AP Biology Transport of Nutrients and Water in Plants

Welcome to Educator.com.0000

Today, we are going to talk about the transport of nutrients and water in plants.0002

And this will be specifically a discussion of the transport of nutrients and water in vascular plants, so plants containing xylem and phloem.0007

I am going to start out with a review of plant cell structure.0016

So, cell structure was covered in detail in an early lecture in this course.0019

However, there are certain elements of plant structure that you need to understand in order to understand how materials move through a plant.0023

Recall that plants have a cell wall, and the cell wall is made of cellulose embedded in a matrix of polysaccharides and proteins.0031

The cell wall provides strength and maintains the shape of the cell, and it also prevents the cell from lysing in a hypotonic environment.0041

Recall from our discussion on osmosis that water is going to move from an area of lower solute concentration to a higher solute concentration.0051

If there is a higher solute concentration inside the cell, then, outside the cell, water will move into the cell.0062

And eventually, if water keeps moving in the cell, and the cell continues to expand, and the volume increases, the cell could lyse.0074

The cell wall prevents this from happening. It provides a counterpressure that prevents water from continuing to rush in.0082

And we will revisit this topic in a little bit.0091

So, just for right now, the cell wall helps provide structure, rigidity and strength and prevents lysis from water entering the cell through osmosis.0093

The cell wall is located outside of the plasma membrane, so this line in black is the plasma membrane, the cell membrane.0106

Some types of plant cells have what is called a secondary cell wall.0116

This makes the cells more rigid, stronger, even less flexible, but that is OK because plants do not need to walk around and move around.0121

So, they trade loss of flexibility with greater strength, so the secondary cell wall.0131

This is just where the cell wall is often called the primary cell wall.0153

And there is a specialized region associated with the cell walls of plants, and it is sometimes considered actually a separate additional components.0158

And it is between the plant cells. It is what is called the middle lamella.0166

This is a layer on top of the primary cell wall, which is rich in pectin, and pectin is sticky. It is a thickener that is found in things like jam.0175

So, this layer helps to hold neighboring cells firmly together.0183

Now, within the cell are the typical components of eukaryotic cell. We have the cytoplasm, the nucleus, the other organelles.0189

Here is another example of the mitochondria. Plants, of course, also have chloroplast, the site of photosynthesis.0201

There is also the large central vacuole, which is the site of storage, another function in the plant.0210

And one important structure that I am going to really be talking about more today is the plasmodesmata.0219

And the plasmodesmata are these openings right here that connect the cytoplasm of one cell to the cytoplasm of the neighboring cell.0226

And that allows these filaments of cytoplasm to connect the two cells, and it allows for transport of material and communication from cell to cell.0239

Again, to really closely review structures and particularly organelles in some cellular structures, you should watch the video earlier in the lecture.0250

This is just a brief overview focusing on plasmodesmata and cell wall and those components0258

of the plant cell that we need to understand when we talk about transport in the plant cell.0264

We are going to start out by transport of water beginning with absorption of water from the soil.0268

So, in vascular plants, vascular plants have true roots and leaves, and water is absorbed by the roots as are dissolved minerals.0274

So, the water and minerals are absorbed by the root, and then, they are transported to the rest of the plant via the xylem.0282

Now, in order to absorb water effectively, roots need to have a large surface area.0288

This is accomplished a couple of ways. For one thing, there are root hairs.0295

Recall that root hairs are projections in the epidermal layer of the plant.0299

So, the epidermal covering of the plant in the area of the root projects out to increase the surface area.0305

Extremely important also is mycorrhiza. These are the symbiotic relationships that plant roots have with certain types of fungi.0311

The fungi benefit the plant by increasing the surface area available for absorption of water.0322

The fungi benefit because they take carbohydrates. They take nutrients from the roots of the plant.0329

So, this is a mutualistic relationship that benefits the plant via the increase in surface area.0335

Water enters the roots via osmosis, and before we go any farther, we are going to just talk about osmosis and the concept of water potential.0344

As I mentioned in the last slide and as you probably remember, in osmosis,0355

the water is going to go from a region of higher water concentration to a region of lower water concentration.0361

It is going to move down its concentration gradient.0370

If you looked at it the other way, which is somewhat easier, if you looked at it the other way,0374

you can just look at solutes and say "OK, the water is going to go from a region of low solute concentration to high solute concentration".0378

