Chapter 14

Energy conversion From Foods: Mitochondria

Energy conversion & Free energy Works in hydroelectricity

Heat

Potential E

Kinetic E

Heat

Electrical E

FREE ENERGY : Energy available for this work (conversion)

Designs for ATP synthesis Chemiosmosis Chemiosmosis is the name given to the generation of ATP from a proton gradient. It occurs in all living things:

Energy

A Design for ATP synthesis:

Oxidative phosphorylation

Pyruvate  CO2 + NADH NADH + O2 ATP + H2O

Another Design for ATP synthesis:

Light-induced phosphorylation Cyclic phosphorylation PS-I only

Non-cyclic phosphorylation PS-I and II

Cyclic Photophosphorylation • Process for ATP generation associated with some Photosynthetic Bacteria • Reaction Center => 700 nm

Noncyclic Photophosphorylation

Photosystem II regains electrons by splitting water, leaving O2 gas as a by-product Primary electron acceptor Primary electron acceptor

Photons

Energy for synthesis of PHOTOSYSTEM I PHOTOSYSTEM II

by chemiosmosis

Chapter 14 Energy conversion From Light:

Chloroplast

Green plants in Ecosystem

THE SUN: MAIN SOURCE OF ENERGY FOR LIFE ON EARTH

Autotrophs (self + nutrition in Greek) : an organism that produces complex organic compounds from simple inorganic molecules using energy from light (by photoynthesis) or inorganic chemical reactions.

The proximate cause…

Why are green? green? Why plants plants are

Why plants are green? Gamma rays

X-rays

UV

Infrared & Microwaves

Visible light

Wavelength (nm)

Radio waves

THE COLOR OF LIGHT SEEN IS THE COLOR NOT ABSORBED

The thylakoid membrane of the chloroplast is impregnated with photosynthetic pigments

Light

Reflected light

Transmitted light

Chloroplast

Absorbed light

(i.e., chlorophylls, carotenoids).

• The location and structure of chloroplasts Chloroplast LEAF CROSS SECTION

MESOPHYLL CELL

LEAF Mesophyll

CHLOROPLAST

Intermembrane space Outer membrane

Granum Grana

Stroma

Inner membrane Stroma

Thylakoid

Thylakoid compartment

The location and structure of chloroplasts A chloroplast contains:

-stroma, a fluid -grana, stacks of thylakoids

The thylakoids contain chlorophyll -Chlorophyll is the green pigment that captures light for photosynthesis

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

outer membrane intermembrane space inner membrane (1+2+3: envelope) stroma (aqueous fluid) thylakoid lumen (inside of thylakoid) thylakoid membrane granum (stack of thylakoids) thylakoid (lamella) starch ribosome plastidial DNA plastoglobule (drop of lipids)

Absorbance of pigments in chloroplasts

– Chlorophyll-a – Chlorophyll-b Carotenoids

Chlorophyll a & b •Chl a has a methyl group •Chl b has a carbonyl group Porphyrin ring delocalized e-

Phytol tail

Chlorophyll-a

(type-a in green plants and algae)

Beta-carotene (Mostly in algae)

Chlorophyll-b

(type-b in green plants and algae)

Chloroplast

Dark reactions

for Photosynthesis

PHOTOSYNTHESIS Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water

Carbon dioxide

Water

Glucose PHOTOSYNTHESIS

Oxygen gas

Photosynthesis

Light and Dark reactions Light reactions: 12H2O + 12NADP + 18ADP → 6O2 + 12NADPH + 18ATP

Dark reactions: 6CO2 + 12NADPH + 18ATP → C6H12O6 + 12NADP + 18ADP + 6H2O Two distinct stages, the light reactions, which convert light energy to ATP and NADPH; and the dark reactions, which convert CO2 to carbohydrate using ATP and NADPH. Both occur in the chloroplasts.

