Welcome back, everyone! In this video, we'll be discussing photosynthesis.
Today, we'll be covering what photosynthesis is, why it occurs, key processes, and of
course all of the steps. Before we start, two important notes about this video...
First, a heads up that these processes are going to be pretty complex and we're
going to be creating diagrams that look like this and like this. Not everyone
loves drawing as much as I do, so I've created a custom "BOGOnotes" study guide
that is available at the link in the description below. It includes
note-taking templates that match the video, standards alignment for teachers, a
vocabulary bank and, of course, an answer key. This is an experiment and I hope
that you'll check it out and let me know what you think of these notes by leaving
a comment below. Second we should know that this video covers C3 photosynthesis.
Desert plants use different variations of this process known as C4 and CAM
photosynthesis. I have an entire video dedicated to the details of the C4 and
CAM processes. Definitely check it out if that's what you're looking for.
Okay let's get started! In photosynthesis, an organism uses solar or light energy
carbon dioxide, water, and ATP to generate glucose (a very useful sugar) and oxygen.
Note the similarities and differences between photosynthesis and cellular
respiration. With a few tweaks, the products of photosynthesis become the
reagents of cellular respiration and vice versa. In eukaryotes, photosynthesis
takes place inside organelles called chloroplasts. Chloroplasts have both
inner and outer compartments and a portion of photosynthesis takes place in
each one. Photosynthesis is divided into two major processes; the light dependent
reactions and the light independent reactions. The light dependent reactions
are also known as just the "light reactions" and the light independent
reactions are also sometimes called the "Calvin Cycle". The inner compartment
consists of stacks of pigment filled disks called "thylakoids". The first part
of photosynthesis, the light reactions, occurs here. The Calvin Cycle, the second part of
photosynthesis, takes place in the fluid-filled outer compartment which is
known as the "stroma". We'll start with the light dependent reactions.
During the light dependent reactions, we use photons of light and water to
recharge molecules of ADP into ATP, and also reload molecules of NADPH. ATP and
NADPH are very important molecules because later on we'll use them to
manufacture sugar. The light dependent reactions also generate oxygen as a
waste product. It's pretty amazing to think that such
an important gas is really just an afterthought. During the light
independent reactions, we incorporate carbon dioxide and generate sugar.
Let's look at the light dependent reactions a little bit more closely.
The thylakoid membranes are full of proteins, all of which play different roles. The major
proteins we'll be discussing are as follows: Photosystem II, Plastoquinone,
Cytochrome B6F, Plastocyanin, Photosystem I, Ferredoxin, Ferredoxin NADP Reductase,
and finally ATP Synthase. That's a lot of proteins, but don't worry we'll work
through them in a (somewhat) manageable way. You may already be wondering "WTF, why is
Photosystem II appearing before Photosystem I?" That's actually not an
error, it's because Photosystem I was discovered first, even though it's not
actually the first photosystem to be used in the light-dependent reactions.
The first step in the light dependent reactions is photoexcitation. Photons of
light kick off a process called "photolysis". This is a light induced
splitting of water that produces three important components; oxygen, protons and
some low energy electrons. The photons have to pass through the chloroplast's
stroma before they can reach the lumen where photolysis takes place. The oxygen
is a waste product, and it diffuses out through the plant's stomata. So, we've
already taken care of that particular product of the light dependent reactions.
The protons accumulate on the inner side of the thylakoid membrane, building up a
powerful gradient that will be used later on in this process. The electrons
will be sent through a series of proteins called the "electron transport
chain". Photons of light from the sun also strike two photosystems PS II and PS I.
Photosystems are light harvesting complexes that absorb light of a
particular wavelength. Photosystem II contains a pigment called p680,
which absorbs light of 618 nanometers. Photosystem I contains a
different pigment called p700 which absorbs light of 700 nanometers. Next up
is the electron transport chain, part 1. The high-energy "electron" is then
"transported" through a "chain" of proteins from Photosystem II down to Photosystem I.