High water concentration equals low solute concentration.0390

Here, there is a lot of solutes and less water, so water is going to go from higher to low solute concentration.0393

And this is what we focused on when we talked about diffusion with an animal cell.0402

However, in plant cells, we are going to talk more about water potential.0407

And this is because in plants, what is important is also pressure because of that cell wall.0411

So, not only does solute concentration affect the movement of water, pressure affects it as well.0418

Water potential, and this is designated by the Greek letter psi, takes into account factors such as solute concentration and pressure.0424

And what you should remember is that water moves from areas of high or less negative water potential to areas of low or more negative water potential.0436

The water potential of pure water is zero, so the water potential equals zero for pure water.0474

If a solution is more concentrated, it has more solutes in it, there will be more of a tendency for water to want to go there.0486

So, the water potential will be negative, so if I add solutes, water potential is going to decrease.0496

And remember, water wants to go from an area of higher to lower water potential.0505

Now, how does pressure play into this? Remember that in a plant cell, there is not just the cell membrane, but there is the cell wall.0509

So, if water enters the cell, it is going to push against this cell wall, but the cell wall is going to push back.0520

This pressure is going to prevent more water from entering. Therefore, adding pressure is going to increase the water potential.0530

So, if I add solutes, the water potential is going to go down. Now, if I add pressure, the water potential will go up.0544

Adding solutes will lower the water potential. It will make it more negative.0556

Adding pressure will increase the water potential. It will make it less negative.0562

It will move it more towards zero.0567

And when you look at a cell, and you look overall, what is the pressure? What is the solute concentration?0569

And you take all that together, you can figure out a water potential.0575

And if the water potential is more negative inside the cell, water is going to move in that direction.0578

Therefore, if you have a root sitting in the soil, and the water potential inside the root is lower than the water potential of the soil,0584

then, water is going to move into that root cell; and that is what is going to determine the direction of the movement.0593

Once the water actually, and the dissolved minerals, make it into the root cell, how do they move around?0603

Well, they enter that root hair and pass into the cortical cells, so here is a cross-section of the root.0608

Here is the cortex, and the size of these cortical cells is greatly exaggerated.0617

Right here is the vascular cylinder, but I want to show you a close-up of these cortical cells.0622

So, here is the vascular cylinder with the xylem and the phloem inside it.0626

And here in black is the endodermal layer surrounding the vascular cylinder.0633

So, what is going to happen is if the water potential is lower inside the root than it is outside, then, water will enter the cortical cell.0641

And then, it is going to move towards the center of the root into the vascular cylinder, but how does it move there? How does it get there?0656

How does it get to the xylem to even be transported up to the leaves, up through the stem?0665

Well, there are two routes, and these two routes are the apoplast route and the symplast route.0671

And to understand these two different routes, I just want to talk about some terminology.0676

If you put together the cell membrane, the nucleus, the cytoplasm, the organelles, the parts of the cell that are alive are called the protoplast.0684

And the symplast pathway involves movement along the protoplast.0691

By contrast, the apoplast involves movement along non-living parts of the cell such as the cell wall and the intercellular spaces.0698

So, two routes are shown below, the apoplast route - excuse me - the symplast route, symplast pathway is in black.0710

To get into that first cell, let me actually close this off because what is going to happen is let's say this is the plant cell,0721

the cortical cell that is on the very edge.0729

It is a cell that is right next to the most outside cell of the root.0733

So, if there is some water, here is the soil solution, and let's say water is going to go ahead and enter the cell,0739

to get into the symplast, it is going to cross the cell wall.0746

It is going to have to cross the cell membrane, which is very important, and then, it is going to enter the cytoplasm; so the cytoplasm is here in grey.0751

Now, once it has crossed that membrane, this water, these minerals, they are in the cell.0759

They can just travel along the cytoplasm through the plasmodesma between these two cells, go through the cytoplasm,0765

cross through the plasmodesma and keep going from cell to cell just remaining in the cytoplasm once it has crossed that first cell membrane.0775

It is a different story with the apoplast.0784

With the apoplast, what is going to happen is here, we have the soil, and some water will enter the cell wall of that first cell.0786

But, it is not going to cross the cell membrane. The cell membrane is in here.0799

And so, to go to the symplast route, the water, the minerals, had to cross the cell membrane.0804

With the apoplast route, it does not. It just goes in the cell wall, goes along.0811

It could cross some intercellular space between these two cells, goes along the cell wall, crosses intercellular spaces, and it goes on and on that way.0816

