Hey, folks! Welcome back to BOGO biology! This week we're going to explore energy
transformation by comparing the processes of photosynthesis and cellular
respiration. Here's what the AP bio curriculum has to say on the subject.
They want you to know that, quote, "biological systems utilize free energy
and molecular building blocks to grow to reproduce and to maintain dynamic homeostasis."
Now, all of this is true, but unless you can support this statement
with evidence, it's really pretty useless. (And also nobody talks this way...)
So, don't go anywhere, because we're going to be unpacking the processes of
photosynthesis and cellular respiration in order to help you understand them better.
To begin with remember that all organisms require a constant input of energy.
However, they differ in terms of how they get it. Most organisms do
photosynthesis, cellular respiration, or both, though there are a few that buck
the trend and do something called chemosynthesis, which does not require
sunlight. In photosynthesis, the goal is to store energy, and in cellular
respiration the goal is to release the energy for use. Today we'll begin digging
into which organisms do each process, the reagents and products, where each set of
reactions takes place, and the major steps involved in each type of energy
conversion. Most people know that plants do photosynthesis and that animals
undergo cellular respiration, but this is actually an incomplete picture. A number
of different kinds of organisms can do each process, and some of them can even
do both. For instance, plants use cellular
respiration when they're germinating from their seeds and don't have any
leaves yet. Now let's talk reagents and products. One of the most important
molecules in biology is a rechargeable reusable energy source called
ATP. The energy stored in ATP's bonds powers the vast majority of cellular
processes, but in the process of powering them, it gets reduced to its low energy
form called ADP. However, through a complex series of chemical reactions and
a little glucose, we can actually recharge the ADP molecule back into ATP.
We call this recharging process "cellular respiration". Another way to think about
this is that photosynthesis depletes our supply of ATP,
whereas cellular respiration recharges the molecules. Looking at the reagents
and products often helps make the relationship seem more clear.
In photosynthesis, we put sunlight, ATP, carbon dioxide, and water in special
compartments in the chloroplasts. Photosynthesis uses these reagents to
convert the carbon from CO2 into a sugar called glucose in a process known as
carbon fixation. We also produce oxygen as a waste product. Now let's compare the
reagents and products of photosynthesis to those of cellular respiration.
In cellular respiration, the reagents are glucose and oxygen, and the process takes
place in the cytoplasm and the mitochondria, rather than in the
chloroplasts. The process produces ATP, carbon dioxide, water, and a little bit of
heat. Notice how the products of one set of
reactions become the reagents in the other. This is exactly what happens in an
ecosystem. Here's a quick look at our chart so far.
Next up, we'll tackle the location of the
two processes. As we have previously mentioned, photosynthesis
takes place in the chloroplasts and respiration takes place in both
cytoplasm and the mitochondria. There's more to it than that, though. Both the
chloroplasts and the mitochondria have inner and outer compartments that are
crucial to the reactions functioning correctly. In the chloroplasts, the inner
compartment is called the thylakoid, and the outer compartment is called the
stroma. In the mitochondria, the inner compartment is called the matrix, and the
outer compartment is called the inter membrane space. Here's what a chloroplast looks like.
Thylakoids are pigment-filled discs stacked up on top of each other. Under a
microscope, they do look a little bit like pancakes. The stroma is a fluid
filled space surrounding the thylakoids. The membrane that separates these two
structures is a crucial part of photosynthesis. This membrane is studded
with several key proteins including light absorbing complexes called
photosystems, and a protein called ATP synthase. Let's check out the membranes
in the mitochondria, and see if we can spot any similarities. This membrane is
also straddled by key proteins including our good friend ATP synthase. If we look
a little bit more closely, we will see that the ATP synthase protein works closely
with something called the proton pump. The pump transports protons from one
side of a membrane to another in order to create a gradient. In order to travel
back to the other side of the membrane and re-establish equilibrium, the protons
have to flow through the ATP synthase. When they do, they reattach inorganic
phosphate to ADP in order to create a newly charged molecule of ATP.
Harnessing protons and ATP synthase happens in both photosynthesis and cellular respiration.
We call this process chemiosmosis. Here's another look at our table.
To finish up, we're going to go over the major steps in each process. We'll start
by splitting up photosynthesis into the "photo" part and the "synthesis" part. The
"photo" part is going to encompass the light dependent reactions, and the
"synthesis" part is going to be the light independent reactions also known as the
Calvin cycle. The reagents for the light dependent reactions are photons of light,
and water. These reactions generate ATP, NADPH, and some oxygen. The oxygen passes
into the atmosphere, but we use the ATP and NADPH in the light independent
reactions. The light independent reactions perform a process called
carbon fixation, where they take carbon from carbon dioxide in the atmosphere
and transform it into a sugar. The sugar produced is a three carbon molecule called
PGAL, and it's essentially half a glucose molecule. Now let's shift over to
talking about respiration. When oxygen is present, aerobic respiration is capable
of generating large amounts of ATP. The three major steps are glycolysis, the
Krebs cycle, and the electron transport chain. Glycolysis runs on glucose as well
as an electron carrier called NAD+, and we also need to kick-start the
reaction with a little bit of ATP. Glycolysis produces a molecule called
pyruvate, a little more ATP, and some recharged electron carriers called NADH
that we'll use later on. The Kreb's cycle produces a little bit more NADH, and it
also produces another electron carrier called FADH2. The Krebs cycle is kept
turning by the input of carbon atoms from glycolysis, and it also kicks out
a couple more molecules of ATP and some carbon dioxide. Finally, the electron
carriers deliver their payload to the electron transport chain.
The electrons trigger the same process we saw earlier. The electrons carried by
NADH and FADH2 trigger the pumping of protons from the inner to the outer
compartments inside the mitochondria. These protons then flow back into the
inner compartment through the ATP synthase, generating ATP.
Oxygen serves is the final electron acceptor in this process, and the
electron transport chain also kicks out a molecule of water. So, to summarize,
these are both very complicated processes, but they actually do have a
lot of things in common. Hopefully this video has given you a
better idea of how to compare and contrast these two processes, and about
how different organisms harvest energy from their environment. That pretty much
wraps up this week's video, but I hope you'll check out some of my other work
on Instagram, Twitter, and of course, YouTube. If you'd like to learn more
about photosynthesis, click here. And, if you like to learn more about cellular
respiration, click here. Thanks again for watching, and please remember to
subscribe!