0:00:00.000,0:00:00.370


0:00:00.370,0:00:03.130
I think we're now ready to learn[br]a little bit about the

0:00:03.130,0:00:04.570
dark reactions.

0:00:04.570,0:00:07.300
But just to remember where we[br]are in this whole scheme of

0:00:07.300,0:00:12.750
photosynthesis, photons came[br]in and excited electrons in

0:00:12.750,0:00:14.520
chlorophyll in the[br]light reactions.

0:00:14.520,0:00:17.500
and as those photons went to[br]lower and lower energy

0:00:17.500,0:00:20.250
states-- we saw it over here[br]in the last video-- as they

0:00:20.250,0:00:22.860
went to lower and lower energy[br]states, and all of this was

0:00:22.860,0:00:26.130
going on in the thylakoid[br]membrane right over here.

0:00:26.130,0:00:27.920
You can imagine-- Let me do[br]it in a different color.

0:00:27.920,0:00:29.980
You can imagine it occurring[br]right here.

0:00:29.980,0:00:32.619
As they went into lower and[br]lower energy states, two

0:00:32.619,0:00:33.360
things happened.

0:00:33.360,0:00:37.470
One, the release of energy was[br]able to pump the hydrogens

0:00:37.470,0:00:38.640
across this membrane.

0:00:38.640,0:00:40.800
And then when you had a high[br]concentration of hydrogens

0:00:40.800,0:00:44.380
here, those went back through[br]the ATP synthase and drove

0:00:44.380,0:00:47.350
that motor to produce ATP.

0:00:47.350,0:00:50.440
And then the final electron[br]acceptor, or hydrogen

0:00:50.440,0:00:51.840
acceptor, depending on how[br]you want to view it.

0:00:51.840,0:00:55.540
The whole hydrogen atom[br]was NAD plus.

0:00:55.540,0:00:59.575
So the two byproducts, or the[br]two byproducts that we're

0:00:59.575,0:01:03.550
going to continue using in[br]photosynthesis from our light

0:01:03.550,0:01:06.540
cycle, from our light[br]reactions I guess.

0:01:06.540,0:01:09.780
I shouldn't call it the light[br]cycle-- were-- I wrote it up

0:01:09.780,0:01:13.290
here-- ATP and NADPH.

0:01:13.290,0:01:16.040
And then the byproduct was that[br]we needed the electron to

0:01:16.040,0:01:17.950
replace that first[br]excited electron.

0:01:17.950,0:01:19.730
So we take it away from water.

0:01:19.730,0:01:22.740
And so we also produce oxygen,[br]which is a very valuable

0:01:22.740,0:01:24.420
byproduct of this reaction.

0:01:24.420,0:01:28.960
But now that we have this ATP[br]and this NADPH, we're ready to

0:01:28.960,0:01:31.690
proceed into the[br]dark reactions.

0:01:31.690,0:01:34.190
And I want to highlight again,[br]even though it's called the

0:01:34.190,0:01:37.280
dark reactions it doesn't mean[br]that it happens at night.

0:01:37.280,0:01:40.510
It actually happens at the same[br]time as light reactions.

0:01:40.510,0:01:42.970
It occurs while the[br]sun is out.

0:01:42.970,0:01:45.470
The reason why they call it[br]the dark reactions is that

0:01:45.470,0:01:46.860
they're light independent.

0:01:46.860,0:01:48.640
They don't require photons.

0:01:48.640,0:01:53.920
They only require ATP, NADPH,[br]and carbon dioxide.

0:01:53.920,0:01:55.660
So let's understand what's[br]going on here

0:01:55.660,0:01:56.350
a little bit better.

0:01:56.350,0:01:58.910
So let me go down to[br]where I have some

0:01:58.910,0:02:01.290
clean space down here.

0:02:01.290,0:02:03.735
So we had our light reactions.

0:02:03.735,0:02:08.639


0:02:08.639,0:02:13.190
And they produced-- I just[br]reviewed this-- produced some

0:02:13.190,0:02:19.870
ATP and produced[br]some and NADPH.

0:02:19.870,0:02:22.880
And now we're going to take some[br]carbon dioxide from the

0:02:22.880,0:02:24.130
atmosphere.

0:02:24.130,0:02:27.160


0:02:27.160,0:02:30.080
And all of this will go into[br]the-- I'll call it the light

0:02:30.080,0:02:31.170
independent reactions.

0:02:31.170,0:02:33.710
Because dark reactions[br]is misleading.

