PhotosynthesisPhotosynthesisChapter 7Chapter 7

We depend on the solar energy of a star 93 We depend on the solar energy of a star 93 million miles away—our Sun!million miles away—our Sun!

Almost all living things are somehow Almost all living things are somehow dependent, directly or indirectly, on the dependent, directly or indirectly, on the amount of solar energy that photosynthesizers amount of solar energy that photosynthesizers can convert into chemical energycan convert into chemical energy

PHOTOSYNTHESIS converts solar energy into the PHOTOSYNTHESIS converts solar energy into the chemical energy of a carbohydratechemical energy of a carbohydrate

The organisms that The organisms that undergo photosynthesis undergo photosynthesis are called are called AUTOTROPHSAUTOTROPHS

For most plants, For most plants, photosynthesis occurs in photosynthesis occurs in the the leavesleaves

Water is absorbed Water is absorbed through the through the rootsroots

Carbon dioxide enter Carbon dioxide enter the leaf through the leaf through openings called openings called stomatastomata——the same carbon the same carbon dioxide that we exhale dioxide that we exhale (this is called(this is called transpirationtranspiration))

After entering the leaf, After entering the leaf, carbon dioxide and carbon dioxide and water diffuse into water diffuse into chloroplasts (the chloroplasts (the organelles that carry on organelles that carry on photosynthesis)photosynthesis)

The chloroplastThe chloroplast

The The stromastroma, a double , a double membrane, surrounds membrane, surrounds the chloroplastthe chloroplast

ThylakoidsThylakoids, sometimes , sometimes stacked to form stacked to form granagrana, , are also located inside are also located inside the chloroplastthe chloroplast

Chlorophyll and other pigments that are part of Chlorophyll and other pigments that are part of the thylakoid membrane are capable of the thylakoid membrane are capable of absorbing solar energy—the energy that drives absorbing solar energy—the energy that drives photosynthesisphotosynthesis

Photosynthetic pigmentsPhotosynthetic pigments

Pigments are molecules Pigments are molecules that absorb or reflect that absorb or reflect wavelengths of lightwavelengths of light

There are pigments in There are pigments in chloroplasts that absorb chloroplasts that absorb light, called their light, called their absorption spectrumabsorption spectrum

Chlorophyll Chlorophyll aa, , chlorophyll chlorophyll bb, and , and carotenoids are all carotenoids are all pigments that participate pigments that participate in photosynthesisin photosynthesis

They absorb violets, They absorb violets, blues, and reds and blues, and reds and reflect green (which is reflect green (which is why leaves appear green why leaves appear green to us)to us)

Chlorophyll a and chlorophyll b are Chlorophyll a and chlorophyll b are primary primary pigmentspigments and play an accessory role and play an accessory role ((accessory pigmentsaccessory pigments))

In autumn, chlorophyll In autumn, chlorophyll disappears from leaves disappears from leaves and carotenoids remainand carotenoids remain—which is why leaves —which is why leaves appear yellow, orange, appear yellow, orange, and red at this timeand red at this time

Photosynthetic reactionPhotosynthetic reaction

Remember that photosynthesis is a redox Remember that photosynthesis is a redox reaction: carbon dioxide is reduced and water reaction: carbon dioxide is reduced and water is oxidizedis oxidized

Revisit the equation for photosynthesis: you Revisit the equation for photosynthesis: you recall that glucose and oxygen are the end recall that glucose and oxygen are the end productsproducts

Two separate reactions occur during Two separate reactions occur during photosynthesis: one to produce the chemical photosynthesis: one to produce the chemical energy of ATP and NADPH (“Light energy of ATP and NADPH (“Light Reactions”) and one reaction to produce the Reactions”) and one reaction to produce the glucose (“Calvin Cycle Reactions”)glucose (“Calvin Cycle Reactions”)

Two types of reactions: light and Two types of reactions: light and Calvin Cycle reactionsCalvin Cycle reactions

There are two sets of reactions that occur There are two sets of reactions that occur during photosynthesis: Light reactions (for during photosynthesis: Light reactions (for ATP and NADPH, chemical energy, ATP and NADPH, chemical energy, production) and Calvin Cycle reactions (for production) and Calvin Cycle reactions (for glucose production)glucose production)

These two reactions are needed because These two reactions are needed because enzymes are needed to produce carbohydratesenzymes are needed to produce carbohydrates

Light Reactions and the Calvin Light Reactions and the Calvin Cycle: working togetherCycle: working together

Light reactionsLight reactions

1. Chlorophyll located within the 1. Chlorophyll located within the thylakoidthylakoid membranesmembranes absorbs solar energy and energizes absorbs solar energy and energizes electronselectrons

