Photosynthesis

PhotosynthesisPhotosynthesis

The conversion of light energy into The conversion of light energy into chemical energy stored in organic chemical energy stored in organic

compounds.compounds.

Biochemical pathway involves a complex series Biochemical pathway involves a complex series of chemical reactions in which the product of one of chemical reactions in which the product of one reaction is consumed in the next reaction.reaction is consumed in the next reaction.

Chlorophyll occurs in all photosynthetic Chlorophyll occurs in all photosynthetic eukaryotic cells and is considered to be essential eukaryotic cells and is considered to be essential for photosynthesis of the type carried out by for photosynthesis of the type carried out by plants.plants.

It functions in the capture of light energy by It functions in the capture of light energy by either directly or indirectly absorbing or receiving either directly or indirectly absorbing or receiving it in the form of high energy electrons from the it in the form of high energy electrons from the accessory pigments.accessory pigments.

Photosynthesis

Cellular Respiration

Autotrophs andheterotrophs

Autotrophs

LightEnergy

Carbon Dioxide + Water

Organic Compounds + Oxygen

Light Absorption in ChloroplastsLight Absorption in Chloroplasts Light ReactionsLight Reactions

Absorption of light in chloroplasts.Absorption of light in chloroplasts. A cell of a plant may have as many as 50 A cell of a plant may have as many as 50

chloroplasts.chloroplasts. ChloroplastsChloroplasts

Surrounded by a pair of membranes.Surrounded by a pair of membranes. Inside the inner membrane are flattened disks called Inside the inner membrane are flattened disks called

thylakoids.thylakoids. Thylakoids form stacks called grana.Thylakoids form stacks called grana. A solution called stroma surrounds the thylakoids.A solution called stroma surrounds the thylakoids. Stroma contains the enzymes responsible for the dark Stroma contains the enzymes responsible for the dark

reactions of photosynthesis.reactions of photosynthesis.

GranaGrana Each grana is composed structurally of layers of Each grana is composed structurally of layers of

protein molecules alternating with layers of protein molecules alternating with layers of chlorophyll, carotenes, and other pigments, and chlorophyll, carotenes, and other pigments, and special types of lipids containing galactose or sulfur special types of lipids containing galactose or sulfur but only one fatty acid.but only one fatty acid.

The surface active lipids are believed to be absorbed The surface active lipids are believed to be absorbed between the layers and serve in stabilizing the between the layers and serve in stabilizing the lamellae composed of one of the alternate layers of lamellae composed of one of the alternate layers of protein and pigments.protein and pigments.

This lamellar structure is important in permitting the This lamellar structure is important in permitting the transfer of energy captured from the sun.transfer of energy captured from the sun.

Light and PigmentsLight and Pigments Light from the Sun is composed of many colors.Light from the Sun is composed of many colors. The array of colors is called the visible spectrum.The array of colors is called the visible spectrum. ROYGBIVROYGBIV Light travels through space as waves of energy.Light travels through space as waves of energy. Light waves are measured in terms of their Light waves are measured in terms of their

wavelength.wavelength. A pigment is a compound that absorbs a specific A pigment is a compound that absorbs a specific

band of wavelengths.band of wavelengths. By absorbing certain colors, a pigment reflects the By absorbing certain colors, a pigment reflects the

remaining colors of the spectrum.remaining colors of the spectrum.

ChlorophyllChlorophyll Chlorophylls are a variety of pigments located in Chlorophylls are a variety of pigments located in

the membrane of the thylakoids.the membrane of the thylakoids. The two most common types of chlorophyll are The two most common types of chlorophyll are

chlorophyll achlorophyll a and and chlorophyll bchlorophyll b..

Chlorophyll aChlorophyll a Absorbs less blue light.Absorbs less blue light. Absorbs more red light.Absorbs more red light. Directly involved in the light reactions of photosynthesisDirectly involved in the light reactions of photosynthesis

ChlorophyllChlorophyll Chlorophyll bChlorophyll b

Absorbs more blue lightAbsorbs more blue light Absorbs less red light.Absorbs less red light. Assists chlorophyll a in capturing light energy. Assists chlorophyll a in capturing light energy.

Therefore it is called an accessory pigment.Therefore it is called an accessory pigment. Other compounds found in the thylakoid membraneOther compounds found in the thylakoid membrane

Yellow, Orange, and Brown Carotenoids.Yellow, Orange, and Brown Carotenoids. Carotenoids serve as accessory pigments.Carotenoids serve as accessory pigments.

