Ch 10 Notes (part 2): The Light Reactions



NOTES: CH 10, part 2: The Light Reactions (10.2)

10.2 - The light reactions convert solar energy to the chemical energy of ATP and NADPH

● Chloroplasts are

● The conversion of light energy into chemical energy occurs in the .

PROPERTIES OF LIGHT:

● ( )

● light behaves like a wave;

● Wavelength =

● wavelengths of light important to life = ( )

● The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation

● light also behaves as though it consists of discrete bundles of energy called

(amt. of energy in 1 photon is inversely proportional to wavelength)

● are most effectively absorbed by chlorophyll & other pigments

Photosynthetic Pigments: The Light Receptors

● Pigments are

● Different pigments absorb different wavelengths

● Wavelengths that are not absorbed are

● Leaves appear green because

EXPERIMENTAL EVIDENCE:

● A spectrophotometer measures a pigment’s ability to absorb various wavelengths

● This machine sends light through pigments and measures the fraction of light transmitted at each wavelength

● An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength

● The absorption spectrum of chlorophyll a suggests that and

● An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process

● The action spectrum of photosynthesis was first demonstrated in 1883 by Thomas Engelmann

● In his experiment, he exposed different segments of a filamentous alga to different wavelengths

● Areas receiving wavelengths favorable to photosynthesis

● He used aerobic bacteria clustered along the alga as a measure of O2 production

PHOTOSYNTHESIS PIGMENTS:

● Chlorophyll a is the

● Accessory pigments, such as chlorophyll b, used for photosynthesis

● Accessory pigments called carotenoids (yellows and oranges) absorb excessive light that would damage chlorophyll

( )

*as chlorophyll and other pigments absorb photons of light, electrons become excited and move from ground state to excited state…

Excitation of Chlorophyll by Light

● When a pigment absorbs light, it goes from a to an , 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

PHOTOSYSTEM = an organized group of pigment molecules and proteins

Photosystem I: ( )

Photosystem II: ( )

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

● Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions

● In a chloroplast, excited electrons are passed from molecule to molecule until it reaches the (the part of the antenna that converts light energy into chemical energy…the pigment molecule here is always )

● There are two types of photosystems in the thylakoid membrane: and

● 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

* light drives ATP and NADPH production by energizing the 2 photosystems

* energy transformation occurs by electron flow, which can be: or

NONCYLIC ELECTRON FLOW: (a.k.a. “Linear Electron Flow”)

● Noncyclic electron flow, the primary pathway, involves and

● also called

STEPS of Noncyclic Electron Flow:

1) Photosystem absorbs LIGHT (ground-state electrons are “excited”); excited electrons in photosystem II are passed to the chlorophyll-a molecule in the ;

2) an enzyme splits water, extracting electrons which fill the electron “hole” of chlorophyll; the oxygen atoms from the split H2O .

Equation:

3) electrons flow from photosystem II to photosystem I via an

4) the E.T.C. uses chemiosmosis to drive ATP formation (NONCYCLIC PHOTOPHOSPHORYLATION)

-the ATP generated here will be used to !

5) as electrons reach the end of the E.T.C. they fill the electron “hole” of P700 of photosystem I;

6) the reaction center of photosystem I passes photoexcited electrons

down a second E.T.C. which transmits them to NADP+, reducing it

and forming

(which is also used to run the Calvin Cycle!)

CYLIC ELECTRON FLOW:

-

-

-

A Comparison of Chemiosmosis in Chloroplasts and Mitochondria:

● chloroplasts and mitochondria generate ATP by chemiosmosis, but use

● mitochondria transfer chemical energy from ;

● chloroplasts transform into the chemical energy of ATP

● The spatial organization of chemiosmosis differs in chloroplasts and mitochondria

● The current model for the thylakoid membrane is based on studies in several laboratories

● Water is split by photosystem II on the side of the membrane

● The diffusion of H+ from the thylakoid space back to the stroma

● ATP and NADPH are produced on the side facing the stroma,

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download