Photosynthesis: Light Reactions
Slide 2
The term “photosynthesis” normally has students thinking about the green
plant tissue that is present all around us. It should also be remembered that at least fifty
percent of global carbon dioxide is fixed into organic molecules by trillions upon trillions
of microscopic autotrophic organisms which form part of the plankton community found in
the surface waters of the ocean and freshwater! Autotrophs (phototrophs) are also found
in extreme environments such as hot springs.
All photosynthetic tissue converts light energy from the sun to chemical energy in the form
of bonds between atoms, as in ATP and NADPH formed during the “light reactions” of
photosynthesis. Photosynthetic tissue also reduces carbon dioxide from the atmosphere
into carbohydrates during the “carbon fixation reactions”. These organic molecules are the
building blocks for all organisms. Because of this unique ability to produce all the
molecules necessary for life, photosynthetic organisms are termed autotrophs, or self
feeders. Heterotrophs, on the other hand, are organisms that are dependent on the carbon
products produced by autotrophs. Thylakoid membranes contain the pigments associated
with photosynthesis and may be distributed throughout the cell, as in the prokaryotic
cyanobacteria (blue-green algae) and purple and green sulfur bacteria. In eukaryotic
organisms the thylakoid membranes and associated pigments are bound into a chloroplast.
Organisms that evolve oxygen contain chlorophyll a but may differ in the types of
accessory pigments that are present
Slide 3
When plant cells are observed with a light microscope the most obvious
structure is the green disc shaped chloroplast. At high magnification the most obvious
structure inside the chloroplast is the elaborate arrangement of thylakoid membranes and
the matrix that surrounds it, called the stroma. The thylakoid membrane forms stacks of
disc like structures called grana (singular: granum) which are all interconnected by ribbons
of thylakoid membrane, the stroma lamellae. All of the pigments associated with
photosynthesis are anchored in the thylakoid membrane. Now let’s look at how pigments
that are involved in photosynthesis perform this function. [See Photosynthesis: Properties
of light if you haven’t already done so].
Slide 4
The process of photosynthesis occurs in two stages. The first stage is
referred to as the “light” or “energy transduction reactions”. During this stage, photons of
light energy are absorbed by chlorophyll molecules and used indirectly in the production of
ATP and NADPH. During the second stage of photosynthesis, the “carbon fixation
reactions”, carbon dioxide is reduced to sugars during the Calvin cycle. The organic
molecules produced also form carbon skeletons for other macromolecules, such as lipids
and proteins, which are synthesized in an organism. ATP and NADPH produced during
the light reactions are used to power the carbon fixation reactions. The light reactions are
associated with the thylakoid membranes; the carbon fixation reactions take place in the
chloroplast stroma.
Slide 5
Pigments involved in photosynthesis are embedded in the thylakoid
membrane. The main photosynthetic pigments in eukaryotic plant cells are chlorophyll a,
chlorophyll b and the carotenoid pigments. These pigments are in close proximity to each
other forming individual photosystems. Each photosystem has an “antenna complex”
which gathers light energy and funnels it to the reaction center at the heart of the
photosystem, much like a satellite dish focuses transmitted signals to your TV allowing
you to watch your favorite show. Carotenoid pigments and both chlorophyll a and b are
associated with the antenna complex. When a pigment absorbs a photon of light the
pigment acquires the energy of that photon causing the transition of that pigment to an
excited, or higher energy state. This energy is transferred to neighboring pigments by
resonance energy transfer. No electrons are exchanged causing a chemical change, only
the energy is transferred. Think of a resonating tuning fork, when placed close to a second
tuning fork it can cause the second fork to resonate. Energy is transferred causing the
sound but no chemical reaction takes place. Energy is funneled through the antenna
complex this way until it reaches the reaction center of the photosystem. [Pause for a
moment and relate this information to the figure]. Two chlorophyll a molecules are
associated with the reaction center. When either of these molecules absorbs the energy
funneled toward it, one of its electrons is boosted to a higher energy level and the electron
is transferred to an electron acceptor that initiates electron flow. A chemical reaction has
now taken place; the chlorophyll a molecule of the reaction center has been oxidized. The
arrangement of pigments in the photosystem allows a continual harvest of light photons
and their energy is continually funneled towards the reaction center allowing uninterrupted
electron flow. Hundreds of photosystems embedded in the thylakoid membrane function
like this one, to harvest light energy.
