Fig. 10-1 Photosynthesis. Photosynthesis is the process that feeds the biosphere by converting solar...

Fig. 10-1

Photosynthesis

• Photosynthesis is the process that feeds the biosphere by converting solar energy into chemical energy

• Directly or indirectly, photosynthesis nourishes almost the entire living world

• The summary equation is the reverse of the cellular respiration equation, but the processes are not simply the opposites or reverse of each other!

• 6 CO2 + 6H2O + Light energy C6H12O6 + 6 O2

• Photosynthesis can be summarized as the following equation:

6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2O

6 CO2 + 6H2O + Light energy C6H12O6 + 6 O2

• Algae is responsible for most of the photosynthesis on the planet. Plants are responsible for a large portion of photosynthesis that occurs and some takes place in other protists. A few prokaryotes also perform photosynthesis.

• These organisms feed not only themselves but also most of the living world

• Autotrophs sustain themselves without eating anything derived from other organisms

• Autotrophs are the producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules

• Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from H2O and CO2

Fig. 10-2a

(a) Plants

Fig. 10-2b

(b) Multicellular alga

Fig. 10-2c

(c) Unicellular protist10 µm

Fig. 10-2d

40 µm(d) Cyanobacteria

Fig. 10-2e

1.5 µm(e) Purple sulfur bacteria

• Heterotrophs aka consumers obtain their organic material from other organisms and are dependent on photoautotrophs for food and O2

• Only the anaerobic chemoheterotrophs can live independent of the services of photoautotrophs!

• Chloroplasts are the functional organelle of photosynthesis likely evolved from photosynthetic bacteria. (Endosymbiotic Theory)

Fig. 10-3b

Thylakoidspace

Chloroplast

GranumIntermembranespace

Innermembrane

Outermembrane

Stroma

Thylakoid

Fig. 10-3a

Mesophyll cell

StomataCO2 O2

Chloroplast

Mesophyll

Leaf cross section

Fig. 10-5-1

Chloroplast

LightReactions

Fig. 10-5-2

Chloroplast

LightReactions

Fig. 10-5-3

Chloroplast

LightReactions

CalvinCycle

Fig. 10-5-4

Chloroplast

LightReactions

CalvinCycle

[CH2O]

(sugar)

Mitochondrion

Substrate-levelphosphorylation

Cytosol

Glucose Pyruvate

Glycolysis

Electronscarried

via NADH

Substrate-levelphosphorylation

Electrons carriedvia NADH and

Oxidativephosphorylation

Citricacidcycle

Oxidativephosphorylation:electron transport

andchemiosmosis

Compare Cellular Respiration to Photosynthesis!

Fig. 10-5-4

Chloroplast

LightReactions

CalvinCycle

[CH2O]

(sugar)

Reactants:

Fig. 10-4

Products:

12 H2O

6 O26 H2OC6H12O6

• Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

• Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced

Reactants:

Fig. 10-4

Products:

12 H2O

6 O26 H2OC6H12O6

Fig. 10-7

Reflectedlight

Absorbedlight

Chloroplast

Transmittedlight

Granum

Fig. 10-6

Visible light

InfraredMicro-waves

RadiowavesX-raysGamma

103 m1 m

(109 nm)106 nm103 nm1 nm10–3 nm10–5 nm

380 450 500 550 600 650 700 750 nm

Longer wavelength

Lower energyHigher energy

Shorter wavelength

• A spectrophotometer measures a pigment’s ability to absorb various wavelengths

• This machine sends light through pigments and measures the fraction of light transmitted at each wavelength

Galvanometer

Slit moves topass lightof selectedwavelength

Whitelight

Greenlight

Bluelight

The low transmittance(high absorption)reading indicates thatchlorophyll absorbsmost blue light.

The high transmittance(low absorption)reading indicates thatchlorophyll absorbsvery little green light.

