Photosynthesis: Fun in the Sun

>>Hello, and welcome to
the “Penguin Prof Channel”. In today’s episode, I’m going to
try to explain photosynthesis. It’s tough. This is a tough subject. So I need you to take a
nice, deep breath and relax. And now think about where
did that oxygen come from. If you said plants,
you’re on the right track. If you said rainforests, that’s where most people
think their oxygen comes from. About twenty percent
of the oxygen that you’re breathing right
now was made by plants from a rainforest, but most of the oxygen was not
made by plants at all. So who’s doing all
the photosynthesis? Well, it’s happening
in the oceans by little tiny microscopic
critters called phytoplankton. All these little green blobs. If you blow them up, they’re
actually really pretty. Photosynthesis allows life
as we know it to exist. Photosynthesis produces
oxygen that we need to breathe and produces carbohydrates
that we use to burn for fuel. There’s two parts of
photosynthesis, and luck for us, there’s two parts to
the word photosynthesis. The word photo, the photo
part is about the capturing of light energy into
chemical bond energy. The synthesis part
of photosynthesis is about the production
of carbohydrates. So the photo part is going to
involve light energy captured by photosynthetic pigments of
the plant, and these photons of energy will be converted
to chemical bond energy in the form of NADPH and ATP. If these molecules and
the concepts of oxidation and reduction are really
foreign to you, please check out my video on redox reactions. I will put the link in
the description box below. These high-energy molecules
are going to be used as energy sources to fix carbon, atmospheric carbon
into carbohydrates. Not into glucose directly
but into molecules that can be used
to make glucose. So basically what we do is we’re
going to divide the processes of photosynthesis into
the light-capturing part, which conveniently are
called the light reactions, and then the carbon
fixation part, which we refer to as the Calvin cycle, or the
light-independent reactions. So let’s talk about light first. Here’s the electromagnetic
spectrum. There’s a teeny tiny
little slice in there that is actually visible to
us, and that’s the same sliver that plants can use
for photosynthesis. Now how plants capture this
light is with pigment molecules like chlorophyll A. Here’s
chlorophyll B. There are other pigments, though. This is beta carotene. Beta carotene is what gives
things like carrots their color. Now, what you can do is look
at the absorbance spectra of different pigments, and
you’ll see that they all peak in different places, and this
actually allows photosynthetic organisms to be very
efficient in their capturing of different wavelengths, but most photosynthetic pigments
have a low in absorbance right around here, and that, as you
can see in the color bar below, corresponds to the color green. So plants do not use green
light very efficiently at all, and that is why green is
transmitted and reflected, and that’s why plants
appear green. You can actually extract
chlorophyll yourself in your own kitchen. I like to use spinach. You’re going to need a
blender and some cheesecloth, and you’re going to need to
blend it up not in water, that won’t work, but
in an organic solvent like pet ether, acetone,
methanol. I know you got that
in your kitchen. Don’t drink this smoothie. If you filter it through the
cheesecloth, you’re going to get a nice solution of
chlorophyll, and what you can do with it is shine light
on it, and you’re going to see something really cool. It’s going to glow red, and
this is called fluorescence, and in order to understand
the light reactions, it’s actually important that
you understand why chlorophyll fluoresces when you shine
light on it like this. So here’s what’s happening. If you isolate a chlorophyll
molecule, and you shine light on it, what’s going to happen
is electrons at the center of the chlorophyll are going to
increase in their energy state. They’re going to get excited. Ooh, I like that. It’s very exciting. Now, as you know, anything
that goes up must come down, and in isolated chlorophyll
molecule, there’s nowhere for those electrons to go. So as soon as they get excited,
they immediately fall back down to what we call the ground
state, and when they do that, they give off some energy in
the form of heat and light. And that light, we
call fluorescence, and that’s why it glows. Now if you go out
into the world, hopefully you have never seen
a glowing plant unless you take some very interesting
medication. So plants don’t naturally
fluoresce, and the reason is
plants capture the energy of the electron before
it falls back down. So you have the striking
of the chlorophyll, and just like before,
the electrons in the chlorophyll
molecule, they increase in their energy state, but
instead of being allowed to fall back down and having
all that energy wasted, the chloroplast has proteins
which capture the energy of the electron as it falls,
kind of like water going down a, a water wheel, and that energy
is then used to do work. And that’s basically what the
light reactions are about. These chlorophyll molecules
are actually organized in a complex system that we call
a photosystem, and the photons of light actually bounce around until they hit
a reaction center, and the whole thing is very
beautiful and complicated. This is what is actually looks like with all those green
blobs representing chlorophyll in there, and it’s
actually quite beautiful. In order to understand how
this works, we’re going to have to understand the
structure of a chloroplast. A chloroplast is an organelle
with not one but two membranes, an inner and an outer membrane,
and then inside, there’s going to be fluid that we call
stroma, and that’s where all of the carbon fixation
is going to happen. And then most of the
chloroplast is actually made of yet more membranes that
we call thylakoids, and they’re organized in
stacks that we call grana. So this would be one granum,
and in the thylakoid membranes, this is where all of the light
reactions are going to occur. So we’re going to talk about
the light reactions first, and then later we’re going to
talk about this carbon fixation. OK, so the first
thing that we’re going to need is a source
of electrons. The electrons come from water. Believe it or not, photosynthetic organisms
use sunlight to split water. So water molecules are going
to be split, and, of course, you’re going to get out of that
oxygen, hydrogen, and electrons. This is called photo oxidation. What’s going to happen is
