Photosynthesis

Photosynthesis1IntroductionAutotrophs: any organism that can make their own food.Photoautotrophs : use light energyChemoautotrophs change inorganic chemicals such as hydrogen sulfate into chemical energy2Important structures

Chloroplast: Site of photosynthesis in eukaryotic cells.Thylakoids: Disk shaped membranes containing photosynthetic pigments. Site of light dependent reactions.Grana: Stacks of thylakoids.Stroma: Fluid filled space surrounding grana. Site of light independent reactions.3Properties of LightWhite light from the sun is composed of a range of wavelengths.Chlorophyll is the main pigment that absorbs sunlight.Chlorophyll absorbs blue light and to a lesser extent, red light.It reflects green light hence its intense green color.44What do chlorophyll and fireworks have in common?Fireworks are made up of various metal salts that go through oxidation and reduction reactions, producing great heat that causes electrons to move from their usual level around the nucleus (ground state) up to a higher position further away from the nucleus (excited state). The energy from heat has been imparted to the electron(s), which now has more potential energy. These excited electrons move almost instantaneously back down to their ground state, releasing their stored potential energy in the form of light. The greater the electrons movement away from the nucleus to the excited state, the more potential energy it has and the more energy it will release. It should be pointed out that chlorophyll absorbs light energy, specifically its electrons, and this energy is used to pump protons instead of producing light. This proton gradient in turn is used to produce ATP, which along with other energized electrons are transferred to the Calvin cycle. 55Suppose a large meteor hit the earth. How could smoke and soot in the atmosphere wipe out life far beyond the area of direct impact?Because any particles in the atmosphere can block the light from the sun and reduce the excitation of electrons in chlorophyll molecules, photosynthesis depends on a relatively clean atmosphere. Any reduction in the available sunlight can have serious effects on plants. Scientists believe that if a large meteor hit the earthas one did when the dinosaurs were wiped out 65 million years agosmoke, soot, and dust in the atmosphere could block sunlight to such an extent that plants in the region, or even possibly all of the plants on earth, could not conduct photosynthesis at high enough levels to survive. And when plants die off, all of the animals and other species that rely on them for energy die as well. As dire as it sounds, all life on earth is completely dependent on the continued excitation of electrons by sunlight.

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stromathylakoid compartmentthylakoid membrane systemtwo outer membranesChloroplastOrganelle of photosynthesis in plants and algae

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a A look inside the leafb One of the photosynthetic cells inside leafleafs upper epidermisphotosynthetic cell in leafleaf veinleafs lower epidermisLeaf Structure88Photosynthesis Equation

6H2O + 6CO26O2 + C6H12O6 watercarbon dioxideoxygenglucoseLIGHT ENERGY9PhotosynthesisTwo stages:light-dependent reactions require light to worklight-independent reactions do not require light10

sunlightWhere the two stages of photosynthesis occur inside the chloroplast light-dependent reactionslight-independent reactionsCO2sugarsNADPH, ATPNADP+, ADPO2H2OTwo Steps in Photosynthesis1111Photosynthesis: Light-DependentPhotosystems - pigments surrounding a central chlorophyll a molecule....the reaction centerEach pigment absorbs a different wavelength of light & transfers its energy to the reaction center which in turns energizes an electron.Only the chlorophyll that is the reaction center can give the energized electrons to the electron acceptors! The energized electron is then used elsewhere to make ATP or NADPH12Two Potential Fates of Excited ElectronsElectron returns to resting, unexcited state.Excited electrons are passed to other atoms.1313An electron in a photosynthetic pigment that is excited to a higher energy state generally has one of two fates (refer to the next two slides also): (1) The electron returns to its resting, unexcited state. In the process, energy is released, some of which may be transferred to a nearby molecule, bumping electrons on that molecule to a higher energy state (and the rest of the energy is dissipated as heat) or (2) The excited electron itself is passed to another molecule.14

Photosynthesis: Light-Dependent

1515Figure 4-15 (part 1) Capturing light energy with excited electrons. Chlorophyll electrons are excited to a higher energy state by light energy.

1616Figure 4-15 (part 2) Capturing light energy with excited electrons.Chlorophyll electrons are excited to a higher energy state by light energy. The Passing of Electrons in Their Excited StateChief way energy moves through cells

Molecules that gain electrons always carry greater energy than before receiving themCan view this as passing of potential energy from molecule to molecule

1717The passing of electrons from molecule to molecule is one of the chief ways that energy moves through cells. Many molecules carry or accept electrons during cellular activities. All that is required is that the acceptor have a greater attraction for electrons than the molecule from which it accepts them. Photosynthesis: Light-Dependent

18Photosynthesis: Light-DependentAs these pigments absorb photons from the sunlight that hits the leaves, electrons in the pigments become excited and then return to their resting state. As the electrons return to their resting state, energy (but not the electrons) is transferred to neighboring pigment molecules. This process continues until the transferred energy from many pigment molecules excites the electrons in a chlorophyll a molecule at the center of the photosystem. 19Photosynthesis: Light-DependentThis is where the electron journey begins. The special chlorophyll a continually loses its excited electrons to a nearby molecule, called the primary electron acceptor, which acts like an electron vacuum. Why must plants get water for photosynthesis to occur?As electrons keep getting taken away from the special chlorophyll a molecule, the electrons must be replaced. The replacement electrons come from water.