By absorbing colors that By absorbing colors that chlorophyll achlorophyll a cannot cannot absorb, the accessory pigments convert the absorb, the accessory pigments convert the remaining energy.remaining energy.

Chlorophyll cChlorophyll c or or d d take the place of take the place of Chlorophyll bChlorophyll b in some plants.in some plants.

CarotenoidsCarotenoids Red, orange, or yellow, fat-soluble pigments found Red, orange, or yellow, fat-soluble pigments found

in almost all chloroplasts.in almost all chloroplasts. Carotenoids that do not contain oxygen are called Carotenoids that do not contain oxygen are called

carotenes. They are deep orange in color.carotenes. They are deep orange in color. Those that contain oxygen are called xanthophylls Those that contain oxygen are called xanthophylls

and are yellowish in color.and are yellowish in color. Carotenoids are bound to proteins within the Carotenoids are bound to proteins within the

lamellae of the chloroplast.lamellae of the chloroplast.

CarotenoidsCarotenoids In the green leaves, the color of the carotenoids is In the green leaves, the color of the carotenoids is

masked by the much more abundant chlorophylls.masked by the much more abundant chlorophylls. In some tissues , such as the ripe tomato or the In some tissues , such as the ripe tomato or the

petals of flowers, the carotenoids predominate.petals of flowers, the carotenoids predominate. The carotenoids function in absorbing light not The carotenoids function in absorbing light not

usable by the chlorophylls and in transferring the usable by the chlorophylls and in transferring the absorbed energy to absorbed energy to chlorophyll achlorophyll a..

PhytochromesPhytochromes

A pigment that is a light sensitive, blue A pigment that is a light sensitive, blue phytochrome.phytochrome.

They play a fundamental role in allowing the They play a fundamental role in allowing the plant to detect whether it is in a light or dark plant to detect whether it is in a light or dark environment.environment.

Electron TransportElectron Transport

Photosystem is a cluster of a few hundred pigment Photosystem is a cluster of a few hundred pigment moleculesmolecules

Light reactions begin when accessory pigment Light reactions begin when accessory pigment molecules in both photosystems absorb light.molecules in both photosystems absorb light.

Those molecules acquire energy from the light Those molecules acquire energy from the light wave.wave.

The energy is passed to other molecules until it The energy is passed to other molecules until it reaches reaches chlorophyll achlorophyll a molecules. molecules.

Electron TransportElectron Transport

Step One:Step One: Light energy forces electrons to enter a higher Light energy forces electrons to enter a higher

energy level in the two energy level in the two chlorophyll achlorophyll a molecules of molecules of photosystem II.photosystem II.

These electrons are said to be These electrons are said to be excitedexcited..

Electron TransportElectron Transport

Step TwoStep Two The excited electrons have enough energy to leave The excited electrons have enough energy to leave

chlorophyll achlorophyll a molecules. molecules. Because they have lost electrons, the Because they have lost electrons, the chlorophyll achlorophyll a

molecules have undergone an oxidation reaction.molecules have undergone an oxidation reaction. The electrons are accepted by a molecule in the The electrons are accepted by a molecule in the

thylakoid membrane called the thylakoid membrane called the primary electron primary electron acceptor.acceptor.

Electron TransportElectron Transport Step ThreeStep Three

The primary electron acceptor then donates the The primary electron acceptor then donates the electron to the first of a series of molecules located electron to the first of a series of molecules located in the thylakoid membrane.in the thylakoid membrane.

This series of molecules is called an This series of molecules is called an electron electron transport chaintransport chain..

As the electrons move from molecule to molecule As the electrons move from molecule to molecule in the electron transport chain they lose energy.in the electron transport chain they lose energy.

The energy they lose is harnessed to move proteins The energy they lose is harnessed to move proteins into the thylakoid.into the thylakoid.

Electron TransportElectron Transport Step FourStep Four

At the same time light is absorbed by photosystem At the same time light is absorbed by photosystem II it is also absorbed by photosystem I.II it is also absorbed by photosystem I.

Electrons move from a pair of chlorophyll a Electrons move from a pair of chlorophyll a molecules in photosystem I to another primary molecules in photosystem I to another primary electron acceptor.electron acceptor.