Photosynthesis

Light and Dark reactions Light

NADP

Light reactions

Calvin cycle

Photosynthesis

Two types of photosystems in the light reactions By theory, it appears to takes just four electrons (and four protons) to reduce CO2 to carbohydrate. However, we find we need eight photons per CO2, implying that two photochemical reactions are needed per electron, and that there are two kinds of photosystem operating in series, each physically separate in its own kind of particle. ATP mill

Water-splitting Photosystem II

NADPH-producing Photosystem I

PSII: Plants produce O2 gas by splitting H2O

The O2 liberated by photosynthesis is made from the oxygen in water (H+ and e-)

Photosystem II

PSII Chemiosmosis for ATP production Thylakoid compartment (high H+)

Light

Light

Thylakoid membrane

Antenna molecules Stroma (low H+)

Reaction center

ELECTRON TRANSPORT CHAIN

PHOTOSYSTEM II

PHOTOSYSTEM I

ATP SYNTHASE

PSII

(PSII+ LHC)

Resonance transfer of electrons to PSII

PSII

(water oxidase system)

Antenna a complex of

the pigment molecules are arranged in

blocks of about 50

The reaction centre, where the photochemical reaction occurs. The excited chlorophyll-a ejects an electron, becoming an extremely strong oxidising agent, capable of pulling electrons out of water. The antenna plus the reaction centre taken together are termed a photosystem.

P680, Chlorophyll-b mainly absorb 60 nm light

LHC-II

(Light-harvesting complexes) 1) Resonance transfer of photons

From LHC to PSII 2) Preventing ‘Back-up’ of electrons This is important because if photosystem I receives too little energy compared to PSII, electrons will 'back up' the transport chain, and prevent excited electrons from escaping photosystem II.

2) Preventing ‘Photoinhibition’ Excited chlorophyll molecules in photosystem II will not be quenched by electrons from water, and will cause oxidative damage to the reaction centre. This causes the destruction of photosystem II,

LHC-II phosphorylated  Association with PSII

Green, Chlorophyll-a/b; Yellow, Cartenoids

Too much light  LHC-II de-phosphorylated  Dissociation with PSII

An Energy spacer?

Light pumping to P680 When a chlorophyll molecule within the LHC contacts a photon of light, resonance energy is produced. This resonance energy is transferred through several more chlorophyll molecules until it reaches the P680 chlorophyll molecules at the heart of the Photosystem II reaction center. The resonance energy causes the loss of an electron from the P680 molecules. This electron is then transferred to a pheophytin molecule, then to Qa and finally to Qb. P680 is then reduced by the splitting of a water molecule which replaces the electron lost in this process.

Why photons in LHC are channeled to PSII?

An Energy spacer?

Resonance transfer of photons LHC

P680 or P700 LHC P680

E=hc⁄λ h, Planck's constant, 6.6 × 10−34 J s. c, speed of light, 3 × 108 m s−1.

Blue light is more energetic

The ultimate cause…

Why plants green? Plants must are be green? LHC

P680, 700

Transfer of electrons Primary electron acceptor Primary electron acceptor

Energy to make

NADP

3

2 Light

Light Primary electron acceptor

1

Reactioncenter chlorophyll

Water-splitting photosystem 2 H + 1/2

P680 become hungry for e- and take it from water

NADPH-producing photosystem

PSI

(ferredoxin reductase system)

Chemiosmosis for ATP production Thylakoid compartment (high H+)

Light

Light

Thylakoid membrane

Antenna molecules Stroma (low H+)

Reaction center

ELECTRON TRANSPORT CHAIN

PHOTOSYSTEM II

PHOTOSYSTEM I

ATP SYNTHASE

Chemiosmosis

in PSII + electron transport chains

• A Photosynthesis Road Map Chloroplast Light Stroma NADP

Stack of thylakoids

ADP +P Light reactions

Calvin cycle

Sugar used for  Cellular respiration  Cellulose  Starch  Other organic compounds

The Calvin Cycle 3 CO2 + 6 NADPH + 5 H2O + 9 ATP → C3H5O3-PO32- + 2 H+ + 6 NADP+ + 9 ADP + 8 Pi

RUBISCO

From Photosynthesis

Photosynthesis uses light energy to make food molecules Chloroplast Light

Photosystem II Electron transport chains Photosystem I

CALVIN CYCLE

A summary of the chemical processes of photosynthesis

Stroma

Cellular respiration Cellulose Starch LIGHT REACTIONS

CALVIN CYCLE

Other organic compounds