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  1. 1. Chloroplast 1
  2. 2. Chloroplast•the site of photosynthesis in eukaryoticcells•disk-like structures•composed of a single membrane•surrounding a fluid containing stacks ofmembranous disks 2
  3. 3. •SOLAR energy radiated from the sunis captured by plants(chloroplast)•Then it is instantaneously changed Chloroplastinto ELECTRICAL energy•Then packaged as CHEMICALenergy 3
  4. 4. The structure of the chloroplast , function andphotosynthetic membranes• Inner membrane- highly impermeable, substances movingthrough this membrane with the aid of a variety oftransporters• Outer membrane- outer covering of the chloroplast, itcontains several different porins (proteins with largechannels, 1 nm), exhibit selectivity towards varioussolutesThe thylakoid is the structural unit of photosynthesis.• Intermembrane space- a narrow space between inner andouter membrane• Thylakoids- flattened membranous sacs which containsthe photosynthetic chemicals• Granum/Grana- are stacks of thyllakoids• Stroma- the areas between grana which contains theenzymes responsible for carbohydrate synthesis. It containssmall, double-stranded, circular DNA molecules andprokaryotic-like ribosomes 5
  5. 5. Chloroplast•photosynthesis takes place inside the chloroplast the process in which plant use watePhotosynthesis- carbon dioxide, and energy form the sun to make food 6
  6. 6. Photosynthesis is the conversion of light energy to chemicalenergy or the production of carbohydrates from carbon dioxide and waterin the presence of chlorophyllthe overall reaction of this process is: 6H2O + 6CO2 ----------> C6H12O6+6O2Leaves and Leaf StructurePlants are the only photosynthetic organisms to have leaves (and not allplants have leaves). A leaf may be viewed as a solar collector crammedfull of photosynthetic cells.The raw materials of photosynthesis, water and carbon dioxide, enter thecells of the leaf, and the products of photosynthesis, sugar and oxygen,leave the leaf.
  7. 7. Photosynthesis is a two stage process.1. Light Dependent Process (Light Reactions), requires the direct energy of light to make energy carrier molecules that are used in the second process. it occurs in the grana of the chloroplast the energy (light) is absorbed and stored in ATP & NADPH2. The Light Independent Process (or Dark Reactions) occurs when the products of the Light Reaction are used to form C-C covalent bonds of carbohydrates. The Dark Reactions can usually occur in the dark, if the energy carriers from the light process are present. Recent evidence suggests that a major enzyme of the Dark Reaction is indirectly stimulated by light, thus the term Dark Reaction is somewhat of a misnomer. It takes place in the stroma of the chloroplasts.* ~ 500t kg of CO2 to carbohydrates/year
  8. 8. The Nature of Light White light is separated into the differentcolors (=wavelengths) of light by passing it through a prism.Wavelength is defined as the distance from peak to peak (ortrough to trough). The energy of is inversely proportional to thewavelength: longer wavelengths have less energy than doshorter ones.
  9. 9. The order of colors is determined by the wavelength of light.Visible light is one small part of the electromagneticspectrum. The longer the wavelength of visible light, the morered the color. Likewise the shorter wavelengths are towardsthe violet side of the spectrum. Wavelengths longer than redare referred to as infrared, while those shorter than violet areultraviolet.Light behaves both as a wave and a particle. Wave propertiesof light include the bending of the wave path when passingfrom one material (medium) into another (i.e. theprism, rainbows, pencil in a glass-of-water, etc.). The particleproperties are demonstrated by the photoelectric effect. Zincexposed to ultraviolet light becomes positively chargedbecause light energy forces electrons from the zinc. Theseelectrons can create an electrical current. Sodium, potassiumand selenium have critical wavelengths in the visible lightrange. The critical wavelength is the maximum wavelength of
  10. 10. WHAT IS PHOTOELECTRIC EFFECT?Photoelectric Effect is the formation and liberation ofelectrically charged particles in matter when it is irradiated bylight or other electromagnetic radiation. The termphotoelectric effect designates several types of relatedinteractions. In the external photoelectric effect, electrons areliberated from the surface of a metallic conductor byabsorbing energy from light shining on the metals surface.The effect is applied in the photoelectric cell, in which theelectrons liberated from one pole of the cell, thephotocathode, migrate to the other pole, the anode, underthe influence of an electric field.
  11. 11. Light travels in packets or quanta of energy called photons, which can bethought of as “particles” of light. The absorption of light is the first step in a photochemicalprocess. When a photon is absorbed by a molecule, an electronbecomes sufficiently energetic to be pushed from an inner to an outerorbital. The molecule is said to have shifted from the ground state to anexcited state. Because the number of orbitals in which an electron canexist is limited and each orbital has a specific energy level, it follows thatany given atom or molecule can absorb only light of certain specificwavelengths. The excited state of a molecule is unstable and can be expectedto last only about .0000000009 second. There is a tendency that theelectron of an excited chlorophyll molecule drops back to a lowerorbital, the energy it had absorbed will be dissipated or released as heator reemitted at a longer wavelength. Instead, the excited electrons of chlorophyll molecules aretransferred to electron acceptors within the chloroplast membranesbefore they have a chance to drop back to lower energy orbitals.Thus, the chloroplasts are able to harness the absorbed energy before itdissipates.
