Video: Eukaryotic cell

Eukaryotic cells are the building blocks of all multicellular organisms. From plants to animals to humans, we're all made of the same basic structure – this one cell. But how can multicellular organisms made from the same basic cell look so different from each other? Well, variable amounts of different structures within each cell is what makes each one different. It's what differentiates muscle cells from those that can detect color in the eye. The function of the cell defines its structure. Remember form follows function.

Today, we're going to go back to basics by exploring the eukaryotic cell.

So, how is it that the tissues and the cells making various organs can look so different and yet all be made from the same eukaryotic cells? Much of the secret lies in the structures within the cell known as organelles. They may appear in different combinations and quantities, but all the cells in eukaryotes draw from the same pool of cellular components, and that's what we're looking at today.

Within humans, there's a large degree of variation seen in the eukaryotic cell. We already know that there are four classifications of tissues – epithelia, nervous cells, muscle cells, and connective cells. All of these cells are eukaryotic, but each have a different structure based on their function, and even within these classifications, there exist many different specializations.

Eukaryotic cells of many different appearances and functions can come together to form tissues, which then combine to form organs. It's when all of these organs work together that we're able to maintain homeostasis which means preserving the internal environment of the body at a constant functional state. But the maintenance of homeostasis is very much dependent on each of these individual eukaryotic cells doing their job correctly.

So, during this tutorial, we'll look at the common features of a generic eukaryotic cell. We'll look at the plasma membrane surrounding the cell and the fluid within it which is called cytoplasm. We'll also have a look at the organization and appearance of cell organelles like the mitochondria, the endoplasmic reticulum, and digestive organelles known as lysosomes. We'll then delve into the nucleus to see what's enclosed within the nuclear envelope and appreciate the cytoskeletal structure that provides support and holds the cell together.

Okay, let's get started with the outermost structure – the plasma membrane.

All eukaryotic cells contain a plasma membrane enclosing the cytoplasm and the nucleus with the exception of red blood cells, which are anucleate. This cell membrane forms an important barrier between the internal and external environments. The plasma membrane is made of a phospholipid bilayer with two layers of hydrophilic phosphate heads facing externally, and attached to each of these heads, hydrophobic lipid bodies projecting internally between the heads. The unique composition of this membrane means it is selectively permeable – an important feature in maintaining the internal and external environments of the cell.

The phospholipid bilayer is permeable to small molecules such as oxygen, carbon dioxide, water, and lipid soluble chemicals. Like steroids, they can pass right through the membrane. All other molecules must be transported into the cell. Some substances are carried in via a protein channel, like this one here. This is called an integral protein, one that is incorporated within the bilayer with both internal and external surfaces. Integral proteins can also be receptors and anchors for the internal structural components. Other molecules are carried into the cell via endocytosis – the taking in of matter by a living cell by invagination of its membrane to form a vacuole.

There are some smaller proteins embedded in the plasma membrane, like this one here. These are called peripheral proteins and bind transiently to the membrane to convey messages to the cell. They might open protein channels or regulate cell signaling. The presence of proteins on the cell surface is also highly specialized depending on the type of cell and its function.

Depending on the function of the cell, the plasma membrane may contain specialized structures. For example, cells that absorb, like the ones you might find lining the intestine, have a convoluted plasma membrane which increases their surface area with the external environment. These extensions of the plasma membrane that are highlighted now are called microvilli.

Another type of projection extension from the cell membrane is called the cilium. We have two types of cilia here. Non-motile or primary cilia are found on almost all types of human cells and often have a sensory function. Normally, you'll find only one of these on a cell. Motile cilia, in contrast, occur in large numbers and beat in a coordinated manner. You'll find them in the trachea, for example, moving mucus along the airways.

Another motile projection is called flagellum. It's much longer than cilia and its main function is to move the actual cell around as opposed to materials on its surface. You'll probably be very familiar with one example of it because the tail of a sperm is actually a flagellum.

Let's now move on to the internal structures of the eukaryotic cell. Here, we can see this gel-like substance called cytosol. It comprises part of the cytoplasm along with the other water, salts, and proteins. Held inside the plasma membrane and suspended within it are all of the other bits and bobs inside the cell. The gel-like nature of cytosol allows movement of the cellular organelles and provides a supportive medium for all of the cell's functions.

So, if it's just water and a few salts and proteins inside this plasma membrane, then what gives the cell its shape? That, my friend, would be the cytoskeleton. Wait, cytoplasm? Cytoskeleton? What's this cyto- stuff we keep talking about? Well, cyto- comes from the Greek word kytos which means “vessel or container,” and in modern-day English, we use the prefix cyto- to refer to a cell – possibly the most important container within all of us.

Now that we know cyto- means cell, it makes sense that the substance giving the cell support and structure would be called the skeleton. Within the typical cell, the cytoskeleton is comprised of three different types of fibers, each with a different role in supporting the cell.