Now, there is a third route. I am focusing on these two routes.0824

But, I do want to point out that there is also the transmembrane route, transmembrane meaning crossing the membranes.0827

So, in addition to crossing this membrane, there are other membranes.0832

If let's say that some fluid is travelling along, and it does not go through the plasmodesma, for some reason, it, instead, crosses the cell membrane,0836

goes across the cell wall, crosses the cell membrane of the adjacent cell and then, goes back in the symplast route.0846

Or it could even cross and then, stay in the apoplast route, so a material could start in one, go on the other, go back and forth.0853

And then, also, there is another membrane around the vacuole.0862

So, if the material enters the vacuole, or it leaves the vacuole, it is also going transmembrane.0867

But, really focusing mainly in apoplast versus symplast right now, you would be aware that there is also the transmembrane route.0873

Now, taking this and looking at it in the context of the root, this movement is lateral.0883

At this point, the water is not going up to the leaves. It is just going from the outside of the root towards the middle of the root.0890

Apoplast and symplast pathways are good for lateral movement like this because it is not0901

very far just going from the outside to trying to get to the xylem, to the vascular cylinder.0906

But, to move up through the xylem, apoplast and symplast pathways are too slow. These are only for short distances.0913

We will talk about what happens when they enter the vascular cylinder, and then, that is bulk flow; and that is a completely different method.0921

So, what happens is, the material, the water, it goes either apoplast route or symplast route until it gets to the endoderm, this barrier.0927

And now, I am going to talk about what happens when it gets to the barrier.0940

Although, I do wanted to just give one more caveat, and that is this is not all just diffusion.0944

So, I mentioned that water does move to this area of more negative water potential.0950

However, sometimes, active transport is involved. It could be from this is outside the root to inside.0957

Diffusion can occur, but active transport may also occur for example with potassium.0964

In order to get potassium from the soil solution into root cells, it may require active transport.0972

Remember that from an earlier lecture, if a molecule is moving against its concentration gradient,0977

it is moving from a lower to a higher region of concentration.0984

Then, that will require the input of energy that requires active transport.0987

And in plant cells, that energy often comes from a proton pump that...0992

So, active transport, often, there is a proton pump that creates a hydrogen ion gradient.0998

And then, the energy is utilized, harnessed from a hydrogen ion that is going down their concentration1006

gradient to get a molecule such as potassium into the cell, into the root via active transport.1013

So, one way or another, the minerals, the water has made it in, and now, it has gone through apoplast or symplast or transmembrane route.1022

And it gets to this final layer of cells, so I will draw this endodermal layer; and this endodermal layer is a very important barrier.1031

There would be one more cell here. It cannot just go symplast, symplast, symplast and then, just pass through.1041

There is this one more layer, and it is not going to have that plasmodesma opening that it can just pass through.1046

There is a blockage because remember, when we talked about the endodermal layer, I mentioned that there is a Casparian strip around these cells.1052

And there is a waxy substance called suberin in certain areas of the cell walls of the endodermal cells.1064

The endodermal cells are very tightly packed together, and they have this Casparian strip.1074

And the thing that this strip is going to do is it is going to prevent water or minerals from just slipping by, from slipping between the cells.1081

Now, this is really to block any kind of material from entering the xylem without ever having crossed a cell membrane.1096

Whereas, with the symplast pathway, the materials have already crossed a cell membrane.1111

So, fluid and minerals that have gone the symplast route are OK. They crossed a cell membrane to get in.1118

They are in this cytoplasm. They can actually just continue on.1123

There is plasmodesma.1129

I want to clarify that there is the plasmodesma it can go through, but that is only if there is a symplast route can it just keep going that route.1132

So, it is going to go through the plasmodesma of the endodermal cell.1139

It is going to cross that cytoplasm and then, enter the vascular cylinder from the symplast route.1144

Now, the apoplast route, though, could have been minerals that got into the cell wall,1149

went along the cell wall, went along the cell wall and never crossed the plasmodesma.1155

I mean, it never crossed a cell membrane, but there needs to be some type of screen.1163

So, this is going to ensure that rather than just slipping through and continuing1167

the apoplast route and slipping between cells, the Casparian strip blocks that.1171

The only way to get across the endodermal layer for the apoplast route materials is to actually cross through the endodermal cell.1177

This is a final screening. It is a final check to make sure that anything making its way into the xylem has passed through a cell membrane.1187