Slide 6
The electron lost from the chlorophyll a molecule in the reaction center
needs to be replenished for the photosystem to continue functioning. These electrons are
extracted from water molecules. Water molecules are split to produce oxygen, protons and
electrons. Oxygen is released to the atmosphere through the stomata, protons contribute to
a proton gradient across the thylakoid membrane facilitating the production of ATP and the
electrons replenish those that were lost from chlorophyll a when it was oxidized. This
process is not fully understood but involves a water-splitting enzyme complex that is
located on the inside of the thylakoid membrane. This is the only place in nature where
water molecules can be split to produce oxygen, protons and electrons!
Slide 7
The thylakoid membrane houses two types of photosystems, photosystem I
(PSI) and photosystem II (PSII). Photosystem II can donate electrons to photosystem I
through a series of redox reactions that occurs in the electron transport chain. The electron
transport chain contains many electron carriers – molecules that will accept and donate
electrons- the main ones to consider for this class are plastoquinone, plastocyanin,
ferredoxin and the cytochrome complex. The reaction center chlorophyll of photosystem
II is known as P680 because it is more efficient at absorbing light of wavelength 680 nm;
the chlorophyll in the reaction center of photosystem I is designated P700 since it best
absorbs light of wavelength 700nm. This is light in the red and far-red region of the
visible spectrum respectively. Now let’s follow the path of an electron as it participates in
non-cyclic electron flow. As previously discussed, the photons of light energy harvested
by the antenna complex cause enough excitement of the chlorophyll a molecule in the
reaction center of photosystem II for it to release an electron. This electron is passed to the
primary electron acceptor of the electron transport chain and then on to plastoquinone,
through the cytochrome complex and plastocyanin eventually arriving at the reaction
center of photosystem I. The electron loses energy with each successive transfer, this
energy is utilized to produce ATP at the thylakoid membrane. We will return to the
process of ATP production shortly. The antenna complex of photosystem I harvests
photons of light energy, funneling the energy to the reaction center P700 where there is
sufficient energy to excite the electron in the reaction center to a higher level than in
photosystem II. This high energy electron can now be transferred through a second
electron transport chain, eventually reducing NADP+ to NADPH. The oxidized
chlorophyll a molecule in the reaction center of photosystem I needs an electron to
continue functioning. This electron is donated from photosystem II after it has passed
down the first electron transport chain. The structure of the photosystems and their
arrangement in the thylakoid membrane allows a continual stream of electrons to pass
through the process generating ATP and NADPH. These energy producing reactions are
complicated but keep in mind the overall function – light energy from the sun is used to
produce ATP and NADPH, the chemical energy required for the carbon fixation reactions.
Slide 8
As we shall see in the next section, the Calvin cycle requires more ATP
than NADPH to reduce carbon dioxide to sugars. When ATP is in low supply, more ATP
is produced through cyclic electron flow. Now follow the path of the excited electrons
produced in photosystem I which are shunted through ferredoxin to the cytochrome
complex in the first electron transport chain, associated with photosystem II and
photosystem I, and then back to the reaction center of photosystem I. This cyclic electron
flow does not produce any NADPH nor does it involve photosystem II, therefore no
oxygen is released during this process. ATP is the only product of cyclic electron flow.
Slide 9
This figure shows the organization of the different photosystems and
electron carriers in the thylakoid membrane. Now, think of each disk in a grana stack as
having the same structure as a Reeses peanut butter cup® The chocolate is equivalent to
the thylakoid membrane of the disk, the peanut butter center is equivalent to the thylakoid
space which contains a high concentration of protons resulting from the splitting of water
molecules. As electrons move down the electron transport chain that connects
photosystem II and photosystem I protons are continually pumped across the membrane
into the thylakoid lumen (space) helping maintain the high proton concentration there. The
protons naturally want to diffuse down their electrochemical gradient. This is facilitated
by the ATP synthase complex which produces ATP as the protons diffuse through the
complex from the thylakoid space into the stroma. To see how ATP synthase complex
functions, refer to the virtual cell link or the lesson on respiration. Note that molecules of
ATP and NADPH are released into the stroma where they will be used as an energy source
for the reactions of the Calvin cycle.