All three are protein filaments that extend through the cell cytoplasm. Firstly, microfilaments are the thinnest type. They are composed of actin and are found in the peripheral regions of the cell. They give the cell its shape and are involved with its movement, so it will come to no surprise that microfilaments will be particularly abundant in motile cells. The shape of actin can be influenced by a variety of actin-binding proteins. It's important to note that myosin is another important type of motor filament mostly found in muscle cells.

The second type of cytoskeleton filaments are intermediate filaments. These are thicker than microfilaments and are involved with and embedded within the cell membrane. They are involved in stabilizing the shape of the cell and provide a scaffolding within the cell for other structures to attach to.

And, finally, microtubules. These are large hollow tubes made of tubulin. They're found in most cells, but are absent in red blood cells. Their primary function is to guide the movement of chromosomes during mitosis. They're also found in cilia – the surface projections which we spoke about earlier that sway back and forth to move extracellular matrix along the external surface of the cell, and flagella, long locomotive projections on the cell surface.

Centrioles are tubular structures present within eukaryotic cells particularly animal cells, and like microtubules, are also formed from the protein tubulin. The primary function of the centriole is to organize the spindle fibers during cell division. Another function of centrioles is in the formation of microtubules for cilia, and flagella.

Microtubules grow from a pair of centrioles called the centrosome, which replicates itself and migrates to opposite poles of the cell before mitosis. These play an important role in guiding chromosomes and organelles into the daughter cells of cell division.

Okay, let's delve into the organelles floating within the cell cytoplasm and we'll start with the largest cell organelle – the nucleus.

Most cells contain one nucleus; however, there are some exceptions like skeletal or cardiac muscle cells which contain numerous nuclei and are called multinucleated. Red blood cells are the opposite in that they are anucleate, which means that they have no nucleus at all.

The nucleus is a bit like a cell within a cell. It has a membrane barrier around the outside and is filled with smaller structures suspended in a matrix. The nuclear envelope is what we call the membrane surrounding the nucleus. This membrane consists of an outer and inner membrane separated by a perinuclear space. The outer layer is continuous with the endoplasmic reticular network and is studded with ribosomes. The internal layer contains its own specific integral proteins.

In addition, associated with the internal nuclear membrane, we see the nuclear lamina. It is the filament network supporting the integrity and shape of the nucleus. At intervals around the surface of the nucleus, the inner and outer membranes fuse and create holes in the envelope, which are called nuclear pores. These pores allow for passage of metabolites, macromolecules, and ribosomal subunits between the nucleus and the cytoplasm.

Chromatin makes up the majority of the nucleus and is a mass of genetic material. DNA, RNA, and proteins are all compactly stored within the chromatin and suspended within the nuclear matrix. The most abundant of these proteins are histones, responsible for packaging of chromosomal DNA. The DNA and histone complexes are arranged in tight coils. Chromatin can be further classified as heterochromatin, which represents the tightly-coiled complexes forming a solid mass, and more loosely packed euchromatin. During cell division, the chromatin is especially tightly packed to form chromosomes.

Each nucleus contains one or more nucleoli. The nucleolus is involved in ribosomal RNA synthesis and ribosome production. During the interphase part of the mitotic cycle, it appears as this dense structure, but disassembles itself during other phases of mitosis.

Having looked at some of the largest structures in the cell, let's now discover what else is living within the confines of the cell membrane. We'll start with the ribosome. We've already heard that they can be found embedded in the outer membrane of the nuclear envelope and they can also be found inside the nucleus and roaming around the cell cytoplasm, but where they're most abundant is on the surface of the rough endoplasmic reticulum – this structure here which we'll talk about in a moment.

Ribosomes are minute particles that have an important role in protein synthesis and the most abundant in protein-secreting cells. The ribosome is formed of two subunits – a smaller one which binds an mRNA chain and a larger one responsible for protein synthesis. Their primary function is to decode genetic messages in RNA for amino acid sequencing.

Let's move on now to the mitochondrion – the power plant of the cell. There are varying amounts of mitochondria roaming around the cell cytoplasm providing energy in the form of ATP for the different processes and cycles the cell is going through. The number of mitochondria in a cell can vary based on – you guessed it – the function and size of the cell. For example, cardiac muscles have about five thousand mitochondria in each cell. Other less energy-demanding many cells like those in the skin have much fewer.

This is the endoplasmic reticulum – a large folded network of sacs, vesicles, and interconnected tubes responsible for the production and transportation of molecules within the cell. Think of the endoplasmic reticulum as the manager of a train station telling everyone what's going where and deciding how many trains need to go to each location depending on demand. It sends much needed supplies to other organelles providing them with a reliable source of nutrients and amino acids. The reticulum essentially divides the cytoplasm into two parts. The cytoplasm within the lumen of the reticulum is called the vacuoplasm whereas the cytoplasm outside of the reticulum is referred to as hyaloplasm or cytosol.

