PHOTOSYNTHESIS
LIGHT REACTIONS
thylakoid membranes
A pigment molecule in a light-harvesting complex absorbs a photon of light. The energy is
passed to other pigment molecules and finally to the reaction center of photosystem II, where
it excites an electron of chlorophyll P680 to a higher state.
This electron is captured by the primary electron acceptor.
Water is split, and its electrons are supplied one by one to P680, each replacing an electron
lost to the primary electron acceptor. The oxygen atom combines with an oxygen from another
split water molecules to form a molecule of O₂.
Each photoexcited electron passes from photosystem II to photosystem I via an electron
transport chain (ETC). The exergonic fall of electrons provides energy for the synthesis of ATP
by pumping H+ across the membrane.
Light energy excites an electron of chlorophyll P700 in the reaction center of photosystem I.
The primary electron acceptor captures the electron, and an electron from the bottom of the
ETC replaces the lost electron in P700.
The excited electron of photosystem I is passed through a short electron transport chain to
NADP+, reducing it to NADPH.
PHOTOSYNTHESIS
LIGHT REACTIONS
thylakoid membranes
Summary:
The light reactions of photosynthesis are the steps that absorb solar energy
and convert it to chemical energy stored in ATP and NADPH. Notice that
these reactions produce no sugar; sugar is not made until the Calvin cycle,
the second stage.
Tracing the light reactions above, there is a flow of electron from ________
molecules to _________, which is reduced to _________, the source of electrons
for sugar synthesis in the ________ cycle.
water ... NADP+ ...NADPH ... Calvin
PHOTOSYNTHESIS
DARK REACTIONS
stroma of the chloroplast
Release of one molecule of G3P
Carbon Fixation
An enzyme called rubisco combines CO2 with a five-carbon
sugar called RuBP. The unstable product splits into two
molecules of the three-carbon organic acid, 3-PGA.
3 CO2 = six 3-PGA
Two chemical reactions consume energy from six molecules of
ATP and oxidize six molecules of NADPH. Six molecules of 3PGA are reduced, producing six molecules of the energy-rich
three-carbon sugar, G3P.
Reduction
Release
of
one
molecule of G3P
Regeneration
Five of the G3Ps from step 2 remain in the cycle. The single
molecule of G3P you see leaving the cycle is the net product of
photosynthesis. A plant cell uses G3P to make glucose and
other organic compounds.
A series of chemical reactions uses energy from ATP to
rearrange the atoms in the five G3P molecules (15 carbons
total), forming three RuBP molecules (15 carbons). These can
turn another of the cycle.
PHOTOSYNTHESIS
DARK REACTIONS
stroma of the chloroplast
Release of one molecule of G3P
Summary:
The Calvin cycle, occuring in the stroma, consists of carbon fixation, reduction,
release of G3P, and regeneration of RuBP. Using carbon from CO2, electrons from
NADPH, and energy from ATP, the cycle constructs G3P, which is used to buil
glucose and other organic molecules.
To synthesize one glucose molecule, the Calvin cycle must turn six times, using 6
CO2, 18 ATP, and 12 NADPH. Explain why this high number of ATP and NADPH
molecules is consistent with the value of glucose as an energy source.
Glucose is a valuable energy source because it is highly reduced, storing lots of potential energy in its electrons. The more energy a molecule stores, the
more energy and reducing power required to produce that molecule.
Complete this summary map of photosynthesis.
PHOTOSYNTHESIS
CONVERTS
INCLUDES
BOTH
TO
IN WHICH
LIGHT EXCITED
ELECTRONS OF
CHLOROPHYLL
CHEMICAL
ENERGY
WATER IS
SPLIT
IN WHICH
CO2 IS FIXED TO
RUBP
AND THEN
ARE PASSED
REDUCE NAD+ TO
AND
USING
TO PRODUCE
PRODUCING
SUGAR (G3P)
BY
CHEMIOSMOSIS
A. light energy; B. light reactions; C. Calvin cycle; D. Oxygen is released; E. ETC; F. NADPH; G. ATP; H. 3-PGA is reduced
This diagram compares the chemiosmotic synthesis of ATP
in mitochondria and chloroplasts. In both cased, label the
structures involved and indicate which side of the
membrane has the higher H+ concentration. Then label on
the right locations within the chloroplast.
Mitochondrion
structure
Intermembrane
space
Chloroplast
structure
c. ________________
Membrane
d. ________________
Matrix
e. ________________
a. electron transport chain; b. ATP synthase; c. thylakoid space (higher H+ concentration; d. stroma; e. ATP