The electrons that are lost by these chlorophyll a The electrons that are lost by these chlorophyll a molecules are replaced by electrons that have molecules are replaced by electrons that have passed through the electron transport chain from passed through the electron transport chain from photosystem II.photosystem II.

Electron TransportElectron Transport Step FiveStep Five

The primary electron acceptor of photosystem I donates The primary electron acceptor of photosystem I donates electrons to a different electron transport chain.electrons to a different electron transport chain.

This chain brings the electrons to the side of the This chain brings the electrons to the side of the thylakoid membrane that faces the stroma.thylakoid membrane that faces the stroma.

There the electrons combine with a proton and NADPThere the electrons combine with a proton and NADP++.. NADPNADP++ is an organic molecule that accepts electrons is an organic molecule that accepts electrons

during redox reactions.during redox reactions. This reaction causes NADPThis reaction causes NADP++ to be reduced to NADPH. to be reduced to NADPH.

Electron TransportElectron Transport If the electrons from photosystem II were not If the electrons from photosystem II were not

replaced, both electron transport chains would stop replaced, both electron transport chains would stop and photosynthesis would not occur.and photosynthesis would not occur.

The replacement electrons are provided by water The replacement electrons are provided by water molecules.molecules.

An enzyme inside the thylakoid splits the water An enzyme inside the thylakoid splits the water molecules into protons, electrons, and oxygen.molecules into protons, electrons, and oxygen.

For every two water molecules that are split, four For every two water molecules that are split, four electrons become available to replace those lost by electrons become available to replace those lost by chlorophyll molecules in photosystem II.chlorophyll molecules in photosystem II.

Electron TransportElectron Transport The protons that are produced are left inside the The protons that are produced are left inside the

thylakoid, while the oxygen diffuses out of the thylakoid, while the oxygen diffuses out of the chloroplast and can then leave the plant.chloroplast and can then leave the plant.

Oxygen is not needed for photosynthesis to occur.Oxygen is not needed for photosynthesis to occur.

Oxygen is a byproduct of the light reactions and is Oxygen is a byproduct of the light reactions and is essential for cellular respiration in most essential for cellular respiration in most organisms.organisms.

ChemiosmosisChemiosmosis

Chemiosmosis is a process in chloroplasts and Chemiosmosis is a process in chloroplasts and mitochondria in which the movement of mitochondria in which the movement of protons down their concentration gradient protons down their concentration gradient across a membrane is coupled to the synthesis across a membrane is coupled to the synthesis of ATP.of ATP.

Adenosine Triphosphate (ATP) is a molecule Adenosine Triphosphate (ATP) is a molecule present in all living cells and acts as an energy present in all living cells and acts as an energy source for metabolic processes.source for metabolic processes.

ChemiosmosisChemiosmosis

Some protons are produced from the Some protons are produced from the breakdown of water molecules inside the breakdown of water molecules inside the thylakoid.thylakoid.

Other protons are pumped from the stroma to Other protons are pumped from the stroma to the interior of the thylakoid.the interior of the thylakoid.

The energy to pump those protons is supplied The energy to pump those protons is supplied by the excited electrons as they pass along the by the excited electrons as they pass along the electron transport chain of photosystem II.electron transport chain of photosystem II.

ChemiosmosisChemiosmosis These actions act to build up a concentration These actions act to build up a concentration

gradient of protons.gradient of protons. The concentration is higher inside the thylakoid than The concentration is higher inside the thylakoid than

in the stroma.in the stroma. The concentration gradient of protons represents The concentration gradient of protons represents

potential energy.potential energy. That energy is harnessed by a protein called ATP That energy is harnessed by a protein called ATP

synthase which is located in the thylakoid synthase which is located in the thylakoid membrane.membrane.

ChemiosmosisChemiosmosis

ATP synthase makes ATP by adding a phosphate ATP synthase makes ATP by adding a phosphate group to adenosine diphosphate (ADP).group to adenosine diphosphate (ADP).

The energy that drives this reaction is provided by The energy that drives this reaction is provided by the movement of protons from inside of the the movement of protons from inside of the thylakoid to the stroma.thylakoid to the stroma.

ATP synthase converts the potential energy of the ATP synthase converts the potential energy of the proton concentration gradient into chemical energy proton concentration gradient into chemical energy stored in ATP.stored in ATP.

ChemiosmosisChemiosmosis

ATP synthase is a multifunctional protein.ATP synthase is a multifunctional protein. By allowing protons to cross the thylakoid By allowing protons to cross the thylakoid

membrane, ATP synthase functions as a membrane, ATP synthase functions as a carrier protein.carrier protein.

