Photosynthesis
I. Chemical Energy and ATP
A. Energy is the ability to do work. Nearly every
activity, whether in society or organisms, depends on
energy. When a car runs out of gas (chemical energy),
it comes to a stop. When the electricity in our homes is
out, we come to realize just how many things depend
on that electrical energy. Organisms are no different –
nearly every activity requires energy. Without energy,
life would cease to exist.
1. Energy comes in many forms – light, heat,
chemical, electrical.
2. Cells use many different chemical compounds for
energy. Chemical energy is stored in the bonds
between the atoms of compounds. When the
bonds are broken and rearranged, energy can be
released as new bonds are formed that are at a
lower energy state than the original bonds.
3. One of the most important energy compounds used
by cells is ATP, which is adenosine triphosphate.
a. ATP consists of adenine, a 5 carbon sugar called
ribose, and three phosphate groups.
b.
c.
d.
e.
f.
g.
Adenine Ribose 3 phosphate groups
b. Energy is released from ATP by breaking
the bond between the last two phosphate
groups, which results in the compound ADP
(adenosine diphosphate).
c. In this way, ATP is like a rechargeable
battery. As the cell has energy available, it
can store it by adding phosphates onto ADP
molecules, making ATP. When it needs
energy, it can remove a phosphate from
ATP to release energy.
d. This makes ATP exceptionally useful as a
basic energy source for all cells.
4. What are some of the activities that require ATP?
a. Active transport – For example, ATP drives
the sodium-potassium ion pump.
b. Movement –ATP provides the energy for
motor proteins that contract muscles and
power cilia and flagella.
Adding a phosphate to
ADP to make ATP is like
recharging a battery
c. Making Molecules – ATP powers the
production of proteins and other
macromolecules; it also powers other chemical
responses in the cell.
d. Light – fireflies, bioluminescence
5. Ironically, cells aren’t jammed with huge amounts
of ATP at once. It’s not really great at storing
large amounts of energy over long periods of time.
Other molecules, like sugars, are better at that
task. So it’s more efficient to keep a small amount
of ATP on hand and keep cycling between ATP
and ADP as needed by using energy from those
sugar molecules.
B. Where do you get ATP?
1. Cells must produce ATP. They don’t have it to
start. So how do you get ATP? From the chemical
energy stored in the food we eat.
a. If you are a heterotroph, you eat other
organisms, either plants or animals or both, to
get the chemical energy you need. Fungi and
many bacteria decompose the remains of
organisms to get chemical energy.
Remember: Food = chemical energy
b. If you are an autotroph, you are able to make
your own food, usually from the energy of the
sun. Plants, algae, and some bacteria are able
to use photosynthesis to convert energy from
the sun and store it in molecules that make up
food (glucose). This is a major achievement:
solar energy (sunlight) is converted to
chemical energy (food molecules).
c. Think about the word photosynthesis. From
the Greek, photo means “light” and synthesis
means “putting together” or making
something. So photosynthesis means “using
light to put something together.”
1. Not all autotrophs use photosynthesis.
Some organisms (so far, mostly some
bacteria) use a process called
chemosynthesis. What do you think the
word chemosynthesis means?
2. Chemosynthetic organisms
(chemoautotrophs) are able to produce
carbohydrates from inorganic molecules
such as hydrogen sulfide.
3. Many of these organisms are found in
very extreme environments – bottom of
the ocean, deep sea volcanic vents, acidic
hot springs. But they’ve also been found
in tidal marshes and in the bottom of
swamps.
d. Remember, all energy in food, whether it’s
made in plants or consumed, originates from
the sun. It’s the producers’ ability to capture
that energy and convert it into food that
makes all other life forms dependent on them.
II. Photosynthesis: An Overview
A. Chlorophyll and Chloroplasts – How is light energy
captured?
1. Energy from the sun travels to the Earth in the
form of light. White light, or what we call
sunlight, is actually a mixture of many different
wavelengths of energy. Some of these wavelengths
are visible to us – ROYGBIV. Others are not –
infrared and ultraviolet for example.
2. Pigment: molecules that absorb certain
wavelengths of light and reflect others
a. Whatever wavelength of light that is reflected
is the color that you see
b. Plants use different pigments to absorb light
and capture the energy.
3. The main pigment used by plants in
photosynthesis is chlorophyll. There are two types
of chlorophyll found in plants, a and b.
a. Chlorophyll a absorbs light very well in the
violet and orange-red areas of the light
spectrum, and a little bit in the blue areas.
b. Chlorophyll b absorbs light very well in the
blue and orange-red areas, and a little bit in
the violet areas.
c. What’s missing? Neither type of chlorophyll
absorbs light very well in the green region of
the spectrum.
d. This gives plants their green color –
remember, what isn’t absorbed is reflected,
and that is what you see.
e. What happens in the fall to give us the
beautiful colors? Chlorophyll breaks down
first, and now the other pigments in the plants,
called carotenoids, can be seen. Carotenoids
are typically red, orange, or yellow pigments.
They are usually masked by the abundant
amount of chlorophyll in the plants.