Refractingprism

Photoelectrictube

Chlorophyllsolution

Spectrophotometer

• An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength

• The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis

• An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process

Wavelength of light (nm)

(b) Action spectrum

(a) Absorption spectra

(c) Engelmann’s experiment

Aerobic bacteria

Filamentof alga

Chloro- phyll a Chlorophyll b

Carotenoids

500400 600 700

700600500400

Absorption Spectrum

• Chlorophyll a is the main photosynthetic pigment

• Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis

• Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll

• Chlorophyll a bright green

• Chlorophyll b dull (khaki) green

• Carotene orange

• Xanthophyll yellow

• Phycoerythrin red

Fig. 10-10

Porphyrin ring:light-absorbing“head” of molecule;note magnesiumatom at center

in chlorophyll aCH3

Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts; H atoms notshown

CHO in chlorophyll b

Fig. 10-11

(a) Excitation of isolated chlorophyll molecule

Excitedstate

(b) Fluorescence

Photon Groundstate

Photon(fluorescence)

Chlorophyllmolecule

• Photosynthesis occurs in the thylakoid membrane and in the stroma.

• The terms light reactions and dark reactions were once used.

• The ‘light dependent’ reactions occur in the thylakoid membrane and the ‘light independent’ reactions occur in the stroma.

• Now there is evidence that the ‘light independent’ reactions don’t occur in the dark!?

Cytochromecomplex

ATPsynthase

ToCalvinCycle

STROMA(low H+ concentration)

Thylakoidmembrane

THYLAKOID SPACE(high H+ concentration)

Photosystem II Photosystem I

LightNADP+

reductase

NADP+ + H+

The ‘Light Reactions’

Fig. 10-12

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

STROMA

Pigmentmolecules

Photon

Transferof energy

Special pair ofchlorophyll amolecules

Photosystem

Primaryelectronacceptor

Reaction-centercomplex

Light-harvestingcomplexes

ChlorophyllIn the

ThylakoidMembrane

of achloroplast

Photosystem II(PS II)

Primaryacceptor

A photosystem (previous slide)consists of a reaction-center complex (protein) surrounded by light-harvesting complexes.

The light-harvesting complexes (pigment molecules bound to proteins as seen on the previous slide) funnel the energy of photons to the reaction center complex

Reaction Center Complex

• A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a

• Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions

Pigmentmolecules

Primaryacceptor

Fig. 10-13-2

There are two types of photosystems in the thylakoid membrane

Photosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm

The reaction-center chlorophyll a of PS II is called P680

• P680+ (P680 that is missing an electron) is a very strong oxidizing agent

• H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680

• O2 is released as a by-product of this reaction

Pigmentmolecules

Primaryacceptor

Cytochromecomplex

Electron transport chain

Fig. 10-13-3

Pigmentmolecules

Primaryacceptor

Cytochromecomplex

Photosystem I(PS I)

Primaryacceptor

Fig. 10-13-4

• Photosystem I (PS I) is best at absorbing a wavelength of 700 nm

• The reaction-center chlorophyll a of PS I is called P700

• After leaving (PS 1) electrons travel down the second electron transport chain.

Pigmentmolecules

Primaryacceptor

Cytochromecomplex

Photosystem I(PS I)

Primaryacceptor

reductase

Fig. 10-13-5

Cytochromecomplex

ATPsynthase

ToCalvinCycle

Thylakoidmembrane

LightNADP+

reductase

NADP+ + H+

The ‘Light Reactions’ of Photosynthesis

Protein complexof electroncarriers

FADH2 FAD

NAD+NADH

(carrying electronsfrom food)

2 H+ + 1/2O2H2O

ADP + P i

Chemiosmosis

Oxidative phosphorylation

ATP synthase

Compare the ETC of cellular respiration to the two ETC’s of photosynthesis!

