</h6><ul><li><p><strong>Light−DependentReactions</strong></p><ul><li><p><strong>Location</strong>:Thylakoidmembranesofthechloroplast.</p></li><li><p><strong>Reactants</strong>:Water(</p></li></ul><h6 id="fb2dce8f-8d85-4932-a37c-b42187691347" data-toc-id="fb2dce8f-8d85-4932-a37c-b42187691347" collapsed="false" seolevelmigrated="true">Major Phases of Photosynthesis</h6><ul><li><p><strong>Light-Dependent Reactions</strong></p><ul><li><p><strong>Location</strong>: Thylakoid membranes of the chloroplast.</p></li><li><p><strong>Reactants</strong>: Water (H_2O),lightenergy,ADP,), light energy, ADP,NADP^+.</p></li><li><p><strong>Products</strong>:Oxygen(.</p></li><li><p><strong>Products</strong>: Oxygen (O_2),ATP,NADPH.</p></li></ul></li><li><p><strong>CalvinCycle(Light−IndependentReactions)</strong></p><ul><li><p><strong>Location</strong>:Stromaofthechloroplast.</p></li><li><p><strong>Reactants</strong>:Carbondioxide(), ATP, NADPH.</p></li></ul></li><li><p><strong>Calvin Cycle (Light-Independent Reactions)</strong></p><ul><li><p><strong>Location</strong>: Stroma of the chloroplast.</p></li><li><p><strong>Reactants</strong>: Carbon dioxide (CO_2),ATP,NADPH.</p></li><li><p><strong>Products</strong>:Glucose(orG3P,whichleadstoglucose),ADP,), ATP, NADPH.</p></li><li><p><strong>Products</strong>: Glucose (or G3P, which leads to glucose), ADP,NADP^+.</p></li></ul></li></ul><h6id="59c0fed4−ce1c−4cc1−8b4c−cf1d5fd50c8c"data−toc−id="59c0fed4−ce1c−4cc1−8b4c−cf1d5fd50c8c"collapsed="false"seolevelmigrated="true">Pigments</h6><ul><li><p><strong>Pigments</strong>aresubstancesthatabsorbspecificwavelengthsoflightandreflectothers.</p><ul><li><p>Thecolorweseeisthewavelengthreflected.</p></li></ul></li><li><p><strong>MainPhotosyntheticPigmentsinPlants</strong>:Chlorophylla(theprimarypigment),chlorophyllb,andcarotenoids.</p><ul><li><p><strong>WhyPlantsAppearGreen</strong>:Chlorophyllpigmentsprimarilyabsorbredandblue−violetwavelengthsoflightandreflectgreenlight.</p></li></ul></li><li><p><strong>WavelengthsforPhotosynthesis</strong>:Wavelengthsthatare<em>absorbed</em>bypigmentscontributeenergytophotosynthesis.</p></li></ul><h6id="92de6d3e−c528−49b6−8ebf−bc2d97224f04"data−toc−id="92de6d3e−c528−49b6−8ebf−bc2d97224f04"collapsed="false"seolevelmigrated="true">Photosystem</h6><ul><li><p>Aphotosystemisacomplexinthethylakoidmembraneconsistingof:</p><ul><li><p>Areaction−centercomplex(proteinsassociatedwithchlorophyllaandaprimaryelectronacceptor).</p></li><li><p>Surroundedbylight−harvestingcomplexes(pigmentmoleculesboundtoproteins).</p></li></ul></li><li><p>Thepurposeofthelightenergyabsorbedbypigmentsinphotosystemsistoexciteelectronstoahigherenergylevel,initiatingtheflowofelectronsthroughtheelectrontransportchain.</p></li></ul><h6id="610dcbd9−71b4−493d−a996−b64e439e2d24"data−toc−id="610dcbd9−71b4−493d−a996−b64e439e2d24"collapsed="false"seolevelmigrated="true">ElectronPathwayinLightReactions</h6><ul><li><p>LightenergyexciteselectronsinphotosystemII(PSII)pigments.</p></li><li><p>ExcitedelectronsarecapturedbytheprimaryelectronacceptorofPSII.</p></li><li><p>Waterissplit(.</p></li></ul></li></ul><h6 id="59c0fed4-ce1c-4cc1-8b4c-cf1d5fd50c8c" data-toc-id="59c0fed4-ce1c-4cc1-8b4c-cf1d5fd50c8c" collapsed="false" seolevelmigrated="true">Pigments</h6><ul><li><p><strong>Pigments</strong> are substances that absorb specific wavelengths of light and reflect others.</p><ul><li><p>The color we see is the wavelength reflected.</p></li></ul></li><li><p><strong>Main Photosynthetic Pigments in Plants</strong>: Chlorophyll a (the primary pigment), chlorophyll b, and carotenoids.