  12. 12. Chlorophyll and Accessory PigmentsA pigment is any substance that absorbs light. The color of thepigment comes from the wavelengths of light reflected (in other words,those not absorbed). Chlorophyll, the green pigment common to allphotosynthetic cells, absorbs all wavelengths of visible light exceptgreen, which it reflects to be detected by our eyes. Black pigmentsabsorb all of the wavelengths that strike them. White pigments/lightercolors reflect all or almost all of the energy striking them. Pigmentshave their own characteristic absorption spectra, the absorptionpattern of a given pigment.Absorption and transmission of different wavelengths of light by ahypothetical pigment.
  13. 13. Chlorophyll is a complex molecule. Several modifications of chlorophyll occuramong plants and other photosynthetic organisms. All photosynthetic organisms(plants, certain protistans, prochlorobacteria, and cyanobacteria) have chlorophylla. Accessory pigments absorb energy that chlorophyll a does not absorb.Accessory pigments include chlorophyll b (also c, d, and e in algae andprotistans), xanthophylls, and carotenoids (such as beta-carotene). Chlorophyll aabsorbs its energy from the Violet-Blue and Reddish orange-Red wavelengths,and little from the intermediate (Green-Yellow-Orange) wavelengths.
  14. 14. Molecular model of chlorophyll. The above image isfrom Molecular model of carotene.
  15. 15. Carotenoids and chlorophyll b absorb some of the energy in the greenwavelength. Why not so much in the orange and yellow wavelengths? Bothchlorophylls also absorb in the orange-red end of the spectrum (with longerwavelengths and lower energy). The origins of photosynthetic organisms inthe sea may account for this. Shorter wavelengths (with more energy) donot penetrate much below 5 meters deep in sea water. The ability toabsorb some energy from the longer (hence more penetrating)wavelengths might have been an advantage to early photosynthetic algaethat were not able to be in the upper (photic) zone of the sea all the time The molecular structure of chlorophylls
  16. 16. The action spectrum of photosynthesis is the relative effectiveness of differentwavelengths of light at generating electrons. If a pigment absorbs light energy,one of three things will occur. Energy is dissipated as heat. The energy maybe emitted immediately as a longer wavelength, a phenomenon known asfluorescence. Energy may trigger a chemical reaction, as in photosynthesis.Chlorophyll only triggers a chemical reaction when it is associated withproteins embedded in a membrane (as in a chloroplast) or the membraneinfoldings found in photosynthetic prokaryotes such as cyanobacteria andprochlorobacteria.Absorption spectrum of several plant pigments (left) and action spectrum ofelodea (right), a common aquarium plant used in lab experiments aboutphotosynthesis.
  17. 17. PHOTOSYNTHETIC UNITS AND REACTION CENTERS Several hundred chlorophyll molecules act together asone photosynthetic unit in which only one member of thegroup- the reaction center chlorophyll- actually transferselectrons to an electron acceptor. Pigment molecules are responsible for lightabsorption. These pigment molecules form a light-harvestingantenna that absorbs photons of varying wavelength andtransfers that energy very rapidly to the pigment molecule atthe reaction center. The energy can only be passed to a pigment moleculethat absorbs light of equal or longer wavelength (lowerenergy) than that absorbed by the donor molecule. Theenergy is ultimately transferred to a chlorophyll of thereaction center, which absorbs light of longer wavelengththan any of its neighbors. Once the energy is received by thereaction center, the electron excited by light absorption canbe transferred to its acceptor.
  18. 18. PHOTOSYTEMSThe light absorbing reactions of photosynthesis occur in largepigment-protein complexes called photosystems.Photosystems are arrangements of chlorophyll and otherpigments packed into thylakoids.2 Types:1. PSI- raises electrons from a midway point to an energy level well above that of NADP+ - the reaction center is chlorophyll a with a wavelength of P700 nm2. PSII- boosts electrons from an energy level below that of water to a midway point - the reaction center is a chlorophyll a with a wavelength of P680 nm
  19. 19. Photophosphorylation is the process of converting energyfrom a light-excited electron into the pyrophosphate bond ofan ADP molecule. This occurs when the electrons from waterare excited by the light in the presence of P680. The energytransfer is similar to the chemiosmotic electron transportoccurring in the mitochondria. Light energy causes theremoval of an electron from a molecule of P680 that is part ofPhotosystem II. The P680 requires an electron, which istaken from a water molecule, breaking the water into H+ ionsand O-2 ions. These O-2 ions combine to form the diatomic O2that is released. The electron is "boosted" to a higher energystate and attached to a primary electron acceptor, whichbegins a series of redox reactions, passing the electronthrough a series of electron carriers, eventually attaching it toa molecule in Photosystem I.
  20. 20. Chloroplast•No energy transformation is 100% efficient•Not all the solar energy captured isconverted to electrical and then chemicalenergy.•Some of it gets lost as heat or other formsof energy (light) 21