0:02:33.710,0:02:45.010
So the light independent[br]reactions, the actual

0:02:45.010,0:02:47.270
mechanism is called[br]the Calvin Cycle.

0:02:47.270,0:02:50.110
And that's what this video[br]is really about.

0:02:50.110,0:02:57.380
It goes into the Calvin Cycle[br]and out pops-- whether you

0:02:57.380,0:03:00.180
want to call it PGAL-- we talked[br]about it in the first

0:03:00.180,0:03:03.440
video-- or G3P.

0:03:03.440,0:03:06.350
This is glyceraldehyde[br]3-phosphate.

0:03:06.350,0:03:09.890
This is phosphoglyceraldehyde[br]They are the exact same

0:03:09.890,0:03:12.420
molecule, just different[br]names.

0:03:12.420,0:03:15.970
And you can imagine it as[br]a 3-carbon chain with a

0:03:15.970,0:03:19.260
phosphate group.

0:03:19.260,0:03:23.160
And then this can then be used[br]to build other carbohydrates.

0:03:23.160,0:03:25.640
You put two of these together[br]you can get a glucose.

0:03:25.640,0:03:28.950
You might remember in the first[br]stage of glycolysis, or

0:03:28.950,0:03:31.730
the first time we cut a glucose[br]molecule we ended up

0:03:31.730,0:03:34.490
with two phosphoglyceraldehyde[br]molecules.

0:03:34.490,0:03:35.970
Glucose has six carbons.

0:03:35.970,0:03:36.990
This has three.

0:03:36.990,0:03:40.350
Let's study the Calvin[br]Cycle in just a

0:03:40.350,0:03:42.120
little bit more detail.

0:03:42.120,0:03:51.420
So let's say exiting the light[br]reactions, let's say we have--

0:03:51.420,0:03:54.000
well let's start off with[br]six carbon dioxides.

0:03:54.000,0:03:56.780
So this is independent of[br]the light reactions.

0:03:56.780,0:03:58.560
And I'll show you why I'm[br]using these numbers.

0:03:58.560,0:04:00.570
I don't have to use these[br]exact numbers.

0:04:00.570,0:04:03.450
So let's say I start[br]off with six CO2s.

0:04:03.450,0:04:05.760
And I could write a CO2 because[br]we really care about

0:04:05.760,0:04:06.760
what's happening[br]to the carbon.

0:04:06.760,0:04:09.320
We can just write it as a single[br]carbon that has two

0:04:09.320,0:04:11.000
oxygens on it, which[br]I could draw.

0:04:11.000,0:04:12.150
But I'm not going to draw[br]them right now.

0:04:12.150,0:04:13.600
Because I want to really[br]show you what

0:04:13.600,0:04:15.090
happens to the carbons.

0:04:15.090,0:04:17.490
Maybe I should draw this[br]in this yellow.

0:04:17.490,0:04:18.920
Just to show you only[br]the carbons.

0:04:18.920,0:04:21.370
I'm not showing you the[br]oxygens on here.

0:04:21.370,0:04:30.170
And what happens is the CO2,[br]the six CO2s, essentially

0:04:30.170,0:04:33.240
react with-- and I'll talk a[br]little bit about this reaction

0:04:33.240,0:04:39.850
in a second-- they react with[br]six molecules-- and this is

0:04:39.850,0:04:41.640
going to look a little bit[br]strange to you-- of this

0:04:41.640,0:04:45.100
molecule, you could[br]call it RuBP.

0:04:45.100,0:04:48.630
That's short for ribulose[br]biphosphate.

0:04:48.630,0:04:52.000
Sometimes called ribulose-1[br]5-biphosphate.

0:04:52.000,0:04:54.430
And the reason why it's called[br]that is because it's a

0:04:54.430,0:04:57.630
5-carbon molecule.

0:04:57.630,0:04:59.710
So, three, four five.

0:04:59.710,0:05:02.840
And it has a phosphate on[br]the 1 and 5 carbon.

0:05:02.840,0:05:05.830
So it's ribulose biphosphate.

0:05:05.830,0:05:12.400
Or sometimes, ribulosee-1-- let[br]me write this-- that's the

0:05:12.400,0:05:14.906
first carbon.

0:05:14.906,0:05:16.460
5-biphosphate.

0:05:16.460,0:05:17.850
We have two phosphates.

0:05:17.850,0:05:20.710
So that's ribulose-1[br]5-biphosphate.

0:05:20.710,0:05:23.760
Fancy name, but it's just[br]a 5-carbon chain with 2

0:05:23.760,0:05:24.950
phosphates on it.

0:05:24.950,0:05:27.570
These two react together.

0:05:27.570,0:05:31.500
And this is a simplification.

0:05:31.500,0:05:32.800
These two react together.

0:05:32.800,0:05:34.780
There's a lot more going on[br]here, but I want you to get

0:05:34.780,0:05:35.960
the big picture.

0:05:35.960,0:05:45.430
to form, 12 molecules of PGAL,[br]of phosphoglyceraldehyde or

0:05:45.430,0:05:53.340
glyceraldihyde 3-phosphate of[br]PGAL, which you can view as

0:05:53.340,0:06:01.000
a-- it has three carbons and[br]then it has a phosphate group.

0:06:01.000,0:06:03.300
And just to make sure we're[br]accounting for our carbons

0:06:03.300,0:06:06.560
properly, let's think[br]about what happens.

0:06:06.560,0:06:08.780
We have 12 of these guys.

0:06:08.780,0:06:10.520
You can think of it that[br]we have-- 12 times

0:06:10.520,0:06:12.510
3-- we have 36 carbons.

0:06:12.510,0:06:14.510
Now did we start with[br]36 carbons?

0:06:14.510,0:06:16.440
Well we have 6 times 5 carbons.

0:06:16.440,0:06:17.260
That's 30.

0:06:17.260,0:06:18.640
Plus another 6 here.

0:06:18.640,0:06:19.060
So, yes.

0:06:19.060,0:06:20.580
We have 36 carbons.

0:06:20.580,0:06:25.260
They react with each other[br]to form this PGAL.

0:06:25.260,0:06:28.690
The bonds or the electrons in[br]this molecule are in a higher

0:06:28.690,0:06:32.020
energy state than the electrons[br]in this molecule.

0:06:32.020,0:06:33.900
So we have to add energy[br]in order for

0:06:33.900,0:06:35.420
this reaction to happen.

0:06:35.420,0:06:37.480
This won't happen[br]spontaneously.

0:06:37.480,0:06:40.620
And the energy for this[br]reaction, if we use the

0:06:40.620,0:06:44.160
numbers 6 and 6 here, the energy[br]from this reaction is

0:06:44.160,0:06:51.890
going to come from 12 ATPs-- you[br]could imagine 2 ATPs for

0:06:51.890,0:06:54.040
every carbon and every ribulose

0:06:54.040,0:07:02.990
biphosphate; and 12 NADPHs.

0:07:02.990,0:07:04.900
I don't want to get you confused[br]with-- it's very

0:07:04.900,0:07:07.220
similar to NADH, but I don't[br]want to get you confused with

0:07:07.220,0:07:08.560
what goes on in respiration.

0:07:08.560,0:07:17.100
And then these leave as 12 ADPs[br]plus 12 phosphate groups.

0:07:17.100,0:07:25.460
And then you're going to have[br]plus 12 NADP pluses.

0:07:25.460,0:07:27.970
And the reason why this is a[br]source of energy is because

0:07:27.970,0:07:30.420
the electrons in NADPH, or you[br]could say the hydrogen with

0:07:30.420,0:07:33.320
the electron in NADPH, is at[br]a higher energy state.

0:07:33.320,0:07:35.290
So as it goes to lower[br]energy state, it

0:07:35.290,0:07:36.680
helps drive a reaction.

0:07:36.680,0:07:40.360
And of course ATPs, when they[br]lose their phosphate groups,

0:07:40.360,0:07:42.372
those electrons are in a very[br]high energy state, they enter

0:07:42.372,0:07:45.550
a lower energy state, help drive[br]a reaction, help put

0:07:45.550,0:07:47.010
energy into a reaction.

0:07:47.010,0:07:50.630
So then we have these[br]12 PGALs.

0:07:50.630,0:07:54.150
Now the reason why it's called[br]a Calvin Cycle-- as you can

0:07:54.150,0:07:55.680
imagine-- we studied[br]the Kreb Cycle.

0:07:55.680,0:07:58.140
Cycles start reusing things.

0:07:58.140,0:08:00.960
The reason why it's called the[br]Calvin Cycle is because we do

0:08:00.960,0:08:04.570
reuse, actually, most[br]of these PGALs.

0:08:04.570,0:08:12.660
So of the 12 PGALs, we're going[br]to use 10 of them to--

0:08:12.660,0:08:15.400
let me actually do[br]it this way.

0:08:15.400,0:08:17.780
So we're going to[br]have 10 PGALs.

