Photosynthesis: Intro, the Light reactions and the Calvin Cycle
The Basics:
All organisms, big and small, require energy. Organisms that eat organic compounds and then transfer the
energy in these compounds to ATP are called heterotrophs. Other organisms, called autotrophs, make their
own organic compounds before transferring the energy in these compounds to ATP. How the autotrophs make
their organic compounds falls into one of two categories. Chemoautotrophs build organic compounds using
energy that they’ve extracted from inorganic chemicals in a process called chemosynthesis. Photoautotrophs
build organic compounds using solar energy in a process called photosynthesis. The important thing to
remember is that, regardless whether the organic compounds are consumed or synthesized, ALL ORGANISMS
must eventually go through cell respiration to transfer the energy from the organic compounds to ATP.
ALL
ORGANISMS.
Chemosynthesis, while interesting, is less relevant to us because the organisms that use this process mostly live
in rather hostile environments, like deep sea vents. Needless to say, unless you live at the bottom of the ocean,
you aren’t benefiting much from the organic compounds these organisms are producing. For this reason we will
be focusing on photosynthesis.
First let’s get some perspective on where this is all happening. Most people think that the entire plant is capable
of photosynthesis when, in reality it is restricted to the leaves. Within the leaves there is a layer of cells called
mesophyll cells. These cells are LOADED with chloroplasts. Within the chloroplasts there are stacks1 of
membranous sacs called thylakoids. It is in these thylakoids that photosynthesis takes place. Embedded in the
membrane of each thylakoid are all the proteins, enzymes and other molecules needed for phase one of
photosynthesis (the light reactions) and floating around the inner space of the thylakoids are all the enzymes
and cofactors needed for phase two (the Calvin cycle). See figure 1 for a better idea.
Figure 1 Location of Photosynthesis
1
Each stack of thylakoids is called a granum. pl. grana
The Process:
Photosynthesis is a rather complex process that can be summarized by the equation in figure 2 below.
Figure 2: Summary Reaction for Photosynthesis
There are many steps to photosynthesis and MANY molecules involved in all these steps. Most of the molecules
involved are permanent fixtures in the plant and, while they participate in the process, they are returned to
their original state by the end so they are considered to be neither reactants nor products. So, there are really
only two reactants, materials the plant needs to take in and use entirely in the process, and two products,
materials the plant produces that it didn’t have before. These reactants and products are what we show in the
summary reaction (fig. 2).
Two Phases of Photosynthesis:
The process of photosynthesis can be divided into two phases. The first phase, called the light reactions,
involves the absorption of light energy and the conversion of the light energy into chemical energy in the form
of ATP and NADPH. (uses water, produces oxygen). The second phase, called the Calvin cycle, involves using
the ATP and NADPH generated during the light reactions to build sugar molecules out of CO2. Each phase
involves some complex chemistry and numerous steps, but we will try to break it down as simply as possible.
(fig. 3)
Figure 3 Two phases of photosynthesis
Phase 1: THE LIGHT REACTIONS
The light reactions, as the name suggests, are the reactions of photosynthesis that actually involve the sun. This
is the phase where the water shown in the summary reaction is used up and where the O2 is produced. The
other major players in this phase are: pigments (such as chlorophyll), electrons, electron carriers, enzymes, and
hydrogen ions (H+). There are a lot of details, but don’t lose sight of the goal: absorb light energy and store it in
chemicals like NADPH and ATP. This is what the light reactions need to do. At the end of the light reactions we
will not have any sugar, but we will have the energy we need to build the sugar (in the Calvin cycle).
Figure 4: The light Reactions
Take a look at figure 4 above. We are going to follow the numbers, step by step, to see what is happening in this
process. Remember where we are: you are looking at a diagram of all the molecules embedded in the thylakoid
membrane within the chloroplast. This is where the light reactions will take place. Find each number on the
diagram as you are reading about it. ALSO read the entire heading which gives you the big idea of each step.
