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Plants are AUTOTROPHS in that they make their own food and thus sustain themselves

without consuming organic molecules derived from any other organisms. Plant cells without consuming organic molecules derived from any other organisms. Plant cells

capture light energy, and convert it to chemical energy. Using this energy, plants make their

own organic molecules and are the ultimate source of organic molecules for almost all

other organisms. They are often referred to as the PRODUCERS of the biosphere because

they produce its food supply. All organisms that produce organic molecules from inorganic

molecules using the energy of light are called PHOTOAUTOTROPHS. In this chapter we

focus on photosynthesis in plants, which takes place in chloroplasts. The process of

photosynthesis most likely originated in a group of bacteria that had infolded regions of the

plasma membrane containing such clusters of enzymes and molecules. Chloroplasts appear

to have originated from a photosynthetic prokaryote that lived inside a eukaryotic cell.

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All green parts of a plant have chloroplasts in their cells and can carry out photosynthesis.

Their green color is from CHLOROPHYLL, a light absorbing pigment in the chloroplasts that Their green color is from CHLOROPHYLL, a light absorbing pigment in the chloroplasts that

plays a central role in converting solar energy to chemical energy. Chloroplasts are

concentrated in the cells of the mesophyll, the green tissue in the interior of the leaf.

Carbon dioxide enters the leaf and oxygen exists, by way of tiny pores called STOMATA.

Water absorbed by the roots is delivered to the leaves in veins.

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An envelope of two membranes encloses an inner compartment in the chloroplast, which

is filled with a thick fluid called STROMA. Suspended in the stroma is a system of is filled with a thick fluid called STROMA. Suspended in the stroma is a system of

interconnected membranous sacs, called THYLAKOIDS, which enclose another

compartment, called the thylakoid space. In some places, thylakoids are concentrated in

stacks called GRANA. Built into the thylakoid membranes are the chlorophyll molecules

that capture light.

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In the 1800’s most scientists assumed that plants produce O2 by extracting it from CO2. In

the 1950’s, scientists tested this hypothesis by using a heavy isotope of oxygen,18O, to the 1950’s, scientists tested this hypothesis by using a heavy isotope of oxygen,18O, to

follow the fate of oxygen atoms during photosynthesis.

EXPERIMENT 1: a plant given carbon dioxide containing 18O gave off no labeled oxygen gas

(18O containing).

EXPERIMENT 2: a Plant given water containing 18O did produce labeled O2.

These experiments showed that the O2 produced during photosynthesis comes from water

and not from CO2. It takes two water (H2O) molecules to make each molecule of O2.

Additional experiments have revealed that the oxygen atoms in CO2 and the hydrogens in

the reactant H2O molecules end up in the sugar molecule and in water that is formed

anew.

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Photosynthesis is a redox (oxidation-reduction) process, just as cellular respiration is. In

photosynthesis, water molecules are split apart, yielding O2, they are actually oxidized; that photosynthesis, water molecules are split apart, yielding O2, they are actually oxidized; that

is, they loose electrons, along with hydrogen ions (H+). Meanwhile, CO2 is reduced to sugar

as electrons and hydrogen ions are added to it.

Overall Cellular respiration harvests energy stored in a glucose molecule by oxidizing the

sugar and reducing O2 to H2O. This process involves a number of energy-releasing redox

reactions, with electrons losing potential energy as they travel down an energy “hill” from

sugar to O2.

In contrast, the food-producing redox reactions of photosynthesis involve an uphill climb.

As water is oxidized and CO2 is reduced during photosynthesis, electrons gain energy by

being boosted up an energy hill. The light energy captured by chlorophyll molecules in the

chloroplast provides the boost for the electrons. Photosynthesis converts light energy to

chemical energy and stores it in the chemical bonds of sugar molecules, which can provide

energy for later use or raw materials for biosynthesis.

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Photosynthesis occurs in two stages, each with multiple steps.The LIGHT REACTIONS include the steps that convert light energy to chemical energy and The LIGHT REACTIONS include the steps that convert light energy to chemical energy and produce O2. The light reaction occur in the thylakoid membranes. Water is split, providing a source of electrons and giving off O2 gas as a by-product. Light energy absorbed by chlorophyll molecules built into the membranes is sued to drive the transfer of electrons and H+ from water to NADP+, reducing it to NADPH. NADPH is an electron carrier similar to NADH that transports electrons in cellular respiration. In summary the light reactions of photosynthesis are the steps that absorb solar energy and convert it to chemical energy stored in ATP and NADPH. Notice that these reactions produce no sugar; sugar is not made until the CASLVIN CYCL, the second stage of photosynthesis.The CALVIN CYCLE occurs in the stroma of the chloroplast. It is a cyclic series of reactions that assemble sugar molecules using CO2 and the energy-containing products of the light reactions. In the 1940’s, Calvin and his colleagues traced the path of carbon in the cycle, using the radioactive isotope 14C to label the carbon in CO2. The incorporation of carbon from CO2 into organic compounds is called CARBON FIXATION. After carbon fixation, enzymes of the cycle make sugars by further reducing the carbon compounds. It is NADPH produced by the light reactions that provides the electrons for reducing carbon in the Calvin cycle. And ATP from the light reaction provides chemical energy that powers several of the steps of the Calvin cycle. The Calvin cycle is sometimes referred to as the dark reactions, or light-independent reactions, because none of the steps requires light directly.