This allows for regulation of water movement, that water and minerals cannot just enter the xylem unregulated, and it is controlled.1199

And the final control here is this endoderm,1209

which prevents passage of water and minerals because of this impenetrable Casparian strip that prevents fluids from slipping between cells.1212

Now, once in the vascular cylinder and moving through the xylem, there is no symplast route for the xylem because the xylem are not living cells.1224

Protoplast is the living parts of the cell, and that constitutes the symplast route.1239

However, since xylem are not alive, there is no symplast route with the xylem.1245

Now that the water and minerals have finally gotten from the soil solution through the cortex into the vascular cylinder,1251

we are going to talk about how water and minerals move up the xylem to the rest of the plant.1257

And we are going to begin by reviewing the structure of the xylem.1264

Recall that the xylem is composed of two cell types in the angiosperms: tracheids and vessel elements.1267

And these are arranged into tube-type structures or chains of cells one after the other.1277

The tracheids are longer and thinner, whereas, a vessel element tends to be shorter and wider.1286

Also, the cell walls in the tracheids are generally thicker than the cell walls of vessel elements.1293

Xylem has a secondary cell wall, so in xylem, there is this, remember, the secondary cell wall, which contains lignin.1299

And lignin makes these cells much stronger, and they provide support for the plant as well as transporting water and minerals.1309

In the tracheids, water flows from cell to cell through regions called pits, and these regions lack the secondary cell wall.1317

In vessel elements, the vessel elements are lined up one on top of the other, end to end.1334

And at the end of a vessel element are regions that have openings in them, and the water can pass through those openings.1338

Here, the water is passing through the pits.1347

Flowering plants developed more recently in evolution. They have both tracheids and vessel elements, whereas, gymnosperms have only tracheids.1350

So, gymnosperms do not have vessel elements, just tracheids.1360

This is the cellular structure of xylem. At last, the water has gone from outside the root through the epidermis through the cortex,1368

finally, entered the vascular cylinder, made it into the xylem, then, what happens?1378

Well, transport of water and solutes through the xylem relies on bulk flow.1382

It is much faster than the apoplast and symplast routes that I talked to you that1387

are good for movement across short distances that are good for lateral movement.1392

But, to move water from the root all the way up to the top of a very tall tree1395

would be far too slow with the apoplast, symplast or transmembrane routes.1401

Bulk flow is the result of a pressure gradient, and this pressure gradient is caused by transpiration.1407

Transpiration is the loss of water from the leaves, so water is lost primarily out the stomata and then, evaporates,1417

so evaporation of water from the leaves.1426

Recall that leaves have stomata that open up and allow for gas exchange, so the plant can get CO² and then, let oxygen out.1436

However, a lot of water is lost through an open stoma, so water exits the leaf through the stoma because it wants to move to the drier air outside.1445

And if you think about it this way, outside the leaf especially if it is a dry day,1456

a dry windy day especially, outside the leaf, the water potential is going to be lower.1461

It is going to be more negative. Water is going to want to move down into that area of lower water potential.1467

It is going to leave the stoma, go outside, evaporate.1475

As a drop of water evaporates from the leaf, a column of water is pulled up.1480

And you can think of it as one drop goes out the stoma. Another drop is pulled in via the root.1487

And this is due to the negative pressure created by transpiration, by the evaporation of water at the very edge from the leaf.1493

It creates a negative pressure that creates a force that can pull a column of water all the way up from the roots.1503

Now, this is possible because of the properties of water.1511

The properties of water were covered in a lecture on the chemistry of life, but it is important to review this and to understand this.1514

Remember that water is both cohesive and adhesive.1521

Cohesion refers to the property through which molecules stick together. Water is very cohesive.1527

It is highly cohesive because recall with the structure of water, there is hydrogen bonding between neighboring water molecules.1536

And when one water molecule is lost from a leaf via transpiration, there is a negative pressure created that pulls the next molecule up.1544

And that water molecule pulls the one behind it and so on and so on all the way1552

down until the column of water is pulled up through what is called a capillary action.1557

Bulk flow is a result of this capillary action.1564

In addition, water is adhesive. Adhesion refers to the ability of a molecule to stick to other substances.1571

So, molecules stick to other substances just as when you have seen water stick to the side of a glass. That is adhesion.1579

So, water not only sticks to other water molecules, but through hydrogen bonding, it also can stick to the walls of the xylem.1592