</p><ul><li><p><strong>Why Plants Appear Green</strong>: Chlorophyll pigments primarily absorb red and blue-violet wavelengths of light and reflect green light.</p></li></ul></li><li><p><strong>Wavelengths for Photosynthesis</strong>: Wavelengths that are <em>absorbed</em> by pigments contribute energy to photosynthesis.</p></li></ul><h6 id="92de6d3e-c528-49b6-8ebf-bc2d97224f04" data-toc-id="92de6d3e-c528-49b6-8ebf-bc2d97224f04" collapsed="false" seolevelmigrated="true">Photosystem</h6><ul><li><p>A photosystem is a complex in the thylakoid membrane consisting of:</p><ul><li><p>A reaction-center complex (proteins associated with chlorophyll a and a primary electron acceptor).</p></li><li><p>Surrounded by light-harvesting complexes (pigment molecules bound to proteins).</p></li></ul></li><li><p>The purpose of the light energy absorbed by pigments in photosystems is to excite electrons to a higher energy level, initiating the flow of electrons through the electron transport chain.</p></li></ul><h6 id="610dcbd9-71b4-493d-a996-b64e439e2d24" data-toc-id="610dcbd9-71b4-493d-a996-b64e439e2d24" collapsed="false" seolevelmigrated="true">Electron Pathway in Light Reactions</h6><ul><li><p>Light energy excites electrons in photosystem II (PSII) pigments.</p></li><li><p>Excited electrons are captured by the primary electron acceptor of PSII.</p></li><li><p>Water is split (H2O \rightarrow 2H^+ + 1/2O2 + 2e^-)toreplacetheelectronslostbyPSII,releasing) to replace the electrons lost by PSII, releasingO_2asabyproduct.</p></li><li><p>ElectronspassfromPSIItophotosystemI(PSI)viaanelectrontransportchain(ETC)inthethylakoidmembrane.</p></li><li><p>AselectronsmovedowntheETC,theirenergyisusedtopumpprotons(as a byproduct.</p></li><li><p>Electrons pass from PSII to photosystem I (PSI) via an electron transport chain (ETC) in the thylakoid membrane.</p></li><li><p>As electrons move down the ETC, their energy is used to pump protons (H^+)fromthestromaintothethylakoidlumen,creatingaprotongradient.</p></li><li><p>LightenergyexciteselectronsinPSIpigments.</p></li><li><p>ExcitedelectronsfromPSIarecapturedbyitsprimaryelectronacceptor.</p></li><li><p>Theseelectronsthenpassdownasecond,shorterETC.</p></li><li><p>Finally,) from the stroma into the thylakoid lumen, creating a proton gradient.</p></li><li><p>Light energy excites electrons in PSI pigments.</p></li><li><p>Excited electrons from PSI are captured by its primary electron acceptor.</p></li><li><p>These electrons then pass down a second, shorter ETC.</p></li><li><p>Finally,NADP^+reductasecatalyzesthetransferofelectronstoreductase catalyzes the transfer of electrons toNADP^+,reducingittoNADPH(, reducing it to NADPH (NADP^+ + 2e^- + H^+ \rightarrow NADPH).</p></li></ul><h6id="66445516−40ac−4376−82ed−be53c5da2112"data−toc−id="66445516−40ac−4376−82ed−be53c5da2112"collapsed="false"seolevelmigrated="true">PurposeofWaterinLightReactions</h6><ul><li><p>WaterservesastheelectrondonorforphotosystemII.</p><ul><li><p>Itreplacestheelectronslostwhenlightexciteschlorophyll.</p></li></ul></li><li><p>Itssplittingalso:</p><ul><li><p>Releasesprotons().</p></li></ul><h6 id="66445516-40ac-4376-82ed-be53c5da2112" data-toc-id="66445516-40ac-4376-82ed-be53c5da2112" collapsed="false" seolevelmigrated="true">Purpose of Water in Light Reactions</h6><ul><li><p>Water serves as the electron donor for photosystem II.</p><ul><li><p>It replaces the electrons lost when light excites chlorophyll.</p></li></ul></li><li><p>Its splitting also:</p><ul><li><p>Releases protons (H^+)intothethylakoidlumen,contributingtotheprotongradient.