0:08:17.780,0:08:23.320
10 phosphoglyceraldehydes 10[br]PGALs we're going to use to

0:08:23.320,0:08:26.150
recreate the ribulose[br]biphosphate.

0:08:26.150,0:08:27.440
And the counting works.

0:08:27.440,0:08:29.530
Because we have ten 3-carbon[br]molecules.

0:08:29.530,0:08:30.910
That's 30 carbons.

0:08:30.910,0:08:33.200
Then we have six 5-carbon[br]molecules.

0:08:33.200,0:08:34.309
30 carbons.

0:08:34.309,0:08:36.580
But this, once again, is[br]going to take energy.

0:08:36.580,0:08:41.520
This is going to take the[br]energy from six ATPs.

0:08:41.520,0:08:45.590
So you're going to have six ATPs[br]essentially losing their

0:08:45.590,0:08:46.260
phosphate group.

0:08:46.260,0:08:47.740
The electrons enter[br]lower energy

0:08:47.740,0:08:49.482
states, drive reactions.

0:08:49.482,0:08:54.660
And you're going to have six[br]ADPs plus six phosphate groups

0:08:54.660,0:08:56.000
that get released.

0:08:56.000,0:08:57.590
And so you see it as a cycle.

0:08:57.590,0:09:00.050
But the question is, well[br]gee I used all of these.

0:09:00.050,0:09:01.250
What do I get out of it?

0:09:01.250,0:09:03.410
Well I only used 10[br]out of the 12.

0:09:03.410,0:09:05.895
So I have 2 PGALs left.

0:09:05.895,0:09:09.280


0:09:09.280,0:09:12.700
And these can then be used--[br]and the reason why I used 6

0:09:12.700,0:09:14.920
and 6 is so that[br]I get 12 here.

0:09:14.920,0:09:16.150
And I get 2 here.

0:09:16.150,0:09:18.790
And the reason why I have 2 here[br]is because 2 PGALs can be

0:09:18.790,0:09:22.900
used to make a glucose.

0:09:22.900,0:09:25.860
Which is a 6-carbon molecule.

0:09:25.860,0:09:31.010
It's formula, we've seen[br]it before, is C6H12O6.

0:09:31.010,0:09:34.140
But it's important to remember[br]that it doesn't have to just

0:09:34.140,0:09:34.660
be glucose.

0:09:34.660,0:09:36.770
It can then go off and generate[br]longer chained

0:09:36.770,0:09:39.400
carbohydrates and starches,[br]anything that

0:09:39.400,0:09:41.140
has a carbon backbone.

0:09:41.140,0:09:42.180
So this is it.

0:09:42.180,0:09:43.350
This is the dark reaction.

0:09:43.350,0:09:47.310
We were able to take the[br]byproducts of the light

0:09:47.310,0:09:51.130
reactions, the ATP and the[br]NADHs-- there's some more ATP

0:09:51.130,0:09:54.670
there-- and use it[br]to fix carbon.

0:09:54.670,0:09:56.700
This is called carbon[br]fixation.

0:09:56.700,0:10:00.060
When you take carbon in a[br]gaseous form and you put it

0:10:00.060,0:10:03.890
into a solid structure, that[br]is called carbon fixation.

0:10:03.890,0:10:08.170
So through this Calvin Cycle we[br]were able to fix carbon and

0:10:08.170,0:10:11.200
the energy comes from these[br]molecules generated from the

0:10:11.200,0:10:12.280
light reaction.

0:10:12.280,0:10:14.720
And of course, it's called a[br]cycle because we generate

0:10:14.720,0:10:18.160
these PGALs, some of them can[br]be used to actually produce

0:10:18.160,0:10:21.580
glucose or other carbohydrates[br]while most of them continue on

0:10:21.580,0:10:26.250
to be recycled into ribulose[br]biphosphate, which once again

0:10:26.250,0:10:27.920
reacts with carbon dioxide.

0:10:27.920,0:10:30.970
And then you get this cycle[br]happening over and over again.

0:10:30.970,0:10:33.060
Now we said it doesn't[br]happen in a vacuum.

0:10:33.060,0:10:35.040
Actually if you want to know the[br]actual location where this

0:10:35.040,0:10:39.510
is occurring, this is all[br]occurring in the stroma.

0:10:39.510,0:10:42.770
In the fluid, inside the[br]chloroplast but outside of

0:10:42.770,0:10:43.950
your thylakoid.

0:10:43.950,0:10:47.130
So in your stroma, this is where[br]your light independent

0:10:47.130,0:10:49.850
reactions are actually[br]occurring.

