Chloroplast

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Chloroplast

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•the site of photosynthesis in eukaryotic cells

•disk-like structures

•composed of a single membrane

•surrounding a fluid containing stacks of membranous disks

Chloroplast

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•SOLAR energy radiated from the sun is captured by plants(chloroplast)

•Then it is instantaneously changed into ELECTRICAL energy

•Then packaged as CHEMICAL energy

Chloroplast

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The structure of the chloroplast , function and photosynthetic membranes • Inner membrane- highly impermeable, substances moving through this membrane with the aid of a variety of transporters• Outer membrane- outer covering of the chloroplast, it contains several different porins (proteins with large channels, 1 nm), exhibit selectivity towards various solutesThe thylakoid is the structural unit of photosynthesis. • Intermembrane space- a narrow space between inner and outer membrane• Thylakoids- flattened membranous sacs which contains the photosynthetic chemicals • Granum/Grana- are stacks of thyllakoids• Stroma- the areas between grana which contains the enzymes responsible for carbohydrate synthesis. It contains small, double-stranded, circular DNA molecules and prokaryotic-like ribosomes• Lumen- the space inside a thylakoid sac

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Chloroplast•photosynthesis takes place inside the

chloroplastPhotosynthesis-

the process in which plant use water, carbon dioxide, and energy form the sun to make food

Photosynthesis is the conversion of light energy to chemical energy or the production of carbohydrates from carbon dioxide and water in the presence of chlorophyll

the overall reaction of this process is: 6H2O + 6CO2 ----------> C6H12O6+ 6O2

Leaves and Leaf StructurePlants are the only photosynthetic organisms to have leaves (and not all plants have leaves). A leaf may be viewed as a solar collector crammed full of photosynthetic cells.The raw materials of photosynthesis, water and carbon dioxide, enter the cells of the leaf, and the products of photosynthesis, sugar and oxygen, leave the leaf.

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 chloroplastthe energy (light) is absorbed and stored in ATP & NADPH

2. 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

The Nature of Light White light is separated into the different colors (=wavelengths) of light by passing it through a prism. Wavelength is defined as the distance from peak to peak (or trough to trough). The energy of is inversely proportional to the wavelength: longer wavelengths have less energy than do shorter ones.

The order of colors is determined by the wavelength of light. Visible light is one small part of the electromagnetic spectrum. The longer the wavelength of visible light, the more red the color. Likewise the shorter wavelengths are towards the violet side of the spectrum. Wavelengths longer than red are referred to as infrared, while those shorter than violet are ultraviolet.

Light behaves both as a wave and a particle. Wave properties of light include the bending of the wave path when passing from one material (medium) into another (i.e. the prism, rainbows, pencil in a glass-of-water, etc.). The particle properties are demonstrated by the photoelectric effect. Zinc exposed to ultraviolet light becomes positively charged because light energy forces electrons from the zinc. These electrons can create an electrical current. Sodium, potassium and selenium have critical wavelengths in the visible light range. The critical wavelength is the maximum wavelength of light (visible or invisible) that creates a photoelectric effect.

WHAT IS PHOTOELECTRIC EFFECT?

Photoelectric Effect is the formation and liberation of electrically charged particles in matter when it is irradiated by light or other electromagnetic radiation. The term photoelectric effect designates several types of related interactions. In the external photoelectric effect, electrons are liberated from the surface of a metallic conductor by absorbing energy from light shining on the metal's surface. The effect is applied in the photoelectric cell, in which the electrons liberated from one pole of the cell, the photocathode, migrate to the other pole, the anode, under the influence of an electric field.

Light travels in packets or quanta of energy called photons, which can be thought of as “particles” of light.

The absorption of light is the first step in a photochemical process. When a photon is absorbed by a molecule, an electron becomes sufficiently energetic to be pushed from an inner to an outer orbital. The molecule is said to have shifted from the ground state to an excited state. Because the number of orbitals in which an electron can exist is limited and each orbital has a specific energy level, it follows that any given atom or molecule can absorb only light of certain specific wavelengths.

The excited state of a molecule is unstable and can be expected to last only about .0000000009 second. There is a tendency that the electron of an excited chlorophyll molecule drops back to a lower orbital, the energy it had absorbed will be dissipated or released as heat or reemitted at a longer wavelength.

Instead, the excited electrons of chlorophyll molecules are transferred to electron acceptors within the chloroplast membranes before they have a chance to drop back to lower energy orbitals. Thus, the chloroplasts are able to harness the absorbed energy before it dissipates.