</p></li><li><p>Releasesoxygen() into the thylakoid lumen, contributing to the proton gradient.</p></li><li><p>Releases oxygen (O_2)asabyproduct.</p></li></ul></li></ul><h6id="8e0e459e−062d−4ee2−9200−2a24cc963bba"data−toc−id="8e0e459e−062d−4ee2−9200−2a24cc963bba"collapsed="false"seolevelmigrated="true">ATPProductioninETCbetweenPSIIandPSI</h6><ul><li><p>TheelectrontransportchainbetweenphotosystemIIandphotosystemIpumpsprotons() as a byproduct.</p></li></ul></li></ul><h6 id="8e0e459e-062d-4ee2-9200-2a24cc963bba" data-toc-id="8e0e459e-062d-4ee2-9200-2a24cc963bba" collapsed="false" seolevelmigrated="true">ATP Production in ETC between PSII and PSI</h6><ul><li><p>The electron transport chain between photosystem II and photosystem I pumps protons (H^+)fromthestromaintothethylakoidlumen.</p></li><li><p>Thisbuildsupahighconcentrationofprotonsinsidethelumen,creatinganelectrochemicalgradient.</p></li><li><p>AsprotonsflowbackoutofthelumenintothestromathroughATPsynthase(chemiosmosis),theenergyofthisflowisusedtosynthesizeATPfromADPand) from the stroma into the thylakoid lumen.</p></li><li><p>This builds up a high concentration of protons inside the lumen, creating an electrochemical gradient.</p></li><li><p>As protons flow back out of the lumen into the stroma through ATP synthase (chemiosmosis), the energy of this flow is used to synthesize ATP from ADP andP_i(photophosphorylation).</p></li></ul><h6id="3211ffad−34e0−42c5−9a7f−186bea1e151e"data−toc−id="3211ffad−34e0−42c5−9a7f−186bea1e151e"collapsed="false"seolevelmigrated="true">NADPHProductioninLightReactions</h6><ul><li><p>NADPHisproducedattheendofthesecondelectrontransportchain(afterPSI).</p></li><li><p>Electrons,alongwith(photophosphorylation).</p></li></ul><h6 id="3211ffad-34e0-42c5-9a7f-186bea1e151e" data-toc-id="3211ffad-34e0-42c5-9a7f-186bea1e151e" collapsed="false" seolevelmigrated="true">NADPH Production in Light Reactions</h6><ul><li><p>NADPH is produced at the end of the second electron transport chain (after PSI).</p></li><li><p>Electrons, along withH^+fromthestroma,aretransferredtofrom the stroma, are transferred toNADP^+bytheenzymeby the enzymeNADP^+reductase,formingNADPH.</p></li></ul><h6id="c381d4d3−35c9−4de2−a9e3−3dea47d80d94"data−toc−id="c381d4d3−35c9−4de2−a9e3−3dea47d80d94"collapsed="false"seolevelmigrated="true">CalvinCycleas"Anabolic"</h6><ul><li><p>TheCalvincycleisdescribedas"anabolic"because:</p><ul><li><p>Itusesenergy(ATP)andreducingpower(NADPH).</p></li><li><p>Itbuildscomplexorganicmolecules(G3P/sugars)fromsimplerinorganicmolecules(reductase, forming NADPH.</p></li></ul><h6 id="c381d4d3-35c9-4de2-a9e3-3dea47d80d94" data-toc-id="c381d4d3-35c9-4de2-a9e3-3dea47d80d94" collapsed="false" seolevelmigrated="true">Calvin Cycle as "Anabolic"</h6><ul><li><p>The Calvin cycle is described as "anabolic" because:</p><ul><li><p>It uses energy (ATP) and reducing power (NADPH).</p></li><li><p>It builds complex organic molecules (G3P/sugars) from simpler inorganic molecules (CO_2).</p></li><li><p>Thisprocesssynthesizesratherthanbreaksdown.</p></li></ul></li></ul><h6id="f7f7f7db−60a7−42d1−a661−de32fbe4a909"data−toc−id="f7f7f7db−60a7−42d1−a661−de32fbe4a909"collapsed="false"seolevelmigrated="true">PurposeofCarbonFixationPhase</h6><ul><li><p>Thepurposeofthecarbonfixationphaseistoincorporateatmosphericcarbondioxide().</p></li><li><p>This process synthesizes rather than breaks down.</p></li></ul></li></ul><h6 id="f7f7f7db-60a7-42d1-a661-de32fbe4a909" data-toc-id="f7f7f7db-60a7-42d1-a661-de32fbe4a909" collapsed="false" seolevelmigrated="true">Purpose of Carbon Fixation Phase</h6><ul><li><p>The purpose of the carbon fixation phase is to incorporate atmospheric carbon dioxide (CO_2)intoanexistingorganicmolecule.