Photosynthesis
The break down
What is Photosynthesis?
Photosynthesis is a process made up of a series of
complex chemical reactions that form a variety of
energy rich molecules that each serve a specific
purpose in the cell
Molecule
Function
ATP
- principle energy molecule for cellular functions
of all living cells
- provides an immediate source of energy for
cellular processes, such as growth and
movement
NADPH
- electron donor (NADPH) involved in energy
transfer
Glucose
- transport molecule (blood sugar)
- medium-term energy storage in most cells
ATP
ATP (adenine triphosphate) is one of the most
important energy rich molecules in cells.
It is used by all cells to provide immediate energy for
cellular actions
It is needed for the synthesis of chemicals as well as
the transport of materials across cell membranes
ATP
ATP is formed when an additional phosphate group
is added to the lower-energy molecule ADP (adenine
diphosphate)
+
NADP and NADPH
NADP+ is an electron acceptor
NADPH is an electron donor
NADP+ can accept one hydrogen atom and two
electrons to become NADPH
The loss and gain of electrons is important in the
process of photosynthesis
Overview
1
2
3
Overview
1. Capturing solar energy and transferring its electrons
2. Using captured solar energy to make ATP and transfer high energy
electrons to NADP+ → yields NADPH which is used as a highenergy electron carrier molecule
3. Using energy stored in ATP and high energy electrons carried by
NADPH to form energy rich organic molecules (glucose) from CO2
Light Dependant Reactions
The first set of reactions of photosynthesis in which
light energy excites electrons in chlorophyll
molecules, powers ATP synthesis, and results in the
reduction of NADP+ and NADPH
These reactions require chlorophyll and occur in the
thylakoid membranes in the chloroplasts
Carbon Fixation
Incorporating CO2 into organic compounds like
glucose
These reactions take place in the stroma of the
chloroplast and utilize energy of ATP and high
energy electrons carried by NADPH
Uses the Calvin cycle to accomplish the task
Stage 1: Capturing Solar Energy
Chlorophyll can be found in the thylakoid
membranes in chloroplasts in clusters called
photosystems
These photosystems absorb light energy
There are two distinct, but interconnected
photosystems (I and II)
Stage 1: Capturing Solar Energy
Stage 1: Capturing Solar Energy
Solar energy is captured when an electron in a
chlorophyll molecule absorbs a photon
Once a photon is absorbed, an electron has a high
amount of energy and is now said to be in an excited
state
The photon can now be converted into chemical
energy
Stage 1: Capturing Solar Energy
The electrons move on from the photosystem onto
the electron transport chain
As each electron leaves, it needs to be replaced
Replacement electrons come from water and
undergo a process called photolysis
Photolysis
The solar energy being absorbed is used to split
water into hydrogen ions and oxygen gas
This occurs in the thylakoid lumen
Two water molecules are used for every 4 electrons
transferred from the photosystem:
2H2O + energy → 4H+ + 4e- + O2
Photolysis
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Stage 2: Electron Transfer and
ATP Synthesis
Now that the energy has been captured, it must be
used to help form more stable, energy rich
molecules (ATP)
This is achieved by two different mechanisms:
1. Electron Transport Chain
2. Oxidation-Reduction Reactions
Electron Transport Chain
As mentioned, solar energy excites electrons
This energy lifts the electrons up to the top of the energy stairway
Potential energy is then gradually released as the electrons travel down
the stairs to their original state
Some of the released energy is used to create ATP
Electron Transport Chain
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Oxidation-Reduction Reactions
How does this transfer of electrons release energy?
At each step of the transport chain, a higher energy
electron is donating an electron to a lower energy
electron acceptor
Oxidation: when an ion loses electrons
Reduction: when an ion gains electrons
Oxidation-Reduction Reactions
Electron donors like NADPH lose electrons while electron
acceptors NADP+ gain electrons
NADPH donates electrons to NADP+
When an element gains electrons (is reduced) it releases energy
and becomes more stable
When NADP+ is converted to NADPH, energy is released
Oxidation-Reduction Reactions
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Break
it
down
Break it Down
Electrons are not only passed
through the transport chain, but
also carried across the thylakoid
membrane toward the thylakoid
lumen (inside part)
Break it Down
As e- move, they release energy
which is used to ‘pull’ positively
charged H+ ions across the
membrane into the lumen
e-
The e- have lost energy but they
don’t stop here
Break it Down
e- get transferred to photosystem I
These e- that are transferred are
replacing the e- that have been
energized by light and moved on to
the next step already
e-
Break it Down
The e- are not passed across the
thylakoid membrane like in
photosystem II but instead are
transferred to NADP+
Each NADP+ can accept 2 e- and
one H+ from the surroundings to
change it (redox reaction) to
NADPH
e-
Summary of Key Steps