Chlorophyll and Accessory PigmentsA pigment is any substance that absorbs light. The color of the pigment comes from the wavelengths of light reflected (in other words, those not absorbed). Chlorophyll, the green pigment common to all photosynthetic cells, absorbs all wavelengths of visible light except green, which it reflects to be detected by our eyes. Black pigments absorb all of the wavelengths that strike them. White pigments/lighter colors reflect all or almost all of the energy striking them. Pigments have their own characteristic absorption spectra, the absorption pattern of a given pigment.

Absorption and transmission of different wavelengths of light by a hypothetical pigment.

Chlorophyll is a complex molecule. Several modifications of chlorophyll occur among plants and other photosynthetic organisms. All photosynthetic organisms (plants, certain protistans, prochlorobacteria, and cyanobacteria) have chlorophyll a. Accessory pigments absorb energy that chlorophyll a does not absorb. Accessory pigments include chlorophyll b (also c, d, and e in algae and protistans), xanthophylls, and carotenoids (such as beta-carotene). Chlorophyll a absorbs its energy from the Violet-Blue and Reddish orange-Red wavelengths, and little from the intermediate (Green-Yellow-Orange) wavelengths.

Molecular model of chlorophyll. The above image is from http://www.nyu.edu:80/pages/mathmol/library/photo

Molecular model of carotene.

Carotenoids and chlorophyll b absorb some of the energy in the green wavelength. Why not so much in the orange and yellow wavelengths? Both chlorophylls also absorb in the orange-red end of the spectrum (with longer wavelengths and lower energy). The origins of photosynthetic organisms in the sea may account for this. Shorter wavelengths (with more energy) do not penetrate much below 5 meters deep in sea water. The ability to absorb some energy from the longer (hence more penetrating) wavelengths might have been an advantage to early photosynthetic algae that were not able to be in the upper (photic) zone of the sea all the time

The molecular structure of chlorophylls

The action spectrum of photosynthesis is the relative effectiveness of different wavelengths of light at generating electrons. If a pigment absorbs light energy, one of three things will occur. Energy is dissipated as heat. The energy may be emitted immediately as a longer wavelength, a phenomenon known as fluorescence. Energy may trigger a chemical reaction, as in photosynthesis. Chlorophyll only triggers a chemical reaction when it is associated with proteins embedded in a membrane (as in a chloroplast) or the membrane infoldings found in photosynthetic prokaryotes such as cyanobacteria and prochlorobacteria.

Absorption spectrum of several plant pigments (left) and action spectrum of elodea (right), a common aquarium plant used in lab experiments about photosynthesis.

PHOTOSYNTHETIC UNITS AND REACTION CENTERSSeveral hundred chlorophyll molecules act

together as one photosynthetic unit in which only one member of the group- the reaction center chlorophyll- actually transfers electrons to an electron acceptor.

Pigment molecules are responsible for light absorption. These pigment molecules form a light-harvesting antenna that absorbs photons of varying wavelength and transfers that energy very rapidly to the pigment molecule at the reaction center.

The energy can only be passed to a pigment molecule that absorbs light of equal or longer wavelength (lower energy) than that absorbed by the donor molecule. The energy is ultimately transferred to a chlorophyll of the reaction center, which absorbs light of longer wavelength than any of its neighbors. Once the energy is received by the reaction center, the electron excited by light absorption can be transferred to its acceptor.

PHOTOSYTEMSThe light absorbing reactions of photosynthesis occur in large pigment-protein complexes called photosystems. Photosystems are arrangements of chlorophyll and other pigments 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 nm

2. 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

Photophosphorylation is the process of converting energy from a light-excited electron into the pyrophosphate bond of an ADP molecule. This occurs when the electrons from water are excited by the light in the presence of P680. The energy transfer is similar to the chemiosmotic electron transport occurring in the mitochondria. Light energy causes the removal of an electron from a molecule of P680 that is part of Photosystem II. The P680 requires an electron, which is taken from a water molecule, breaking the water into H+ ions and O-2 ions. These O-

2 ions combine to form the diatomic O2 that is released. The electron is "boosted" to a higher energy state and attached to a primary electron acceptor, which begins a series of redox reactions, passing the electron through a series of electron carriers, eventually attaching it to a molecule in Photosystem I.

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•No energy transformation is 100% efficient

•Not all the solar energy captured is converted to electrical and then chemical energy.

•Some of it gets lost as heat or other forms of energy (light)

Chloroplast