AP Biology Subcellular Structure

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We are going to continue our discussion of cell structure and function with the topic of subcellular structure.0002

We will start out by talking about the subcellular structure of prokaryotic cells and then, go on to talk about two types of eukaryotic cells- plant cells and animal cells.0010

Looking at prokaryotic cells, the first thing is they come in different shapes.0022

One shape is cocci, for example, streptococcus.0028

Cocci is plural, and this is spherical in shape, so round cells versus bacilli, which are rod-shaped and vibrio, which are actually curved rods.0034

They are shaped, sort of, like a comma; and cholerae the causative agent for the disease cholera, the severe gastrointestinal illness, is actually called vibrio cholerae.0058

That bacteria has this curved rod shape.0069

You look at a bacterial cell, you, first, will notice the shape, and this actually has a shape of a bacillus. It is rod-shaped.0072

The next thing you will notice is the cell wall. Cell walls in bacteria contain peptidoglycan, and the name tells you what it is composed of.0080

It is composed of short peptides, so peptido and polysaccharides, that is the glycan part of the word.0096

And there are two general types of cell walls- gram-positive and gram-negative.0105

Gram-positive cell walls contain a greater amount of peptidoglycan.0116

Gram-negative cells have only a thin layer of peptidoglycan, and the names gram-positive and gram-negative come from a technique called gram staining.0121

In gram staining, cells are stained with a purplish dye called crystal violet.0131

The crystal violet is, then, rinsed off with alcohol, and the cells are counter stained with a pinkish dye like safranin.0139

Gram-positive cells, because of their thick layer of peptidoglycan, they retain the crystal violet. They retain the purple dye, and they appear purple.0156

Gram-positive cells on gram stain look purple under the microscope.0168

Gram-negative cells, when you rinse the crystal violet, it does not really rinse out of the gram-positive cells.0174

But the gram-negative cells do not retain that purple dye, as well, because they just have a thin layer of peptidoglycan.0181

What happen is, when you counter stain, the cells retain that dye and they appear pink, so gram-negative cells appear pink.0187

Gram-positive cells appear purple, and that is due to a difference in the composition of their cell walls.0196

In addition to the cell walls, some bacteria are covered by a layer called a capsule.0204

This is usually composed of polysaccharides, and what the capsule does is it helps the bacterial cell to evade the immune system.0214

It is sort of a covering that allows it to slip in to a host cell.0229

Some bacterial cells have projections from the surface that are called pili. Another name for these, you may hear, is fimbriae.0235

Pili or fimbriae can have a couple of purposes. One is that they allow for attachment.0250

They allow the bacteria to attach to surfaces. Also, a specialized type of pili called sex pili, allow for the exchange of genetic materials, exchange of DNA between cells.0256

Some bacteria are modal and one means of motility is flagella.0277

Flagella are structures that can move the bacteria around, and although eukaryotic cells can have flagella, they are actually a different composition in structure.0284

The flagella in bacteria have several parts. They actually have three different parts.0304

The first part is the basal apparatus, and that is embedded in the cell membrane in the cell wall.0309

The basal apparatus should be right around here, and that is the motor for the flagella.0324

It contains a pump that is powered by ATP, which allows the flagella to actually move. It provides the energy for that movement.0329

The second part is the hook, and it lies between the basal apparatus and this last part, which is called the filament; and it connects the two.0340

The hook connects the basal apparatus in this third section called the filament.0349

The filament is made up of a protein called flagellin. This is a part that actually moves, and there is usually one or maybe a few flagella on a modal bacteria.0354

OK, we talked about the outside of the cell, the shape of the cell.0372

Looking at the internal structures of a prokaryotic cell, the genetic material is in a region called the nucleoid region.0376

The DNA is actually circular. There can be additional DNA and circles outside this nucleoid region, and these are called plasmids.0388

Plasmids contain only a few genes, and they are usually not essential to survival.0407

Typical would be virulence gene, genes that allow the bacteria to more easily or effectively infect a host, and bacteria can actually can exchange plasmids.0414

Plasmids are also very important in biotechnology.0425

In addition, prokaryotic cells contain ribosomes. Recall that ribosomes are structures where protein synthesis takes place.0431

Prokaryotic and eukaryotic cells both contain ribosomes, but the ribosomes in bacteria are smaller.0444

They are 70S size, which has to do with their sedimentation, and these are found free in the cytoplasm. They are not bound.0451

Notice that unlike eukaryotic cells, prokaryotic cells do not have a bunch of other membrane-bound organelles.0461

They lack endoplasmic reticulum and Golgi apparatus. They are much less complex than eukaryotic cells.0469