Step 1: Light energizes chlorophyll’s electrons
In the thylakoid membrane there are clusters of chlorophyll molecules (photosystems). Chlorophyll is a
pigment and when a photon (packet of light) hits these pigment molecules the energy from the light excites
two of the electrons from the chlorophyll. These electrons become so energized that they start bouncing
around from pigment molecule to pigment molecule.
Step 2: Movement of electrons creates a H+ ion gradient
Eventually these electrons bounce away from the chlorophylls and into an electron transport chain. This
just means that there is a series of molecules each of which can accept electrons from the previous molecule
in the series. BUT with each transfer of electrons, some of the electrons’ energy is used to actively transport
H+ into the thylakoid. We’ll come back to these H+ a little later.
Step 3: Electrons are reengergized by light
Since the energy of the electrons is being used as they are passed through the chain, by the time the
electrons reach the end of the chain (Photosystem 1) they are back to the ground level energy that they
started with. Luckily photosystem 1 is another cluster of chlorophyll molecules so more light can be
absorbed. Once again the electrons are raised to a higher energy level and once again they are passed from
chlorophyll to another molecule.
Step 4: Reenergized electrons are passed to an electron carrier
Rather than going through another electron transport chain, this time the reenergized electrons will be
transferred to the electron carrier NADP+ to make NADPH. NADPH will then carry those energized electrons
to the reactions in phase 2 of photosynthesis.
Step 5: H+ ions flow through ATP synthase to make ATP
Remember we said that in step 2, the energy from the electron transfers was being used to pump H+ into
the thylakoid. This required energy because there is a low concentration of H+ outside the thylakoid and a
high concentration inside. Transport from low to high requires energy. The bouncing electrons provided
this energy.
The reason behind creating a high concentration of H+ on the interior of the thylakoid is that now those H+
can be allowed to passively (high to low) flow back out of the thylakoid through a protein called ATP
synthase. This protein has a dual job. It acts as both a transport protein (moving H+) and an enzyme. As the
H+ flow through the ATP synthase it propels the enzyme and allows it to add the third phosphate on to ADP
to make ATP. Think of it as a water wheel. Water flows naturally over the wheel causing the wheel to turn
and the turning wheel generates energy for the mill or whatever it is powering. Here, H+ flows through ATP
synthase and ATP is generated. Now we have the ATP we need to run the Calvin reactions as well as the
NADPH discussed in step 4.
Step 6: Electrons taken from chlorophyll are replaced by electrons from H2O
One more thing to do, and while we are discussing it last, it pretty much happens as soon as the first pair of
electrons is energized by the first photon. Remember we said that when the electrons are energized, they
actually leave the chlorophyll. But at the same time, electrons act like the glue that keeps the atoms of a
molecule together. So if we keep removing electrons from chlorophyll without replacing them, all of the
atoms making up chlorophyll molecules would just fall apart. No chlorophyll, no photosynthesis. So we
need to replace the electrons each time a pair is removed. This is where H2O finally comes into the picture.
H2O is enzymatically split into three components: Oxygen gas, H+ ions, and electrons. The oxygen is released
as a byproduct of photosynthesis (this, by the way is why we can breathe, so thank you Mr. Plant!) The H+
ions become part of the gradient flowing through ATP synthase and the electrons are passed to chlorophyll
to replace the pair of electrons that were taken away and ultimately transferred to NADP+.
And voila! The light reactions are done. We have used light energy and water to generate ATP and NADPH
and now we have what we need to continue on to the Calvin cycle. It may seem overwhelming, but if you
look at the big picture we can break it down into six big ideas. Notice that for each step above, there is a
statement written in bold next to where it says the step number. THIS is the take home message for each
step. THIS is what you MUST know.
Discussion Questions: The answers to some of these questions require past knowledge and thinking in
conjunction with the information in the reading above.
1. While we differentiate between photoautotrophs and heterotrophs, why is it safe to say the energy for
BOTH of these types of organisms really comes from the sun?
2. It is a common misconception that plants go through photosynthesis to make their glucose and
therefore they do not need to do cell respiration. EXPLAIN why this is INCORRECT.
