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Photosynthesis
Chapter 10
What is photosynthesis…
 Photosynthesis transforms light energy into
chemical bond energy stored in sugar and other
organic molecules.
 Energy-rich organic molecules made from
energy-poor molecules, CO2 and H2O.
 Directly or indirectly supplies energy to most
living organisms.
 Autotrophic organisms require an energy from
light (photoautotrophs) or from the oxidation of
inorganic substances (chemoautotrophs).
 Photoautotrophs -- plants, algae and some
bacteria.
 Chemoautotrophs -- some bacteria.
The Nature if Light and
Pigments
 Sun emits electromagnetic radiation, the energy of which
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depends on the wavelength of light.
A wavelength is the distance between the crests of
electromagnetic waves.
Visible light is only a small portion of the electromagnetic
spectrum and ranges from about 380 to 750 nanometers
in wavelength.
Blue and red are the colors (wavelengths) most useful as
energy for photosynthesis.
Pigments -- Substances that absorb visible light.
Different pigments absorb different wavelengths of light.
Color you see is the color most reflected or transmitted
by the pigment.
A leaf appears green because it reflects green light.
Chlorophyll and other
pigments
 Chlorophyll a – blue-green pigment that participates
directly in the light reactions.
 Other accessory pigments can absorb light and
transfer the energy to chlorophyll a, expanding the
range of wavelengths available for photosynthesis.
 Chlorophyll b -- yellow-green pigment with a minor
structural difference that gives the pigment slightly
different absorption spectra.
 Carotenoids -- yellow and orange pigments that can
transfer energy to chlorophyll a.
 We see these in the fall as chlorophyll breaks down.
Photoexcitation of Pigments
 When light is absorbed, electrons in the pigment
molecule are boosted from its lowest-energy
state (ground state) to a higher energy level
(excited state).
 The light energy absorbed is converted to
potential energy of an electron elevated to the
excited state.
 This state is unstable, so electrons quickly fall
back to the ground state, releasing energy. This
energy may:
 1. Be lost as heat.
 2. Be re-emitted as light of lower energy (longer
wavelength) -- fluorescence.
 3. Trigger another reaction if nearby electron
acceptor molecules trap excited electrons.
Leaf Structure
 Leaves are the major organs of photosynthesis
in most plants.
 Photosynthetic pigments are found in
chloroplasts which are concentrated in leaf’s
interior.
 Mesophyll -- green tissue inside the leaf.
 Stomata – microscopic pores in the leaf through
which CO2 enters and O2 exits.
 Vascular bundles (veins) – transport water
absorbed by the roots to leaves; also export
sugar from leaves to other parts of the plant.
Chloroplasts
 Intermembrane Space – narrow space which
separates the two membranes of the chloroplast.
 Thylakoids -- Flattened membranous sacs inside
the chloroplast; Chlorophyll is found in the
thylakoid membranes.
 Grana -- (Singular = granum) Stacks of
thylakoids.
 Thylakoid Space – space inside the thylakoid
 Stroma -- viscous fluid outside the thylakoids.
 Photosynthetic prokaryotes lack chloroplasts,
but have chlorophyll built into the plasma
membrane or into membranes of vesicles within
the cell.
Photosystems: Light-Harvesters of
the Thylakoid Membrane
 Chlorophyll a, chlorophyll b and the carotenoids are assembled
into photosystems located within the thylakoid membrane. Each
photosystem is composed of:
 1. antenna complex -- Pigment molecules (200-300) absorb
photons of light and pass the energy from molecule to molecule
to the reaction center.
 2. reaction-center chlorophyll -- One of the many chlorophyll a
molecules transfers an excited electron to initiate the light
reactions.
 3. primary electron acceptor -- Molecule traps excited state
electrons released from the reaction center chlorophyll; powers
the synthesis of ATP and NADPH later.
 Two types of photosystems:
 • Photosystem I has a specialized chlorophyll a molecule
known as P700, which absorbs best at 700 nm.
 • Photosystem II has a specialized chlorophyll a molecule
known as P680, which absorbs best at a wavelength of 680 nm.
Part 1: The light-dependent
reactions
 Light excites electrons from P680 (reaction center
chlorophyll in photosystem II).
 Electrons ejected from P680 are trapped by the
photosystem II primary electron acceptor.
 The electrons are then transferred to an electron
transport chain embedded in the thylakoid
membrane.
 Carriers: plastoquinone (Pq)  2 cytochromes 
plastocyanin (Pc) to P700 of photosystem I (noncyclic electron flow).
 Electrons lost from the P680 reaction center must be
replaced; 2 H2O in the thylakoid space split; 4 H+ are
pumped into the membrane; 4 e- are transferred to
the chlorophyll; O2 is produced as a by-product.
Non-cyclic Photosynthetic
Phosphorylation
 Excited electrons lose potential energy along the
transport chain as they fall back to P700.
 This flow of electrons is coupled to reactions that
phosphorylate ADP to ATP (another example of
chemiosmosis).
 Protons are pumped from the stroma to the
thylakoid space as the electrons move along the
transport chain, creating a proton gradient.
 ATP synthase enzyme in the thylakoid membrane
uses this proton-motive force to make ATP as H+
flows back across the membrane.
Part 1: The light-dependent
reactions continued
 Light excites electrons from P700 (reaction center
chlorophyll in photosystem I).
 Excited electrons are transferred to the primary
electron acceptor for photosystem I, then passed
to ferredoxin (Fd), an iron-containing protein.
 An enzyme catalyzes the reduction of NADP+,
transferring electrons from ferredoxin and
producing NADPH (electron carrier for the second
part of photosynthesis, the Calvin Cycle).
 The electron "holes" in P700 are filled by
electrons supplied by photosystem II.
Cyclic Photo-phosphorylation
 Involves only photosystem I and generates ATP without
producing NADPH or evolving oxygen; this system probably
evolved first.
 Called cyclic because excited electrons that leave from
chlorophyll a at the P700 reaction center return to the same
place.
 Photons are absorbed by Photosystem I; P700 chlorophyll
releases electrons to the primary electron acceptor, which
passes them to ferredoxin.
 Electrons them move down the electron transport chain (same
one from P680 to P700).
 H+ are pumped across the membrane, setting up the proton
gradient for ATP production by chemiosmosis.
 This cyclic pathway supplements the ATP required for the
Calvin cycle and other metabolic pathways. The noncyclic
pathway does not produce enough ATP to meet demand.
 NADPH concentration may influence whether electrons flow
through cyclic or noncyclic pathways.