Appropriately we call this next step the "electron transport chain". At every
step, the electron's energy level is reduced, and this energy is harvested and
used to do useful things. First, the electron is transported to a protein
called Plastoquinone. Plastoquinone works much like a conveyor belt; it
delivers the electron to the next protein, Cytochrome B6F, then to
Plastocyanin, and finally to Photosystem I. Cytochrome B6F is unique among these
electron transport proteins because it's a "proton pump", not just a conveyor belt.
When energized by the electron, Cytochrome B6F sucks a proton out of the
stroma and into the thylakoids. This creates a powerful concentration
gradient, but since the pump only works one way, the proton can't reverse course
and return to the stroma. The "exhausted" electron from the Plastocyanin replaces
the high-energy electron that will move on, so Photosystem I has plenty of
electrons. Now we're in the electron transport chain part 2. The new
high-energy electron moves through another protein called Ferredoxin and
then to the enzyme Ferredoxin NADP Reductase. We can tell this last protein
is an enzyme because it contains the ending "ase". Next we're going to recharge
NADP+. Derredoxin NADP Reductase loads NADP+ up with two electrons and a
hydrogen also attaches to the molecule as well. NADPH is known as a
"mobile electron carrier". Its job is to move hydrogen and electrons from one place to
another. Each NADPH molecule delivers a proton
and two electrons to the Calvin cycle where they're needed. Then, the "empty"
NADP+ molecule returns back to pick up some more. Years ago one of my students
compared it to a truck; it picks up a load in one location, transports it
somewhere else, drops off the items, and then returns for
a new load and does it all over again. In honor of my biology students from the
2013 to 2014 year, I've continued to draw NADPH as a dumptruck ever since.
Next we're going to recharge some ATP. The chloroplast then exploits
the powerful proton gradient we've been creating with the protons from
Photolysis, and the protons from Cytochrome B6F.
There are now many protons and a strong positive charge on the inside of the
thylakoids compared to the outside. The protons want to move down their
electrochemical gradient, a process we call "chemiosmosis". They pass through a
protein called ATP Synthase in an attempt to re-establish equilibrium.
This "recharges" ADP into a high-energy ATP by reconnecting a molecule of
inorganic phosphate to it. We call the recharging of ATP "phosphorylation".
The charged ATP is now in the stroma and ready to be sent on to the Calvin Cycle
along with the NADPH. To recap, so far we've harnessed photons of light and used
it to break up molecules of water, stimulated photosystems,
we loaded up molecules of NADPH, recharged ATP, and created oxygen as a
waste product. This brings us to the second phase of photosynthesis; the light
independent reactions, also known as the Calvin Cycle. The Calvin Cycle is the
"synthesis" part of photosynthesis, where we assemble important products.
In the Calvin Cycle, we'll use some of the products of the light dependent
reactions, specifically ATP and NADPH, and we'll add in carbon dioxide and water.
The goal of this whole process is to finally generate sugars. Dirst we're
going to insert all the reagents and connect them to either the light
dependent reactions or to the Calvin Cycle. Photons are very important in
creating the products that drive the Calvin Cycle, but they aren't directly
involved in the process. Because of this, we will connect our photons of light to the
LDR like this. The next reagent, water, will be crucial in both the light
dependent reactions and in the Calvin Cycle. It comes from the soil, enters
the plant through the roots, and travels through special tubes called "xylem".
6 water molecules will be added to the Calvin Cycle in the first step.
The hydrogen and oxygen atoms in it will become part of the molecules that we're
creating. The carbon dioxide comes in from the air and enters via tiny gated
openings in the leaves called "stomata". It's also added to the Calvin Cycle in
the first step. CO2 is an important reagent because it's a
key source of carbon, which is necessary for the production of sugars.
