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,
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