By catalyzing the synthesis of ATP from ADP, By catalyzing the synthesis of ATP from ADP, ATP synthase functions as an enzyme.ATP synthase functions as an enzyme.

The Calvin CycleThe Calvin Cycle The Calvin Cycle is a biochemical pathway that The Calvin Cycle is a biochemical pathway that

produces organic compounds, using energy stored in produces organic compounds, using energy stored in ATP and NADPH during light reactions.ATP and NADPH during light reactions.

Carbon atoms from COCarbon atoms from CO22 are bonded into organic are bonded into organic

compounds and is called compounds and is called carbon fixationcarbon fixation..

The Calvin Cycle occurs within the stroma of the The Calvin Cycle occurs within the stroma of the chloroplast.chloroplast.

The Calvin CycleThe Calvin Cycle

Step OneStep One COCO22 diffuses into the stoma from the surrounding diffuses into the stoma from the surrounding

cytosol.cytosol. An enzyme combines a COAn enzyme combines a CO22 molecule with a five- molecule with a five-

carbon carbohydrate called RuBP.carbon carbohydrate called RuBP. The product is a six-carbon molecule that splits The product is a six-carbon molecule that splits

immediately into a pair of three-carbon molecules immediately into a pair of three-carbon molecules known as PGA.known as PGA.

The Calvin CycleThe Calvin Cycle Step TwoStep Two

PGA is converted into another three-carbon molecule PGA is converted into another three-carbon molecule called PGAL in a two part process.called PGAL in a two part process.

Each PGA molecule receives a phosphate group from a Each PGA molecule receives a phosphate group from a molecule of ATP.molecule of ATP.

The resulting group then receives a proton from NADPH The resulting group then receives a proton from NADPH and releases a phosphate group thereby producing PGAL.and releases a phosphate group thereby producing PGAL.

In addition ADP, NADP, and phosphate are produced In addition ADP, NADP, and phosphate are produced where they are then used again in the light reactions to where they are then used again in the light reactions to synthesize additional molecules of ATP and NADPH.synthesize additional molecules of ATP and NADPH.

The Calvin CycleThe Calvin Cycle Step 3Step 3

Most of the PGAL is converted back into RuBP in a Most of the PGAL is converted back into RuBP in a complicated series of reactions.complicated series of reactions.

This requires a phosphate group from another molecule This requires a phosphate group from another molecule of ATP, which is changed into ADP.of ATP, which is changed into ADP.

By regenerating RuBP that was consumed in step one, By regenerating RuBP that was consumed in step one, the reactions of step three allow the Calvin Cycle to the reactions of step three allow the Calvin Cycle to continue operating.continue operating.

Some PGAL molecules are not converted into RuBP. Some PGAL molecules are not converted into RuBP. Instead they leave the Calvin Cycle to be used by the Instead they leave the Calvin Cycle to be used by the plant to make other organic compounds.plant to make other organic compounds.

Alternate PathwaysAlternate Pathways Plants that fix carbon exclusively through the Calvin Plants that fix carbon exclusively through the Calvin

Cycle are known as Cycle are known as CC33 plants plants because of the three- because of the three-

carbon compound PGA that is initially formed.carbon compound PGA that is initially formed. Other plant species fix carbon through alternative Other plant species fix carbon through alternative

pathways and then release it to enter the Calvin pathways and then release it to enter the Calvin Cycle.Cycle.

These plants are generally found in hot, dry climates These plants are generally found in hot, dry climates where plants can rapidly lose water to the air.where plants can rapidly lose water to the air.

Most water loss is through small pores called Most water loss is through small pores called stomatastomata which are usually located on the underside which are usually located on the underside of the leaf.of the leaf.

Alternate PathwaysAlternate Pathways

Plants can partially close their stomata when the air is Plants can partially close their stomata when the air is hot and dry thereby reducing water loss.hot and dry thereby reducing water loss.

Stomata are also the major path for COStomata are also the major path for CO22 to enter and to enter and

OO22 to leave a plant. to leave a plant. When the stomata are partially closed, the level of When the stomata are partially closed, the level of

COCO22 in the plant falls as CO in the plant falls as CO22 is consumed in the is consumed in the

Calvin Cycle.Calvin Cycle. At the same time the level of OAt the same time the level of O22 in the plant rises as in the plant rises as

the light reactions split water and generate Othe light reactions split water and generate O22..