</p><ul><li><p>Thismoleculeisribulose−1,5−bisphosphate,orRuBP.</p></li><li><p>TheenzymeRuBisCOcatalyzesthisprocess.</p></li></ul></li><li><p>Thisconvertsinorganiccarbonintoanorganicform.</p></li></ul><h6id="f33c6f72−4fd3−4c3f−8eb5−480abafea097"data−toc−id="f33c6f72−4fd3−4c3f−8eb5−480abafea097"collapsed="false"seolevelmigrated="true">WhytheSecondPhaseisCalled"Reduction"</h6><ul><li><p>ThesecondphaseoftheCalvincycleiscalled"reduction"becausethefixedcarboncompound(3−phosphoglycerate)isreducedtoglyceraldehyde−3−phosphate(G3P).</p></li><li><p>ThisreductionsteprequiresenergyfromATPandelectronsfromNADPH.</p></li></ul><h6id="33d7787f−8d64−431b−88d2−321ee9be8c5f"data−toc−id="33d7787f−8d64−431b−88d2−321ee9be8c5f"collapsed="false"seolevelmigrated="true">ProductLeadingtoGlucoseinReductionPhase</h6><ul><li><p>Attheendofthereductionphase,glyceraldehyde−3−phosphate(G3P)moleculesareproduced.</p></li><li><p>Foreverythree) into an existing organic molecule.</p><ul><li><p>This molecule is ribulose-1,5-bisphosphate, or RuBP.</p></li><li><p>The enzyme RuBisCO catalyzes this process.</p></li></ul></li><li><p>This converts inorganic carbon into an organic form.</p></li></ul><h6 id="f33c6f72-4fd3-4c3f-8eb5-480abafea097" data-toc-id="f33c6f72-4fd3-4c3f-8eb5-480abafea097" collapsed="false" seolevelmigrated="true">Why the Second Phase is Called "Reduction"</h6><ul><li><p>The second phase of the Calvin cycle is called "reduction" because the fixed carbon compound (3-phosphoglycerate) is reduced to glyceraldehyde-3-phosphate (G3P).</p></li><li><p>This reduction step requires energy from ATP and electrons from NADPH.</p></li></ul><h6 id="33d7787f-8d64-431b-88d2-321ee9be8c5f" data-toc-id="33d7787f-8d64-431b-88d2-321ee9be8c5f" collapsed="false" seolevelmigrated="true">Product Leading to Glucose in Reduction Phase</h6><ul><li><p>At the end of the reduction phase, glyceraldehyde-3-phosphate (G3P) molecules are produced.</p></li><li><p>For every threeCO_2moleculesfixed,oneG3Pmoleculeexitsthecycle.</p></li><li><p>TwoG3Pmoleculescanthenbecombinedtoformoneglucosemolecule.</p></li></ul><h6id="57442183−abae−489c−bde3−05b43d4b25db"data−toc−id="57442183−abae−489c−bde3−05b43d4b25db"collapsed="false"seolevelmigrated="true">PurposeofRegenerationPhase</h6><ul><li><p>Thepurposeoftheregenerationphaseistoregeneratetheinitialmolecules fixed, one G3P molecule exits the cycle.</p></li><li><p>Two G3P molecules can then be combined to form one glucose molecule.</p></li></ul><h6 id="57442183-abae-489c-bde3-05b43d4b25db" data-toc-id="57442183-abae-489c-bde3-05b43d4b25db" collapsed="false" seolevelmigrated="true">Purpose of Regeneration Phase</h6><ul><li><p>The purpose of the regeneration phase is to regenerate the initialCO_2acceptormolecule,RuBP,fromtheremainingG3Pmolecules.</p></li><li><p>ThisprocessrequiresATPandensuresthecontinuousoperationoftheCalvincycle.</p></li></ul><h6id="444fb84c−15bb−40b4−b2bb−cca225692347"data−toc−id="444fb84c−15bb−40b4−b2bb−cca225692347"collapsed="false"seolevelmigrated="true">FateofG3PMolecules</h6><ul><li><p>TheG3PmoleculesproducedfromtheCalvincyclehaveseveralfates:</p><ul><li><p>OneG3Pmolecule(foreverythreefixedacceptor molecule, RuBP, from the remaining G3P molecules.</p></li><li><p>This process requires ATP and ensures the continuous operation of the Calvin cycle.