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photosynthesis

the chloroplasts of plants caputre light energy that has traveled 150 million kilometers from teh sun and convert it to chemical energy stored in sugar and other organic molecules; produces sugar from inorganic compounds and light energy 6CO2 + H2) --> C6H12)6 + 6O2; broken in two portions: light reactions and light independent reactions

autotrophs

"self feeders"; they sustain themselves without eating anything derived from other organisms; produce theri organic molecules from CO2 and other inorganic raw materials obtained from the enviroment; producers of the biosphere; almost all plants; only nutreints they require are water and minerals from the soil and carbon dioxide from the air

photoautotrophs

organisms that use light as a source of energy to synthesize organic substances

heterotrophs

obtain their organic material by the second major mode of nutrition; unable to make their own food; biosphere's conusmers; live on compounds produced by other organisms

chloroplasts

the sites of photosynthesis in plants;

chlorophyll

the green pigment located within chloroplasts; light energy absorbed by chlorophyll drives the syntehsis of organic molecules inside the chloroplast; reside in the thylakoid membranes

mesophyll

chloroplasts are found mainly in the cells of the mesophyll; the tissue in the interior of the leaf

stomata

microscopic pores that carbon dioxide enters the leaf through

stroma

dense fluid within the chloroplast;

thylakoids

an elaborate system of interconnected membranous sacs; segregates the stroma from another compartment=thylakoid space

grana

thylakoid stacks (singlular= granum)

role of water in photosynthesis

oxygen given off by plants through their stomata is derived from water and not from carbon dioxide; the chloroplast splits the water into hydrogen and oxygen; discovered by Van Neil who hypothesized that plants split water as a source of electrons from hydrogen atoms releasing oxygen as a byproduct

photosynthesis as a redox process

similar to cellular respiration (in cellular respiration energy is released from sugar when electrons associated wit hydrogen are transported by carriers to oxygen forming water as a byproduct, the electrons lose potential energy as they fall down the electron transport chain toward electronegative oxygen and the mitochondrion harnesses that energy to synthesize ATP); photosynthesis reverses the direction of electron flow, water is split and electrons are transferred along with hydrogen ions from the water to Carbon dioxide, reducing it to sugar; because electrons increase in potential energy as they move from water to sugar this process requires energy, the energy boost is provided by light

the two stages of photosynthesis

Light reactions, dark reactions

light reactions

harvest sunlight to produce ATP in thylakoid membranes; ETC produces an H+ gradient, H+gradient generates ATP; converts solar energy to chemical energy; light absorbed by chlorophyll drives a transfer of electrons and hydrogen from water to an acceptor caleld NADP which temporarily stores the energized electrons. Water is split in the process and thus it is the light reactions of photosynthesis that give of 02 as a byproduct; the light reactions use solar power to reduce NADP+ to NADPH by adding a pair of electrons along with a hydrogen nucleus or H+; the light reactions also generate ATP using chemiosmosis to power the addition of a phosphate group to ADP (photophosphorylation); thus light energy is initially converted to chemical energy in teh form of two compounds: NADPH (a source of energized electrons, reducing power) and ATP (the versitile energy currency of cells); light reactions produce no sugar

photophosphorylation

the light reactions also generate ATP using chemiosmosis to power the addition of a phosphate group to ADP

Light independent reactions

dark reactions; calvin cycle: begins by incorporatin CO2 from the air into organic molecules already present in the chloroplast= carbon fixation; then reduces teh fixed carbon to carbohydrate by the addition of electrons (reducing power is provided by NADPH); to convert CO2 to carbohydrate the calvin cycle also requires chemical energy in the form of ATP; makes sugar but it can only do so with the help of the NADPH and ATP produced in the light reactions; occurs in the stroma

light

form of energy known as electromagnetic energy/ electromagnetic radiation; travels in rythmic waves analogous to those created by dropping a pebble into a pond; disturbances of electrical and magnetic fields rather than disturbances of a material medium such as water

wavelength

the distance between the crests of electromagnetic waves is called the wavelength; range from less than a nanometer to more than a kilometer

visible light

380nm (violet) to 750nm (red)

