AP Biology Photosynthesis

Welcome to Educator.com.0000

In this last lecture in the series on cellular energetics, we will be focusing on the process of photosynthesis.0002

Photosynthesis is the process by which light energy is converted into chemical energy.0011

Plants, algae, cyanobacteria, and some protists are all capable of photosynthesis.0018

Photosynthetic organisms are known as autotrophs. These are producers of organic compounds.0025

In contrast, organisms such as animals and fungi that cannot produce organic compounds must consume other organisms for nutrients.0036

For examples, animals consume plants, fungi and other animals.0047

These consumers are called heterotrophs, and these are consumers of organic compounds.0052

Before we delve into the details of photosynthesis, it is important to understand overall what happens.0066

Here is the overall reaction for photosynthesis: six carbon dioxides plus six waters are used to form one molecule of glucose plus six oxygens.0072

CO₂ is used, and oxygen is produced as a by-product.0086

Something else important to have here is energy.0092

This is an energy requiring the process, and that energy comes in the form of light, so light energy is used to fuel this process.0095

To survive, plants need to take in only water, minerals and CO₂, and then, they can create the glucose that is used for cellular respiration.0108

Because energy must be input, this reaction is endergonic.0119

There are two processes in photosynthesis.0127

The first set of processes is called the light reactions or the light-dependent reactions.0132

The second are the light-independent reactions. These are also called the dark reactions.0142

The dark reactions or light-independent reactions do not directly require light.0157

However, the products of the light reactions are needed for the light-independent reactions to occur, so light is needed, but it is indirect.0162

Let's start out by talking about where photosynthesis takes place.0176

Here is a leaf, and on the right is a chloroplast, which is the organelle where photosynthesis takes place; and this was described in electron cell structure, but we are going to review it here.0182

Just starting out with the anatomy of the leaf, there is a waxy covering that protects the leaf and prevents water loss, and this is called the cuticle.0197

This cuticle layer is produced by a cell layer called the upper epidermis, and we will talk more about plant anatomy in the plants lecture.0208

What we are going to focus on now is layer of cells known as the mesophyll.0221

In the mesophyll layer, photosynthesis takes place.0231

Here, on the underside of a leaf, you will see openings, and these openings or pores are known as stomates.0241

Gas can enter and leave the cell through the stomate.0251

What is going to happen is CO₂ is going to enter. Photosynthesis will take place, and oxygen can leave.0254

You notice that there is gaps between. This is called spongy mesophyll right here, and these cells in the spongy mesophyll are separated out.0261

And these air spaces that are created allow for the diffusion of gases.0272

The CO₂ enters, can diffuse into the mesophyll cells where photosynthesis takes place.0278

The opening and closing of the stomates is controlled by what is called guard cells.0286

The guard cells surround these, and they can close it off, open up.0292

In addition to gases, something else that can be lost is water.0302

Stomates are a major source of water loss through a process called transpiration, so water loss through transpiration.0307

This is especially a problem in a climate that is hot and dry.0318

Here, we have what is called a vascular bundle, and the vascular tissues of the plant in here, there are two types- xylem and phloem.0321

The xylem is used to transport water and minerals from the roots up to the rest of the plants.0338

The phloem is used to transport the products of photosynthesis, glucose, to the rest of the cell.0348

Photosynthesis takes place in the leaves primarily, and the glucose can, then, be transported to other parts of the plants such as the roots.0356

This is leaf structure, and the mesophyll cells are the location of photosynthesis.0366

Within the cells where photosynthesis actually takes place is the chloroplast. This is just showing a chloroplast within a cell.0372

There is a fluid called stroma within a chloroplast, and the chloroplast are surrounded by a double membrane.0382

Like the mitochondria, this double membrane is taken as support for the endosymbiosis theory.0392

Recall the endosymbiosis theory says that organelles are a result of a symbiotic relationship between early prokaryotes.0399

In this case, an early prokaryote might have engulfed another photosynthetic prokaryote.0407

And eventually, that other prokaryote became dependent on the larger prokaryote and became part of it and is the chloroplast.0414

Like mitochondria, chloroplasts also have their own DNA. Again, that is also support for the endosymbiosis theory.0426

We have this double membrane. We have this fluid called stroma, and then, we have these stacks of discs called grana.0437

These discs, which are membranous, are called thylakoids, and we will talk in more detail about what happens, where each process takes place.0452

But one thing in general to remember is that the dark reactions take place in the stroma or the light-independent reactions, whereas the light reactions take place in the grana.0467

And the photosynthetic enzymes are embedded in the membranes of the thylakoid.0486

Cyanobacteria are also capable of undergoing photosynthesis, yet, they do not contain chloroplast.0494

Instead, photosynthesis takes place in infoldings of the cell membranes of these prokaryotes.0502

The infoldings in the cyanobacteria cell membrane are also called thylakoid membranes because they are the site of photosynthesis.0507

