electron transport /photosynthesis

chapter 14

electron transport• Reduced coenzymes

• NADH and FADH2

• used to produce ATP-

• oxidative phosphorylation• energy from redox

reactions used to produce ATP

• chemiosmosis

electron transport• electron tranport chain

• multiprotein complexes• accept and donate

electrons• each time, electrons

lose free energy• H+ pumped out

• chemiosmosis• potential energy

stored in H+ gradient used to synthesize ATP

electron transport• types of electron carriers

• flavoproteins• cytochromes• three copper atoms• ubiquinone• iron sulfur proteins

electron transport• complex I

• NADH dehydrogenase• flavin

mononucleotide• iron-sulfur protein

• complex II (TCA cycle)• succinate

dehydrogenase• FADH2 - lower energy

• complex III• cytochrome bc1

• complex IV• cytocrome c oxidase

2 H+2 H+

-2

4 4 2

proton differential = 12 ( per pair of e- )

proton motive force• ATP-synthase

• F0

• 10-14 subunits• H+ flows into half channel• binds with rotor subunits• rotor turns

• F1 • internal rod spins• catalytic portion fixed

F1

F0

INTERMEMBRANE SPACE

Rotor

StatorH+

Internal rod

Catalytic knob

ADP

+P

iATP

MITOCHONDRIAL MATRIX

F1

F0

H+

binding change mech.• catalysis of ATP

• three binding sites• three conformations (active site)

• loose• binds with ADP and P

• tight• ATP formed

• open• ATP released

energy accounting• about 32 ATP molecules produced from glucose

• uncoupling proteins• brown fat

2 2 26

2 2 26 or 28TOTAL: ~30-32 max

• what if there is no O2?• fermentation

• glycolysis -> pyruvate / NADH• pyruvate ->

• reduced to lactate• no release of CO2

• OR• pyruvate ->

• reduced to EtOH• release of CO2

anaerobic anaerobic• eukaryotes

• if O2 not available - • pyruvate undergoes fermentation

photoautotrophs• photosynthesis

• low energy electrons converted to high energy electrons• used to chain carbon

atoms together• electron donors

• H2S• H2O

• cyanobacteria

plant structure• leaves

• eudicots• two layers of

mesophyll• palisade• spongy

• stomata• monocots

• single layer of mesophyll

eudicot

monocot

palisade mesophyll

spongy mesophyll

chloroplasts• plastids

• special organelles found in plants

• involved in various metabolic processes and storage

• develop from proplastids in meristem

• chloroplasts• specialized plastids involved

in the process of photosynthesis

Mesophyll

Leaf cross sectionChloroplasts Vein

Stomata

Chloroplast Mesophyll cell

CO2

O2

20 μm

Outer membrane

Intermembrane space

Inner membrane

1 μm

Thylakoid space

ThylakoidGranumStroma

chloroplasts• outer membrane

• intermembrane space• inner membrane

• stroma• thylakoid

• grana• thylakoid space

Outer membrane

Intermembrane space

Inner membrane

1 μm

Thylakoid space

ThylakoidGranum

Stroma

Chloroplast

photosynthesis• light-dependent

• energy from sunlight absorbed• stored as ATP and NADPH

• light-independent• CO2 converted to carbs• uses energy from ATP and NADPH

Light

Light Reactions

Calvin Cycle

[CH2O]

(sugar)

CO2

Chloroplast

ATP

NADPH

NADP+

ADP+ P

i

H2O

O2

Figure 10.17

Mitochondrion Chloroplast

MITOCHONDRION STRUCTURE

CHLOROPLAST STRUCTURE

Intermembrane space

Inner membrane

Matrix

Thylakoid space

Thylakoid membrane

Stroma

Electron transport

chain

H+ Diffusion

ATP synthase

H+

ADP + P i

Key Higher [H+ ]

Lower [H+ ]

ATP

harvesting light• chlorophyll structure

• porphyrin ring• hydrocarbon tail

• hydrophobic• carotenoids

• accessory pigments• absorb blue & green light

Phytol tail

Porphyrin ring

CH3

CH3 in chlorophyll a

CHO in chlorophyll b

harvesting light• photosystems

• photosynthetic unit• antenna

• reaction-center

Thy

lako

id m

embr

ane

Photon

Photosystem

STROMA

Light- harvesting complexes

Reaction- center

complex

Primary electron acceptor

Transfer of energy

Special pair of chlorophyll a

molecules

Pigment molecules

THYLAKOID SPACE (INTERIOR OF THYLAKOID)

e−

photosystems• photosystem II

• boosts electrons energy level to midpoint• reaction center (P680)