photons

behaves as a wave and as though it consists of discrete particels called photons; not tangible objects but they act like objects in that each of them has a fixed quantity of energy; the amount of energy is inversely related to wavelength of the light (the shorter the wavelength the greater energy of each photon of that light, thus a photon of violet light packs nearly twice as much energy has a photon of red light

spectrophometer

the ability of a pigment to absorb various wavelengths of light can be measured with an instrument called a spectrophotometer

absorption spectrum

frequencies absorbed by a particular pigment; a graph plotting a pigments light absorption versus wavelength

chlorophyll a

the absorption spectra of chloroplast pigments provide clues to the relative effectiveness of different wavelengths for driving photosynthesis since light can perform work in chloroplasts only if it is absorbed; absorption spectrum of chlorophyll a suggests that violet blue and red light works best for photosynthesiss since they are absorbed while green is the least effective color;

action spectrum

profiles the relative effectiveness of different wavelengths of radiation in driving the process; prepared by illuminating chlorplasts with light of different colors and then plotting wavelength against some measure of photosynthetic rate such as CO2 consumption of O2 release

chlorophyll b

almost identical to chlorophyll a but a slight structural difference between them is enough to give the two pigments slightly different absorption spectra and as a result have different colors (a=blue-green, b=yellow-green

carotenoids

hydrocarbons that are various shades of yellow and orange because they absorb violoet and blue-green light; accessory pigment; may broaden the spectrum of colors that can drive photosynthesis; most important function=photoprotection

photoprotection

carotenoids absorb and dissipate exessive light energy that would otherwise damage chlorophyll or interact with oxygen forming reactive oxidative molecules that are dangerous to the cell;

photosynthetic pigment

uses light energy to excite an electron in a pigment molecule; higher enenergy electron then jumps off the pigment molecule and enters the ETC where its energy pumps H+ into the inner-thylakoid space; as these H+ ions diffuse out through ATP synthase they generate ATP for the cell (same ATP synthase as in mitochondrion)

photosystem

collection of chlorophyll molecules (pigment), proteins and small organic molecules; located in the thylakoid membrane of chloroplasts (in eukaryotes)

light harvesting complex

consists of pigment molecules bound to particular proteins; the number and variety of pigment molecules enable a photosystem to harvest light over a larger surgace and a larger portion of the spectrum than any single pigment moleucel alone could; when a pigment molecule absorbs a photon the energy is transferred from pigment molecule to pigment molecule within a light harvesting complex until it is funneled into the reaction center

reaction center

a protein complex that includes two special chlorophyll a molecules and a molecule called the primary electron acceptor

structure of a photosystem (and function)

light harvesting complexes absorb energy and pass that energy on to the reaction center; energy is passed along pigment molecules in light harvestin complexes into the reaction center; energy absorbed from light causes an electron to exit the reaction center chlorophyll alpha's; reaction center chlorophyll molecules in PSII and PSI are identical but because of their surroundings these PS's absrob slightly different frequencies of light (PSII p680, PSI p700)

primary electron acceptor

takes high energy electrons from center chlorphyll and passes it on to ETC;

Noncyclic electron flow

see sheet

cyclic electron flow

under certain conditions the photoexcited electrons take an alternative path called cyclic electron flow; uses photosystem I but not photosystem II

The Calvin Cycle

uses ATP and NADPH to convert CO2 to sugar; anabolic= building sugar from smaller molecules and consuming energy;

glyceraldehyde-3-phosphate (G3P)

the carbohydrate produced directly from the Calvin cycle is actually not glucose, but a three carbon sugar named G3P; for the net synthesiss of one molecule of this sugar the cycle must take place three times fixing three molecules of CO2

3 Phases of Calvin Cycle

1.) Carbon fixation 2) Reduction 3) Regeneration of the CO2 acceptor (RuBP)

Net synthesis of one G3P molecule in the Calvin Cycle

nine molecules of ATP consumed; six molecules of NADPH consumed; G3P spun off from the Calvin Cycle becomes the starting material for metabolic pathways that synthesize other organic compounds including glucose and other carbohydrates; neither the light reactiosn nor the Calvin cycle alone can make sugar from CO2; photosynthesis is an emergent property of the intact chloroplast which integrates the two stages of photosynthesis

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