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Photosynthesis: The Light Reactions and The Calvin Cycle

October 13, 2019

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!

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  • Reply Parmvir Singh January 14, 2019 at 1:31 pm

    Wow great

  • Reply Rafael January 14, 2019 at 3:18 pm

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    I like the idea of study guides, do you do videos on Honors Chemistry? Or AP Chemistry?

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    Great videos!:)

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  • Reply AwaitingTheDrop February 24, 2019 at 8:36 pm

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  • Reply Savannah Weaver April 4, 2019 at 8:04 pm

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  • Reply Arthur Greenbull August 19, 2019 at 8:27 am

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