-
If plant cells are performing both photosynthesis and cell respiration what two organelles must be present
in their cells?
3. Draw a diagram of a chloroplast and label the following parts: inner membrane, outer membrane,
thylakoids, stroma and thylakoid space.
4. Based on the summary reaction, if it were given 360 molecules of CO2, how many glucose molecules
could a plant make? How many O2 molecules would be produced?
5. How can such a complex process, that uses many, many different molecules, be reduced to such a
simple chemical equation?
6. What is the “connection” between the light reactions and the Calvin cycle? (what molecules actually
move from one to the other and why?)
7. From the summary reactions, which reactants/products have been accounted for by the time we get to
the end of the light reactions?
Ok, now, we gave you ALL the details to read through for curiosity sake but you aren’t responsible for every little
detail. IN YOUR WORDS, provide a single sentence that describes each of the following.
a. Summarize the light reactions in one sentence that includes reactants, products, energy source AND
purpose.
b. Summarize the Calvin cycle in one sentence that includes reactants, products, energy source AND
purpose.
c. In one sentence, summarize the importance of electrons in the light reactions including the source,
purpose and where they end up.
d. In one sentence, summarize the importance of H+ ions in the light reactions including source, and
purpose.
e. In one sentence, summarize the role chlorophyll plays in the light reactions include its relationship to
the electrons.
Phase 2: The Calvin Cycle
Now that we have converted solar energy into chemical energy in the form of ATP and NADPH, we can use these
molecules to drive the chemical reactions needed to build glucose. The Calvin cycle happens in the stroma of
the chloroplast. This is the open space outside of the thylakoids. There a numerous molecules and enzymes
floating around here but no chlorophyll molecules. For this reason, the Calvin cycle cannot be directly powered
by sunlight. Sometimes the Calvin cycle is referred to as the dark reactions. This simply means it doesn’t need
light to happen, not that it only happens in the dark.
Like the light reactions, there are a number of players in the Calvin cycle that don’t show up in the summary
reaction. They, too, are molecules that are permanent fixtures in the chloroplasts and are recycled back to their
original form by the end of the process. The major players are: CO2, ATP, NADHP, and RuBisCO.
Step 1: Carbon
Fixation
Step 3:
Regeneration
of RuBP
Step 2: Energy
Investment
phase
Figure 5 The Calvin Cycle
Once again we’re going to break this down into steps. Follow the steps in the diagram above as we go through
the descriptions below. Just a note: the molecules in this cycle have been simplified to show JUST the carbons
and the phosphates. Each one is really composed of C, H, O, and P.
Step 1: CO2 is incorporated into an organic compound with the enzyme RuBisCO.
Remember from our biochemistry unit that an organic compound is one that contains C and H. CO2 doesn’t
contain H so it is considered inorganic. The first step of the Calvin cycle is incorporating three molecules of CO2
into an organic molecule already present in the chloroplast. This process of turning inorganic carbon into
organic carbon is called carbon fixation. Most of the carbon fixation in the world is done by the enzyme
RuBisCO and is therefore RuBisCO is estimated to be one of the most abundant proteins on the planet.
Each step of the Calvin cycle requires several chemical reactions that allow atoms to be rearranged. For step 1,
we start with 3 molecules of a 5 carbon compound (15 carbons total). We then add 3 CO2 molecules and
rearrange the atoms into 6 molecules of a 3 carbon compound (18 carbons total). Once we have incorporated
the CO2 we can move on to step 2.
Step 2: “Charge” the carbon compounds with the ATP and NADPH generated during the light reactions
Building sugar is a highly endergonic process. Energy needs to be invested. That energy is going to require a
combination of energy coupling and electron transfer. Let’s start with the energy coupling. Remember how this
works: highly exergonic reactions that release energy are paired with endergonic reactions that absorb energy.
What is the most useful exergonic reaction we have? The hydrolysis of ATP!!! By breaking off that third
phosphate we free up a huge amount of energy that can be used to build the higher energy carbon compounds.