20Photosynthesis: Light-Dependent

21Photosynthesis: Light-DependentThink of a pump pushing water into an elevated tank, creating a store of potential energy that can run out of the tank with great force and kinetic energy, which can be harnessed to do work, such as moving a large paddle wheel.Similarly, the protons eventually rush out of the thylakoid sacs with great forceand that force is harnessed to build energy-storing ATP molecules, one of the two products of the photo portion of photosynthesis.22Photosynthesis: Light-DependentProduct #1 of the Photo Portion of Photosynthesis: ATPProduct #2: NADPHProduct #3 (waste): O2

23Also known as the Calvin CycleThey occur whether or not light is present.Occur in the stroma of the chloroplast.The purpose of the reactions is to take the energy from ATP and energized ions from NADPH and add them to carbon dioxide to make glucose or sugar.The reactions reduce carbon dioxide by adding energize electrons and protons to it and removing one oxygen atom. This effectively converts the carbon dioxide into CH2O.24Photosynthesis: Light-inDependentPhotosynthesis: Light-inDependent25

Photosynthesis: Light-inDependent26

Factors that Affect Rates3 factors can limit the speed of photosynthesis: light intensity, CO2 concentration, temperatureWithout enough light, a plant cannot photosynthesize very quickly, even if there is plenty of water and CO2 .

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Factors that Affect Rates28Sometimes photosynthesis is limited by the concentration of carbon dioxide in the air. Even if there is plenty of light, a plant cannot photosynthesize if there is insufficient carbon dioxide.

Factors that Affect RatesIf it gets too cold, the rate of photosynthesis will decrease. Plants cannot photosynthesize if it gets too hot.

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Summary of Photosynthesis

12H2OsunlightCalvin-Bensoncycle6O2LightDependentReactionsLightIndependentReactionsNADP+ADP + Pi6 RuBP12 PGALPend products (e.g., sucrose, starch, cellulose)phosphorylated glucose6H2O6CO2ATPNADPHLinked ProcessesPhotosynthesisEnergy-storing pathway Releases oxygenRequires carbon dioxideAerobic RespirationEnergy-releasing pathwayRequires oxygenReleases carbon dioxidePhotosynthesis: The MovieAnimation32The battle against world hunger can use plants adapted to water scarcity.

33The Sudan. Ethiopia. India. Somalia. Many of the worlds regions with the highest rates of starvation also are places with the hottest, driest climates. This is not a coincidence. These climate conditions present difficult challenges for sustaining agriculture. In the absence of stable crop yields, food production is unpredictable and the risk of starvation high.

Figure 4-22 Nowhere to hide.StomataPores for gas exchange

34These openings are the primary sites for gas exchange in plants: carbon dioxide for photosynthesis enters through these openings and oxygen generated as a by-product in photosynthesis exits through them. When open, the stomata also allow water to evaporate from the plant. Closing their stomata, however, solves one problem for plants (too much water evaporation) while it creates another: with the stomata shut, oxygen from the photo reactions of photosynthesis cannot be released from the chloroplasts and carbon dioxide cannot enter them. If there are no carbon molecules for sugar production, the Calvin cycle tries to fix carbon but instead finds only oxygen. Plant growth comes to a standstill and crops fail.

Figure 4-23 Plant stomata.

How to get CO2 when stomata are shut?Primary sites for gas exchange in plantsCO2 for photosynthesis O2 generated as a by-product in photosynthesis exits water vapor evaporates Closed stomata: prevents water evaporation O2 cannot be released from the chloroplasts CO2 cannot enter them No carbon = no Calvin cycle plant growth comes to a standstill; crops fail

35Evolutionary AdaptationsSome plants are able to thrive in hot, dry conditions. Evolutionary adaptations along with recent agricultural (technological) advances help battle world hungerplants close their stomata to combat water loss through evaporation

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37Figure 4-24 (part 2a) C3, C4, and CAM photosynthesis. See next slide for discussion.C4 PhotosynthesisC4 plants produce an enzyme that is better at attracting carbon (in addition to RUBISCO) called PEP Carboxylase allows CO2 to be taken into the plant very quickly "delivers" the CO2 directly to RUBISCOCalled C4 because the CO2 is first incorporated into a 4-carbon compoundStomata are open during the day

38Some plants, including corn and sugarcane, have evolved a process that minimizes water loss but still enables them to make sugar when the weather is hot and dry. In the process called C4 photosynthesis, these plants add an extra set of steps to the usual process of photosynthesis .