the electrons are going to fuel the entire system
of the light reactions. So we’re going to watch the
electrons here in just a second. We’re also going to get
oxygen, and this is the source for atmospheric oxygen, and
you’re going to get protons, and the protons are
going to contribute to the proton gradient in the
thylakoid lumen that we’re going to see in a little bit. That’s going to be
used to make ATP. OK. So let’s see what
happens to the electrons. When light strikes photosystem
two, it’s going to get bounced around until it hits the
reaction center, and, there, the electrons in the center
are going to be increased in energy [inaudible],
just like we saw before, but instead of the electron
just falling back down and fluorescing, that electron’s
energy is going to be captured by proteins in the thylakoid
membrane called the electron transport chain. Kind of like water wheels. As the electron passes
through these proteins, the energy in the
electron is going to be harnessed to do cell work. So the electron is going to
fall through these proteins, and the energy is going
to be used to pump protons against their gradient
into the thylakoid lumen. So we’re creating a
gradient of protons here. Now another photon of
light strikes the center of photo system one,
and that’s going to re-energize the same electron
to a high-energy state again. Where the electron ends up is in what’s called photo
reduction of NADP. So NADP gets reduced to NADPH. So you should see overall
the flow of electrons goes from water through
photo system two, through the electron transport
chain, through photo system one, and it ends up in NADPH. Wow. Isn’t that incredible? What about the production
of ATP? Well, that’s where the
proton gradient comes in. So the proton gradient comes
from photo oxidation of water as well as the electron
transport chain, constantly pumping protons
into the thylakoid lumen. Now the protons are
not happy here. They desperately
want to get out, but the only place they fit
is through a little slot in a protein called
the ATP synthase, and what will happen is the
energy of the proton going down its gradient, going
in the direction it wants to go fuels the phosphorylation
of ADP to make ATP. Those are the light reactions. Now what we’re going to do is
focus in on the Calvin cycle and see what all of that ATP and
NADPH is going to be used for. The Calvin cycle goes by a
lot of names, as you can see. Melvin Calvin, yes, that was
his real name, is the person who is responsible
for figuring this out. He used carbon 14 to trace
carbon through photosynthesis. He wrote an autobiography
about his journey, which is a nice little read,
and he won the Nobel Prize in chemistry for this discovery. The star of the show turns out
to be an enzyme called RuBisCO, and RuBisCO is the most
abundant protein on Earth. At any given time, there’s
probably forty billion tons of it on the planet,
and it is responsible for incorporating atmospheric
carbon from carbon dioxide into an organic molecule. I know you want to
know how this works. RuBisCO is the enzyme that is
the key to carbon fixation. That doesn’t mean
that carbon is broken. It just means that the
carbon from CO2 is going to be incorporated
into organic compounds which will be made into glucose. This really is the key to
life on Earth as we know it. So I can’t overemphasize enough
the importance of this enzyme. So here’s what it actually does. RUBP is a five-carbon
molecule that actually cycles through the Calvin cycle,
as you’ll see in a minute, and the addition of CO2,
that additional carbon, this carboxylation of RUBP
happens because of RuBisCO, and what you form is an unstable
intermediate that’s six carbons in length. It immediately gets
split into two molecules of three phosphoglycerate,
and this happens at the beginning of
the Calvin cycle. Now the whole of
the Calvin cycle, the sum of all the
reactions looks like this. Things to notice. There’s the source of the carbon
that’s going to be incorporated into the new carbohydrate. This NADPH, that was made
in the light reactions. We need some water,
and we need some ATP. What’s going to come out
of it is not glucose. Now, a lot of textbooks will
say that glucose is the result of the Calvin cycle, and
that’s actually not true. What you’re making is G3P,
glyceraldehyde-3-phosphate. That is actually the
carbohydrate that gets made, and that will be used
to build glucose, or it could be incorporated into
storage molecules like starch, structural molecules
like cellulose, or anything else
that the plant needs. There are three phases
to the Calvin cycle. Phase one is the incorporation
of carbon dioxide to RUBP. This guy right here,
and that’s done because of the amazing
enzyme RuBisCO. Phase two is reduction phase,
and we are using up some of the ATP and some of
the NADPH that we made in the light reactions. Phase three is the
regeneration of RUBP, and this is why it’s a cycle. OK, so let’s just talk
about numbers for a second. If you have three RUBP
molecules entering the cycle, you can make six
glyceraldehyde-3-phosphates, six G3P’s, but it
cost you five of those to make the three RUBP’s. So that’s actually the,
the cyclical part of it. The only thing that gets
to leave is one G3P, right, because you make six,
but it cost you five. This is kind of confusing. So if you want to see how
expensive this really is to get one G3P released
from the cycle, it’s going to cost nine
ATP’s and six NADPH’s. So this is a really
expensive process. So here is a really good
summary of all of this. We’re going to input solar
energy in water, and we’re going to run the light
reactions, and the products of the light reactions
include oxygen gas, right. That comes from the
splitting of water and the high energy molecules
ATP and NADPH, which is going to provide reducing power. Those, along with
carbon dioxide, will feed into the Calvin cycle, the outputs of which include
the sugars, it’s actually G3P, which will be converted
into glucose. That’s the chemical energy,
and ADP and NADP plus, which will feed back
into the light reactions. Here’s another view of
the same thing that kind of adds the location
of everything in the chloroplast,
and that’s it. Wow. So the next time
you look at plants, man, they deserve a lot of respect. Photosynthesis is an
unbelievably complex process, but it makes life
on Earth possible. As always, I hope
that was helpful. Thank you so much for visiting
the “Penguin Prof Channel”. Please support by clicking
like, share, and subscribe, visit on Facebook,
follow on Twitter. Good luck.