Some of the endoplasmic reticulum has ribosomes embedded on its surface. This is rough endoplasmic reticulum and is continuous with the nuclear envelope. Its lumen is continuous with the perinuclear space and the smooth endoplasmic reticulum. It's within the rough endoplasmic reticulum that proteins are assembled in a process called translation. The remainder of the structure is called the smooth endoplasmic reticulum. The only difference in appearance is the lack of ribosomes on the surface. Functionally, the primary role of the smooth endoplasmic reticulum is to make cellular products like lipids and hormones and to further process the proteins synthesized in the rough endoplasmic reticulum.

The structure we're looking at next isn't dissimilar to the endoplasmic reticulum and it is called the Golgi apparatus. The Golgi apparatus is an interconnected network of folded and tubular membrane usually found close to the nucleus. Although similar to the endoplasmic reticulum, the Golgi apparatus is a separate structure and the exchange of materials between the two structures happens via vesicles. It is primarily associated with the transportation of molecules to either the internal or the external environment. It receives products that have been synthesized to the endoplasmic reticulum and repackages them into vesicles that will be transported to the cell membrane for release to lysosomes within the cell or to be stored. We can see some of those vesicles budding off the endoplasmic reticulum in the illustration and also some products being released by exocytosis into the external environment.

Let's have a look at those lysosomes which are one type of vesicle budding from the Golgi apparatus. These subcellular structures are highly variable in shape and size and store hydrolyzing and digestive enzymes. When they first bud off the Golgi apparatus, they are called primary lysosomes. Their main function is to digest unwanted items in the cell; for example, phagocytosed microorganisms, cell debris, and damaged worn-out cell organelles.

Because the acidic hydrolases within the lysosome is so potent, they must be separated from the cell cytoplasm by a membrane. When a lysosome fuses with another vesicle containing external materials, it becomes a secondary lysosome. Sometimes, the materials cannot be digested completely and some residual particles are left behind. Vesicles containing such particles are called tertiary lysosomes or residual bodies. Lysosomes are abundant in cells that are involved with phagocytosis such as macrophages and specific white blood cells.

Peroxisomes are small vesicular structures similar to the lysosomes we just discussed, but contain several oxidases rather than digestive enzymes. Oxidases are enzymes that can break down many types of organic material and toxic substances especially long-chain fatty acids. Peroxisomes are abundant in the cells of the liver and the kidney where most of the toxic substances are removed from the body.

And that wraps up the structural components of a eukaryotic cell.

You didn't think we'd leave you without your favorite clinical notes section, did you? Today, we're going to be talking about cell suicide. Now, don't be afraid. It sounds much more gruesome than it is. We're, of course, talking about apoptosis or programmed cell death. Apoptosis is just a normal part of the life of a cell in a multicellular organism and is actually really important in making sure we function the way that we're supposed to. Other than everyday cell turnover, it plays a role during fetal development.

In the womb, our fingers and toes are connected by webbing, which disappears prior to birth due to apoptosis. Otherwise, we'd come out resembling our little duck pals.

Apoptosis can be stimulated by both internal and external factors. They cause the cell to shrink and the nucleus and chromosomes to fragment forming apoptotic bodies and the cytoplasm to surrounding phagocytes. The macrophages then engulf the apoptotic cell, making apoptosis a quick and clean process. If the process is not carried out due, for example, to a fault in signaling, it can lead to cancers, inflammatory infections, or autoimmune diseases.

Alright, so we've come to the end of our tutorial now. Let's run through a quick summary of everything that we learned today.

Surrounding the cell is the plasma membrane – a double layer of protective material that is selectively permeable to molecules and separates the internal and external environments. It can have surface projections such as microvilli, which increase the surface area. Contained within the plasma membrane is cell cytoplasm – a gel-like matrix that suspends all of the subcellular components. The cell itself has an internal structural framework referred to as the cytoskeleton. It is made up of microfilaments, intermediate filaments, and microtubules, which maintain the shape of the cell, attach it to its neighbors, or allow it to become mobile.

There are a number of subcellular organelles within the cytoplasm, the largest of which is the nucleus. This is like a cell within a cell as it is surrounded by a nuclear envelope that has pores to allow for the passage of material in and out. Within the nucleus is an abundant of genetic material stored as chromatin and a nucleolus. Attached to the nuclear envelope are the rough endoplasmic reticulum with ribosomes dotted on its surface and smooth endoplasmic reticulum where proteins and lipids are produced. These products are then transported off to the Golgi apparatus for packaging and are sent on their merry way to their destination. Some of these products become lysosomes or peroxisomes which are both involved in the breakdown of waste and unwanted substances within the cell.

Alright, that's it. We've covered the elements of a generic eukaryotic cell. But, remember, the cells you'll see in real life will be very different to this one. Cells are highly variable depending on their role, allowing us as mammals and all other species to be highly functional and resilient in life.

On that thought, let's wrap up this tutorial. Hope you enjoyed it and see you next time!