Where did this ATP come from? The light reactions!
But ATP alone cannot provide all the energy for these reactions. Electron carriers also participate. As we build
these organic compounds, NADPH will donate its higher energy electrons to the carbon compounds. NADPH will
return to its original NADP+ state and the carbon compounds will gain the energy of the electrons. At the end of
step 2 we will have 6 molecules of a three carbon compound called G3P. One G3P will leave the carbon cycle
and will be used to build glucose, and the other 5 will move on to step 3 of the Calvin cycle.
Step 3: Rebuilding the CO2 acceptor, RuBP to repeat the cycle
The last step of the Calvin cycle is really about resetting the molecules so they are ready for another round. The
remaining molecules of G3P will be rearranged back into 3 molecules of the 5 carbon compound, RuBP. This is
the same molecule that accepted the 3 molecules of CO2 in the very beginning. This will require some input of
energy (ATP) but benefit of being able to incorporate more CO2 in future rounds of the Calvin cycle far exceeds
the small expense.
At the end of the Calvin cycle we managed to create one molecule of G3P out of 3 CO2 molecules. Clearly this is
not glucose, BUT G3P from multiple rounds of the Calvin cycle can be combined into glucose. This glucose can
then be used immediately (and by “used” we mean run through cell respiration to transfer the energy to ATP) or
it can be stored as starch for later use. Either way, we have completed the process of photosynthesis and
successfully built glucose using energy from the sun.
Discussion questions: Use a combination of past material, the material in this packet and your brilliant minds to
answer the following questions IN YOUR OWN WORDS.
1. What is carbon fixation and why is it important to living things?
2. Describe in general what it means to “couple reactions.
a. How specifically are reactions coupled in the Calvin cycle?
3. How many rounds of the Calvin cycle would be needed to generate a starch molecule that is 10 glucoses
long?
4. What happens to the ADP and NADP+ after the Calvin cycle?
5. For each step of the Calvin cycle, write a one sentence summary that encompasses the PURPOSE of the
step and any key molecules that might be involved (by KEY we mean CO2, G3P, RuBisCO, ATP, and
NADPH)
a. Step 1:
b. Step 2:
c. Step 3:
Complete the following Comparison table:
Light Reactions
Calvin Cycle
Which reactant(s) from the
summary reaction are used here?
Which products(s) from the
summary reaction are made here?
Where in the chloroplast does it
take place
What is the energy SOURCE?
What is the purpose?
What is a key enzyme used in this
phase?
Students are doing an experiment in which they are testing the effect of different variables on the rate of
photosynthesis in plant cells. For each of the experimental change below, describe whether the change will
result in an increase or a decrease in the rate of photosynthesis, whether it specifically affects the light
reactions, the calvin cycle or BOTH and an explanation why.
1. Instead of using cells from the leaf, students use cells from the stem of the plant.
a. Effect on rate of PS?
b. Phase(s) of PS affected?
c. Explanation:
2. Instead of using cells from a moist environment they use cells from a dry environment whose stomata are
closed
a. Effect on rate of PS?
b. Phase(s) of PS affected?
c. Explanation:
3. Instead of using a 100W light bulb, students use a 60W light bulb (both white light)
a. Effect on rate of PS?
b. Phase(s) of PS affected?
c. Explanation:
4. Instead of using a red light, students use a blue light of equal intensity.
a. Effect on rate of PS?
b. Phase(s) of PS affected?
c. Explanation:
5. Instead of using cells at room temperature, students boil the cells first
a. Effect on rate of PS?
b. Phase(s) of PS affected?
c. Explanation:
6. Instead of using cells grown in your average atmosphere, cells are exposed to higher levels of CO2
a. Effect on rate of PS?
b. Phase(s) of PS affected?
c. Explanation:
7. Instead of using normal cells, students use cells that were exposed to a RuBisCO inhibitor.
a. Effect on rate of PS?
b. Phase(s) of PS affected?
c. Explanation: