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PHOTOSYNTHESIS – the complete sentence version
Photosynthesis is a complex yet important process. Organisms that can do it, called autotrophs, are
the only ones capable of converting the sun’s energy into a form that heterotrophs can use. That means that
almost all organisms on earth get all of the energy in the food they eat because of photosynthesis. The overall
goal of the process is to make sugar using the energy of the sun. The overall chemical equation for the process
is:
6CO2 + 12H20
SUN’S ENERGY
C6H12O6 + 6O2
The action of photosynthesis happens within the cells of plants, algae, some protists, and bacteria. Three
out of four of these types of organisms are eukaryotes and carry out this essential process within the cellular
organelles (parts of the cell) called chloroplasts (we’re going to avoid bacterial photosynthesis). Chloroplasts
are compartments within the cell that have a phospholipid bilayer membrane. Inside of this compartment are
stacks of other compartments. Each of these smaller spaces is called a thylakoid and has its own phospholipid
bilayer membrane (they’re kind of like a bunch of small pita breads stacked inside of one large pita bread).
There are two main parts to photosynthesis: the Light Dependant Reactions (LDR) and the Calvin
cycle (sometimes called carbon fixation). The Calvin cycle is where the sugar actually gets made and can be
thought of as a factory producing a product. All factories need an input of raw materials, energy, and some type
of machinery to produce their outputs (desired and waste). The Calvin cycle is not able to use the sun’s energy
directly and relies upon the LDR to provide power (like the power plants to which all factories must be
connected).
The LDR take place on and within the thylakoid membrane. Each small compartment has many (1000s)
of repeats of the set up seen below. The area below the membrane is the inside of the thylakoid and above is the
outside.
(see next page for a more detailed diagram.)
The blobs in the membrane marked with arrows are (from left to right) Photosystem II (PII), Photosystem I
(PI), and ATP synthase. All of these blobs are either protein mixtures or pure protein (ATPase).
Plastoquinone (PQ) is the electron acceptor for Photosystem II. Ferrodoxin (FD) is the electron acceptor for
Photosystem I. The photosystems are where the chlorophyll of a photosynthetic organism is located.
Chlorophyl is special because when it is hit by light its electrons can capture the light’s energy and get
“excited.” The set up in the thylakoid is special because it allows for the harnessing of the energy of these
excited electrons.
When the sun is shining on a plant (a good example) the sun is hitting the photosystems of the plant
cell’s thylakoids. This excites the electrons. The machinery between PII and PI takes those electrons and uses
their energy to pump lots of Hydrogen ions (H+) into the thylakoid. The electrons that leave from PII are
replaced by electrons taken from water, thereby releasing oxygen. The electrons are then passed into PI where
they get energized by more sunlight. The molecules to the right of PI take these excited electrons and pass them
off to a molecule known as NADPH. This molecule will act as a shuttle to take these high energy electrons to
where the Calvin cycle is taking place in order to provide energy.
The electrons carried by NADPH are not the only molecule that the sun’s energy is converted into.
Remember that there are lots of H+ on the inside of the thylakoid as a result of sun powered pumping. These
are dying to get out in order to reach equilibrium. They can’t get through the membrane because they are
charged, BUT ATP synthase is a protein pump that allows them through. It’s a special channel though,
because as H+ move through it, it spins like a waterwheel and that spinning energy is used to make ATP (hence
the name ATP synthase, because it synthesizes [makes] ATP). ATP is the other energy containing molecule
that the Calvin cycle will use as a source of energy to make sugar.
The Calvin cycle (a.k.a. the dark reaction or light independent reaction) takes place in the liquid outside of
the thylakoids, but inside of the chloroplasts. This is called the stroma of the chloroplast. Lurking out here are
a bunch of enzymes (protein machines that do jobs inside of cells) that will make the sugar. The overall
scheme of the process looks like this:
CO2
Rubisco
RuBP
3-Phosphoglycerate
ATP
SUGAR (G3P)
ATP
NADPH
What’s happening is that the enzyme Rubisco is taking CO2 from the atmosphere and combining it with RUBP
and making 3-Phosphoglycerate. The resulting molecule is then split and rearranged using the energy
contained in ATP and NADPH (where’d they come from?) to make sugar (G3P, which can be used to make
glucose) and another molecule of RUBP to start the cycle again. The sugar is used by the plant as an energy
source to carry out endergonic reactions and as a building block to make other larger molecules that it needs.
PHOTOSYNTHESIS – the outline
The following descriptions are meant to accompany and help explain the notes you took on the
diagrams/animations of the light reactions and Calvin cycle. The descriptions move in sequence from the first
diagram to the last. BE AWARE THAT ALL THREE OF THESE THINGS ARE HAPPENING AT ONCE IF
LIGHT IS HITTING THE PLANT.
LIGHT DEPENDANT REACTIONS
1. Photosystem II - The photosystem itself is a complex of chlorophyll molecules.
a. Light hits the photosystem
b. An electron gets excited by this light energy
c. This high energy electron leaves the photosystem and moves to the hydrogen pump next door
(B6-f complex).
d. The electron’s energy is used to pump hydrogen ions from the stroma of the chloroplast into
the thylakoid interior
e. Water is split to replace the electron lost from the photosystem, this releases oxygen (a waste
product).
f. Animation links
http://www.stolaf.edu/people/giannini/flashanimat/metabolism/photosynthesis.swf
http://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120
072/bio13.swf::Photosynthetic%20Electron%20Transport%20and%20ATP%20Synthesis
highly detailed description of Light reaction
2. Photosystem I
a. Light hits the photosystem
b. An electron gets excited by this light energy
c. This high energy electron leaves the photosystem and moves through a complex of molecules
and enzymes that attach the electron to a molecule of NADP+ making NADPH
d. The NADPH is a shuttle for these high energy electrons (they’ll be used in the Calvin cycle).
3. Animation links: see 1st link above
4. ATP generation – using ATP synthase
a. Hydrogen ions are built up in high concentration in the thylakoid by the workings of
Photosystem II.
b. This concentration gradient means that those H+ really want to get out.
c. Unfortunately, being charged particles the thylakoid membrane won’t let them out.
d. The only place through which they can pass is ATP synthase. ( a protein pump)
e. The movement of the hydrogen through ATP synthase causes it to spin.
f. This spinning energy chemically bonds ADP and P together to make ATP (to be used in the
Calvin cycle).
g. Animation links:
http://www.stolaf.edu/people/giannini/flashanimat/metabolism/atpsyn1.swf
http://www.stolaf.edu/people/giannini/flashanimat/metabolism/atpsyn2.swf
CALVIN CYCLE (CARBON FIXATION) a.k.a The Dark Reaction
1. Takes place in the stroma of the chloroplast.
2. Rubisco (enzyme) takes carbon dioxide out of the atmosphere.
3. This is combined with RuBP to make 3-Phosphoglycerate (3 PGA)
4. ATP and NADPH from the light dependant reactions power the splitting and rearranging of 3Phosphoglycerate.
5. The splitting and rearranging results in glyceraldehyde 3-phosphate (6-G3P) Sugar (for the plant to
use for energy and building other molecules) and RuBP (to start the next round of the Calvin cycle).
6. There are lots (1000s+) of the enzymes responsible for sugar production in a chloroplast.
https://highered.mheducation.com/sites/9834092339/student_view0/chapter39/calvin_cycle.html
There is a quiz below the animation-give it a try! You never know when these questions may show up
again!
Figure 8-10
The light reactions involve two photosystems connected by an electron transport chain.
http://www.youtube.com/watch?v=g78utcLQrJ4
Bozeman science photosynthesis tutorial video
Online Activity 8.1 pages 2-4
Page 2
What structure transports food and water?
What is the name of the opening in the leaf that allows exchange of gases?
What are the photosynthetic cells in the interior of the leaf called?
What is the outer protective cell layer called?
What structure allows for rapid diffusion of gases?
Page 3-Label the parts of a plant cell
Page 4 Label the parts of the chloroplast
Online Activity 8.2 –page 3, closer look (found on bottom of page)
Watch the animation a few times to fully understand what occurs during the light reaction of
photosynthesis
Besides oxygen, what two molecules are produced by the light reactions?
Where in the chloroplast do the light reactions take place?
Water (H2O) is split in photosystem II to form two protons (H+), two electrons (e–) and oxygen (O). What roles do each of
these products play in photosynthesis?
Online Activity 8.3
How many 3-PGA molecules are produced from each reaction of RuBP with CO2?
What type of molecule is rubisco?
Name the high-energy molecule that is required for the regeneration of RuBP?
What is the primary function of the Calvin cycle?
What are the inputs and outputs of the Calvin cycle?
What molecule is the direct product of photosynthesis? How is that molecule then used by plant cells?
The light reactions, which take place in the thylakoid membranes, convert light energy to the chemical energy
of ATP and NADPH. The light reactions use the reactant water from the equation and release the product
oxygen. The Calvin cycle, which takes place in the stroma, uses ATP and NADPH to convert carbon dioxide to
sugar. By converting light energy to chemical energy, photosynthesis is the first step in the flow of energy
through an ecosystem.