Photosynthesis

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5

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

Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll

Slide 40

Fig. 10-10

Porphyrin ring:

light-absorbing

“head” of molecule;

note magnesium

atom at center

in chlorophyll a

CH3

Hydrocarbon tail:

interacts with hydrophobic

regions of proteins inside

thylakoid membranes of

chloroplasts; H atoms not

shown

CHO

in chlorophyll b

Slide 41

Excitation of Chlorophyll by Light

When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable

When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence

If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat

Slide 42

Fig. 10-11

(a) Excitation of isolated chlorophyll molecule

Heat

Excited

state

(b) Fluorescence

Photon

Ground

state

Photon

(fluorescence)

Energy of electron

e–

Chlorophyll

molecule

Slide 43

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

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

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

Slide 44

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

Slide 45

Fig. 10-12

THYLAKOID SPACE

(INTERIOR OF THYLAKOID)

STROMA

e–

Pigment

molecules

Photon

Transfer

of energy

Special pair of

chlorophyll a

molecules

Thylakoid membrane

Photosystem

Primary

electron

acceptor

Reaction-center

complex

Light-harvesting

complexes

Slide 46

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