0:10:49.850,0:10:54.640
And it's not just happening with[br]the ADP and the NADPH.

0:10:54.640,0:10:59.600
There's actually a fairly decent[br]sized enzyme or protein

0:10:59.600,0:11:00.560
that's facilitating it.

0:11:00.560,0:11:02.790
That's allowing the carbon[br]dioxide to bond at certain

0:11:02.790,0:11:05.900
points and the ribulose[br]biphosphate and the ATP to

0:11:05.900,0:11:08.440
react at certain points, to[br]essentially drive these two

0:11:08.440,0:11:10.380
guys to react together.

0:11:10.380,0:11:15.560
And that enzyme, sometimes it's[br]called RuBisCo, I'll tell

0:11:15.560,0:11:16.870
you why it's called RuBisCo.

0:11:16.870,0:11:19.000
So this is RuBisCo.

0:11:19.000,0:11:23.690
So rub-- let me get the[br]capitalization right--

0:11:23.690,0:11:29.950
ribulose biphosphate rub--[br]bis-- co-- carboxylase.

0:11:29.950,0:11:31.040
And this is what[br]it looks like.

0:11:31.040,0:11:34.490
So it's a pretty big protein[br]enzyme molecule.

0:11:34.490,0:11:37.710
You can imagine that you have[br]your ribulose biphosphate

0:11:37.710,0:11:39.070
bonding at one point.

0:11:39.070,0:11:41.690
You have your carbon dioxide[br]bonding at another point.

0:11:41.690,0:11:43.120
I don't know what[br]points they are.

0:11:43.120,0:11:45.550
ATP bonds at another point.

0:11:45.550,0:11:46.640
It reacts.

0:11:46.640,0:11:50.400
That makes this thing twist and[br]turn in certain ways to

0:11:50.400,0:11:55.590
make the ribulose biphosphate[br]react with the carbon dioxide.

0:11:55.590,0:11:57.460
NADPH might be reacting[br]at other parts.

0:11:57.460,0:12:01.040
And that's what facilitates[br]this entire Calvin Cycle.

0:12:01.040,0:12:06.600
And you might-- I told you over[br]here-- that this R U B P,

0:12:06.600,0:12:11.410
this is ribulose-1[br]5-biphosphate.

0:12:11.410,0:12:16.800
This RuBisCo, this is short for[br]ribulose-1 5-biphosphate

0:12:16.800,0:12:18.290
carboxylase.

0:12:18.290,0:12:19.880
I won't write it all out;[br]you could look it up.

0:12:19.880,0:12:23.260
But it's just telling you, it's[br]an enzyme that's used to

0:12:23.260,0:12:28.040
react carbon and ribulose-1[br]5-biphophate.

0:12:28.040,0:12:28.900
But now we're done.

0:12:28.900,0:12:30.750
We're done with photosynthesis.

0:12:30.750,0:12:36.070
We were able to start off with[br]photons and water to produce

0:12:36.070,0:12:40.100
ATP and NADPH because we had[br]those excited electrons, we

0:12:40.100,0:12:45.610
had the whole chemiosmosis to[br]drive the-- that allowed the

0:12:45.610,0:12:47.840
ATP synthase to produce ATP.

0:12:47.840,0:12:50.660
NADPH was the final[br]electron acceptor.

0:12:50.660,0:12:54.020
These are then used as the fuel[br]in the Calvin Cycle, in

0:12:54.020,0:12:54.990
the dark reaction.

0:12:54.990,0:12:56.820
Which is badly named, it should[br]be called the light

0:12:56.820,0:12:57.650
independent reaction.

0:12:57.650,0:12:59.270
Because it actually does[br]happen in the light.

0:12:59.270,0:13:02.070
You take your fuel from the[br]light reactions with some

0:13:02.070,0:13:05.840
carbon dioxide and you can fix[br]it using your-- I like to call

0:13:05.840,0:13:08.090
it-- the RuBisCo enzyme[br]in the Calvin Cycle.

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And you end up with your[br]phosphoglyceraldehyde which

0:13:11.210,0:13:14.000
could also be called your[br]glyceraldehyde 3-phosphate,

0:13:14.000,0:13:17.860
which can then be used to[br]generate glucose, which we all

0:13:17.860,0:13:21.400
use to eat and fuel[br]our bodies.

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Or we learn in cellular[br]respiration, that can then be

0:13:23.780,0:13:27.550
converted into ATP[br]when we need it.