4. Chloroplasts – remember that this is the organelle
where photosynthesis will take place.
a. Thylakoids: saclike membranes inside
chloroplasts
b. Thylakoids are interconnected and arranged
in stacks called grana (singular stack is a
granum).
c. Chlorophyll is located in the thylakoid
membranes.
d. Outside of the thylakoids is a fluid called the
stroma.
5. Since chlorophyll absorbs light, it is absorbing
energy. When it absorbs the light, some of that
energy is transferred directly to electrons in the
chlorophyll molecule itself. This raises the energy
level of the electrons, so light energy can be used to
supply a steady supply of high-energy electrons.
THIS IS WHAT MAKES PHOTOSYNTHESIS
WORK. If those electrons didn’t jump energy
levels and become “excited,” photosynthesis
wouldn’t happen and life on the planet would be
very different indeed!
B. You wouldn’t grab a hot metal pan right out of the
oven with your hands – the pan is too hot for your
hands to handle and you would be burned. So you use
an oven mitt to handle the heat of the pan to protect
your hand. In the same way, high energy electrons
made by chlorophyll are too “hot” to handle and
require a special “carrier” to move them from place to
place. The plant cell’s “oven mitts” are called electron
carriers.
1. Electron carrier: a compound that can accept
a pair of high energy electrons and transfer
them, along with most of their energy, to
another molecule
2. NADP+ (nicotinamide adenine dinucleotide
phosphate) is one of these electron carriers.
NADP+ accepts and holds 2 high energy
electrons, along with a hydrogen ion (H+).
3. This converts NADP+ to NADPH. This is one
way to trap some of the energy from sunlight
into chemical form. NADPH can then carry
the high-energy electrons (produced when
chlorophyll absorbed light) to chemical
reactions elsewhere in the cell.
4. High-energy electron carriers are used to help
build a variety of molecules the cell needs,
including carbohydrates like glucose.
C. Photosynthesis is complicated. There are many steps.
But the overall result is not. In a nutshell:
1. Photosynthesis uses the energy of sunlight to
convert water and carbon dioxide into high
energy sugars (glucose) and oxygen.
2. The reaction of photosynthesis is below.
Please note which compounds are the products
and which are the reactants.
6 CO2 + 6H2O light
C6H12O6 + 6O2
carbon dioxide water glucose oxygen
3. Photosynthesis consists of two sets of
reactions, the light-dependent reactions and
the light-independent reactions.
a. The light-dependent reactions require the
direct involvement of light and the light-
absorbing pigments. They use energy
from sunlight to produce energy rich
compounds like ATP and NADPH. Light-
dependent reactions happen in the
thylakoid membranes of the chloroplast
and require water.
b. The light-independent reactions use the
ATP and NADPH molecules from the
light-dependent reactions to produce high
energy sugars from carbon dioxide. No
light is required, and these reactions take
place in the stroma.
As you can see, the two sets of reactions work together to capture the
energy of the sunlight and transform it into energy rich carbohydrates.
III. Details of Photosynthesis : The Light Dependent
Reactions – What happens?
A. The light dependent reactions use the energy of
sunlight to produce oxygen and convert ADP and
NADP+ to the energy carriers ATP and NADPH.
1. Thylakoids contain clusters of chlorophyll and
proteins known as photosystems.
2. Photosystems absorb sunlight and generate high-
energy electrons. The electrons are then passed to
a series of electron carriers embedded in the
thylakoid membrane.
B. There are two photosystems (I and II). They are
named in the order that they were discovered. This
part of photosynthesis is all about following the
electrons. Ironically, we begin with photosystem II.
1. As light is absorbed by photosytem II, electrons
are energized in the chlorophyll. More and more
high energy electrons are passed to the electron
transport chain.
2. Electron transport chain: a series of electron
carrier proteins that shuttle high energy electrons
during ATP generating reactions
3. Why doesn’t the chlorophyll in photosystem II run
out of electrons? At the same time these high
energy electrons are passed onto the electron
transport chain, enzymes of photosystem II are
splitting water molecules.
a. Water is split into hydrogen ions (H+),
electrons, and oxygen.
b. The electrons replace the high energy
electrons that have been passed from the
chlorophyll to the electron transport chain.
c. As the electrons are taken from the water, the
oxygen that is left behind is released into the
atmosphere. (This is the main source of
oxygen in the atmosphere and we need it to
survive.)
d. What about the H+ ions from the water? They
are released inside the thylakoid.
4. As electrons move down the electron transport
chain, their energy is used by the protein
molecules to pump H+ ions from the stroma into
the thylakoid space.
5. At the end of the electron transport chain, the
electrons are then passed to a second photosytem
called photosystem I.
6. When the electrons get to photosystem I, they
don’t have as much energy as they did before since
some was used to actively transport hydrogen ions
into the thylakoid space. So the pigments in
photosystem I use the energy of the sun to
reenergize the electrons.
7. The reenergized electrons enter a second electron
transport chain that is very short. At the outer
surface of the thylakoid membrane, this chain
transfers the electrons to NADP+ which is in the
stroma. In addition to the electrons, NADP+ picks
up H+ ions in the stroma, making NADPH. This
NADPH (full of energy now) becomes very
important in the second part of photosynthesis, the
light-independent reactions.
8. Wait a minute! What’s going on with all those H+
ions that were left behind when water split? And
didn’t we just pump a bunch more into the
thylakoid space?
a. All those H+ ions in the thylakoid space make
that area positively charged and the stroma is
negatively charged in comparison.
b. This gradient, both in charge (positive and
negative) and in H+ ion concentration, between
the stroma and the thylakoid space provides
the energy to make ATP.
c. H+ ions can’t cross the thylakoid membrane
directly. They must use a protein in the
membrane called ATP synthase which allows
H+ ions to pass through it.
1. When the H+ ions pass through, they
force the ATP synthase to rotate.
This rotation causes ATP synthase to
bind an ADP and a phosphate group
together, making ATP.
2. This means that at the end of the light
dependent reactions, you have both
ATP and NADPH. These energy rich
compounds will be needed to make
the carbohydrates during the light-
independent reactions.
IV. Photosynthesis: The Light-Independent Reactions –
Producing Sugars
FYI – Also known as the Calvin Cycle or sometimes the
Dark Reactions
A. Recall that the light-dependent reactions resulted in
ATP and NADPH. During the light-independent
reactions (Calvin Cycle) plants use the energy that
Light-Dependent Reactions: take place in the
thylakoid of the chloroplast. They use energy
from sunlight to make ATP and NADPH.
ATP and NADPH contain to build high energy
carbohydrate compounds (sugars).
1. Why not just use the APT and NADPH for energy
directly? Why bother making another compound?
a. ATP and NADPH are not stable enough to
store energy for a long period of time. They
are only good for a few minutes.
b. Carbohydrates like glucose and other sugars
are very stable and can be stored for a long
time.
B. The Calvin cycle begins with carbon dioxide entering
from the atmosphere. Recall that CO2 has one carbon
atom. This part is all about following the carbon.
1. An enzyme in the stroma combines 6 carbon
dioxide molecules with 6 other carbon compounds
that were already present in the chloroplast. Each
of these carbon compounds has 5 carbon atoms
(for a total of 36 carbon atoms).
2. The same enzyme rearranges all these atoms to
make 12 molecules that contain 3 carbons each
(still 36 carbon atoms).
3. Other enzymes are going to take these compounds
with 3 carbons each and convert them into higher
energy forms in the rest of the cycle. The energy
to do this comes from ATP and the high energy
electrons in NADPH.
4. In the middle of the cycle, 2 of the 12 carbon
compounds are removed. Why is this important to
mention? These removed carbon compounds will
be the building blocks that the plant cell will use to
make glucose, one of the main sugars used for
energy.
a. These same carbon compounds will be used to
make other sugars, lipids, amino acids, and
many other compounds.
b. In this way, the Calvin cycle contributes to all
the other products needed for the plant to grow
and perform the basic needs of metabolism.
5. The 10 remaining carbon molecules (with 3
carbons each, so a total of 30) are converted back
into 6 carbon molecules each with 5 carbons.
Remember them? We needed these molecules to
combine with CO2 at the beginning of the cycle to
get the process started. In this way, the next cycle
is begun again.
C. Summary of the Calvin Cycle
1. The Calvin cycle uses 6 molecules of carbon
dioxide to produce a single 6 carbon sugar
molecule, typically glucose.
2. Recall the overall reaction of photosynthesis:
6CO2 + 6H2O LIGHT
C6H12O6 + 6O2
carbon dioxide water glucose oxygen
3. The reactions involved are made possible from the
energy in the ATP and NADPH produced in the
light-dependent reactions.
4. When you eat a plant or something else that ate a
plant, you are getting the energy and raw
materials stored in these compounds.
V. Factors that Affect Photosynthesis
A. Many factors can impact the rate of photosynthesis.
The most important are temperature, light intensity,
and the availability of water.
1. Temperature: photosynthesis depends on
enzymes that work best between 0° C and 35° C.
a. If the temperature is above or below, the
enzymes may be affected and photosynthesis
can slow down. At extremely low
temperatures, photosynthesis may stop
completely.
b. Think about what can happen to enzyme
shape at temperatures outside their range –
can it function if shape changes?
2. Light Intensity: high light intensity increases the
rate of photosynthesis
a. There is a maximum rate that once
reached can’t be surpassed.
b. No light = no photosynthesis
3. Water: it’s a raw material, so a lack of water can
slow or even stop photosynthesis
a. water loss can damage plant tissues
b. plants in dry conditions, such as the desert,
have waxy coatings to reduce water loss
c. other biochemical adaptations impacting the
efficiency of photosynthesis are also possible
B. Extreme Conditions and Photosynthesis
1. Some plants under extreme conditions such as
intense light and high heat have further
adaptations that allow photosynthesis to happen in
these extreme conditions.
2. Some regulate the openings in the leaves (called
stomata) to remain shut during the day and open
at night (cooler conditions) to reduce water loss.
But it also reduces the availability of carbon
dioxide. So these plants have different chemical
pathways that allow them to perform
photosynthesis while still minimizing water loss.
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