Alright, two parts of photosynthesis: light reactions and dark reactions.0517

We are going to focus first on the light reactions or the light-dependent reactions.0522

This is a stage of photosynthesis during which light energy is used to produce ATP.0528

ATP is needed for the dark reactions and that is when glucose is produced.0534

Light reactions produce the ATP. The ATP is, then, used in the dark reactions to actually form glucose.0541

To understand the light reactions, you need to understand a little bit about light and pigments.0552

Pigments are substances that absorb visible light, so pigments absorb visible light.0560

There are several pigments that we will talk about with photosynthesis, for example chlorophyll a, which participates in the light reactions,0577

also, chlorophyll b and another group of pigments called the carotenoids.0587

These are actually considered the accessory photosynthetic pigments. We will talk more about the functions of each of these.0598

We have chlorophyll a, chlorophyll b and carotenoids.0608

Different pigments absorb different wavelengths of light, and different wavelengths of light are different colors.0611

You notice here shorter wavelengths of light, for example 400nm appears violet.0617

Longer wavelength, here in the 500 range appears blue and then, so on, and then, red light has a much longer wavelength.0628

Several things can happen. A pigment can absorb light, or it can transmit light and reflect the light.0638

The reason plants appear green is that chlorophylls absorb violet, blue and red light, so they are absorbing all of this.0648

They transmit and reflect green light.0659

When you look at a plant, that plant, the pigments inside the chloroplast have absorbed this part of the spectrum.0662

And what you are seeing is what is reflected back by then, which is the green light.0672

These absorb blue, violet, red light, and they reflect and transmit green light.0676

Carotenoids actually absorb violet, blue and green light.0692

They are absorbing violet, blue and green light, and that leaves the red and orange part of the spectrum to be transmitted and reflected.0698

Red and orange are transmitted and reflected.0707

Sweet potatoes and carrots are very high in carotenoids.0712

And that is why when we look at them, they appear orange or reddish-orange or yellowish because they are reflecting yellow or orange light.0716

Something else to understand about light is that it behaves as both a wave and a particle.0731

Light has aspects to it that are like a wave and then, other aspects that are like a particle.0739

Photons are particles of light that contain a particular amount of energy, so a photon is a particle of light that contains a certain amount of energy.0756

The shorter the wavelength, the greater the energy in the photon, so violet light has a shorter wavelength than red light.0773

Therefore, violet light is going to contain more energy, and I am going to use light and light energy and photon interchangeably so you should be familiar with these terms.0783

OK, before we go on to talk about exactly what happens in the light reactions and how ATP is produced,0794

you need to understand how the pigments are arranged and how what is called photosystems are set up in the thylakoid membrane.0800

Recall that we have a chloroplast, and we have thylakoids stacked up into what is called a grana; and this is the location of the light reactions.0809

Now, looking at the close up, photosystems are located in the thylakoid membranes of the chloroplast. This is where the light reaction takes place.0823

These photosystems consist of two parts, a reaction center complex surrounded by several light-harvesting complexes.0833

This whole blue area is the reaction center complex, and it is surrounded by light-harvesting complexes, so here is one. Here is one.0843

Light-harvesting complex is just a group of pigment molecules.0860

Another name for light-harvesting complexes is antenna complexes, and the reason that these are called antenna complexes is because they pick up electromagnetic radiation.0867

They absorb electromagnetic radiation, and what light-harvesting complexes consist of is multiple types of pigments.0878

Different types of pigment molecules absorb different wavelengths of light more effectively0895

like I talked about the carotenoids absorbing certain wavelengths of light and reflecting others- chlorophyll a, chlorophyll b.0900

Different types of pigments absorb different light wavelengths.0908

Therefore, by having multiple different types of these pigment molecules, the antenna complexes can absorb light from a much wider range from the electromagnetic spectrum.0912

The job of a light-harvesting complex is to absorb light and - which we will talk about in a second - the absorption of that light excites these molecules.0927

These molecules become excited, and that energy is transferred to the reaction center.0940

Here in the reaction center are two...there is only one shown here. It is very schematic, but there are actually two chlorophyll a molecules.0946

And these chlorophyll molecules have special properties that allow them to actually donate an electron when the electrons are excited by this energy that has been transmitted.0957

And notice it says P680. I will talk in the next slide about what that means, but there are two different types of photosystems; and one, it will say P680 here, and the other is P700.0973

This is just a particular type of one of the two types of photosystems.0984

We have the reaction center complex. Light is exciting those pigment molecules.0989

The energy is being transmitted. Electrons are not transmitted.0993

It is just energy. One excites the next.0998

Next excites the next until it gets to this reaction center chlorophyll molecule.1000

And at that point, the excited electron from the reaction center chlorophyll molecule, two electrons are transferred to what is called the primary electron acceptor.1006

What has happened here is that light energy is turned into chemical energy through this transfer of electrons.1017

That is what the photosystem is doing. It is turning light energy into chemical energy.1031

Let's go ahead and look at a couple ways in which these electrons can be transferred because the first step here is the transfer of the electrons from P680 to this primary electron acceptor.1042

This is the redox situation transferring...that we are going to see a series of redox reactions.1055

This primary acceptor has the electrons, and then, we are going to see a series of redox reactions, and let's go ahead and look at this.1061

There are two ways in which electrons can be transferred in photosynthesis: linear or noncyclic photophosphorylation and cyclic photophosphorylation.1067

This is linear electron transfer, and you will see in this sketch compared with the next one that the electrons take a linear path from one to another or another, another and on.1079

This orange, or excuse me, the orange is the light.1089

The purple is the path the electrons are taking.1092

And you will what cyclic electrons transfer or cyclic photophosphorylation you are going to see the electrons actually flow in a circle.1097

First of all, what is photophosphorylation?1106

Well, photo means light. Phosphorylation means to add a phosphate group or phosphorus to a molecule, and what we want to end up here with is ATP.1109

We are using light energy to phosphorylate ADP, so ADP plus phosphate to make ATP, and light energy is used.1120

Light energy is harnessed for this reaction, so it is photophosphorylation, and here, it is with the noncyclic or linear electron transfer.1134

We already discussed the first step of this process, but I will recap it.1144

The first step is that light or photons hit the pigment.1148

Right here, which shown all of this, is the light-harvesting complex. Here is the reaction center.1156

Light hits the light-harvesting complex, and these photons are absorbed by the pigments here, and that excites the electrons in these pigments.1165

These electrons move to a higher energy state.1176

That energy, then, is transferred from pigment molecule to pigment molecule and then, eventually to the reaction center.1179

The reaction center here in photosystem II is called P680, and that is because this type of chlorophyll most effectively absorbs light energy of the wavelength of 680.1191

Over here, the other photosystems is photosystem II P700, and it most effectively absorbs light of a wavelength of 700.1205

Energy transferred from molecule to molecule in the light-harvesting complex eventually makes its way to transfer1219

the energy to these special chlorophyll molecules in the reaction center complex called P680.1225

When this molecule becomes excited, and its electrons are transferred to a higher energy level, the result is going to be the transfer of an electron.1233

And a total of two electrons are transferred by these chlorophyll molecules here to the primary electron acceptors, so this is the primary electron acceptor.1244

Light energy is being converted to chemical energy, so primary electron acceptor.1253

Those electrons are, then, going to be transferred through an electron transport chain.1261

And we talked in detail under the aerobic respiration lecture about what an electron transport chain is and how it works, and these electrons are transferred down in a series of redox reactions.1268

Let’s back up before we go any further with that and look at what is happening here.1280

P680 has lost two electrons. It needs to replace those electrons.1284

Where are those electrons going to come from?1289

Well, remember that water is required for photosynthesis, and what happens is the reaction center can perform photolysis.1290

Photolysis, if you look at the word water - excuse me - of photo for light and lysis for split or break, so this is the splitting of a molecule of water.1300

The reaction center, in addition to transmitting this energy around and donating these electrons to the primary electron acceptor, something else that happens in the reaction center is water is split.1314

Water is H₂O. It is two hydrogens, one oxygen, and two hydrogens are composed of two electrons and two protons or two H⁺ molecules.1325

When water is split, you end up with two electrons, two protons and an oxygen.1345

Well, the oxygen is a by-product of photosynthesis, and it ends up just leaving the plant.1351

Notice that oxygen is written as 1/2 O₂, and the reason is oxygen, as soon as it is split off, immediately combines with another oxygen molecule to form O₂.1356

That is the state that it usually exists in instead of just saying "oh, we call it 1/2 O₂ because it is usually floating around as O₂".1367

OK, H₂O, we have accounted for the oxygen. We have two electrons and two protons.1375

The two electrons are going to go ahead and be transferred to P680, and they will replace the electrons that P680 transferred to the primary acceptor.1382

P680 transferred two electrons. The reactions that are in a complex splits water, and two electrons from the water are used to replace electrons in P680.1398

In addition, this should actually be two protons are released.1408

The two electrons are transferred to P680. In addition, 1/2 O₂ is released, and two protons are released.1415

OK, P680 had its electrons replaced. Now, let's look at what is happening here.1424

Electrons were transferred to the primary electron acceptor, and now, they are being passed along down the electron transport chain.1429

This electron transport chain is found in the thylakoid, and it consists of a series of protein complexes and carrier molecules and here where the first one, PQ is plastoquinone.1441

You do not have to memorize the whole name, but just so you know, this is plastoquinone. That is what that stands for- a cytochrome complex and plastocyanin.1457

These are the components of the electron transport chain, and recall with the electron transport chain in oxidative phosphorylation, that electrons are passed from one carrier molecule to the next.1467

And each electron carrier molecule is more electronegative than the one preceding it.1480

These electrons are going to a more electronegative molecule to even more to even more.1486

Because they are being passed to more electronegative molecules, the free energy is decreasing.1491

The free energy is decreasing as they go down here, and that release of energy can be used to generate a proton gradient.1497

Just as with oxidative phosphorylation, a proton gradient is created.1505

Remember that in photolysis, these protons were released.1511

Well, these protons are picked up by the electron transport chain and pumped.1515

They create a proton gradient in which there is a higher concentration of hydrogen ions in the lumen or in the space inside the thylakoid than there is out in the stroma.1521

This creates a proton. This electron transport chain creates a proton gradient between the thylakoid space and the stroma.1536

Recall in the chloroplast, we have the thylakoid stacked into grana and then, the stroma out here.1555

And that is going to end up being a gradient between these two areas, where there is a higher concentration in the thylakoid of protons and a lower one in the stroma.1565

Just like with oxidative phosphorylation, energy is released as these protons diffuse down their concentration gradient, and the energy is harnessed to phosphorylate ADP.1574

That is what we see here. ADP plus inorganic phosphate are combined to form ATP.1586

Remember the enzyme ATP synthase can couple hydrogen ions going down their concentration gradient with the syntheses of ATP because ATP synthase contains a proton channel.1593

This is another example of chemiosmosis, very similar to what we saw earlier that was happening in the mitochondria with oxidative phosphorylation.1605

The transfer of electrons down the electron transport chain to more electronegative molecules releases energy. The energy is used to pump protons.1618

Those protons form a gradient, and then, energy is harnessed as the protons flow down their concentration gradient through the channel ATP synthase, and ATP is formed.1630

Backing up for a second and realizing, the purpose of the light reactions in photosynthesis is the formation of ATP and NADPH.1642

Both of which are needed for the dark reactions, for the light-independent reactions.1650

With ATP and NADPH and CO₂, it is possible to make glucose during the second part of photosynthesis.1655

Water is also needed for photosynthesis, and we see that here because those electrons are needed to replace the electrons transferred from P680; and it is a source of hydrogen ions for that gradient.1666

Aright, what is going on over here in photosystem I?1679

We focused on photosystem II, electron transport chain and ATP being made.1683

Meanwhile, while light is hitting photosystem II, it is also hitting photosystem I.1688

Again, light energy is absorbed by the pigment molecules, so these photons are absorbed.1696

The pigment molecule electrons become excited, and this energy is transferred from pigment molecule to pigment molecule in the light-harvesting complex; and then, it excites the electrons in P700.1702

Two electrons from these chlorophyll molecules are transferred to the primary electron acceptor in photosystem I so very similar to what happened in photosystem II so far.1716

Those electrons from the primary electron acceptor also go down in electron transport chain, so we have another electron transport chain here.1730

However, there is no proton gradient created here. There is no ATP created here.1738

Instead, the end point for these electrons is that they are donated to NADP⁺ to form NADPH.1744

NADP⁺ is an electron carrier similar to NAD or FAD that we talked about in earlier lectures.1753

And this is needed for the redox reaction in the Calvin cycle or in the light-independent reaction we will talk about in a few minutes.1764

Alright, the only thing that is missing here is how do we place the electrons that were transferred from P700?1774

Recall that in P680, it loses two electrons, but that is OK.1781

It splits water in photolysis, and those electrons are replaced. That does not happen here.1784

Instead, look where we stopped here.1790

Electrons transferred in photosystem I to the primary electron acceptor and down the ETC, and then, what happens to them?1793

Well, actually, those two electrons are donated to P700 to replace the electrons that were transferred to the primary electron acceptor.1801

This is a pretty complex cycle, but the important things to know is that there are two photosystems involve that light energy is used and collected by the antenna complex or the light-harvesting complex.1814

It is transferred to the reaction center complexes.1828

The chlorophyll molecules there donate electrons to the primary electron acceptors, and those electrons proceed down two electron transport chains.1835

The first electron transport chain uses chemiosmosis. It generates a proton gradient, and the energy from that is harnessed to make ATP.1846

The electron transport chain associated with photosystem I does not have a proton-motive force, a proton gradient.1856

It does not make ATP instead, it reduces NADP⁺ to NADPH.1867

We have these two things that are needed for the light-independent reactions.1874

The electrons in photosystem II are replaced by two electrons gained from splitting water, from photolysis.1878

The electrons in photosystem I that were donated to the primary acceptor are replaced by these electrons from photosystem II.1886

This is noncyclic photophosphorylation. The flow of electrons is linear.1897

In contrast, in cyclic photophosphorylation, there is only one photosystem involved, and this is photosystem I.1906

And you can already see that there is not a second photosystem here donating electrons linearly.1913

There is a single photosystem, and the electrons go in a cyclical pattern.1919

The first step is the same. Sunlight is used as the energy source.1926

Those photons excite the electrons in the light harvesting complex, in the antenna complex.1934

That energy is transferred from pigment molecule to pigment molecule and then, eventually to the new pigment molecule in the reaction center complex, the pigment molecules.1941

Same as before, two electrons are, then, donated to the primary electron acceptor.1956

What happens next is those two electrons are passed along the ferredoxin, a carrier of electrons.1963

And you recall in photosystem I in the last slide, this was the first electron carrier that photosystem I donated to in noncyclic photophosphorylation as well.1971

The second step is different, however.1982

The next step is ferredoxin donates those electrons through a redox reaction to cytochrome complex, then, to plastocyanin.1985

Then, those electrons go back and replace the electrons lost from P700 in the first place, so it goes in a cycle.1995

This is different than what we saw in the previous slide with noncyclic phosphorylation because there is no NADPH formed, and the flow of electrons is circular rather than linear.2008

However, there is still the production of ATP.2024

Some photosynthetic bacteria use this system only. Others use both.2032

It is possible that this is actually an older system than the linear electron flow that we talked about.2039

But either way, the cell ends up with the ATP that it needs in order for the light-independent reactions to occur and glucose to be produced.2046

There is no NADPH. However, there is a proton gradient, so check.2057

There is a proton gradient produced, and through chemiosmosis, ATP is produced- so much simpler system than the previous one.2062

OK, the light-independent reactions are the step of photosynthesis in which glucose is formed, and this is called - this light-independent reaction we are focusing on - the Calvin cycle.2075

And during the Calvin cycle, a sugar called glyceraldehyde-3-phosphate is formed, so glucose is not directly formed by the Calvin cycle.2087

Instead, G3P or PGAL...this is all the same sugar, glyceraldehyde-3-phosphate, PGAL and G3P are all the same, and 2 PGAL molecules can be used to form glucose.2094

Even though in photosynthesis, the Calvin cycle does not directly form glucose, it forms a sugar that can be used to make glucose.2110

Let's look at what is happening.2120

CO₂ is input in this cycle, and it is fixed.2122

When we talk about being fixed, that a carbon molecule is fixed, we mean that it is being incorporated into an organic molecule, into an organic compound.2127

We start out with ribulose bisphosphate or RuBP, which is a 5-carbon molecule.2137

A CO₂ is added, and this is catalyzed by the enzyme RuBisCO to form the six, a very unstable intermediate molecule.2142

It is six carbons. It is unstable.2152

It quickly breaks down into two 3-phosphoglycerate molecules.2154

Now, you will notice that I wrote CO₂ x 3 here.2160

The CO₂s enter one at a time, but it is good to look at this cycle in terms of three turns of the cycle2164

because it would take a total of three CO₂s to get enough new carbon in there to generate the equivalent of one PGAL.2170

After each turn, a PGAL comes out, but you have only added one carbon.2179

So, if you ask how many CO₂s would you have to put in to really have enough carbon to create PGAL? It is three turns.2183

And it would require six turns of the cycle to generate glucose because you need two PGALs, six carbons total, for a glucose.2191

In this first step, RuBisCO fixed this carbon into ribulose bisphosphate to form an unstable intermediate, and if this happens three times, we are going to get three of those.2202

This 3-carbon molecule splits into two, so that is 3 x 2, that is 6. Therefore, it is three phosphoglycerate.2214

Here is where the ATP comes in.2223

Remember that the light in the light-dependent reactions, the light reactions that we just talked about, generate ATP, and that ATP is used to phosphorylate 3-phosphoglycerate.2226

This is the phosphate source to form 1,3-bisphosphoglycerate.2240

The next one we have is a redox reaction.2246

Again, from the light reactions, we got NADPH. This NADPH is oxidized and 1,3-bisphosphoglycerate is reduced.2248

In addition, one of the phosphate groups is removed.2262

The phosphate is lost from the substrate molecule, and it is reduced to form glyceraldehyde-3-phosphate or PGAL.2267

After three turns of the cycle, we would end up with six, six, six PGALS. One of these leaves the cycle.2278

OK, we are left with five PGALs. One leaves the cycle, and this one that leaves can be used to form glucose.2289

It can be used as a backbone for other molecules. It can be used to form other organic molecules.2298

This one is gone. That leaves five.2306

Now, let's keep track of the carbons. It is always important in cellular respiration to keep track of the carbons.2309

I have three carbons in this molecule. PGAL is a 3-carbon molecule, and one left the cycle.2316

I have five left. 5 x 3 is 15 carbons.2323

From those 15 carbons, you can get three RuBPs because RuBP is a 5-carbon molecule. 5 x 3 is 15 carbons.2328

By rearranging and phosphorylating these, we get RuBP back, and this phosphorylation, you need a source of phosphate; and the source again, is ATP.2342

Here, we see ATP, ATP and NADPH being used, and those came from the light reaction.2352

Even though the sun does not have to provide energy directly into the Calvin cycle, that energy was needed to generate ATP, which is being used by the Calvin cycle.2358

In order to get one molecule of glucose, what did we have to use?2373

Well, to get one molecule of PGAL, we had to use nine ATPs and six NADPHs per PGAL.2377

Remember though, it takes two PGALs to have enough carbon to make a glucose.2389

Therefore, we have to use eighteen ATPS and six NADPHs per glucose molecule created by photosynthesis.2394

This pathway for the Calvin cycle is called the C3 pathway, and it is used by C3 plants.2407

And the reason is the first true metabolic intermediate created is 3-phosphoglycerate, which is a 3-carbon molecule.2414

This is so unstable and quickly splits. We do not even count that one.2422

We just look at this and say "OK, 3-carbon molecule". This is the C3 pathway.2426

There is another pathway that can occur that is called photorespiration.2433

And it has to do with the fact that that first enzyme I mentioned that catalyzes the first step of the reaction, RuBisCO, RuBisCO can also bind to oxygen.2439

Remember that RuBisCO fixes carbon. It fixes CO₂ to form an organic molecule using RuBP plus CO₂ to form 3-phosphoglycerate.2453

We get RuBP plus CO₂ to get that 3-carbon molecule, 3-phosphoglycerate.2468

That is what we want to happen- photosynthesis. The Calvin cycle occurs.2479

PGAL will be produced. Glucose will be produced.2484

However, RuBisCO also binds oxygen.2486

Normally, under typical conditions, this may not be a problem, but under certain conditions, it can create a problem and in fact, stunt the plant growth.2501

The reason is, if oxygen is fixed then, the plant is not going to be performing photosynthesis.2510

This RuBisCO is, instead, binding O₂.2518

It is going to divert the plant, at least, partly from photosynthesis, and instead, what is going to happen is the process called photorespiration.2521

This is most likely to occur under hot, dry conditions, and the reason is that hot, dry conditions leads to decreased CO₂ concentration. Why is this?2531

Well, remember that the stomates allow for gas to enter and leave the plant.2545

The stomates open. CO₂ enters.2551

Photosynthesis occurs. Oxygen is created.2556

Oxygen leaves. The only problem is a lot of water is lost in the stomates.2559

Under hot, dry conditions, the stomates will close to conserve water.2565

The result is that the CO₂ level drops, and because RuBisCO has an affinity for oxygen, when CO₂ is low,2571

there is a relatively higher concentration of oxygen, lower concentration of CO₂. It grabs on to that oxygen.2578

OK, what happens? Instead of fixing CO₂, it fixes oxygen, and what results in photorespiration is oxygen is fixed; and it forms a 3-carbon compound.2586

What ends up happening in that compound? Well, it exits the chloroplast.2604

The compound is taken up by peroxisomes. Inside the peroxisomes, it is broken down, and CO₂ is released.2610

Seems like a waste because you will notice no ATP was produced.2624

No NADPH is produced. No glucose was produced2631

The point of photosynthesis is to form glucose.2638

It is not like normal photosynthesis. You are not getting glucose out.2644

It is called photorespiration, so it is a type of respiration; but in regular cellular respiration, when we talked about aerobic and anaerobic respiration, ATP was produced.2649

This is not really doing anything useful, and in fact, the plant's growth get stunted.2659

Examples of C3 plants are plants like rice and wheat, and you might ask “well, why does the plant even do this?”.2665

Well, nobody is really sure, but it is possible that this is just an evolutionary left over.2671

It is a remnant from evolution because long ago, evolutionarily, there was a lower oxygen level in the atmosphere2676

So, it is possible that RuBisCO binding oxygen was not a problem because the conditions were higher in CO₂.2688

And even though there was that capability, it never was a positive or a negative. It just did not really have a big effect.2697

Under current conditions, when the oxygen level was higher, it becomes a problem because, then, RuBisCO does bind the oxygen. Photorespiration does occur.2703

That is one theory on why this may occur.2716

What do plants in hot, dry conditions do so that they can continue to grow and thrive and not have this photorespiration diverting the process of photosynthesis?2720

Well, there is a couple of adaptations.2731

There are certain group of plants called C4 plants, and these are plants found in hot, dry environments; and they have an altered leaf anatomy and a slightly different pathway for photosynthesis.2733

Let's first look at the leaf anatomy.2748

A couple things, we still have that upper epidermis and the mesophyll layer where photosynthesis takes place.2750

There is stomates that we talked about before, and here is the vascular bundle.2759

Now, you will notice this group of cells around the vascular bundle, and these are called bundle-sheath cells.2765