Light reactionsLight reactions

2. these energized electrons move down an 2. these energized electrons move down an electron transport chain and energy is captured electron transport chain and energy is captured and later used for ATP productionand later used for ATP production

Light reactionsLight reactions

3. Energized electrons are also taken up my 3. Energized electrons are also taken up my NADP+ , and electron carrier, which turns into NADP+ , and electron carrier, which turns into NADPHNADPH

Light reactions: summaryLight reactions: summary

Summary:Summary: Solar energy Solar energy chemical energy (ATP, NADPH) chemical energy (ATP, NADPH)

How light reactions generate ATP How light reactions generate ATP and NADPH: Photosystemsand NADPH: Photosystems

This pathway uses two This pathway uses two photosystemsphotosystems, , photosystem I (PSI) and photosystem II (PSII)photosystem I (PSI) and photosystem II (PSII)

A A photosystem photosystem consists of a pigment complex consists of a pigment complex and electron acceptor molecules within the and electron acceptor molecules within the thylakoid membranes thylakoid membranes

Photosystem I: Photosystem I:

Photosystem I is a water-splitter photosystem, Photosystem I is a water-splitter photosystem, which uses light energy to extract electrons which uses light energy to extract electrons from Hfrom H22OO

This is the process that releases OThis is the process that releases O22, a waste , a waste product of photosynthesisproduct of photosynthesis

Photosystem II:Photosystem II:

Photosystem II is an NADPH-producing Photosystem II is an NADPH-producing photosytemphotosytem

It produces NADPH by transferring light-It produces NADPH by transferring light-excited electrons from chlorophyll to NADP+excited electrons from chlorophyll to NADP+

An electron transport chain connecting the two An electron transport chain connecting the two photosystems releases energy that the photosystems releases energy that the chloroplast uses to make ATPchloroplast uses to make ATP

However….we still need Glucose! So onward However….we still need Glucose! So onward to the Calvin Cycle!to the Calvin Cycle!

Calvin Cycle reactions: making Calvin Cycle reactions: making sugar from Carbon Dioxidesugar from Carbon Dioxide

This takes place in the stromaThis takes place in the stroma 1. Carbon dioxide is taken up and reduced to carbohydrate1. Carbon dioxide is taken up and reduced to carbohydrate

The ATP and NADPH that was formed during the Light Reaction The ATP and NADPH that was formed during the Light Reaction carries out this reductioncarries out this reduction

Calvin CycleCalvin Cycle

2. After production of a carbohydrate, ADP + 2. After production of a carbohydrate, ADP + P and NADP+ return to the Light Reaction P and NADP+ return to the Light Reaction where they become ATP and NADPH once where they become ATP and NADPH once moremore

Calvin Cycle: summaryCalvin Cycle: summary

Summary:Summary:

Chemical energy Chemical energy chemical energy chemical energy (ATP, NADPH) (carbohydrate)(ATP, NADPH) (carbohydrate)

Calvin Cycle: the big pictureCalvin Cycle: the big picture

The energy yield in Light Reactions is stored The energy yield in Light Reactions is stored in ATP and NADPHin ATP and NADPH

These molecules are used by the Calvin Cycle These molecules are used by the Calvin Cycle reactions to reduce COreactions to reduce CO22 to a carbohydrate, to a carbohydrate, mostly to a carbohydrate called G3P (also mostly to a carbohydrate called G3P (also called PGAL)called PGAL)

G3P is then converted to all other organic G3P is then converted to all other organic molecules that the plant needsmolecules that the plant needs

Stages of the Calvin CycleStages of the Calvin Cycle

1) the enzyme “RuBP carboxylase” fixes CO2 1) the enzyme “RuBP carboxylase” fixes CO2 to RuBP, producing a 6-carbon molecule that to RuBP, producing a 6-carbon molecule that immediately breaks down to two Cimmediately breaks down to two C33 molecules molecules

Stages of the Calvin CycleStages of the Calvin Cycle

2) CO2) CO22 (incorporated into an organic (incorporated into an organic molecule) is reduced to a carbohydrate molecule) is reduced to a carbohydrate (CH(CH22O)O) This step requires NADPH and some of the ATP This step requires NADPH and some of the ATP

from the light reactionsfrom the light reactions

The Calvin CycleThe Calvin Cycle

For every three turns of the Calvin Cycle, the For every three turns of the Calvin Cycle, the net gain is one G3P moleculenet gain is one G3P molecule

The other 5 G3P molecules are used to reform The other 5 G3P molecules are used to reform three molecules of RuBPthree molecules of RuBP This step also requires ATP for energyThis step also requires ATP for energy It takes two G3P molecules to make one glucose It takes two G3P molecules to make one glucose

moleculemolecule