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0:11
Hi, my name is Charles Walker. I teach honors introductory biology at the University of New Hampshire. With my colleagues at UNH in biology, physics, mathematics, and chemistry, we will discuss how light energy is gathered daily from the sun and then utilized during photosynthesis in higher plants. This process ultimately provides us with organic molecules, without which, we could not survive here on Earth. This amazing story is as complicated as
0:38
a Russian novel, with many characters and complex interactions between the players. We will integrate views from all four of our disciplines to provide you with
0:49
an integrated understanding of this classic story in basic biology. Using this video, my colleagues will approach our story by discussing physics of photon production by the sun, definitely stunning stuff, followed by the mathematics of distributing photons on the surface of the earth, fascinating and vital to our story. Then they will discuss the biological and chemical mechanisms that plants use to capture photon energy from the sun,
1:14
release oxygen to our atmosphere, and ultimately to capture the sun's energy and incorporate it into organic molecules like ATP and ultimately sugars. Sugars produced in this way are the source for all of our essential organic molecules, including proteins, carbohydrates, nucleic acids and lipids. Photosynthesis is a vital energy storage process, upon which ninety-nine percent of the living organisms on Earth are totally dependent. Without it, the complex life we
1:43
experience every day would not be possible. Hi, my name is Dawn Meredith, and I teach in the physics department at UNH. I'd like to share with you some of the details of the energy transformations that begin in the sun and end in plants.
2:00
Energy from the sun comes to us in the form of light waves, also known as photons. Some of the light is visible to humans and some is invisible. In this sketch, light between the two vertical lines is what humans can perceive. The other light in the ultraviolet and infrared we cannot see. Light waves are characterized by their wavelength, as indicated by the oscillating curves on this second sketch.
2:31
Higher energy photons that we perceive as violet light have shorter wavelengths than red light.
2:41
The light from the sun will eventually be gathered by two complexes in plants called photosystem I and photosystem II.
2:51
Photosystem II has a reaction center named P680. The 680 refers to the wavelength of light that P680 preferentially absorbs. The units on 680 are nanometers, which are 10 to the -9 meters: about the size of an atom. Light at this wavelength is red. In photosystem I the reaction center is called P700 because it preferentially absorbs 700 nanometer light, and that is also red.
3:22
The energy from the sun has a long journey to be transformed into the potential energy of ATP.
3:29
The first step is that hydrogen ions, that is protons, fuse together into helium-4 in the dense, hot core of the sun. Also in this reaction, photons are emitted with energy about one million times greater than that of visible photons. These so-called gamma rays interact with the other particles in the sun scattering in all directions and transferring energy to the particles.
3:57
Because of these interactions, photons take a diffusive random walk from the center of the sun to its surface. As with all other kinds of diffusion, this is a very slow process, taking an astounding 100,000 years or more. By the time the photon reaches the edge of the sun, it has an energy much lower, and therefore, a longer wavelength.
4:24
Once the light leaves the sun, it travels to the earth in about eight minutes. This trip is much quicker because the space between the earth and the sun is nearly empty, and so the photons have fewer particles to interact with.
4:40
This complicated plot in orange shows the relative intensity of the wavelengths of light emitted from the sun. The X-axis indicates that the sun, the light from the sun, is primarily in the range: 250 nanometers to 2,000 nanometers. The multicolored curve is the relative intensity of light from the sun after it has gone through our atmosphere. The values are different because some of the photons are absorbed by water, carbon dioxide, and other molecules in the atmosphere.
5:15
From the plot, you can see that the most intense wavelengths are in the visible range: from 300 to 800 nanometers, but there is also photons in the ultraviolet and infrared.
5:29
It is no accident that living systems on Earth have evolved to utilize the most abundant photons on Earth.
5:37
The sunlight then falls on the plant and on photosystem II. Photosystem II in plants contain over 100 pigment molecules, each of which can absorb photons from the sun. By having many pigment molecules gathering light, each photosystem II can gather more light than a single reaction center could gather on its own.
6:01
Amazingly, these pigment molecules transfer their energy to neighboring molecules via a dipole dipole interaction with 100 percent efficiency. So the pigments can pass the energy absorbed from photons to other pigment molecules and eventually to the reaction center. Once the energy is transferred to the reaction center, an electron is ejected from the reaction center, and this electron becomes part of the complicated process that Dr. Fagerberg will detail in a few moments.
6:39
Hello, my name is Karen Graham, and I'm a professor in the Department of Mathematics and Statistics. As an interesting exercise, we would like to estimate the number of 680 nanometers on PSII every second. The rate of energy output of the sun is 3.8 x 10 to the 26 watts, or joules per second. Note that this is in units of energy per time, known as power. Since there is a constant stream of energy leaving the sun, it makes
7:10
more sense to describe the energy output in units of energy per time, just like the amount of falling rain can be described as gallons per second. As the light leaves the sun, it spreads out uniformly in a sphere, getting more and more spread out the further it travels, and therefore less intense. The energy intensity decreases as one over R squared, because the area of the spherical shell increases as R squared. Now we have yet another way to describe the energy: energy flow
7:43
each second spread over one square meter of area. That is, the energy is spread out not only over time, but over space as well. Objects with larger area will receive more energy than those with smaller area. To return to the rain analogy, this means I can collect more in a large mouth barrel than in a small cup. The earth is 5 x 10 to the 11th meters from the sun, so the energy from the sun is evenly spread out over
8:17
a spherical shell with that radius and with area 4 Pi x 5 x 10 to the 11 meters, that quantity squared.
8:29
Which is equal to 2.83 x 10 to the 23 meters squared. Using this, we calculate the intensity at the earth or anywhere on this imaginary spear using the formula: intensity at Earth is equal to power over area, which is equal to 3.8 x 10 to the 26 joules per second divided by 2.83 x 10 to the 23 meters squared, which is equal to 1,344 watts per meter squared. Meaning that every second, 1,344 joules of energy from the sun falls
9:11
on a patch of ground at the earth that is one meter squared. From this sketch of spreading light energy we see that only a small fraction of the sun's light falls on the earth. The rest of the light keeps going until it hits something else. For example: another planet or a very distant star. Recall that we want to find the number of 680 nanometer photons incident on PSII every second. We have just calculated the total intensity, but this intensity is spread out over many wavelengths. However,
9:46
we can use the plot of intensity
9:49
that Dr. Meredith showed us
9:53
to estimate the energy near 680 nanometers.
9:59
Let's look at the interval 675 to 685 nanometers because this is centered around 680 nanometers.
10:09
Using calculus and the concept of area under the curve, we can calculate the energy in the 675 to 685 nanometer range to be 12.5 watts per meter squared. This means that every second, 12.5 joules of energy from the sun falls on a one meter square patch of ground on the earth. But our question is how much energy falls on PSII, which is much smaller than one meter squared. It is actually 11 nanometers squared.
10:50
Using this, we have that the energy falling on PSII every second is equal to energy over area x the area of PSII,
11:01
which is equal to 12.5 joules per meter squared x 11 meters squared, which is equal to 1.5 x 10 to the -15 watts. We are almost at our final answer of how many 680 nanometer photons fall on PSII every second. Using the formula: the number of photons falling every second on PSII is equal to the energy falling on PSII per second divided by the energy per photon. But to calculate this number, we need to know how much energy each photon has. Using known relations,
11:40
we can calculate the energy of 680 nanometer photons to be 2.9 x 10 to the -19 joules per photon.
11:51
Plugging this number into the above formula and using the power on PSII we calculated before, we have that the number of photons falling every second on PSII is equal to 1.5 x 10 to the -15 watts divided by 2.9 x 10 to -19 joules per photon, which is equal to 4,350 photons every second.
12:23
Because of a number of factors and efficiency of the photosystem, this number far exceeds the ability of the photosystem to use all of the photons. On a bright sunny day the system is saturated. Hello, my name is Wayne Fagerberg, Professor Emeritus from the Department of Molecular, Cellular, and Biomedical Sciences. In this part of the video, we are going to discuss the light reaction of photosynthesis, which is the biological process of gathering light energy in the preparation for
12:54
CO2 reduction to make sugar molecules. In the plant, this process takes place in organelles called chloroplasts. The chloroplast is surrounded by two outer membranes and contains an internal membrane system called the thylakoids. As we can see in this electron micrograph of the chloroplast, each thylakoid forms a flattened sack enclosing a space called the lumen, and it is surrounded by a watery enzyme- filled region called the stroma.
13:25
The thylakoids may occur in stacks and are called granal thylakoids or non-stacked regions called stromal thylakoids. Each has a different role to play in our story.
13:37
Chlorophyll molecules are grouped within the thylakoid membrane system into complexes called photosystems. Each photosystem contains about 300 chlorophyll molecules and associated proteins. Within each photosystem is a single molecule of chlorophyll, where all the action takes place. This is called the reaction center chlorophyll.
14:02
There are two different photosystems seen in this drawing of the thylakoid membrane, designated photosystem I and II. Each photosystem has a different role to play in gathering light energy.
14:17
Photosystem II is located in the stacked thylakoids and has a reaction center chlorophyll called P680, and it is associated with a very important molecular complex called the OEC, or oxygen evolving complex. Photosystem I is located in the unstacked thylakoids, which are in contact with the stromal region of the chloroplast, and it has a reaction center chlorophyll called P700.
14:47
Linking these two photosystems are a number of electron and electron proton carrying molecules. So, let's follow the pathway of the absorption of light energy by these systems and see what happens and what is accomplished. Many parts of this story are similar to the electron transport story you may already be familiar with from the electron transport in the mitochondria.
15:12
To begin this story, imagine a photon of light entering the chloroplast and being absorbed by one of the 300 chlorophylls in photosystem I.
15:22
The light energy is passed from one chlorophyll to the other in the complex with 100 percent efficiency until it reaches the P700 reaction center chlorophyll.
15:36
When it does, the energy from the photon is used to excite, or energize, an electron in that molecule. This forces the electron to an outer electron orbital. This excited electron is quickly taken away by electron carrier proteins in the photosystem and is transported to the outer stromal side of the photosystem.
16:01
Here ferredoxin Fd takes the electron, because it has a stronger affinity for the electron than the carriers in the photosystems. Fd, with its electron, is then attracted to a nearby enzyme complex called NADP reductase. Here, the electron is taken from Fd and passed to NADP, thus reducing it. At the same time, a proton from the watery stroma is attracted to the negatively charged NADP, and it becomes NADPH.
16:40
Until photosystem I reaction center can replace the electron it lost to NADP, it cannot function again. In other words, it can't function in the oxidized state.
16:53
To replace the electron in photosystem I, we turn to photosystem II.
16:59
A second photon of light comes into the complex and activates it in a manner similar to photosystem I. Energy is passed to the reaction center chlorophyll, P680, and once again, the electron carrier proteins take this excited electron, move it to the outer membrane leaflet, where it is picked up by plastoquinone, or PQ.
17:25
PQ, with an extra electron, now attracts a proton from the stroma.
17:32
PQ, with its extra electron and proton, shuttles along the membrane until it bumps into cytochrome b/f complex in the stromal thylakoids. When it does, it is carried to the lumenal side of the membrane, where the electron is taken away from PQ by cytochrome f. Since the cytochromes can carry only electrons and not protons, this causes the accompanying proton to fall off into the lumenal space.
18:04
Oxidized PQ is then recycled.
18:07
The electron is immediately passed to PC, plastocyanin, which then shuttles the electron back to the reaction center chlorophyll in photosystem I, thus reducing it.
18:20
Now photosystem I is ready to work again. However, at this point photosystem II reaction center has been oxidized and needs to replace its lost electron. Here is where the OEC, located at the base of photosystem II, comes into play.
18:38
The OEC is capable of taking water, H2O, and separating the hydrogens from the oxygens, then the electrons from the hydrogens, leaving a proton behind in the thylakoid lumen, while the electrons are then used to reduce the P680 reaction center chlorophyll of photosystem II.
19:00
At this point the whole system is reset, both photosystem reaction center chlorophylls are reduced. However, NADPH needs a second electron and proton to become fully reduced. And to accomplish, this we have to run the complete cycle again. In so doing, we will use two more photons of light and fully reduce NADP to NADPH2. Therefore, to make a final product NADPH2 we will have used a total of four photons.
19:38
After this system has run twice, a diatomic oxygen molecule will be released from the OEC and may eventually find its way into the atmosphere. Dr. Bauer will talk more about the chemistry involved in this process later.
19:55
During energy capture and electron transport, we have been building up a proton gradient in the thylakoid lumen. As in the mitochondria, these protons can move from the lumen across the membrane into the stroma down an electrochemical gradient through the ATP synthase complex. As they do, they provide enough energy to facilitate the phosphorylation of ADP and inorganic phosphate together to make ATP.
20:31
As a result of the absorption of light, we have been able to create a reducing agent in the form of NADPH2, and we have produced ATP, which provides the ability to change the free energy state of molecules, and thus their reactivity during the assembly of a sugar molecule. This is the light reaction.
20:53
I'm Chris Bauer from the chemistry department.
20:56
Nestled in the cradle of the photosystem II molecular structure, is a small mineral cluster containing four manganese atoms and one calcium atom, with five oxygen atoms linking them all together.
21:09
Not even large enough to be called a crystal, the Mn4CaO5 cluster may be one of the most important molecular structures for supporting life.
21:20
This is the core of the oxygen evolving complex, the OEC, where water is converted to oxygen gas.
21:29
The cluster is tethered in place by about a dozen oxygen and nitrogen atoms on the amino acid side chains of the proteins surrounding the cluster.
21:38
This includes glutamate, aspartate, histidine, and alanine.
21:44
The valence electrons on the oxygen and nitrogen atoms coordinate to the manganese atoms.
21:50
The position of the atoms in this complex array have been measured by X-ray diffraction images, most recently to a precision of 0.2 nanometers, about the size of the atoms themselves.
22:02
Not a perfect picture, but close enough to allow reliable measurements of bond lengths and angles.
22:10
The process by which oxygen is created is still not totally understood, but research is starting to pull back the curtain to reveal how this light dependent process works.
22:22
Overall, the reaction that creates the oxygen we breathe is shown here.
22:28
This is an oxidation reaction, which changes oxygen from a semi-ionic form in water to neutral form in molecular oxygen by removing electrons and hydrogen ions.
22:42
The manganese atoms in the OEC cause this to happen. This makes the OEC an oxidizing agent.
22:50
But the OEC has to be charged-up first; that's where light comes in.
22:56
The absorption of light photons at other locations in the PSI and PSII systems deposits energy in molecular structures, which transfer that energy to the OEC.
23:08
That energy causes reactions that remove electrons from the OEC, specifically from the manganese atoms.
23:16
The manganese atoms are believed to be in a dark state, with two atoms in the plus three oxidation state and two in the plus four oxidation state.
23:26
In discrete steps, the plus three atoms are each raised to the plus four state and perhaps one to the plus five state.
23:35
The OEC is now sufficiently charged to support oxidation of water.
23:40
At this point, we leave experiment and embrace theory.
23:46
Using a molecular orbital calculation that accounts for the spin states of the manganese D electrons, it is possible to calculate likely pathways by which OEC actually turns the oxygen in water into molecular oxygen.
24:03
One possible model is that two water molecules, shown in blue, coordinated by ion dipole attraction to the calcium or dangling manganese ions, are close enough so that one oxygen can link to the other to make an O-O single bond.
24:23
This would be caused by the charged-up manganese cluster removing two electrons and releasing two hydrogen ions, shown here in a simplified way.
24:34
The cluster still has enough oxidizing power to remove two more electrons, allowing the oxygens to form a double bond of molecular O2.
24:44
Besides achieving a joining of the two oxygen atoms, this process also reduces the manganese and the OEC back to the initial dark state, because the manganese atoms have gained electrons.
24:58
This process also acidifies the cytoplasm because of the four released hydrogen ions, which then drives ATP synthese, in another portion of the cell, to make ATP.
25:12
The O2 then releases from the cluster, because it is now a neutral and a non-polar substance, and diffuses out of the cell to the atmosphere.
25:23
The ATP and NADPH2 molecules produced in the light reaction can now be used to reduce CO2 to make sugar molecules using the enzymes in the Calvin Benson cycle, which is located in the stroma.
25:39
To make a single molecule of six carbon sugar, the Calvin Benson cycle will consume 6 carbon dioxides, 18 ATPs, and 12 NADPH2s.
25:53
To make the 18 ATPs needed in the Calvin Benson cycle, will require about 72 protons to pass through the ATP synthase complex. There are mechanisms to account for the movement of the extra protons into the lumen we have not discussed in this video.
26:13
In order to generate the products used to make one 6 carbon sugar, the light reaction will release 6 molecules of oxygen and will consume 12 molecules of water.
26:25
When one molecule of sugar, glucose, is oxidized for metabolic energy, it will use 6 molecules of oxygen and will produce 12 molecules of water, and it will release 6 molecules of carbon dioxide, the exact reverse of photosynthesis. Sugar molecules made in the Calvin Benson cycle can be used in energy metabolism or they can be used as building blocks for all other organic molecules.
26:59
Photosynthesis provides us first, with the oxygen needed to sustain high energy requiring eukaryotic metabolism, second with a mechanism for removal of CO2, a biproduct of oxidative respiration and a toxic greenhouse gas from the atmosphere, and three with the molecular conduit through which energy from the non-living inorganic world of the sun becomes part of the living organic world on Earth. In our joint presentation, we have tried to demonstrate the energetic links between the sun and
27:32
living organisms on Earth. We hope that you have gained a more complete understanding of how our sun is linked to our living world and how science from various disciplines can contribute essential data vital to understanding this complex story.
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