• Chapter 7~Photosynthesis

Photosynthesis in nature

• Autotrophs:

• biotic producers; photoautotrophs; chemoautotrophs; obtains organic food without eating other organisms

• Heterotrophs: biotic consumers; obtains organic food by eating other organisms or their by-products (includes decomposers)

The chloroplast

• Sites of photosynthesis• Pigment: chlorophyll• Plant cell: mesophyll• Gas exchange: stomata• Double membrane• Thylakoids, grana,

stroma

Leaves and Photosynthesis

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Grana

Chloroplast

Leaf cross section

granum

independent thylakoidin a granum

mesophyll

lowerepidermis

upperepidermis

cuticle

leaf veinouter membrane

inner membrane

thylakoid space

thylakoid membrane

overlapping thylakoidin a granum

CO2

O2

stoma

stromastroma

37,000

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7.2 The Process of Photosynthesis

• Light Reactions – take place only in the presence of light– Energy‑capturing reactions – Chlorophyll absorbs solar energy– This energizes electrons– Electrons move down an electron transport chain

• Pumps H+ into thylakoids• Used to make ATP out of ADP and NADPH out of NADP

• Calvin Cycle Reactions – take place in the stroma– CO2 is reduced to a carbohydrate – Use ATP and NADPH to produce carbohydrate

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Photosynthetic Pigments and Photosynthesis

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Wavelengths (nm)

Increasing wavelength

a. The electromagnetic spectrum includes visible light. b. Absorption spectrum of photosynthetic pigments.

Increasing energy

Gammarays X rays UV Infrared

Micro-waves

Radiowaves

visible light

500 600 750

Wavelengths (nm)

380 500 600 750

chlorophyll a

chlorophyll b

carotenoids

Rel

ativ

e A

bso

rpti

on

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Electromagnetic Spectrum

• Visible Light – only a small portion of the spectrum, but it is the radiation that drives photosynthesis.

• Photons – fixed amounts of energy, non-tangible objects. Amount of energy is inversely related to wavelength

Photons and Energy

• When pigments (ex. Chlorophyll) absorb photons, the energy has to go somewhere. The electrons of a pigment jump to an excited state and absorb the photon. The electrons will then have potential energy, but will be very unstable so will fall back to the ground state and when they fall they release energy as heat.

Plants as Solar Energy Converters

• The light reactions consist of two alternate electron pathways:– Noncyclic pathway

– Cyclic pathway

• Capture light energy with photosystems– Pigment complex helps collect solar energy like an antenna

– Occur in the thylakoid membranes

• Both pathways produce ATP

• The noncyclic pathway also produces NADPH

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Photosystems

• Light harvesting units of the thylakoid membrane

• Composed mainly of protein and pigment antenna complexes

• Antenna pigment molecules are struck by photons

• Energy is passed to reaction centers (redox location)

• Excited e- from chlorophyll is trapped by a primary e- acceptor

Figure 10.6 Why leaves are green: interaction of light with chloroplasts

Photosystem I & II

• Photosystem I – the reaction center is known as P700 because it best absorbs light at 700nm

• Photosystem II – the reaction center is known as P680 because it absorbs light best at 680nm

• The two pigments are actually the same, but are associated with different proteins so work differently

Noncyclic electron flow• Photosystem II (P680):

– photons excite chlorophyll e- to an acceptor

– e- are replaced by splitting of H2O (release of O2)

– e-’s travel to Photosystem I down an electron transport chain

– as e- fall, ADP ATP (noncyclic photophosphorylation)

• Photosystem I (P700):

– ‘fallen’ e- replace excited e- to primary e- acceptor

– 2nd ETC ( Fd~NADP+ reductase) transfers e- to NADP+ NADPH (...to Calvin cycle…)

• These photosystems produce equal amounts of ATP and NADPH

Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 1)

Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 2)

Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 3)

Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 4)

Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 5)

Chemiosmosis• Chloroplasts and Mitochondria generate ATP through

chemiosmosis. Just like we learned in Chapter 9 in the electron transport chain and oxidative phosphorylation, the chloroplasts do this much the same way except for:

• In cellular respiration the e- dropped down the etc came from food molecules. In chloroplasts the e- came from captured light. Meaning the mitochondria transforms chemical energy from food molecules to ATP, and chloroplasts transform light energy into chemical energy.

• In mitochondria the protons were pumped into the intercellular matrix then back into the mitochondrial matrix. In chloroplasts the protons are pumped into the thylakoid space and then back into the stroma, so ATP is made in the stroma where it is used to drive sugar synthesis during the Calvin cycle.