They are also smaller.0476

For eukaryotic cells, we are going to start out by talking about a typical animal cell, and then, we are going to go on to look at a typical plant cell.0483

Alright, first of all, you will notice that they have a cell membrane or a plasma membrane, and what they do not have is a cell wall.0493

There is no cell wall in animal cells.0509

The plasma membrane structure is a phospholipid bilayer.0516

And in the next section, we will talk in detail about the structure of cell membranes, the proteins embedded in cell membranes as well as their function.0519

Some eukaryotic cells, some animal cells, have microvilli, and these are projections.0529

The plasma membrane, instead of just forming more of a circular shape, it is going to project out.0535

It is projections of the plasma membrane and then, there will be a cytoplasm in here, and the purpose of microvilli is to increase surface area.0542

Again, form follows function.0563

You would have to think about what tissues would need to have a great surface area, and that would be tissues that function for absorption such as the gastrointestinal tract.0565

And actually, cells in the intestine do have a lot of microvilli, which increases their surface area, and allows them to be more effective in absorbing nutrients.0576

OK, looking within the cell, the genetic material is contained in a nucleus. Here is the nucleus, and it has a double membrane.0588

Within the nucleus are chromosomes, so nucleus, important points, double membrane, and it contains chromosomes.0602

The DNA are organized into chromosomes, and they are bound by proteins called histones.0619

Now, items need to get into the nucleus such as proteins, and things also need to get out of the nucleus.0626

Since the DNA is the nucleus, this is where DNA synthesis takes place. It is also where transcription of messenger RNA takes place.0634

DNA and then, transcription occurs, and there is messenger RNA that needs to be transported out of the cell; and that occurs via pores.0642

Right here are pores, and these are openings through which materials can cross into or out of the nucleus.0651

If you look at a stained cell, you might notice that there are some dark regions within the nucleus, and these are called nucleoli or singular, here is nucleolus.0663

The nucleolus is a region where ribosomal or rRNA synthesis takes place. It is also an area where the ribosomal subunits are assembled.0679

The proteins for the ribosomal subunits, there is actually two subunits per ribosome.0705

The component proteins are manufactured out here in the cytoplasm, and then, they are imported in through pores.0711

Subunits are imported into the nucleus to the nucleolus, and then, they are assembled together to make the two subunits and the ribosomal RNA is added at that point.0720

You have the assembled subunits.0732

They contain rRNA, and then, they are exported back out to come together as one large ribosome comprised of the two subunits.0734

Alright, the next item we are going to talk about, the next organelle, are ribosomes, and there are actually two types of ribosomes: free and bound.0746

Free ribosomes, it is just the way the name suggests is they are out here free in the cytoplasm, and since form follows function, protein synthesis takes place on ribosomes.0770

Proteins that are destined for the cytoplasm are made by the free ribosomes, so cytoplasmic proteins are made here.0785

Free ribosomes are the site of synthesis of cytoplasmic proteins.0797

Bound ribosomes, well, what are they bound to? They are actually bound to another structure called the endoplasmic reticulum.0806

This is the endoplasmic reticulum, all these sacs and tunnels, and these bumps you see on the surface of part of the endoplasmic reticulum are the bound ribosomes.0812

The area of the endoplasmic reticulum that has these bound ribosomes is called the rough endoplasmic reticulum or sometimes just RER.0824

And the reason it is rough appearing is because of these ribosomes.0833

Here, proteins are to synthesize that are going to be headed for export from the cell.0836

Proteins for export and by export it may mean they are actually leaving the cell entirely, or they may end up just embedded in their cell membrane.0843

Proteins made by the bound ribosomes are either headed completely out of the cell, or they are going to at least end up integrated in the cell membrane.0854

And this makes sense because the ER can, then, package these in the vesicles headed for another organelle, that we will talk about in a second, called the Golgi apparatus.0864

And then, they are ready to go for export.0874

Again, two types of ribosomes: free ribosomes make proteins that are destined for the cytoplasm.0876

Bound ribosomes are found on the endoplasmic reticulum, and they make proteins that are destined either for the cell membrane or to be completely exported from the cell.0882

Alright, this is just the same slide continued so that we can cover the rest of structures in here.0894

I just talked about the rough endoplasmic reticulum, and I focused on the ribosomes; but now, I am talking about the endoplasmic reticulum, itself.0902

The endoplasmic reticulum or just the ER, there are two types.0920

I already mentioned the rough ER. The other type is the smooth ER.0924

Both types are composed of a series of flattened sacs and tubules, and these are often called cisterna- these tubules.0936

