The Reactions of
Photosynthesis
• The process of photosynthesis is quite
complex; you will only have to know a
simplified overview of the steps and
stages.
2 Stages
a.Light-dependent reaction (thylakoid)
b.Carbon-fixation reaction (stroma)
Energy products from
photosynthesis
• Photosynthesis produces different
intermediate and final energy rich
products through out its process
• ATP
• NADPH
• Glucose
1. ATP - Adenosine
triphosphate
• Principal energy-supply molecule for
cellular functions of all living cells (most
important and USEABLE form of
chemical energy)
• Provides an immediate source of
energy (for growth and movement)
• ATP is formed when an inorganic
phosphate group (P) is attached to
lower-energy ADP (Adenosine
diphosphate)
ATP
Energy
Input
Energy output
ADP
+
P
• Energy is released when an enzyme
removes a phosphate group thus
forming ADP
• Molecules of ATP are broken down and
regenerated thousands of times each
day
2. NADPH - Nicotinamide
dinucleotide phosphate
• NADPH – electron donor
• NADP+ - electron acceptor
• NADPH contains STORED energy and
RELEASES energy when it donates 2eand one H+ to become NADP+
3. Glucose
• Provides medium-term energy storage in
most cells
• Transport molecule - glucose molecules are
transferred to different parts of the plant for
storage
An overview of
Photosynthesis
Stage 1 - Capturing Solar Energy
• Chlorophyll (and other pigment
molecules) are found in clusters
embedded in the thylakoid membrane
– these are called photosystems
• Chloroplasts have 2 photosystems (PS I
and PS II)
– named for the order in which they were
discovered, not for their sequence in the
photosynthetic process
• Electrons in the chlorophyll molecule
are ‘excited’ when they absorb a
photon.
• This occurs within PSII
• This high-energy electron leaves PSII
and is passed through an electron
transport chain
Electron transport chain…
• a series of electron carrying molecules
• each has a progressively stronger
attraction for electrons
• a small amount of energy is released at
each step
• As electrons pass through the ETC,
they return to lower energy levels,
become more stable and energy is
released.
• Released energy makes ATP
• Electrons that move through the ETC
must be replaced……They are replaced
during a process called photolysis
• Solar energy is used to split water into
hydrogen ions (H+) and oxygen gas
• Photolysis occurs in the thylakoid lumen
2 H2O(l) + energy  4 H+ + 4 e- + O2(g)
• Oxygen gas is released to the
environment as a waste product!!!
Stage 2: Electron Transfer
and ATP synthesis
How does the transfer of
electrons release energy?
• Oxidation - a reaction in which an atom
or molecule loses electrons
• Reduction - a reaction in which an
atom or molecule gains electrons
• When a molecule (such as NADP+)
gains electrons, it becomes more
stable(NADPH) and releases energy
Pathway of other electrons
• Energy released when an electron
passes through the ETC is used to “pull”
hydrogen ions from the stroma into the
lumen (B in diagram)
• The concentration of H+ ions as well as
electrical charge builds up within the
lumen
• Electrons have lost most of their original
energy (received by light) by the time
they reach PS I (electrons begin PS I
inside the membrane)
• The electron again absorbs solar
energy and reaches an excited state
• however… this electron does not enter
an electron transport chain
• The electrons are transferred to a
molecule of NADP+
• Each NADP+ accepts two electrons and
an H+ ion from its surroundings
• Newly formed NADPH
molecules are used in
the Calvin cycle in
stage 3
Summary
Chemiosmosis
• Chemiosmosis is
a process for
synthesizing ATP
using the energy
of an
electrochemical
gradient and the
ATP synthase
enzyme
• Recall - a high concentration of H+ ions
has built up in the lumen
• This creates an electrochemical
gradient
• This gradient can be put to good use, as
H+ ions try to return to equilibrium
• H+ ions are able to escape from the
lumen only through specialized protein
complexes - ATP synthase
complexes.
• As H+ rush through the complex, they
release energy
• This energy is used to combine ADP
with a free phosphate group
• ATP is generated by the energy of the
electrons from PSII
Lumen
Chemiosmosis
uses an
electrochemical
gradient and a
special protein
complex to
generate ATP
Stroma
Summary - Light dependent
Reactions
• Water is consumed (split apart)
• ATP, NADPH, and O2(g) are formed
• ATP and NADPH go on to be used in
stage 3 - carbon fixation
• CO2 and glucose are NOT involved in
these stages!!