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Sunlight is a type of energy called ELECTROMAGNETIC ENERGY or RADIATION.

Electromagnetic spectrum, is the full range of electromagnetic wavelengths from the very Electromagnetic spectrum, is the full range of electromagnetic wavelengths from the very

short gamma rays to the very long-wavelength radio waves. Visible light- the radiation your

eyes see as different colors- is only a small fraction of the spectrum. It consists of

wavelengths from about 380 nm to about 750 nm. The distance between the crests of two

adjacent waves is called a WAVELENGTH. Shorter wavelength have more energy than

longer ones.

The theory of light as waves explains most of light’s properties. However, light also behaves

as discrete packets of energy called photons. PHOTONS is a fixed quantity of light energy,

and as you have just learned, the shorter the wavelength, the greater the energy.

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Light-absorbing molecules called pigments, built into the thylakoid membranes, absorb

some wavelengths of light and reflect or transmit other wavelengths. We do not see the some wavelengths of light and reflect or transmit other wavelengths. We do not see the

absorbed wavelengths; their energy has been absorbed by pigment molecules. What we

see when we look at a leaf are the green wavelengths that the pigment transmits and

reflects.

Different pigments absorb light of different wavelengths, and chloroplasts contain several

kinds of pigments. Chlorophyll a, which participates directly in the light reactions, absorbs

mainly blue-violet and red light. A very similar molecule chlorophyll b absorbs mainly blue

and orange light and reflects yellow-green.

Chloroplasts also contain a family of pigments called CAROTENOIDS, which seem to be

used in photoprotection: They absorb and dissipate excessive light energy that would

otherwise damage chlorophyll or interact with oxygen to form reactive oxidative molecules

that can damage cell molecules.

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When a pigment molecule absorbs a photon, one of the pigment’s electron’s jumps to an

energy level farther from the nucleus. In this location, the electron has more potential energy level farther from the nucleus. In this location, the electron has more potential

energy, and we say that the electron has been raised from a ground state to an excited

state. The excited state is very unstable. Chlorophyll in its native habitat of the thylakoid

membrane, passes off its excited electron to a neighboring molecule before it has a chance

to drop back to the ground state.

In the thylakoid membrane, chlorophyll molecules are organized along with other pigments

and proteins into clusters called photosystems. A photosystem consists of a number of

light-harvesting complexes surrounding a reaction center complex. The light-harvesting

complexes consist of pigment molecules bound to proteins. The REACTION CENTER

COMPLEX contains a pair of chlorophyll a molecules and a molecule called the primary

electron acceptor, which is capable of accepting electrons and becoming reduced.

Two types of photosystems have been identified, and they cooperate in the light reactions.

They are referred to as Photosystem 1 and photosystem 2, with photosystem 2 acting first.

In photosystem 2 the chlorophyll a of the reaction center is called P680 because the light it

absorbs best is red light with a wavelength of 680nm. The reaction center chlorophyll of

photosystem 1 is called P700 nm.

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In the light reactions, light energy is transformed into the chemical energy of ATP and

NADPH. In this process, electrons removed from water molecules pass from photosystem 2 NADPH. In this process, electrons removed from water molecules pass from photosystem 2

to photosystem 1 to NADP+. Between the two photosystems, the electrons move down an

electron transport chain and provide energy for the synthesis of ATP.

1.) A pigment molecule in a light-harvesting complex absorbs a photon of light. The energy

is passed to other pigment molecules and finally to the reaction center of photosystem 2,

where it excites an electron of chlorophyll P680 to a higher energy state.

2.) This electron is captured by the primary electron acceptor.

3.) Water is split, and its electrons are supplied one by one to P680, each replacing an

electron lost to the primary electron acceptor. The oxygen atom combines with an oxygen

oxygen from another split water molecule to form a molecule of O2.

4.) Each photoexcited electron passes from photosystem 2 to photosystem 1 via an

electron transport chain. The exergonic “fall” of electrons provides energy for the synthesis

of ATP by pumping H+ across the membrane.