Starting with the rough ER, proteins are synthesized by ribosomes on the rough ER.0949

And then, right after these proteins are synthesized, they can enter the lumen of the endoplasmic reticulum through pores - OK - exported proteins made here or made in the rough ER.0954

In addition, the endoplasmic reticulum actually makes a cell membrane. It makes a cell membrane for itself.0977

It actually makes a cell membrane for other parts of the cell, other organelles, the plasma membrane, and those get packaged up in vesicles and get transported out.0986

A couple of functions of the endoplasmic reticulum, site of protein synthesis, site of synthesis of plasma membrane components, the smooth ER has some additional functions.0995

Lipids are made here. Those are components of plasma membranes.1010

This also includes items such as steroids, substances such as steroids.1018

The smooth ER is also the site of detoxification. The smooth ER functions for both synthesis and detoxification.1028

Alcohol is a substance that needs to be detoxified by cells, and in particular, the liver plays a major role in detoxification.1039

You would expect the liver cells to have a lot of smooth ER.1047

OK, two types: rough ER, which functions in protein synthesis, smooth ER, where the synthesis of lipids occurs and detoxification.1051

The protein are made here and lipids, so the substances that are made in the endoplasmic reticulum then leave through vesicles, so vesicles butt off.1063

There is another set of sacs right here called the Golgi apparatus or Golgi bodies.1075

This is a series of flattened sacs, which sits right next to the endoplasmic reticulum often, and communicates with it through vesicle; and there are two sides or two phases to the Golgi.1090

There is one side that is a receiving side, and this is called the cis face, so it is the receiving side.1103

After the Golgi apparatus does its work, it sends items out from vesicles on the trans face.1110

Sometimes people compare the Golgi apparatus to a post office because its job is to process, sort and package materials such as proteins and lipids.1127

The Golgi functions to process, sort and package items including proteins and lipids.1139

What do I mean by a process?1151

Well, the Golgi does things like phosphorylate, so that is phosphate groups. It adds sugars, so glycosylation, so some examples to give you are phosphorylation and glycosylation.1153

And sometimes, the purpose of these is to provide a signal so that the cell knows where the protein is going, so it is, kind of, like a label.1168

The protein is made in the endoplasmic reticulum. Vesicles butt off.1176

They fuse with the Golgi apparatus on the cis phase. They enter the Golgi.1181

They are going to be modified inside.1185

Again, a phosphate group might be added, and that particular group might let the cell know that this is a cell membrane protein1187

so that when it butts off the trans phase in the vesicle, then, that vesicle will fuse with the cell membrane, and if the protein is a membrane protein, it will stay there.1194

If it is not a membrane protein, then, it would just be secreted out of the cell, and there needs to be some sort of signal to tell the cell where a particular protein goes.1205

OK, there are a bunch of/several other structures that we need to cover.1217

I mentioned vesicles, and again, these are ways of the various membranes and membranous structures within the cell to communicate with each other.1225

They are also a way for the cell to export or import materials, so those are vesicles.1234

Large vesicles are called vacuoles. For example, this is might a vacuole.1244

Vacuoles actually do not play as large of a role in animal cells as they do in plant cells, fungi and protist. For example, food vacuoles provide a way for protist to eat.1254

A protist might engulf food so the plasma membrane would invaginate. Food particles would come in, and then, the plasma membrane will pinch off and form a food vacuole with the food inside.1273

Protist also have contractile vacuoles, and the purpose of these is to pump out water.1289

This allows the cell to maintain its osmolarity, and osmolarity is a solute concentration of a fluid.1302

Again, there are vacuoles in animal cells, but they play a bigger role often in other types of cells, plant cells and protist, where you will can find food vacuoles and contractile vacuoles.1310

Another type of structure is lysosomes, which contain lysozymes. OK, these contain enzymes.1322

These are just little membranous sacs that contain enzymes that break down old organelles.1337

They break down, I am just going to say particles, because they can breakdown old organelles. They also breakdown pathogens.1348

Cells in the immune system, macrophages in particular, engulf and ingest foreign particles such as bacteria.1356

Once those are ingested, a vesicle is formed, and that vesicle can actually fuse with the lysosome.1366

And then, when the two fuse together, become one, then the bacteria, the pathogen that is inside, will be digested by these enzymes.1372

As I mentioned, old organelles are also ingested inside a lysosome, and the purpose of that is to refresh and renew the cell.1383

A damage organelle can be digested, and then, its component parts are actually reused by the cell, so they are a continual process of renewal going on.1390

The environment inside a lysosome is very acidic, so the pH in there is low, and the enzymes within a lysosome function best at a low pH.1399

This helps to protect the cell because, let's say this was to burst, if the lysosome burst, and it releases all these enzymes, they could just start eating up the cell.1410

But because the cytoplasmic environment has a higher pH, the enzymes are not going to be as active.1419

Now, if many lysosomes burst at ones, and the pH of the cell overall actually decreased, then there could be a problem.1426

But if it is just a single or a few lysosomes burst, they are going to find themselves in an environment where they do not work very effectively, so the cell is protected that way.1433

There is a set of diseases known as lysosomal storage diseases, and in these, there is an accumulation of macromolecules such a proteins, polysaccharides and lipids.1444

And proteins, polysaccharides and lipids are normally digested in the lysosome, but in people with lysosomal storage diseases, some of the enzymes are either absent or dysfunctional.1455

They end up with an accumulation of these products, and neurons, in particular, are susceptible to damage due to the accumulation of these products, and it causes severe neurological problems.1466

Diseases such as Tay-Sachs disease is an example of a lysosomal storage disease.1477

Taken together, the plasma membrane, the nuclear membrane, the endoplasmic reticulum, Golgi apparatus, lysosome, all those taken together are sometimes known as the endomembrane system.1486

And the different parts of the endomembrane communicate through vesicles,1503

so vesicles coming from the ER to the Golgi fused with the plasma membrane or vesicles fused with a lysosome or a vacuole.1509

The endomembrane system are various structures that often work together and communicate through vesicles.1521

And again, that includes the nuclear membrane, the plasma membrane, the endoplasmic reticulum, the Golgi apparatus, lysosomes, vesicles, vacuoles.1526

Mitochondria are structures that are the site of cellular respiration.1541

Cellular respiration occurs here, and the energy from organic molecules is used to make ATP; and ATP provides the energy for most of the cells functions.1552

A typical cell contains hundreds to even thousands of mitochondria. As expected, cells that use a lot of energy like muscle cells have a lot of mitochondria.1566

The structure of the mitochondria is interesting. It actually has a double membrane.1579

When we talked about the endosymbiotic theory of cell origin in the previous lecture,1586

I mentioned that that double membrane is most likely the result of a membrane from the original larger anaerobic bacteria that engulfed the smaller aerobic bacteria.1591

There is an outer membrane from one of those bacteria and then, the inner membrane that evolved from the smaller bacteria.1603

Mitochondria have a double membrane, and the space between the two membranes is called the intermembrane space.1609

There are folds with the inner membrane, and within those folds is a compartment called the matrix, so there is a double membrane.1619

There is a small space between the two membranes called the intermembrane space, and then, there are folds.1628

These folds are often called cristae/crista and the matrix, which is the compartment formed by these infoldings of the inner membrane.1637

When we talk about cellular respiration, you will see how various enzymes and processes are carried out in certain parts of the mitochondria.1650

Mitochondria have their own DNA. Recall that it has circular DNA like bacterial cells.1663

They also have their own ribosomes. Mitochondrial ribosomes are 70S.1670

They are similar to bacterial ribosomes, and if you want to review those similarities between mitochondrial cells - oh, excuse me - mitochondria and bacterial cells,1677

go ahead and check out the lecture on comparison of prokaryotic and eukaryotic cells and the endosymbiotic theory.1688

The final structure we are going to talk about are peroxisomes.1699

Peroxisomes are bound by a single membrane, and they contain enzymes that detoxify substances.1712

The ox in peroxisomes tells you how they achieve this, and they do it by oxidation; and these are found in both plants and animals.1723

What they do is they transfer hydrogen to oxygen to form hydrogen peroxide, which is H₂O₂.1732

The only problem is hydrogen peroxide in large amounts is actually toxic to the cell, so peroxisomes have additional enzymes that continue on.1739

They remove an oxygen and release that oxygen and end up with water, which obviously is not harmful to the cell.1748

Again, the purpose of peroxisomes is to detoxify substances. They do that through oxidation resulting in the formation of a hydrogen peroxide.1756

In order to, then, detoxify the hydrogen peroxide, an oxygen is removed which generates water.1768

And they breakdown substances such as fatty acids, and they also detoxify substances such as alcohol.1774

They also play the role in production of bile. Peroxisomes help with the production of bile, and in plants, they help convert fatty acids into sugar.1781

These are formed by enzymes that are made in the cytosol and by vesicles that butt off the endoplasmic reticulum, so vesicles butt off the endoplasmic reticulum.1797

The enzymes are made separately and placed into the peroxisomes, and those are formed.1814

In contrast, lysosomes butt off the Golgi apparatus with the enzymes already contained in them.1820

OK, that summarizes the many organelles and structures in the eukaryotic cell, typical animal cell.1829

But the structures within a eukaryotic cell in both plants and animals actually, these are held in place by cytoskeleton.1839

OK, we talked about the different organelles, and within the plasma membrane is the cytoplasm.1850

The fluid in the cytoplasm, so the space in here is the cytoplasm, and the fluid is called the cytosol.1861

Even though this is describe as a fluid a lot of times, it actually has a texture that is more like a gel.1868

It is not a thin watery fluid. It is more like a gel.1876

Within the cytoplasm are all those organelles we talked about, the nucleus and the mitochondria, the endoplasmic reticulum.1879

The cytoskeleton holds those organelles in place in addition to supporting the cell and giving it its shape.1889

Finally, the cytoskeleton allows for motility.1897

Cytoskeleton is composed of various filaments, and there is three major types of filaments.1901

The first are microtubules. The second are microfilaments, and the third are intermediate filaments.1906

Microtubules are the largest in diameter, so talking about microtubules, they are composed of a protein called tubulin.1927

Tubulin subunits can be added or taken off the end, so there is these individual subunits, and the cell can add on to one end.1939

It can take off of the other end, and that allows for motility.1948

The cytoplasm is not just a rigid structure that does not move. You can think of it more like scaffolding.1952

If you are building a building, you have scaffolding, and when you are finished with one part of the building, you can take it down; or if you need to build a higher part, you can add to it.1959

The cytoskeleton works more like that.1966

There are several important structures that are composed of microtubules. The first one is centrioles, and this picture shows centrioles.1971

Centrioles are found in centrosomes. Their purpose is to organize microtubules.1984

These are one type of MTOC or microtubule organizing center. What this is showing is division of the chromosomes.1997

This is a spindle apparatus right here, spindle fibers, and these spindle fibers are also composed of microtubules.2011

There is a centrosome out here. Within that, are two centrioles organized at right angles, and here is a cross section showing what a centriole looks like.2020

You see that there is nine triplets, and each of these is a microtubule.2034

If you took a centriole, cross-sectioned it, looked down at it, this is what you would see: microtubules organized in this way.2040

The spindle fibers radiate out of these microtubules and then, attach to the chromosomes, and then, during anaphase, the chromosomes are separated.2050

Centrioles play a key role in miosis and mitosis.2060

Recall that plant cells do not have centrosomes, and they do not have centrioles; but they do have microtubule organizing centers.2065

OK, first structure that is made out of microtubules is the centrioles as well as the spindle fibers, so I am going to put spindles.2072

Second structure is flagella. Third structure is cilia, as well as other components of the cytoskeleton.2086

This picture here, this cross section, shows the structure of flagella and the structure of cilia.2095

Flagella and cilia are both organelles of motility.2103

Some animal cells are modal. For example, sperm is modal, and the difference between flagella and cilia is that flagella tend to be longer.2107

There is usually just one flagella or maybe a few flagella at one end of the cell. Protozoa actually use flagella for motility.2117

Again, prokaryotes also have flagella, as well, but they are made out of different molecules, and they have a different structure.2128

Eukaryotic flagella, though, are made up of microtubules.2136

The second organelle of motility is cilia.2140

Cells that use cilia for motility usually have many cilia, so cilia tend to be shorter and there is more of them.2143

Cilia are very effective for moving fluids.2150

The airway in animals is lined with cilia so that if you breathe in dust, other particles, the cilia on your respiratory tract keeps mucus moving out2153

so that, instead of the dust going down in to your lungs, it is going to get pushed out.2167

Some organisms such as paramecium actually use cilia for locomotion rather than just for pushing fluids.2173

OK, microtubules composed of tubulin, centrioles and spindle fibers are made of microtubules.2181

Flagella and cilia are also made of microtubules.2188

Microtubules are fundamental in allowing for cell motility, and that is the first type of filament in the cytoskeleton.2192

The second type of filament are microfilaments. Microfilaments are composed of actin.2200

Actin interacts with the second protein, which is a globular-shaped protein called myosin, and you might be familiar with the words actin myosin from the study of muscle cells.2212

Muscles use actin and myosin interaction for motility.2224

A second type of motility that occurs is that of amoeba using pseudopods to move.2231

An amoeba cell actually form these, almost arms or something, and move around with them, and these are called pseudopods.2238

And these pseudopods, they move because of the action of microfilaments.2246

During cell division, a cleavage furrow is formed, so the two cell, there is one cell, and then it adds on to the size of its plasma membrane.2259

It doubles its organelles. Mitosis occurs, and then, a cleavage furrow forms, and the cell divides into two.2271

This cleavage furrow is also the result of microfilaments.2278

Finally, cytoplasmic streaming that occurs in plant cell and fungi is a result of microfilaments.2285

And we will talk more about that when we talk about plant cells, but it is a circular movement within the cytoplasm.2294

Microfilament functions: first of all their structure, they are composed of actin, and the actin interacts with myosin to allow for contraction of muscles.2300

The second type of movement that can occur is the formation of a pseudopod, which allows amoeba to move as an example.2313

Microfilaments are essential for forming the cleavage furrow for cell division, and finally, microfilaments play an important role in cytoplasmic streaming.2322

So, that was two. The third one is intermediate filaments.2332

As expected, these are intermediate in size between microfilaments and microtubules, and by size, I mean diameter.2336

Microtubules have the largest diameter. Microfilaments have the smallest diameter, and then, intermediate filaments are in between that.2343

And there is not just a single type of subunit that these are made from.2350

There is actually several different types of intermediate filaments, and they each are constructed from a characteristic protein. However, many of these subunits contain keratin.2354

Keratin helps a cell maintain its shape and provides support, and it is keratin with the intermediate filaments that help hold organelles in place.2367

Intermediate filaments are very important in maintenance of cell shape, support and anchoring organelles into place.2390

Keratin is frequently found in epithelial cells. For example our hair and our nails are formed out of keratin.2398

OK, we talked about organelles found in eukaryotic animal cells. We also talked about the cytoskeleton structure.2409

Many of these elements are the same in plant cells, so I am going to just focus on the differences.2419

Plant cells have a plasma membrane or cell membrane just like animal cells.2431

It is a phospholipid bilayer, but looking at what is different, they also have a cell wall; and bacteria have cell walls as well, but the cell wall in plants is made of a different material.2439

The primary cell wall is made of cellulose.2453

Cellulose is a polysaccharide, and this particular polysaccharide of cellulose is embedded in a matrix of polysaccharides and proteins.2460

And the purpose of the cell wall - actually multiple purposes - one, is to maintain the cell shape.2470

The other is to provide protection. The other is to maintain the turgor of the cell.2479

Turgor is a type of rigidity that occurs as a result of water pushing on the cell wall.2486

This is a central vacuole, and it fills with water especially when the cell is in a hypotonic environment, an environment where there is less solutes than inside the cell.2495

The cell is going to take up water, push against the cell wall, and that will make the cell rigid.2504

Unlike animals, plant cells do not have a skeleton, so they use this turgor or this rigidity to maintain their shape.2512

If a plant does not get enough water, it really dries out. It starts to welt the stem and leaves, and that is because it is not maintaining its turgor.2521

In addition, the cell wall prevents lysing of the cell. If a cell takes up too much water, and it does not have a cell wall, it will burst.2531

However, a plant cell cannot take up too much water because at some point, it is going to pump up against this rigid wall, so it prevents the cell from bursting.2543

Some plant cells actually have a second cell wall called a secondary cell wall between the plasma membrane and this primary cell wall, so that is a secondary cell wall.2553

In addition to cellulose, the secondary cell wall contains lignin. Lignin gives a plant wall or the plant cell even greater strength, and lignin is found in wood.2570

A plant sacrifices, it is very rigid because of that, so it cannot bend as much, but, in return, it gets protection. It maintains its shape.2586

Alright, there is a layer associated with the cell wall. Sometimes it is called a separate component.2601

Sometimes, it is considered part of the cell wall, but this layer is called the middle lamella.2607

The middle lamella contains pectin. Pectin is very sticky.2614

It is a polysaccharide. If you make jelly or preserves or something, you will see that pectin is used as a thickener, as a thickening agent.2619

And since it is very sticky, it helps adjacent plant cells hold together.2629

There will be another plant cell right next to this one, let's say right here, and they will actually share a middle lamella.2636

The other cell will have its cell wall here, and then, right between the two is the middle lamella.2645

OK, I already mentioned briefly the central vacuole. The central vacuole is quite large.2654

It can actually take up a third or even half of the cell.2662

The result is that, especially if it absorbs more water, gets bigger, the organelles are found more at the periphery of the cell.2666

The central vacuole functions for storage, so it stores things. It takes care of waste, so waste disposal.2675

It maintains the pH of the cell, and it has a function in maintaining turgor, as I mentioned.2687

Again, in a hypotonic environment, which we will discuss when we talk about plasma membranes and osmolarity.2695

But in a hypotonic environment, in an environment where there is a dilute substance or dilute liquid outside the cell, the plant will take up more water.2702

The central vacuole will become larger. It will push up against the cell wall, and the cell will be rigid.2710

The central vacuole also takes up waste, and it separates it out. It keeps it separate from the rest of the cell so that it cannot cause harm.2718

It isolates those materials.2727

OK, a couple more structures on the plants, plant cells- chloroplast.2732

Chloroplasts are a type of organelle called plastids. Plastids are a set of organelles that are not found in animal cells.2742

They are found in plant cells, but they are not found in animal cells.2751

There are three types. The first type is leucoplast.2756

The second type is chromoplast, and the third type is chloroplast; and this picture, right here, is a chloroplast.2763

Most type of the plastid, you are probably you most familiar about.2776

Starting with leucoplast. Leucoplasts store starch.2780

Because of that, they are found mostly in the roots of plants and in structures called tubers.2789

The eye of the potato, when that starts growing, that is a tuber, so leucoplasts or starch.2795

Chromoplasts contain pigments. In particular, they contain orange and yellow pigments.2801

The reason a carrot appears orange is because of the pigments that are found inside the chromoplasts in the plant cell.2819

Chloroplasts are the site of photosynthesis.2827

This is a chloroplast, and the fluid inside is called the stroma. This is a fluid found inside the chloroplast.2840

You will notice that it has a double membrane just like the mitochondria. Each of these membranous sacs is called a thylakoid, so each sac is a thylakoid.2851

A stack of these together is called a granum, plural is grana, and the enzymes that carry out photosynthesis are found in the thylakoid membrane.2869

As you will notice, this is green, and it is because the chloroplasts contain green pigment. They give parts of the plant, such as the stem and the leaves, their green color.2883

Like mitochondria, there are features of chloroplast that are similar to bacteria and provide evidence for the endosymbiotic theory of eukaryotic origin.2894

Like mitochondria, they have circular DNA. They have a double membrane.2907

They divide by or reproduce by binary fission, and they have their own ribosomes that are similar in structure to bacterial ribosomes. They are 70S ribosomes.2912

Alright, we can do a few examples now.2931

Example one: match each organelle with its description.2933

We have Golgi apparatus, mitochondria, chloroplast, lysosome and endoplasmic reticulum.2936

Looking down at the descriptions, the first description, the site of photosynthesis, well, we just covered that. The site of photosynthesis is the chloroplast.2945

Chloroplast contain thylakoid stacked up into grana, and the thylakoid membrane contains the enzymes that carry out photosynthesis.2957

B: contains hydrolytic enzymes, which digest old organelles and pathogens. Hydrolytic enzymes are found in lysosomes.2968

Recall that lysosomes are present only in animal cells.2979

They are not found in plant cells, and their function is to breakdown old organelles so their parts can be reused2983

and also to destroy pathogens that have been ingested by the cells of the immune system.2990

OK, C: site of production of ATP from energy released from organic molecules. That is the mitochondria.2999

The mitochondria is the site of cellular respiration. ATP is made there, and ATP provides the energy for most functions in the cell.3008

D: site of production of proteins destined for secretion from the cell and production of lipids and detoxification of substances. That is describing the endoplasmic reticulum.3019

The rough endoplasmic reticulum has ribosomes bound to it, and it is a site of production of proteins that are going to be secreted from the cell or going to end up in the cell membrane.3031

The smooth endoplasmic reticulum is the site of the production of lipids, and it is also the site of detoxification of certain substances.3041

Series of flattened sacs, this is E, site of processing, sorting and packaging of proteins made in the rough ER. It also produces polysaccharides, and this is Golgi apparatus.3051

The Golgi apparatus, remember, is the center where the proteins made in the endoplasmic reticulum are sorted out.3063

They are phosphorylated or they are glycosylated. They are packaged in vesicles, and they are sent off to their destination.3071

Example two: what are the functions of the cell wall?3081

The functions of the cell walls, we talked about cell walls in plants because animals do not have cell walls- multiple functions3086

One is they provide the cell with strength. They maintain the turgor of the cell.3094

They prevent lysis, so they prevent the cell from bursting when it is in an environment where it might absorb a lot of water, and if it over absorbs water it will lyse.3104

Instead of lysing though, if the plant absorbs water in a central vacuole, that is just going to push against the cell wall3115

and maintains the cell's shape and rigidity and maintain the shape of the plant through turgor.3123