And adhesion plays a role in getting that column of water to move up through capillary action.1599

Transpiration occurs, and through cohesion and adhesion, this chain of water molecules is pulled up one after another after another.1607

Factors that would increase transpiration are going to increase this movement, like I said, dry windy condition, more transpiration.1614

However, if the stomata closes, there is going to be less transpiration, less of this negative pressure gradient.1621

So, bulk flow is the means through which water and minerals are transported up hundreds of feet even in some trees.1627

That is transport of xylem sap.1638

Now, we are going to talk about transport of nutrients in plants, and they are transported via the phloem.1640

So, we will start with the review of phloem structure.1645

The phloem conducts nutrients from the leaves, not just the leaves though, also from their site of storage- from leaves or storage sites.1647

To get a little deeper in now, it is not just site of photosynthesis.1658

But, remember that nutrients can be stored in various areas of the plant such as in the roots.1662

Sieve tube elements...here is an example of phloem, and it consist of sieve.1671

The cells are called sieve tube elements. That is one cell type, and the second cell type are the companion cells.1677

Sieve tube elements lack nuclei. Their job is to actually transport the nutrients.1683

And at the ends of these are what is called sieve tube plates, and near the sieve tube plates are companion cells.1689

And the companion cells associated with sieve tube elements, sieve tube plates specifically,1703

they are believed to regulate the flow of nutrients through the phloem, and these actually are nucleated. They have a nucleus.1713

The transport of phloem sap is not through bulk flow. It is through what is called translocation.1725

Phloem sap moves from what is called a sugar source to a sugar sink, and this is translocation.1730

So, first of all, what is a sugar source? A sugar source is a site of sugar production, and you have to think of production more broadly.1740

Production, a lot of it in the plant is literally production of sugar from photosynthesis, but sugar may be also produced by breaking down starch.1748

Sugar can be produced by breakdown of starch, release of a storage.1757

So, sugar can be released because it has been just made, or it can be released because it was stored; and it is being broken down.1765

So, breakdown of starch because anywhere where sugar is released, it is given, it is produced as a sugar source.1770

A sugar sink is a site of the use of sugar. This use could be consumption.1780

The plant cell needs it or to use for growth, to use for energy, or it could be used for storage.1786

So, anytime a structure is taking sugar, it could be for storage or for consumption. That is a sugar sink.1794

Examples of sugar sinks would be an area of the plant that is growing, growing structures like young leaves or fruit. A fruit consumes a lot of sugar.1803

Tubers stores sugar. Potatoes are tubers, so OK, tubers, when they store sugar, when they are taking the sugar, they are a sugar sink.1822

Now, bulbs, that is another area where sugar is stored in the form of starch. They store sugar.1836

These are sugar sinks. They are taking sugar.1844

What is a sugar source? That is an area of production or release of sugar.1849

Leaves: young leaves that are growing, they are mainly taking the sugar, but when they are mature, mature leaves are mainly producing sugar.1854

Mature leaves produce sugar.1864

Now, the same structure can be a sugar sink at one time and a sugar source at another.1866

When the tuber is releasing sugar, when it is breaking down the starch, when it releases sugar,1871

it becomes a sugar source the same with the bulb when it releases sugar.1880

Unlike xylem sap, this is not just one direction of movement. With xylem, we talked about water going from the roots up the plant to the leaves.1886

In this case, sometimes, a sugar could be going towards a tuber.1896

Sometimes you are saying "going away from the tuber" when the plant needs to use its storage sugar.1899

So, this is not just one direction of flow, and this is a big different between the movement of xylem sap and the movement of phloem sap.1905

To give you an example of translocation, sugar is manufactured in the mesophyll of the leaves.1914

And then, it travels from those mesophyll cells to the vascular cylinder. It enters the phloem.1923

It enters the sieve tube elements of the phloem, and in fact, this process may involve active transport.1930

It may actually take active transport to get that sugar from its source into the phloem.1938

Now, it is in the phloem. It travels through this chain of sieve tube elements until it reaches the sugar sink.1943

Once it reaches the sugar sink, let's say an area of the plant that is rapidly growing, a young leaf,1953

it can enter that young leaf cell because it will now be going down its concentration gradient.1960

So, it might take active flow to initially unload the sugar into the phloem.1967

But then, when it is offloaded at its destination, it will usually go just down its concentration gradient to enter passively the cell that requires sugar.1973

