important structures in a leaf two structures important for photosynthesis: stoma (pl. stomata):...
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Important structures in a leaf Two structures important for photosynthesis: Stoma (pl. Stomata): Pores in a plant’s
cuticle through which water and gases are exchanged between the plant and the atmosphere.
Mesophyll cells: Contain a lot of chloroplasts (between 40-200) arranged to receive maximum amount of light.
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Plastids• Chloroplasts -photosynthetic; green due to
chlorophyll content• Chromoplasts: contain pigments other than
chlorophyll (in fruits, leaves, flowers)• Leucoplasts: involved in lipid biosynthesis• Amyloplasts: store starch (colorless)• Etioplasts: intermediate state in production of
chloroplasts, in tissue exposed to light for the first time
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The Chloroplast• Usually lens-shaped, an organelle needed for
photosynthesis
• Has internal membrane system arranged into flattened sacs (thylakoids) 2 compartments: thylakoidspace and stroma
• Thylakoids stacked forming grana (1 granum)
• Contains the green pigment chlorophyll & pigments of other colors (red, blue, yellow/brown)
• Depending on light conditions, chloroplasts can move within the cells e.g. to the surface to catch more light in low light conditions
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ChlorophyllsThe principle photoreceptor in
photosynthesis is Chlorophyll Chlorophyll a& b in plant, bacteriochlorophyll
a& b in bacteriaChlorophyll is similar to the heme group of
globins and cytochromes, but with very significant differences
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• Mg2+ is in the center, not Fe2+• Ring V is fused to pyrrolering III• Hydrocarbon tail• Ring IV is partially reduced
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• Ratio of Chlorophyll a:b in plant (3:1)
• Only chl-a is a constituent of the photosynthetic reaction centers, hence central photosynthesis pigment
• Chlorophyll molecules are bound to chlorophyll-binding proteins. • In a complex with proteins the absorption spectrum of the bound chlorophyll may differ from the absorption spectrum of the free chlorophyll
•The same applies for other light-absorbing substances, (carotenoids, xanthophylls etc)
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Chlorophyll , Pigments & Light
• Free absorbing substances are called chromophore and the chromophore-protein complexes are called pigments.• Pigments are often named after the wavelength of their absorption maximum. • Chlorophyll-a 700 means a pigment of chl-a with an absorption maximum of 700 nm.
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• All photosynthetic organisms have Chlorophyll a
• Chlorophyll a absorbs Light in Red (660 nm) and Blue (450 nm) Wavelengths
• Leaves are green because chlorophyll reflect the Green light (which is detected by our eyes)
• The Color of the pigment comes from the wavelengths of light reflected NOT absorbed
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Accessory Pigments (Light Antenna)Light Harvesting complex are membrane
proteins containing pigments to absorb light energy outside the range of chlorophyll.
The most common pigments are Chlorophyll b, Carotenoids, Xanthophylls& Pilins
(in water-dwelling algae & Bacteria).
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Fall Colors• Leaves contain chlorophyll and other pigments, but
they appear green because chlorophyll is the major component.
• During the fall, the green chlorophyll pigments are greatly reduced revealing the other pigments: Carotenoids and/or Xanthophylls
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When a pigment absorbs light, It goes from a ground state to an excited state, which is unstable
Ground state
Excited state
Heat
Fluorescence
Photon
e-
Chlorophyll molecule
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If a Pigment absorbs light, it must release its energy to return to its ground state,this can be accomplished via four common mechanisms:1.Dissipated as Heat (the most common route in general)
2.Transferred to another molecules (required special arrangements)
3.Emitted as Fluorescence (required special molecules)
4.Trigger a Chemical Reaction (special molecules)
Efficiency of photosynthesis is nearly 100% due to special arrangement of proteins in the thylakoids membrane(Such arrangement prevents dissipation of energy as heat)only the other three mechanisms are important for photosynthesis
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How light is harvested
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PhotosystemI and PhotosystemIIPhotosystemI (PS I) It needs light of longer wave lengths
(lambda > 700 nm)PhotosystemII (PS II) It becomes active when exposed to
shorter wave lengths (lambda < 680 nm)
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Electron TransferWhen Light Strikes
Magnesium (Mg) Atom in Center of Chlorophyll Molecule, the Light Energy Excites a Mg Electron
The Electron Can Be Converted to Useful Chemical Energy
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PhotophosphorylationThe Excited
Electron eventually Provides Energy so a Phosphate Group Can Be Added to (ADP), Yielding (ATP)
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Photosynthesis: The Main Players
In eukaryotes, photosynthesis is carried out by four protein complexes, all located in the thylakoid membrane:
1. Photosystem II or P680: pass electrons & splits H2O2. Cytochromeb6f complex : the electric acceptors (plastoquinone& plastocyanin) 3. Photosystem I or P700 generates NADPH4. Proton translocating ATP synthase complex