• photosystem I• boosts electrons energy level to above

NADP+

• reaction center (P700)• Z scheme -

Primary acceptor

P680

Light

Pigment molecules

Photosystem II (PS II)

1

2

e−

splitting water• photosystem II

• light-harvesting complex II• contains most of the antenna pigments• light energy transmitted to core of PS2

splitting water• photosystem II

• transfers electron to pheophytin1. primary electron acceptor

• pheo + P680+ (powerful oxidizing agent)2. pheo- donates electron to PQA —> PQB

3. with next photon, plastoquinone reduced to plastoquinol (PQH2)

• photolysis • requires 4 electrons (2

for each O atom)

production of NADPH• bridging the gap

• PQH2 -> cytochrome b6f (multiprotein complex)• electrons passed to plastocyanin• plastocyanin passes electrons to P700+

1 μm

Thylakoid space

ThylakoidGranum

production of NADPH• photosystem I

• electron passed -> lumen side to stroma side1. P700 transfers electron to

A0

2. A0- passes to A1

3. A1- passes electron through 3 Fe-S clusters

4. passed to ferredoxin5. 2 ferredoxins interact with

ferredoxin NADP+ reductase

• FAD group - accepts 2 e-

• NADPH

photophosphorylation• production of ATP

• pH gradient established• ATP synthase pumps H+ out of lumen• noncyclic phosphoryation

• cyclic photophosphorylation• sometimes electrons from ferredoxin passed back

plastoquinone• then to cyt b6f

• pump H+ into lumen• mainly in lamellae• cuts PSII out of loop

actual locations• PSI and PS II are not located in the same areas

• PSII is located in the stacked grana• PSI is located outside stacked grana

GRANA

3 Co2 (one at a time) 1

Carbon fixationP P

short-lived intermediate

x3

P

3-Phosphoglycerate

x6

3Regeneration of

CO2 acceptor

3 ADP

3 ATP

2

Reduction

P

Glyceraldehyde 3-phosphate (G3P)

x66 NADPH

6 NADP+

6 P

P

Glyceraldehyde 3-phosphate (G3P)

CALVIN CYCLE

Ribulose bisphosphate

(RuBP)

P Px3

Rubisco

P

Glyceraldehyde 3-phosphate (G3P)

x5

PP1,3-bisphosphoglyceratex6

6 ADP

6 ATP

carbohydrate synthesis• GAP molecules

• exported into cytoplasm• exchanged for

phosphate • used to synthesize

sucrose• can remain in

chloroplast

Light

Light Reactions:

Photosystem II Electron transport chain

Photosystem I Electron transport chain

NADP+

ADP+ P

i

RuBP

ATP

NADPH

3-Phosphoglycerate

Calvin Cycle

G3P

Starch (storage)

Sucrose (export)

Chloroplast

H2O CO

2

O2

photorespiration• when stomata are closed

• rubisco --> O2 --> 2-carbon compound • 2-phosphoglycolate

• consumes O2, releases CO2• glycolate -> glycoxylate ->

glycine (CO2 released)• perhaps evolutionary relic

• limits buildup of damaging light reaction products

• problem for plants when hot and dry

C4 plants• adaptation to hot climate

• C4 pathway works well with low CO2

• CO2 combined with PEP • produce 4-carbon compounds

• passed to bundle sheath cell and calvin cycle• mainly found in monocots

• mainly grasses• corn, sorghum, sugar cane

bundle sheath cells

mesophyll

The C4 pathwayMesophyll

cellPEP carboxylaseCO

2

Oxaloacetate (4C) PEP (3C)

Malate (4C)

Pyruvate (3C)

CO2

Bundle- sheath

cell

Calvin Cycle

Sugar

Vascular tissue

ADP

ATP

CAM plants• crassulacean acid metabolism

• open stomata at night• fix CO2

• close stomata during day• use CO2 from CAM pathway

• examples• orchids, bromeliads, pineapple, ferns, cycads, some dicots

Temporal separation of steps

CO2

Organic acid

CO2

Calvin Cycle

Sugar

Day

Night

CAMCO

2 incorporated

(carbon fixation)

CO2 released

to the Calvin cycle

2

1