5.) Light energy excites an electron of chlorophyll P700 in the reaction center of

photosystem 1. The primary electron center captures the electron, and an electron from

the bottom of the electron transport chain replaces the lost electron in P700.

6.) The excited electron of photosystem 1 is passed through a short electron transport

chain to NADP+, reducing it to NADPH.

NADPH, ATP, and O2 are the products of the light reactions.

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Chemiosmosis is also the mechanism that generates ATP in a chloroplast. Recall that the

process of chemiosmosis drives ATP synthesis using the potential energy of a concentration process of chemiosmosis drives ATP synthesis using the potential energy of a concentration

gradient of hydrogen ions across a membrane. The gradient is created when an electron

transport chain uses the energy released as it passes electrons down the chain to pump

hydrogen ions across a membrane. During the electron transport chain hydrogen ions are

pumped across the membrane from the stroma into the thylakoid space. This generates the

concentration gradient across the membrane.

In photosynthesis, this chemiosmotic production of ATP is called

PHOTOPHOSPHORYLATION because the initial energy input is light energy.

Notice that in the light-driven flow of electrons through the two photosystems, the final

electron acceptor is NADP+, not O2 as in cellular respiration.

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The Calvin cycle functions like a sugar factory within a chloroplast. Inputs to this all-

important food-making process are CO2 (from the air) and ATP and NADPH (both important food-making process are CO2 (from the air) and ATP and NADPH (both

generated by the light reactions). Using carbon from CO2, energy from ATP, and high-

energy electrons from NADPH, the calvin cycle constructs and energy-rich, three carbon

sugar, glyceraldehyde-3-phosphate (G3P). A pplant cell can use G3P to make glucose and

other organic molecules as needed.

The starting material is a five-carbon sugar named ribulose bisphosphate (RuBP).

1.) CARBON FIXATION- In the carbon fixation step, the enzyme rubisco attaches CO2 to

RuBP.

2.) REDUCTION- In the next step, a reduction, NADPH reduces the organic acid 3-PGA to

G3P with the assistance of ATP. To make a molecule of G3P, the cycle must incorporate the

carbon atoms from three molecules of CO2. The cycle actually incorporates one carbon at a

time, but we show it starting with three CO2 molecules so that we end up with a complete

G3P molecule.

3.) RELEASE OF ONE MOLECULE OFG3P- For every three CO2 molecules fixed, one G3P

molecules leaves the cycle as product, and the remaining five G3P molecules are

rearranged.

4.) REGENERATION OF RuBP Using energy from ATP to regenerate three molecules of RuBP.

Note that for the net synthesis of one G3P molecule, the Calvin cycle consumes 9 ATP and 6

NADPH molecules, which were provided by the light reactions.

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The photosystems transfer photo excited electrons through electron transport chain, where

energy is harvested to make ATP, and to NADP+, where electrons are stored in the high-energy is harvested to make ATP, and to NADP+, where electrons are stored in the high-

energy compound NADPH. The chloroplast’s sugar factory is the Calvin cycle, the second

stage of photosynthesis. In the stroma, the enzyme rubisco combines CO2 with RuBP, and

ATP and NADPH are used to reduce 3-PGA to G3P.

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Closing stomata on a hot, dry day is an adaption that reduces water loss, but it also

prevents CO2 from entering the leaf and O2 from leaving. As a result, CO2 levels get very prevents CO2 from entering the leaf and O2 from leaving. As a result, CO2 levels get very

low in the leaf, while O2 from reactions builds up. These conditions reduce called

photorespiration.

Such plants are called C3 PLANTS because the first organic compound produced is the

three-carbon compound 3-PGA. When stomata closes to reduce water loss and O2 builds

up in a leaf, rubisco adds O2 instead of CO2 to RuBP. A two-carbon product of this reaction

is then broken down by the cell to CO2 and H2O, a process known as PHOTORESPIRATION.

Unlike photosynthesis, photorespiration yields no sugar, and unlike cellular respiration, it

produces no ATP.

C4 PLANTS: Precede the Calviin cycle by first fixing CO2 into a four-carbon compound.

When the weather is hot and dry, a C4 plant keeps its stomata mostly closed, thus

conserving water. At the same time, it continues making sugars by photosynthesis

CAM PLANTS: these species are adapted to very dry climates. A CAM plant conserves water

by opening its stomata and admitting CO2 only at night. When CO2 enters the leaves, it is

fixed into a four-carbon compound, as in C4 plants. The four-carbon compound in a CAM

plant banks CO2 at night and releases it to the Calvin cycle during the day. In C4 plants,

carbon fixation and the Calvin cycle occur in different types of cells. In CAM plants, these

process occur in the same cells, but at different times.

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