Cytochromecomplex

ATPsynthase

ToCalvinCycle

Thylakoidmembrane

LightNADP+

reductase

NADP+ + H+

Fig. 10-16

Mitochondrion Chloroplast

CHLOROPLASTSTRUCTURE

MITOCHONDRIONSTRUCTURE

Intermembranespace

Innermembrane

Electrontransport

H+ Diffusion

Matrix

Higher [H+]Lower [H+]

Stroma

ATPsynthase

ADP + P i

Thylakoidspace

Thylakoidmembrane

Both Chloroplasts and Mitochondria perform chemiosmosis!

• Mitochondria:

transfer chemical energy from food to ATP

protons are pumped to the intermembrane space

diffuse back into the matrix

• Chloroplasts:

– transform light energy into the chemical energy of ATP

– protons are pumped into the thylakoid space

– diffuse back into the stroma

Cytochromecomplex

ATPsynthase

ToCalvinCycle

Thylakoidmembrane

LightNADP+

reductase

NADP+ + H+

• Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I

• Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane

• Diffusion of H+ (protons) across the membrane drives ATP synthesis

• In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor

• P700+ (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain

• Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)

• The electrons are then transferred to NADP+ and reduce it to NADPH

• The electrons of NADPH are available for the reactions of the Calvin cycle

Cytochromecomplex

ATPsynthase

ToCalvinCycle

Thylakoidmembrane

LightNADP+

reductase

NADP+ + H+

• Linear electron flow, as previously described is the primary pathway for electrons, it involves both photosystems and produces ATP and NADPH using light energy

• However there is a second possible route for electron flow. The cyclic route.

Cyclic Electron Flow

• Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH

• Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle

• Some organisms such as purple sulfur bacteria have PS I but not PS II

• Cyclic electron flow is thought to have evolved before linear electron flow

• Cyclic electron flow may protect cells from light-induced damage

ATPPhotosystem II

Photosystem I

Primary acceptor

Cytochromecomplex

Primaryacceptor

reductaseNADPH

The Cyclic Route

• Regardless of the route… the light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH

• ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place

• The Calvin Cycle occurs in the stroma

after the ‘light reactions’

Calvin Cycle of Photosynthesis!

Ribulose bisphosphate(RuBP)

3-Phosphoglycerate

Short-livedintermediate

Phase 1: Carbon fixation

(Entering oneat a time)

Rubisco

3-Phosphoglycerate

Rubisco

1,3-Bisphosphoglycerate

66 NADP+

Phase 2:Reduction

Glyceraldehyde-3-phosphate(G3P)

1 POutput G3P

(a sugar)

Glucose andother organiccompounds

CalvinCycle

3-Phosphoglycerate

Rubisco

66 NADP+

Phase 2:Reduction

1 POutput G3P

(a sugar)

CalvinCycle

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)

Acetyl CoA

CoA—SH

Citrate

IsocitrateNAD+

-Keto-glutarate

CoA—SH

CO2NAD+

+ H+SuccinylCoA

CoA—SH

GTP GDP

Succinate

Fumarate

CitricacidcycleH2O

Malate

Oxaloacetate

Comparethe Krebs Cycle of Cellular Respiration to the Calvin Cycle of Photosynthesis

3-Phosphoglycerate

Rubisco

66 NADP+

Phase 2:Reduction

1 POutput G3P

(a sugar)

CalvinCycle

In plants… chloroplasts are concentrated in the leaves but are found in all green portions of the plant including stems.

•Their green color is from chlorophyll, the green pigment within chloroplasts

•Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast

•CO2 enters and O2 exits the leaf through microscopic pores called stomata

• Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf

• A typical mesophyll cell has 30–40 chloroplasts

• The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana

• Chloroplasts also contain stroma, a dense fluid

• Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)

• The light reactions (in the thylakoids):

– Split H2O

– Release O2

– Reduce NADP+ to NADPH

– Generate ATP from ADP by photophosphorylation

• The Calvin cycle (in the stroma)

– forms sugar from CO2, using ATP and NADPH

– begins with carbon fixation, incorporating CO2 into organic molecules

Photosynthesis occurs in different forms! All forms of photosynthesis require a hydrogen source but that source varies.