In C4 plants, the light reactions take place in the mesophyll cells, and the Calvin cycle, the dark reactions or light-independent reactions, take place in the bundle-sheath cells.2774

These two processes are separated out.2793

Let's look at what happens first in the mesophyll cell.2797

The light reactions are taking place here, and, as usual, ATP is being produced; and something else happens in these mesophyll cells. CO₂ is fixed.2803

In C3 plants, the CO₂ fixation initially occurs in the Calvin cycle.2818

That is where it is first integrated into an organic compound, but the problem is, the enzyme that is doing that fixing RuBisCO, can bind to oxygen.2823

The CO₂ comes in, and instead of just letting it go into these bundle-sheath cells where the RuBisCO is waiting, it goes into the mesophyll cells.2833

And there, the carbon is fixed into an organic compound.2846

The good thing about this is that the enzyme that catalyzes the fixation of CO₂ in the mesophyll cell2851

does not bind to oxygen and has a very high affinity for CO₂, even higher than RuBisCO does.2859

What you can think of this as is that the mesophyll cells are just grabbing up the CO₂ and fixing it.2866

They fix it to form an organic compound oxaloacetate that you are probably familiar with from the citric acid cycle and then, used to form malate.2876

This malate is, then, transported into the bundle-sheath cell, where it is broken down into pyruvate, and CO₂ is released.2886

Notice that these bundle-sheath cells are sequestered. They are more towards the center of the leaf.2896

What has happened is a microenvironment is created within these bundle-sheath cells, where there is a high concentration of CO₂.2902

As long as there is a high concentration of CO₂, RuBisCO is fine because it will tend to bind the CO₂, and photosynthesis will occur.2910

Where problems occur, is when there is a high concentration of oxygen.2919

If you have these cells sitting over here, and there is oxygen, then, the RuBisCO might bind that.2923

Instead, the cell depends on an enzyme that binds only to CO₂ instead of O₂ to immediately fix the CO₂2930

and then, shunt it into these sequestered cells, where there is higher CO₂ levels, where a RuBisCO can safely perform the Calvin cycle.2937

There is this separation between the processes. The light processes take place here.2950

The Calvin cycle takes place in the bundle-sheath cell, and initial fixation of the carbon takes place in the mesophyll cell although it is exported out into the other cell.2956

Why is this called C4? Well, the reason is oxaloacetate is a 4-carbon molecule.2970

Remember in C3 plants with the Calvin cycle, that initial intermediate of the Calvin cycle is a 3-carbon molecule, so we call that C3.2976

Here, carbon is not being fixed initially in the Calvin cycle. It is being fixed in this process in the mesophyll cell.2988

And initially, we end up with oxaloacetate, a 4-carbon molecule, so these are called C4 plants; and this export of malate occurs through plasmodesmata.2995

Remember we talked about these openings between plant cells in an earlier lecture on cell structure.3005

The second method by which plants can avoid photorespiration is via the CAM pathway or Crassulacean acid metabolism- CAM.3020

CAM plants are plants like cacti. They are succulents that live in hot, dry dessert environments.3037

And in such an environment, these plants really need to keep their stomates closed during the day, or there will be too much water loss. There is a problem with that though.3043

Normally, the stomates are open during the day. That way, CO₂ can come into the plant.3054

And it is ready for the Calvin cycle at the same time of day when sunlight is available because the Calvin cycle needs the ATP and NADPH from the light reactions.3062

If the sun is out, light reactions are occurring. The stomates are open.3073

CO₂ comes in. The Calvin cycle is occurring.3077

That ATP is being used. The CO₂ is being used.3080

Glucose is produced. However, CAM plants have developed a way to pick up the CO₂ at night and hold on to it until the day time.3084

The way it works with these plants is they keep their stomates closed during the day. At night, the stomates open up, and CO₂ enters the cell.3097

There has got to be a way, though, for them to save that CO₂ until morning when the light reactions can occur.3110

And the way they do that is by fixing the CO₂ into an organic compound called malic acid.3116

CAM plants store carbon. They store the CO₂ in the form of organic acids, and this is all occurring in the same cell.3124

There is not the separation in the two different cell types.3143

The light and dark reactions can all occur in the same cell type. It is different than what we saw in the C4 plants.3146

OK, at night, the stomates are open. CO₂ comes in.3153

It is converted to malic acid.3158

In the morning, the stomate is closed. No CO₂ is coming in, but there is light energy during the day.3160

And the light reactions are forming ATP and NADPH, and now, the Calvin cycle can occur because the ATP and NADPH is available.3168

What happens during the day is the CO₂ is released and then, fixed as part of the Calvin cycle during the day when ATP and NADPH are available.3183

In the end, whether it is a C3 pathway, a C4 plant or a CAM plant, the same thing has happened with photosynthesis.3198

Water, CO₂ and light energy were used to produce glucose.3209

In C4 and CAM plants, we saw some steps prior to the Calvin cycle, different thing were done with the CO₂.3221

