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 O 2 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,