C4 photosynthesis also adds additional energy expense. Outcompeted in mild climates by C3 plants.

C4 PhotosynthesisAdaptive Value: photosynthesizes faster than C3 plants under high light intensity and high temperatures better water use efficiency because PEP Carboxylase brings in CO2 faster doesnt need to keep stomata open as much C4 plants include several thousand species in at least 19 plant families, including corn & sugarcaneC4 photosynthesis also adds additional energy expense; outcompeted in mild climates by C3 plants.

39Some plants, including corn and sugarcane, have evolved a process that minimizes water loss but still enables them to make sugar when the weather is hot and dry. In the process called C4 photosynthesis, these plants add an extra set of steps to the usual process of photosynthesis .

C4 photosynthesis also adds additional energy expense. Outcompeted in mild climates by C3 plants.

C4 Photosynthesis

40Some plants, including corn and sugarcane, have evolved a process that minimizes water loss but still enables them to make sugar when the weather is hot and dry. In the process called C4 photosynthesis, these plants add an extra set of steps to the usual process of photosynthesis .

C4 photosynthesis also adds additional energy expense. Outcompeted in mild climates by C3 plants.

41Figure 4-25 Map of C3 and C4 photosynthesis. Has implications for world hunger: humans may use these adaptations to grow food in the dry, inhospitable climates where starvation rates are highest.Also has implications for global plant distribution responses to climate change!With global warming, many scientists expect to see a gradual expansion of the geographical ranges over which C4 plants occur and expect that non-C4 plants will be pushed farther and farther away from the equator. CAM PhotosynthesisCAM = Crassulacean Acid Metabolism after plant family in which it was first found (Crassulaceae) because CO2 is stored as an acid before use in photosynthesisStomata open at night (evaporation rates usually lower), closed during dayCO2 converted to acid, stored @ nightdaytime: acid broken down CO2 released to RUBISCO for photosynthesis42Some plants, including corn and sugarcane, have evolved a process that minimizes water loss but still enables them to make sugar when the weather is hot and dry. In the process called C4 photosynthesis, these plants add an extra set of steps to the usual process of photosynthesis .

C4 photosynthesis also adds additional energy expense. Outcompeted in mild climates by C3 plants.

CAM PhotosynthesisAdaptive Value: better water use efficiency than C3s under arid conditions open stomata @ night when transpiration rates are lowerno sunlight, lower temperatures, lower wind speeds, etc. May CAM-idle under extremely arid conditionsleave stomata closed night and dayO2 given off in photosynthesis is used for respiration CO2 given off in respiration is used for photosynthesiskind of like a perpetual energy machine, but plant cannot CAM-idle foreverallows plant to survive dry spells, recover very quickly when water is available again (unlike plants that drop their leaves and twigs and go dormant during dry spells)43Some plants, including corn and sugarcane, have evolved a process that minimizes water loss but still enables them to make sugar when the weather is hot and dry. In the process called C4 photosynthesis, these plants add an extra set of steps to the usual process of photosynthesis .

C4 photosynthesis also adds additional energy expense. Outcompeted in mild climates by C3 plants.

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CAM PhotosynthesisCACTUSAGAVEORCHIDBROMELIADAll Three Photosynthetic Pathways

45It is believed that C4 and CAM photosynthesis originally evolved as successful adaptations to hot and dry regions.Researchers are now using these adaptations to fight world hunger. Specifically, they have introduced from corn into rice several genes that code for the C4 photosynthesis enzymes. Once in the rice, these genes increase the rice plants ability to photosynthesize, thus leading to higher growth rates and food yields. The experiments are still in the early stages, and whether the addition of C4 photosynthesis enzymes will make it possible to grow new crops on a large scale in previously inhospitable environments is not certain. Early results suggest, however, that it is a promising approach.

Figure 4-24 C3, C4, and CAM photosynthesis.

Evolutionary Adaptations & World HungerC4 and CAM photosynthesis originally evolved as successful adaptations to hot/dry regionsResearchers are now using these adaptations to fight world hunger. several genes that code for the C4 photosynthesis enzymes have been introduced from corn into riceonce in rice, genes increase rice plants ability to photosynthesize higher growth rates and food yields46Some plants, including corn and sugarcane, have evolved a process that minimizes water loss but still enables them to make sugar when the weather is hot and dry. In the process called C4 photosynthesis, these plants add an extra set of steps to the usual process of photosynthesis .

C4 photosynthesis also adds additional energy expense. Outcompeted in mild climates by C3 plants.