</p></li></ul><h6 id="444fb84c-15bb-40b4-b2bb-cca225692347" data-toc-id="444fb84c-15bb-40b4-b2bb-cca225692347" collapsed="false" seolevelmigrated="true">Fate of G3P Molecules</h6><ul><li><p>The G3P molecules produced from the Calvin cycle have several fates:</p><ul><li><p>One G3P molecule (for every three fixedCO_2)isexportedfromthechloroplasttothecytoplasm.</p><ul><li><p>There,itcanbeusedtosynthesizeglucoseandotherorganiccompounds(e.g.,sucrose,starch,fattyacids,aminoacids).</p></li></ul></li><li><p>TheremainingG3PmoleculesstayinthechloroplasttoregenerateRuBP.</p></li></ul></li></ul><h6id="642cdea6−03ba−422b−9c92−4b6812a7177d"data−toc−id="642cdea6−03ba−422b−9c92−4b6812a7177d"collapsed="false"seolevelmigrated="true">C4andCAMPlantsvs.C3Plants</h6><ul><li><p>C3,C4,andCAMplantsrepresentdifferentphotosyntheticadaptationstohot,dryenvironments:</p></li><li><p><strong>C3Plants</strong></p><ul><li><p>Themostcommontype.</p></li><li><p>Carbonfixationoccursdirectlyintoathree−carboncompound(3−PGA)usingRuBisCOinthemesophyllcells.</p></li><li><p>Underhot,dryconditions,stomataclose,andphotorespirationcanoccur,reducingphotosyntheticefficiency.</p></li></ul></li><li><p><strong>C4Plants</strong></p><ul><li><p>Haveaspatialseparationofcarbonfixation.</p></li><li><p>Lightreactionsoccurinmesophyllcells,where) is exported from the chloroplast to the cytoplasm.</p><ul><li><p>There, it can be used to synthesize glucose and other organic compounds (e.g., sucrose, starch, fatty acids, amino acids).</p></li></ul></li><li><p>The remaining G3P molecules stay in the chloroplast to regenerate RuBP.</p></li></ul></li></ul><h6 id="642cdea6-03ba-422b-9c92-4b6812a7177d" data-toc-id="642cdea6-03ba-422b-9c92-4b6812a7177d" collapsed="false" seolevelmigrated="true">C4 and CAM Plants vs. C3 Plants</h6><ul><li><p>C3, C4, and CAM plants represent different photosynthetic adaptations to hot, dry environments:</p></li><li><p><strong>C3 Plants</strong></p><ul><li><p>The most common type.</p></li><li><p>Carbon fixation occurs directly into a three-carbon compound (3-PGA) using RuBisCO in the mesophyll cells.</p></li><li><p>Under hot, dry conditions, stomata close, and photorespiration can occur, reducing photosynthetic efficiency.</p></li></ul></li><li><p><strong>C4 Plants</strong></p><ul><li><p>Have a spatial separation of carbon fixation.</p></li><li><p>Light reactions occur in mesophyll cells, whereCO_2isfirstfixedintoafour−carboncompoundbyPEPcarboxylase.</p></li><li><p>Thiscompoundisthentransportedtobundle−sheathcells,whereis first fixed into a four-carbon compound by PEP carboxylase.</p></li><li><p>This compound is then transported to bundle-sheath cells, whereCO_2isreleasedandenterstheCalvincycle.</p></li><li><p>Thisreducesphotorespirationandallowsforefficientphotosynthesisinhot,brightconditions.</p></li></ul></li><li><p><strong>CAMPlants(CrassulaceanAcidMetabolism)</strong></p><ul><li><p>Haveatemporalseparationofcarbonfixation.</p></li><li><p>Theyopentheirstomataatnighttotakeinis released and enters the Calvin cycle.</p></li><li><p>This reduces photorespiration and allows for efficient photosynthesis in hot, bright conditions.</p></li></ul></li><li><p><strong>CAM Plants (Crassulacean Acid Metabolism)</strong></p><ul><li><p>Have a temporal separation of carbon fixation.</p></li><li><p>They open their stomata at night to take inCO_2,whichisfixedintoorganicacidsandstoredinvacuoles.</p></li><li><p>Duringtheday,stomataclose,andthestored, which is fixed into organic acids and stored in vacuoles.</p></li><li><p>During the day, stomata close, and the storedCO_2$$ is released to fuel the Calvin cycle.

  • This minimizes water loss in extremely arid conditions.