Each molecule of carbon dioxide contains only 1 carbon atom and we need to add
6 of them to the Calvin cycle. As we go through the cycle, notice how many
carbons are involved in each step. They're indicated by small dark circles
with a letter "C" in the middle. The Calvin cycle begins here with a molecule called
RuBP. Since its full name is "Ribulose 1-5 BiPhosphate", you can probably
guess why we're going to stick to calling it "RuBP". 1 molecule of RUBP
contains 5 carbons. To run the Calvin Cycle, we're going to use 6 molecules of
RuBP with 5 carbons each. This is a total of 30 carbons. Combining 5-carbon RUBP
with the single carbon from CO2 yields an intermediate with 6 carbons. This
addition is done by an enzyme called Rubisco. Like RuBP Rubisco is also a
nickname. It's "legal name", if you will, is "Ribulose 1-5 BiPhosphate Carboxylase/Oxygenase".
This process of adding carbon dioxide is also known as
"carbon fixation", where inorganic carbon from the atmosphere is incorporated into
an organic molecule. Notice how we still have the same number of carbons before
and after this step, we haven't magically created or lost any. The 6 RuBP
molecules contain 30 carbons, and the 6 CO2 molecules contain 6 carbons in all.
When we add them together, we still have 36 carbons but now arranged in 6
groups of 6. The 6-carbon intermediate is highly unstable, and it
breaks apart almost immediately after forming. The resulting 12 molecules are
called "PGA" which is short for "Phosphoglyceric Acid", and they each have
3 carbons. Again note that we haven't mysteriously lost or gained any carbons.
We had 6 times 6 carbons in the previous step, and now we have 12 times 3 carbons
for a total of 36 each time. The PGA molecule that we use to make glucose
also has 3 carbons like PGA, so we're definitely getting closer. To turn PGA
into PGAL, we have to rearrange yet again. This is going to require a boost
of energy from ATP and we're also going to add in a proton and 2 electrons. These
ingredients might sound familiar from earlier in the video, because
this is where we are going to use the products of the light
dependent reactions. Remember also that the oxygen has already diffused out
of the plant through the stomata, so it's not going to be added into this mixture.
12 ATP provides the energy and 12 NADPH each provide a proton and the two
electrons. The ATP reverts to low-energy ADP and then returns to the light
dependent reactions to be "recharged". The NADPH reverts back to NADP+ and also
returns to light dependent reactions for another load. When we combine the 12 PGA,
the electrons, the protons, and charge it with energy, we get 12 PGAL.
PGAL can be converted into one glucose molecule. However, we won't be spending
all of our PGAL here. Remember that the Calvin Cycle is a "cycle"; it comes back to
where it started. We're going to spend 2 of the 12 PGAL to make one glucose,
and then we're going to "reinvest" the other 10. The 10 PGAL contained 30
carbons in total. Remember that we started the cycle with 30 carbons.
However, the process started with 6 groups of 5, not 10 groups of 3.
With a burst of energy and a LOT of complex chemistry, we can rearrange the
molecules. The reshuffling is extremely complicated, so we'll simply refer to
this step as "10 PGAL Get Crazy". The energy needed for this crazy chemistry "dance"
comes from molecules of ATP from the light dependent reactions.
After releasing its energy, the low-energy ADP molecule is sent back to recharge. In the
final step of the Calvin Cycle, we convert 10 3-carbon PGAL back into 6
5-carbon RuBP, and the Calvin Cycle can begin again, of course
provided that we keep the key reagents coming in. So there we have it!
We've taken photons of light, carbon dioxide, water, and some ATP and converted it into
glucose and oxygen. Photosynthesis is definitely complicated, but without it we
wouldn't be able to exist.
Thank you so much for watching everyone! I hope this
video was useful; I had a great time making it and I definitely put a lot of
time and effort into this one. The reason that I made it is because someone
suggested it a couple of months ago and I've been meaning to do it for a long
time, so when you leave comments I actually do read them and respond to
them and I really appreciate hearing from all of you guys all over the world.
So, maybe consider subscribing to my channel, or checking out some of my
other videos. I'd appreciate it and I hope that you guys are all doing well
and are safe wherever you are. Take care, bye!