Strength, turgor prevents lysis, and it also provides protection.3129

One of the primary and secondary walls of cell plant is composed of cell walls of plants composed of.3136

The primary cell wall is mainly made out of cellulose, which is a polysaccharide, and it is embedded in a matrix of various polysaccharides and proteins.3143

Secondary cell walls also contain cellulose, and they contain a second polymer called lignin, which give them additional strength.3153

What are the cell walls of fungi primarily composed of? Fungi have cell walls, as well, but these are composed of chitin.3162

Example three: name three structures composed of microtubules, and list their functions.3176

We talked about multiple structures that are made out of microtubules. The first one was centrioles.3183

Centrioles help to organize microtubules within the cell.3192

They are found in the centrosomes, and the spindle fibers made out of microtubules radiate out from the centrioles and then separate the chromosomes during mitosis and miosis.3196

Remember that microtubules are very important for motility of the cell.3210

The second structure that is composed of microtubules is flagella. Flagella allow cells to have motility.3214

They move back and for by an angulating motion. Again, an example would be the motility of sperm cells.3225

Third, cilia, this also allows for motility. They are shorter than flagella, and there is often many per cell.3233

They are especially effective in moving fluid, so they are found in areas like the respiratory tract.3243

Also, you should be aware that there is another type of cilia called primary cilia. These are found in almost every cell in invertebrates.3250

However, there is only one per cell. The important thing is that these are non-modal.3261

They are different in regular cilia, and that they are non-modal, and they have different functions.3266

The primary cilia actually function for sensory reception and signalling pathways. They also have a different microtubule arrangement than that of cilia and flagella.3273

Centrioles and spindle fibers, flagella, cilia and also primary cilia, which have different functions and structure, they are still composed of microtubules.3294

What are microfilaments composed of? They are composed of actin, and remember that actin interacts with myosin to allow for cell motility.3304

And now, we are asked to give two examples in which microfilaments are involve in cell motility.3314

One of these is muscle contraction. The interaction with actin and myosin allows for muscle contraction.3321

Second would be cytoplasmic streaming. Cytoplasmic streaming is found in plant cells and fungal cells.3330

And let's say in a plant cell, what will happen is the cytoplasm will actually flow around in a circular pattern.3342

And you can think of it as analogous to stirring up something when you are cooking, and the purpose is actually to move things around.3350

This allows for nutrients to get where they needed for exchange of materials between the organelles.3358

Instead of the cytoplasm just sitting there, by, sort of, stirring it up, moving it around, it allows for exchange of materials.3364

Cytoplasmic streaming in plants and fungi are the result of microfilaments.3372

A couple more, I mentioned that amoebas move using pseudopods, and these are the result of microfilaments.3378

That is what allows the pseudopods to move.3390

Finally, cell division, the cleavage furrow.3394

When two cells or excuse me one cell is preparing to divide into two, it forms this furrow, and then, separates of into two cells.3400

Example four: why do some antibiotics that target bacterial cells also affect mitochondria and chloroplast?3416

It is true that there are antibiotics that are meant to just target bacterial cells, but they are also been shown in the lab to affect mitochondria and chloroplast.3423

And the reason for that is because chloroplast and mitochondria have structural similarities to bacterial cells.3435

So, similarities between bacteria and mitochondria and chloroplast- that would be a reason, for example the ribosomes.3446

If an antibiotic is targeting bacterial ribosomes, these are 70S ribosomes, so I would expect the antibiotic to affect the bacteria and the mitochondria and chloroplast.3468

But it is going to leave the other ribosomes in the eukaryotic cell alone because those have a different structure. They are 80S.3479

Other similarities, remember, are the DNA. It is circular.3486

The DNA of mitochondria, bacteria and chloroplast are all circular.3495

Size: mitochondria and chloroplast are smaller. Eukaryotic cells are larger.3501

These similarities could cause an antibiotic to affect all three cell types.3507

How is this relevant to the endosymbiotic theory of eukaryotic origin?3514

Well, the endosymbiotic theory states that organelles from the eukaryotic cell are results of a relationship in the engulfment of smaller aerobic bacteria by larger anaerobic bacteria.3518

There is a larger prokaryotic anaerobic cell. It engulfed a smaller prokaryotic aerobic cell.3536

And eventually, over time, evolution, this became a eukaryotic cell, and the smaller bacterial cell became mitochondria, chloroplast, organelles within the cell.3545

And the fact that mitochondria and chloroplast are similar to bacterial cells, it provides support for this endosymbiosis theory of eukaryotic origin.3560

Thanks for visiting Educator.com, and that concludes this lecture on subcellular structure.3572