Alternate PathwaysAlternate Pathways

Both of these conditions, low COBoth of these conditions, low CO22 levels and levels and

high Ohigh O22 levels, inhibit carbon fixation by the levels, inhibit carbon fixation by the

Calvin Cycle.Calvin Cycle.

Some plants have evolved a way of dealing Some plants have evolved a way of dealing with this problem using alternative pathways with this problem using alternative pathways for carbon fixation.for carbon fixation.

The CThe C44 Pathway Pathway

During the hottest part of the day, the CDuring the hottest part of the day, the C44 plants plants

have their stomata partially closed.have their stomata partially closed. Certain cells in a C4 plants have an enzyme Certain cells in a C4 plants have an enzyme

that can fix COthat can fix CO22 into four-carbon compounds into four-carbon compounds

even when the COeven when the CO22 level is low and the O level is low and the O22

level is high.level is high. These compounds are then transported to other These compounds are then transported to other

cells where the COcells where the CO22 is released and enters the is released and enters the

Calvin Cycle.Calvin Cycle.

The CThe C44 Pathway Pathway

CC44 plants include corn, sugar cane, and plants include corn, sugar cane, and

crabgrass.crabgrass.

CC44 plants lose only about half as much water as plants lose only about half as much water as

CC33 plants when producing the same amount of plants when producing the same amount of

carbohydrate.carbohydrate.

The CAM PathwayThe CAM Pathway

Some plants close their stomata during the day Some plants close their stomata during the day and open them at night opposite of most plantsand open them at night opposite of most plants

Such plants fix carbon through a pathway Such plants fix carbon through a pathway called CAM.called CAM.

At night CAM plants take in COAt night CAM plants take in CO22 and fix it and fix it

into a variety of organic compounds.into a variety of organic compounds. During the day CODuring the day CO22 is released from those is released from those

compounds and enters the Calvin Cycle.compounds and enters the Calvin Cycle.

The CAM PathwayThe CAM Pathway

CAM plants grow fairly slow compared to CAM plants grow fairly slow compared to other plants.other plants.

CAM plants use less water than either CCAM plants use less water than either C33 or C or C44

plants.plants.

Rate of PhotosynthesisRate of Photosynthesis

The rate of photosynthesis increases and then The rate of photosynthesis increases and then reaches a plateau as light intensity or COreaches a plateau as light intensity or CO22 concentration increases.concentration increases.

Below a certain temperature, the rate of Below a certain temperature, the rate of photosynthesis increases as the temperature photosynthesis increases as the temperature increases.increases.

Above that temperature, the rate of Above that temperature, the rate of photosynthesis decreases as temperature photosynthesis decreases as temperature increases.increases.

Harvesting LightHarvesting Light Photosystem I, shown here Photosystem I, shown here

looking from the top, contains an looking from the top, contains an electron transfer chain, colored electron transfer chain, colored here in bright colors, at the center here in bright colors, at the center of each of the three subunits. of each of the three subunits.

Each one is surrounded by a Each one is surrounded by a dense ring of chlorophyll and dense ring of chlorophyll and carotenoid molecules that act as carotenoid molecules that act as antennas. antennas.

In this picture, the protein is In this picture, the protein is transparent so that only the transparent so that only the cofactors are seen. These antenna cofactors are seen. These antenna molecules each absorb light and molecules each absorb light and transfer energy to their neighbors.transfer energy to their neighbors.

Rapidly, all of the energy funnels Rapidly, all of the energy funnels into the three reaction centers, into the three reaction centers, where is captured to create where is captured to create activated electrons.activated electrons.

This picture shows the electron This picture shows the electron transfer chain at the center, drawn transfer chain at the center, drawn in spacefilling spheres. in spacefilling spheres.

Two special chlorophyll Two special chlorophyll molecules, residues 1140 and molecules, residues 1140 and 1239, are also shown in spheres 1239, are also shown in spheres and colored green. and colored green.

These two chlorophyll molecules These two chlorophyll molecules act as a bridge between the act as a bridge between the reaction center in the middle and reaction center in the middle and the many molecules in the the many molecules in the surrounding antenna. surrounding antenna.

The many antenna cofactors are The many antenna cofactors are shown here in bond representation shown here in bond representation with small spheres for the with small spheres for the magnesium ions at the center of magnesium ions at the center of each chlorophylleach chlorophyll