About the author


  1. Please make video lessons on all kinds of subjects…The way you present the subjects is easy to understand and you have a good selection of slides as well.

  2. I'm Italian but I can certainly say that this video was easier to understand than all video (in italian) that I've seen before, THANK YOU VERY MUCH! =D

  3. Wow… I'm doing a research paper on the mechanics of photosynthesis for a botany class and I need 5 scholarly and peer reviewed articles. After watching this, I now feel like I don't need to look up anything else. I don't know how you made it so easy to take in, bravo! Cheers from the drummer in Super Space Nation!!!! Rock on!

  4. I WISH we had used this for class! Seriously, we just had a test over this in my botany class, and I think we would have been WAY more prepared if we watched this video. 100,000 likes!

  5. I'm German and all the German videos on youtube leave important things (like the photosystem I) out or are way too hard to understand. So yeah, that video was very helpful. I finally get it 😀

  6. Thank you so much, I just realized how much my school sucks, they never explained this as clear as this. THANK YOU <3

  7. This is the first video I've watch from this channel, and I fall in love right away.
    Your presentation is neat. And you talk very smoothly.
    Thank you so much..

  8. the words can NOT describe how much Im thankfull for your lectures. Y R the RuBisCo of those lectures :). THANNNNNNK YOUUUUU

  9. why plants dose not become hot with Calvin reaction. All those ATPs and NADPHs are not 100% converted in those reaction, so what about the heat?  

