LE 10-3

LE 10-3Leaf cross section

Vein

Mesophyll

Stomata CO2 O2

Mesophyll cellChloroplast

5 µm

Outermembrane

Intermembranespace

Innermembrane

Thylakoidspace

ThylakoidGranumStroma

1 µm

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Chloroplasts are organelles that are the site of photosynthesis

• Leaves are the major locations of photosynthesis

• 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

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


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

• 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 can be summarized as the following equation:

6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O

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

• Photosynthesis is a redox process in which water is oxidized and carbon dioxide is reduced

LE 10-5_3

H2O

LIGHTREACTIONS

Chloroplast

Light

ATP

NADPH

O2

NADP+

CO2

ADPP+ i CALVIN

CYCLE

[CH2O](sugar)


The Two Stages of Photosynthesis: A Preview

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

• The light reactions (in the thylakoids) split water, release O2, produce ATP, and form NADPH

• The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH

• The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules

LE 10-7

Chloroplast

LightReflected light

Absorbed light

Transmitted light

Granum


Photosynthetic Pigments: The Light Receptors

• Pigments are substances that absorb visible light

• Different pigments absorb different wavelengths

• Wavelengths that are not absorbed are reflected or transmitted

• Leaves appear green because chlorophyll reflects and transmits green light


• 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

LE 10-12

Thylakoid

Photon

Light-harvestingcomplexes

Photosystem

Reactioncenter

STROMA

Primary electronacceptor

e–

Transferof energy

Specialchlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

Thyl

akoi

d m

embr

ane


A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes

• A photosystem consists of a reaction center surrounded by light-harvesting complexes

• The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center


• 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


• There are two types of photosystems in the thylakoid membrane

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

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

• The two photosystems work together to use light energy to generate ATP and NADPH

LE 10-13_5

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADPCALVINCYCLELIGHT

REACTIONS

NADP+

Light

H2O CO2En

ergy

of e

lect

rons

O2

e–

e–

+2 H+

H2O

O21/2

Pq

Cytochromecomplex

Electron transport chain

Pc

ATP

P700

e–

Primaryacceptor

Photosystem I(PS I)

e–e–

ElectronTransportchain

NADP+

reductase

Fd

NADP+

NADPH+ H+

+ 2 H+

Light

Non Cyclic

LE 10-14

ATP

Photosystem II

e–

e–

e–e–

Millmakes

ATP

e–

e–

e–

Phot

on

Photosystem I

Phot

on

NADPH


Noncyclic Electron Flow

• During the light reactions, there are two possible routes for electron flow: cyclic and noncyclic

• Noncyclic electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH

LE 10-15

Photosystem IPhotosystem II ATP

Pc

Fd

Cytochromecomplex

Pq

Primaryacceptor

Fd

NADP+

reductase

NADP+

NADPH

Primaryacceptor


Cyclic Electron Flow

• Cyclic electron flow uses only photosystem I and produces only ATP

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

LE 10-16

MITOCHONDRIONSTRUCTURE

Intermembranespace

MembraneElectrontransport

chain

Mitochondrion Chloroplast

CHLOROPLASTSTRUCTURE

Thylakoidspace

Stroma

ATP

Matrix

ATPsynthase

Key

H+ Diffusion

ADP + PH+

i

Higher [H+]Lower [H+]


A Comparison of Chemiosmosis in Chloroplasts and Mitochondria

• Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy

• Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP

• The spatial organization of chemiosmosis differs in chloroplasts and mitochondria

LE 10-17

STROMA(Low H+ concentration)

Light

Photosystem II Cytochromecomplex

2 H+

LightPhotosystem I

NADP+

reductaseFd

PcPq

H2O O2

+2 H+

1/22 H+

NADP+ + 2H+

+ H+NADPH

ToCalvincycle

THYLAKOID SPACE(High H+ concentration)

STROMA(Low H+ concentration)

Thylakoidmembrane ATP

synthase

ATPADP

+P

H+i

[CH2O] (sugar)O2

NADPH

ATP

ADPNADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

Light


• Water is split by photosystem II on the side of the membrane facing the thylakoid space

• The diffusion of H+ from the thylakoid space back to the stroma powers ATP synthase

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


• The Calvin cycle has three phases:

– Carbon fixation (catalyzed by rubisco)

– Reduction

– Regeneration of the CO2 acceptor (RuBP)

LE 10-18_3

[CH2O] (sugar)O2

NADPH

ATP

ADPNADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

Light Input

CO2

(Entering oneat a time)

Rubisco

3 P PShort-lived

intermediate

Phase 1: Carbon fixation

6 P3-Phosphoglycerate

6 ATP

6 ADP

CALVINCYCLE

3

P PRibulose bisphosphate

(RuBP)

3

6 NADP+

6

6 NADPH

P i

6 P1,3-Bisphosphoglycerate

P

6 PGlyceraldehyde-3-phosphate

(G3P)

P1G3P

(a sugar)Output

Phase 2:Reduction

Glucose andother organiccompounds

3

3 ADP

ATP

Phase 3:Regeneration ofthe CO2 acceptor(RuBP) P5

G3P


The Calvin cycle uses ATP and NADPH to convert CO2 to sugar• The Calvin cycle, like the citric acid cycle,

regenerates its starting material after molecules enter and leave the cycle

• The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH

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

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


Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates

• Dehydration is a problem for plants, sometimes requiring tradeoffs with other metabolic processes, especially photosynthesis

• On hot, dry days, plants close stomata, which conserves water but also limits photosynthesis

• The closing of stomata reduces access to CO2 and causes O2 to build up

• These conditions favor a seemingly wasteful process called photorespiration


Photorespiration: An Evolutionary Relic?

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

• In photorespiration, rubisco adds O2 to the Calvin cycle instead of CO2

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

LE 10-19

Photosyntheticcells of C4 plantleaf

Mesophyll cell

Bundle-sheathcell

Vein(vascular tissue)

C4 leaf anatomy

StomaBundle-sheathcell

Pyruvate (3 C)

CO2

Sugar

Vasculartissue

CALVINCYCLE

PEP (3 C)

ATP

ADP

Malate (4 C)

Oxaloacetate (4 C)

The C4 pathway

CO2PEP carboxylaseMesophyllcell

C4 leaf anatomy and pathway


C4 Plants

• C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells

• These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle


CAM Plants

• CAM plants open their stomata at night, incorporating CO2 into organic acids

• Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle

LE 10-20

Bundle-sheathcell

Mesophyllcell Organic acid

C4

CO2

CO2

CALVINCYCLE

Sugarcane Pineapple

Organic acidsrelease CO2 toCalvin cycle

CO2 incorporatedinto four-carbonorganic acids(carbon fixation)

Organic acid

CAMCO2

CO2

CALVINCYCLE

Sugar

Spatial separation of steps Temporal separation of steps

Sugar

Day

Night


The Importance of Photosynthesis: A Review

• The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds

• Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells

• In addition to food production, photosynthesis produces the oxygen in our atmosphere

LE 10-3

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