- [Instructor] Photosynthesis
is pretty awesome.
It's where the plants and the trees
and all the green stuff that
you find around yourself
take in water and carbon
dioxide from their surrounding,
and then use the energy of the sun
to manufacture their own
carbohydrate molecules.
And why is it called photosynthesis?
Because the plants use photo,
which means light energy,
to synthesize, meaning to create,
carbohydrates from the raw materials.
And why do they do that?
Because carbohydrates is food for them.
But guess what?
Carbohydrates are food for us as well.
Even we need these carbs for our energy.
This means photosynthesis
feeds almost all living
things on this planet.
And on top of that, there is a bonus.
During this process, the plants kick out
a particular molecule into the atmosphere.
Guess which one?
Oxygen.
This means photosynthesis
gives me my carbohydrates, my food,
and it gives me oxygen to breathe.
That's why I love photosynthesis.
But here are my questions.
Why can plants and trees
synthesize their own carbohydrates
but we animals can't?
What do they have that we don't?
And what exactly happens
during this process?
How exactly do they produce
carbohydrates from the raw materials?
These are the questions
that we're gonna try
and answer in this video.
So let's start with the first one.
What do plants and trees
have that we don't?
Well, it turns out that they have
something called chlorophyll.
What's that?
Well, to answer that question,
let's clear this up a little bit.
And then we have to look at
one of these leaves very carefully.
So if we zoom in, we will see a leaf,
which is not surprising,
but if we zoom in even further
maybe using a microscope,
now we see something very interesting.
So these compartments that you are seeing,
they are the cells of that leaf.
And inside these cells
you can see all these green things, right?
These green things are
called chloroplasts.
And guess what?
Photosynthesis happens inside
these chloroplasts, okay?
There are a lot of
complex molecules inside
working together to
carry out this reaction.
Now one of the most
important molecule inside
is a pigment called chlorophyll.
So chlorophyll is a pigment
found in these green stuff,
and chlorophyll is the
reason why they are green.
Okay, chlorophyll gives
them their green color.
It makes all the plants and trees green.
But another important job of chlorophyll
is to absorb the energy from the sunlight.
Without chlorophyll,
energy wouldn't be absorbed
and photosynthesis wouldn't happen.
So here's how I like to think about it.
Imagine we are concentrating
on one of the chloroplasts,
as the sunlight comes and hits it,
the chlorophyll absorbs that energy
and makes it available
for all the molecules.
Now they have the energy to
take in carbon dioxide and water
and then carry out the
photosynthesis reaction
which we'll look at in
detail in a little while.
And then it spits out this
juicy carbohydrate molecules.
And then it also produces oxygen,
and the oxygen gets thrown out.
Now before we dive into this chloroplast
to see exactly how this
chemical reaction happens,
let me tell you a couple of things
that I used to get really
confused about, okay.
First is because we see plants
throwing out oxygen during this process,
I thought they don't need oxygen,
I thought they need carbon dioxide
but they don't need oxygen.
It turns out that's wrong.
Even plants need oxygen just
like how we need it, okay,
and we'll talk more about
this in videos of respiration,
but plants and trees,
they also need oxygen
for the same reasons why we need it.
Then why do they throw it out?
Well, it turns out that
during photosynthesis,
they create a lot of oxygen, okay,
so they keep whatever they want
and the rest they throw out.
So they don't throw out
all the oxygen, okay,
only the ones they don't
want they throw out,
and we are breathing that right now.
Secondly I used to think this process
only happens in the leaves.
But no, they can also happen
in the stem and the bulbs
and sometimes in the fruits as well.
Okay, photosynthesis can happen
in any part of the tree which is green,
because any part which
is green has chloroplast,
and wherever chloroplast is present,
photosynthesis can happen.
So for example you may have seen
these green bananas and apples, right?
Why are they green?
Because of chloroplasts and chlorophyll.
They can also carry out
photosynthesis, okay?
So not just leaves,
but leaves carry out most
of the photosynthesis.
And lastly what about this houseplant
which is in my balcony
which has non-green leaves?
Can they carry out photosynthesis?
I used to think no,
because they are not green
so they don't have chlorophyll
so they can't photosynthesize.
But it turns out that's wrong.
It turns out that even these
plants do have chlorophyll
and so they can photosynthesize.
Then why aren't they appearing green?
Well, because plants can
have other pigments as well.
And in most cases where
green plants are there,
chlorophyll is dominating,
and that's why we see them to be green.
But in this plant for example,
the other pigments like
maybe the red color pigments,
they are dominating,
and that's why when we look at them,
it looks a little purplish reddish.
It doesn't mean chlorophyll is not there.
Chlorophyll is there,
chloroplasts are there,
they can also photosynthesize.
Alright, so now that that's clear,
let's dive into this chloroplast
and see exactly what chemical
reaction is taking place.
So let's begin by writing
a chemical reaction for this process.
How do we do that?
Well, whenever I forget what's going on,
I start by thinking about the products.
Now I know that in photosynthesis
I get carbohydrate, okay.
This helps me remember the reaction.
Here's how.
See, carbohydrate has two words,
it has carbo and hydrate.
Carbo means carbon.
So to manufacture carbohydrate,
I need carbon, plants need carbon.
Where do they get that from?
Well, they get it from carbon dioxide.
That's how I remember carbon dioxide.
Then they also need hydrate.
Hydrate is water, right?
Just like how we say
dehydrated means loss of water.
So they also need water.
Oh, so plus H20.
So these are the two reactants needed.
But of course remember to
carry out this reaction,
they also need energy from the sun.
Photosynthesis, photo
part is over here, okay.
So these are the three things needed.
Now what do we get?
Well, let's see.
Now whenever we write this arrow mark,
we put a chlorophyll on top of that
to signify that energy is
absorbed by the chlorophyll.
Without chlorophyll, this
cannot happen, right?
Okay, anyways, what do we get?
We get carbohydrate, right,
that's the major product,
but how do we write the
formula for carbohydrate?
Again I look at this.
See, carbo means carbon so there's a C.
Hydrate means water so H20.
This is carbohydrate, okay.
But wait, carbohydrates need not have just
one atom of carbon and
one molecule of water,
they can have three and
three or six and six,
or any number they can have, alright.
So there are many carbohydrates.
So in photosynthesis,
which carbohydrate do we get?
Well, we get the carbohydrate in which
there are six atoms of carbon
and six molecules of water.
That's also something we
need to remember, okay.
So the one we get is where
there are six of these, alright.
And this molecule,
this carbohydrate is given a name,
it's called glucose.
So photosynthesis gives
us glucose, alright.
And what else?
We get one more product, which is that?
A byproduct which they
throw out, which is that?
Plus oxygen.
And you will see in a second
why I have colored oxygen this way.
And here is our chemical reaction.
But if you look carefully,
you see that it's not balanced.
For example on the right,
we have six carbon atoms,
on the left we just have one.
Okay, so we need to first balance it.
And so can you try balancing it first?
Great idea to pause the video
and see if you can balance
this whole equation yourself first.
Okay, hopefully you tried.
So I'll just quickly go
ahead and balance this.
Since there are six carbon over here,
we multiply this by six.
There are six water molecules over here,
so I multiply this by six as well.
Now let's look at the oxygen.
The oxygen here is balanced, six and six.
But over here we have 12 oxygens,
so this also will balance
by multiplying by six, we get 12.
Now everything is balanced.
Now I know this was fast
but we've talked a lot about balancing
in great detail in chemistry videos.
So if you need more clarity
on how to balance equations,
great idea to go back and
watch chemistry videos.
Anyways, now let's think about
what's going on in this reaction.
I'll start with what I thought again.
So when I learned this,
I thought that here carbon dioxide
is being broken into carbon and oxygen.
So I thought that plants
take in carbon dioxide,
break it, and throw away oxygen.
That's how we get our oxygen.
And then they take that carbon
and attach it to water
to give us carbohydrate.
That makes sense, right?
Easy, right?
Unfortunately that's now how it works.
Why not?
Well, the main reason for that
is because carbohydrates are not
really carbon attached to water.
So glucose is not six carbons
attached to six water.
I'll show you the structure of glucose.
There are many structures
but one of the structures
of glucose looks like this.
Now I know this looks all scary,
and don't worry, we don't
have to remember any of this.
But what I'm going to show you is
do you see carbon attached
to water anywhere?
No, right?
It's not like that.
It's a little bit complicated, right?
So even though the name
is stuck, carbohydrate,
it's not really a good name actually.
The name is stuck
but it's not really
carbon attached to water.
So you can't attach carbon to water
and expect to get a carbohydrate, okay.
So let me get rid of this.
Carbohydrates are not
really carbon and water.
But it's useful to remember the formula.
I use it to remember the formula, okay.
Let's get rid of this.
So if that's not what's going on,
what's really happening in this reaction?
So here's what's happening.
The chlorophyll absorbs
the energy from the sun
and then uses it to break open water
into hydrogen and oxygen.
And by the way it's not chlorophyll alone,
there are many molecules working together,
but some of their energy is used
to break open hydrogen and oxygen,
and that oxygen is thrown out.
So the oxygen in photosynthesis
comes from water, not from carbon dioxide.
And this process is pretty
important, at least historically,
because that's how oxygen
came into our atmosphere.
So almost all the oxygen
that we are breathing today
was once upon a time locked in water.
Pretty cool, right?
So we need to change the color of this.
This came from water.
And then what about this hydrogen?
Well, then the hydrogen gets attached
to carbon dioxide in a series of steps,
it's not a one-step reaction,
there are many more
molecules involved over here,
but in a series of steps
to finally give us glucose.
And so the oxygen over here
is the oxygen from the carbon dioxide.
So again we need to color that properly.
So again what happens?
Chlorophyll absorbs
the energy from the sun
and uses some of it to break open water
into hydrogen and oxygen.
Oxygen is thrown out
and then the hydrogen is
attached to carbon dioxide
in a series of steps to
finally give us glucose.
And I can stare at this forever
because this is literally
what's keeping me and you alive,
but we need to end this video
so let's quickly go ahead and summarize.
What did we see in this video?
We saw that photosynthesis takes place
inside this green things
called as chloroplasts.
They are green because of a
pigment called chlorophyll,
and chlorophyll helps in
absorbing the energy from the sun.
And what does it do after
absorbing the energy?
It uses it to split open water
into hydrogen and oxygen.
The oxygen gets thrown out,
and then the hydrogen is
attached to carbon dioxide
in a series of steps to give us glucose.
This is what happens
inside the chloroplast.
This is photosynthesis.
This is how we get our
food and our oxygen.