D:\ImageLibrary1-17\10-Photosynthesis\10-17-LightReactions.mov

Figure 10.15 Comparison of chemiosmosis in mitochondria and chloroplasts

Figure 10.16 The light reactions and chemiosmosis: the organization of the thylakoid membrane

Organization of a Thylakoid

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H+

H+

P

e-

O22+2

1

P

H2O CO2

solarenergy

thylakoidthylakoid membrane

thylakoid space

NADPH

ATP

Stroma

chemiosmosis

ATP synthase

e- NADP+

granum

photosystem II

stroma

+ ADP

H+

H+H+H+

H+

H+H+

H+

H+

H+

H2O

electron transportchain

e-

thylakoidmembrane

Calvincycle

reactions

ADP +

NADP +

Lightreactions NADPH

ATP

CH2OO2

photosystem INADP

reductaseH+

Thylakoidspace

H+

H+

H+

H+

H+H+

Pq

e-

e-

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Animation

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The Calvin cycle

• 3 molecules of CO2 are ‘fixed’ into glyceraldehyde 3-phosphate (G3P)

• Phases:

– 1- Carbon fixation~ each CO2 is attached to RuBP (rubisco enzyme)

– 2- Reduction~ electrons from NADPH reduces to G3P; ATP used up

– 3- Regeneration~ G3P rearranged to RuBP; ATP used; cycle continues

D:\ImageLibrary1-17\10-Photosynthesis\10-16-CalvinCycle.mov

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Animation

Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.

Calvin cycle

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are needed to see this picture.

Calvin Cycle, net synthesis

• For each G3P (and for 3 CO2)…….Consumption of 9 ATP’s & 6

NADPH (light reactions regenerate these molecules)

• G3P can then be used by the plant to make glucose and other organic compounds

Cyclic electron flow

• Alternative cycle when ATP is deficient

• Photosystem I used but not II; produces ATP but no NADPH

• Why? The Calvin cycle consumes more ATP than NADPH…….

• Cyclic photophosphorylation

Fate of G3P

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Sucrose (in leaves,fruits, and seeds)

G3P

amino acidsynthesis

glucosephosphate

fatty acidsynthesis

+fructose

phosphate

Cellulose (in trunks,roots, and branches)

Starch (in rootsand seeds)

© Herman Eisenbeiss/Photo Researchers, Inc.

7.5 Other Types of Photosynthesis

• In hot, dry climates– Stomata must close to avoid wilting

– CO2 decreases and O2 increases

– O2 starts combining with RuBP, leading to the production of CO2

– This is called photorespiration

• C4 plants solve the problem of photorespiration– Fix CO2 to PEP (a C3 molecule)

– The result is oxaloacetate, a C4 molecule

– In hot & dry climates• C4 plants avoid photorespiration

• Net productivity is about 2-3 times greater than C3 plants

– In cool, moist environments, C4 plants can’t compete with C3 plants

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Chloroplast Distribution in C4 vs. C3 Plants

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C3 Plant C4 Plant

bundle sheathcell

bundle sheathcell

mesophyllcells

veinveinstomastoma

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CO2 Fixation in C3 and C4 Plants

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C4

CO2

RuBP

Calvincycle

3PG

mesophyll cell

G3P

a. CO2 fixation in a C3 plant, wildflowers

CO2

mesophyllcell

CO2

Calvincycle

bundlesheathcell

G3P

b. CO2 fixation in a C4 plant, corn, Zea maysa: © Brand X Pictures/PunchStock RF; b: Courtesy USDA/Doug Wilson, photographer

Other Types of Photosynthesis

• CAM Photosynthesis– Crassulacean-Acid Metabolism– CAM plants partition carbon fixation by time

• During the night– CAM plants fix CO2

– Form C4 molecules, which are– Stored in large vacuoles

• During daylight– NADPH and ATP are available– Stomata are closed for water conservation– C4 molecules release CO2 to Calvin cycle

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CO2 Fixation in a CAM Plant

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Calvin

cycle

CO2

CO2

C4

G3P

CO2 fixation in a CAM plant, pineapple, Ananas comosus

night

day

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© S. Alden/PhotoLink/Getty Images.

Other Types of Photosynthesis

• Each method of photosynthesis has advantages and disadvantages– Depends on the climate

• C4 plants most adapted to:– High light intensities– High temperatures– Limited rainfall

• C3 plants better adapted to– Cold (below 25°C)– High moisture

• CAM plants are better adapted to extreme aridity– CAM occurs in 23 families of flowering plants– Also found among nonflowering plants

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A review of photosynthesis