  10. thanks a lot ! but, can you explain more detail about calvin cycle ? your explaination is too simple.. btw, this video helps me a lot. 🙂 

  11. Hey I'm just a little confused still… So in the light reactions, it says that the light energizes the electrons… are the electrons ALREADY in the plant, or is the light the electrons???????

  12. The electrons actually come from the excited pair of chlorophylls from the reaction Center of PSII. Here's what happens:

    1) LHC captures light and absorbs photons, bouncing the photons through chlorophyll molecules of LHC;

    2) The photons go from the LHC to the pair of chlorophylls (chls) to the Reaction Center (RC) of PSII, which has a pair of chorophylls called chlorophylls P680, that is, these chls absorb light at a wavelength of 680 nm;

    3) These photons make the chlp680 molecules become excited, i.e., the double bonds from the porphyrin head of chls are broken, RELEASING electrons from the pair of chls. thus, the chlp680 molecules get excited (P680+), releasing (e-) from their broken double bond reactions;

    4) the released electrons from the pair of chl680s move to the next acceptor of electrons called PHEOPHYTIN, which is a molecule of chlorophyll WITHOUT the Mg2+ chelated by the porphyrin head of chl. In summary, Pheophytin is like a molecule of chl lacking its magnesium ion.

    5) the pheophytin by its turn, transfers the electrons to quinones (Q) bound to proteins from the reaction center. These electrons then will migrate from quinones from PSII to  PlastoQuinones (PQ), and then these PQs will transfer the electrons to the Cytochrome B6F complex.

    6) but HEY!! What ABOUT THE PAIR OF CHLS FROM RC THAT LOST THEIR ELECTRONS ?? The PSII complex has also an enzimatic subdivision called WATER OXIDIZING COMPLEX (WOC), which as the name says, it's gonna oxidize H2O to O2, producing electrons (e-) and H+ ions.Keep in mind that this complex has  manganese (Mn) and Calcium (Ca) as cofactors.

    7) the electrons generated from WOC will replace the lost ones by the pair of chls from the Reaction Center of PSII, thus restarting the process.

    8) so the overall process is: LIGHT (photons) >> LHC >> RC chl 680 >> (photons) >> RC chl680+ >> e- >> Pheo >> Quinones attached to proteins in PSII >> electrons leave PSII and go to  PQs >> Cytochrome BCF.  WOC >> H2O => O2 + e- + H+ >> e- => chlP680+ >> chlP680.

  13. This video is quite simple, yet makes you realize basically what photosynthesis is, what is very good when you don't get a goddamn thing of what this messed up process is at first.

  14. Thanks to ThePenguinProf! I seem strangely able to understand and remember a lot of what you say. Not just this video, either. Great info – great clarity – great voice!

    There is such a wonderful selection of bio-science instructional videos on the web. We that love these areas of inquiry are truly blessed. Keep making them – soon I'll be doing brain transplants.

  15. U r totally Awesome..!!
    I can't thnk u enough , this just the video i was looking for …
    explanation is very very nice and clear..

  16. Your the best!! my exam board uses slightly different terms tho. but non-the less I now understand. thank you so much.

  17. watched this when I took AP bio my senior year & helped me get the highest grade on the photosynthesis test… watching this again for my botany class in university and still super helpful!! thank you!!

  18. "You will never see a glowing plant, unless you have taken some interesting medication"
    Medication? Do you mean, illegal medicine? ( ° ʖ °)

  19. What happened to the other elements from the periodic table that sit in the enzymes and act as catalysts to perform magic, or do we just isolate the system to prove a scientific result and forget what makes it work. Add in the life that splits the elements from solids in the soil and prepares them for use by plants to create the whole picture and mention that the elements are present and dissolved in sea water for the plankton to absorb.
    Teaching is about giving knowledge not withholding it because you are too bone idle to include it.

  20. Thanks for sharing your knowledge with the man in the street. Since all of it would stop if the sun didn't shine, could it be that the photons created the leaf in the first place.

  21. The sun is the reason I'm here. photosynthesis, Without it life, would not exist. ( Do people ask you, why are you here?)I tell them the sun. It will be here again tommorow. I guess I'm like a sun worshiper.

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