• Familiar: 6CO2 + 6H2O + Light Energy C6H12O6 + 6O2

• Generally: CO2 + 2H2X [CH2O] + H20 + 2X

• Plants: CO2 + 2H2O [CH2O] + H20 + O2

• Sulfur Bacteria: CO2 + 2H2S [CH2O] + H20 + 2S

• The plants that perform the form of photosynthesis we are most familiar with are called C3 plants.

• An alternate form of photosynthesis which is quite common is called Photorespiration.

• Plants that perform photorespiration are referred to as C4 plants and others as CAM plants!

• Photorespiration may be an evolutionary relic that evolved at a time when the atmosphere had far less O2 and more CO2

• Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle

• In most plants (C3 plants), initial fixation of CO2, catalyzed by rubisco, forms a three-carbon compound

• In photorespiration (C4 and CAM plants), rubisco adds O2 instead of CO2 into the Calvin cycle

• Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar

• The first product of C4 plants in the Calvin Cycle is a 4 carbon compound thus the name C4.

• Includes sugarcane, corn and others.

• CAM plants (Crassulacean acid metabolism) are adapted to arid conditions. Crassulaceae is a family of succulent plants in which the process was first discovered. Includes pineapples, cactus and others.

• Since CO2 is not available during the day because stomata are closed to conserve water…CO2 uptake occurs at night and is stored as organic acids. The next morning when the light reactions are available to supply ATP and NADPH, the organic acids are converted back to CO2 and used in the Calvin Cycle to make carbohydrates.

C4 Plants

C4 leaf anatomy

Mesophyll cellPhotosyntheticcells of C4

plant leafBundle-sheathcell

Vein(vascular tissue)

The C4 pathway

Mesophyllcell CO2PEP carboxylase

Oxaloacetate (4C)

Malate (4C)

PEP (3C)ADP

Pyruvate (3C)

Bundle-sheathcell

CalvinCycle

Vasculartissue

C4 Plants

C4 leaf anatomy

Photosyntheticcells of C4

plant leaf

Vein(vascular tissue)

Bundle-sheathcell

Mesophyll cell

C4 Plants

Bundle-sheathcell

Oxaloacetate (4C) PEP (3C)

PEP carboxylase

Malate (4C)

Mesophyllcell

CalvinCycle

Pyruvate (3C)

Vasculartissue

The C4 pathway.

Fig. 10-20

Sugarcane

Mesophyllcell

Bundle-sheathcell

Organic acidsrelease CO2 to Calvin cycle

CO2 incorporatedinto four-carbonorganic acids(carbon fixation)

Pineapple

SugarSugar

CalvinCycle

Organic acid Organic acid

(a) Spatial separation of steps (b) Temporal separation of steps

CO2 CO2

The Big Idea

Review

Fig. 10-21

LightReactions:

Photosystem II Electron transport chain

Photosystem I Electron transport chain

RuBP 3-Phosphoglycerate

CalvinCycle

G3PATP

NADPHStarch(storage)

Sucrose (export)

Chloroplast

• Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits

Fig. 10-UN1

reductase

Photosystem II

Cytochromecomplex

Primaryacceptor

Photosystem I

Electron transport

H2O Pq

Fig. 10-UN2

Regeneration ofCO2 acceptor

1 G3P (3C)

Reduction

Carbon fixation

CalvinCycle

3-Phosphoglycerate

Rubisco

66 NADP+

Phase 2:Reduction

1 POutput G3P

(a sugar)

CalvinCycle

• Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P)

• For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2

• The Calvin cycle has three phases:

– Carbon fixation (catalyzed by rubisco)

– Reduction

– Regeneration of the CO2 acceptor (RuBP)

Fig. 10-1 Photosynthesis. Photosynthesis is the process that feeds the biosphere by converting solar...

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