That is translocation.1982

What we covered in this lecture was the movement of water and minerals in the xylem and the movement of nutrients through the phloem.1985

In our first example, it is matching. Match the following terms to their descriptions.1999

First is apoplast: channels that connect the cytoplasm of one plant cell to the cytoplasm of an adjacent cell.2006

Well, the apoplast is not channels, and it does not really involve the channels, so that is not correct.2016

The loss of water from a plant through evaporation- that is incorrect.2023

A pathway consisting of the plant cell walls in intercellular spaces that allows movement of water and minerals from cell to cell- this is correct.2028

Remember that the apoplast pathway involves the non-living parts of a cell.2037

And it allows water and minerals to move along cell walls in these intercellular spaces, so that is apoplast.2042

Plasmodesmata: channels that connect the cytoplasm of one plant cell to the cytoplasm of an adjacent cell- that is correct.2050

And the plasmodesmata are part of the symplast pathway.2057

Transpiration: the loss of water from a plant through evaporation- yes, and this is the driving force of bulk flow.2062

And then, finally, Casparian strip must be D: a belt of waxing material within the endodermal cell walls that is impermeable to water and minerals.2075

And this allows the endoderm to regulate the movement of water and minerals from the apoplastic2084

pathway by forcing those to cross the cell membrane that one time before they enter the vascular cylinder.2091

Example two: what effect would an increase in solutes in the cytoplasm have on the water potential inside a cell?2100

Remember that water wants to go from an area of higher water potential, and then, water is going to move to an area of lower water potential.2108

And by a higher water potential, we mean less negative. Lower is more negative.2129

Adding solutes is going to have...let's say this is the cells sitting in the solution, and then, I go and I add a bunch of solutes.2141

That is going to make the water potential lower. Adding solutes will decrease the water potential in that cell.2152

Water will want to move into the cell. It will become more negative.2166

A second major factor in water potential is pressure.2172

Now, what effect would the breakdown of the cell wall in a plant be expected to have on water potential within the cell?2176

Well, the cell wall exerts pressure, and it is possible that the breakdown of the cell wall could decrease some of that pressure.2182

If the result of breakdown of the cell wall is decrease of pressure, that is going to translate to a lower water potential, so a decreased water potential,2194

in other words, a more negative water potential. It will allow more water to enter the cell.2207

Both of these are going to have the effect of decrease in the water potential inside the cell.2212

Example three: what role do the properties of water play in the bulk flow of xylem sap?2219

Bulk flow is dependent on a pressure gradient.2225

And this pressure gradient is created by the transpiration of water, the evaporation of water from the leaves of plants.2228

However, the capillary action that allows that negative pressure to be transmitted from cell to cell and actually2236

move a whole column of water is dependent on the properties of water, in particular, cohesion and adhesion.2244

Cohesion describes the property of molecules sticking together so that when one water molecule is lost out of the leaf, it is pulling a molecule behind it.2253

And that is pulling the one behind it and all the way down to the root.2270

And then, in the root, one more molecule of water is pulled in from the soil solution.2274

So, water molecules sticking together allows for capillary action.2280

Adhesion describes the property through which molecules stick to other substances.2285

Not only are the molecules of water sticking to each other, they are sticking to the walls of the xylem.2293

And this adhesive property also helps move these water molecules from the root through bulk flow all the way up through the entire plant.2299

Example four: define the terms sugar sink and sugar source, and give an example of each.2314

A sugar sink is a structure that consumes sugar. It includes storage.2322

It uses sugar. It can use it for a source of energy, or it can use it to make starch; so either way, it is using sugar.2338

An example would be an area of the plant that is rapidly growing, so a growing structure.2348

It could be growing roots. It could be growing immature leaves, fruit.2355

It could be an area of storage like a tuber that it could be using sugar for storage.2362

So, those are possible sugar sinks.2369

Whereas, sugar sources are structures that produce sugar.2371

They can produce it through photosynthesis or through the breakdown of starch.2386

So, the major site of photosynthesis is the leaves, so a mature leaf would produce sugar through photosynthesis.2390

A tuber, which is a site of storage, if it is releasing sugar, then, it is a sugar source.2398

When it is taking the sugar and making starch out of it, then, it is a sugar sink; so this would be a tuber that is making starch.2406

This is a tuber that is breaking down starch.2416

Those are sugar sink and sugar source and an example of each.2419

Thank you for visiting Educator.com. That concludes this lesson.2425