But in the end, it is all the same, and the plant is going to end up with PGAL molecules and thus, glucose.3228

That glucose can be used in cellular respiration to form ATP, so the energy in the glucose can be released. ATP can be used, and that is going to fuel the cellular processes.3241

This glucose produced or the PGAL can also be used as a carbon skeleton for other organic compounds in the plant.3257

Excess sugar made by the plant via photosynthesis is stored as starch.3265

Alright, let's go ahead and review what has been covered today.3272

In example one: although the Calvin cycle is called a light-independent reaction, in the absence of light the cycle cannot occur in plants. Why not?3276

Well, maybe it could occur for a little while, but it would quickly stop; and the reason is that the Calvin cycle requires ATP and NADPH, and these are produced via the light reactions.3285

Therefore, if you stick a plant in the dark, and you give it a plenty of CO₂, OK, there is plenty of CO₂, there is plenty of water, the plant dies.3314

Well, it is because it does not have the energy source or the NADPH needed for the Calvin cycle to occur, so we will not be reproducing glucose.3326

Even though it is not directly dependent on light, it is dependent on the products of the light reactions.3339

Below is the drawing of the leaf structure in a C4 plant.3350

Label the following: stomate, vascular bundle, cells that are the site of fixation of CO₂ resulting in the formation of the organic acid malate, site where the Calvin cycle occurs.3356

Alright, remember that C4 plants have a different anatomy.3373

This is also called kranz anatomy, and it allows the plant to minimize the photorespiration; and this is found in plants in hot, dry environments.3377

So, stomate right here, this is a stomate. These are pores that allow for gas to enter and leave the leaf.3388

Vascular bundle, that is the structure right here.3399

The vascular bundle contains the vascular tissue xylem and phloem that carries water, minerals and glucose - water and minerals for the xylem and nutrients for the phloem - throughout the plant.3404

Now, were talking about the C4 plant, so in C4 plants, carbon dioxide enters the stomates, enters the mesophyll cells and is initially fixed to form the organic acid malate or malic acid.3421

These cells are the mesophyll cells, and that is where CO₂ is going to initially fixed.3438

And then, that organic acid can be transported into the bundle-sheath cells released, forming pyruvate and CO₂, and the site where the Calvin cycle occurs is the bundle-sheath cells.3447

The RuBisCO is sequestered in these bundle-sheath cells in the interior of the leaf, where there is not going to be so much risk of binding oxygen and photorespiration occurring.3468

The mesophyll is the site of the fixation of CO₂ initially fixed, and then, it is released and fixed again in bundle-sheath cells.3480

The malate is formed in the mesophyll cells, and the bundle-sheath cells are the site of the Calvin cycle.3494

Stomate, vascular bundle, mesophyll cells, where CO₂ is fixed to form the organic acid malic acid and then, bundle-sheath cells where the Calvin cycle takes place.3502

Example three: list three major differences between photosynthesis and photorespiration.3516

Alright, photosynthesis is the process by which CO₂ is fixed - so photosynthesis - and glucose is produced.3525

Oxygen is a by-product, so oxygen is released.3558

In earlier steps of photosynthesis, ATP in the light reactions, they are made, but they are also used; so they are both made and used.3564

Now, photorespiration, photorespiration is a process that occurs when there is a low CO₂ concentration.3578

That occurs usually under hot, dry conditions, and it is not good for the plant, stunts the plant growth, and if we look at the differences, we will see why.3591

Instead of CO₂ being fixed and glucose being produced, oxygen is fixed.3599

CO₂ is produced. No glucose is made, and there is no ATP or NADPH made.3607

Photorespiration is really just a dead end process, whereas photosynthesis results in the formation of glucose, which is necessary for the plant to live.3630

Example four: most plants have their stomates open during the day in order to obtain carbon dioxide when the sunlight is available for the light reactions.3644

CAM plants have stomates open at night and close during the day yet, they are still able to undergo photosynthesis? How is this possible?3654

Alright, normally, stomates are open during the day.3664

The CO₂ enters the cell, and therefore, during the day when there is sunlight available, the light reactions occur.3668

ATP and NADPH are produced, and then, that along with the CO₂, is used for the Calvin cycle.3676

CAM plants have stomates closed during the day.3684

When the sunlight is available, and ATP and NADPH are being made, the stomates are shut, and the plant cannot get CO₂, how does it work?3688

Well, recall that CAM plants obtain CO₂ at night and store it in the vacuoles as malic acid.3698

They pick up the CO₂ at night, fix it into organic acids and then, store it in vacuoles.3718

In the morning, the CO₂ is released from the organic compound and used for the Calvin cycle at a time of day when the light reactions are occurring and ATP and NADPH are available.3727

That concludes this lecture on photosynthesis at Educator.com.3753

Thanks for visiting.3758