Cells

You have learned about cells before. Now you need to refresh your memory, and focus on those aspects of cells that will help you with Anatomy & Physiology.

As you review cells, keep in mind that you only need to focus on animal cells, since humans are animals. You may visit a web site that describes prokaryotic cells or plant cells, and those are not cells you need to be concerned with for this course.

When you look at a cell, like the one in this cartoon, cellcart.jpg (16961 bytes) keep in mind that this is a "model" cell. As we progress through the semester, we will study a variety of cell types. Each cell type is a variation on the theme presented here.

Looking at this fake cell, you probably notice certain prominent aspects of it right away. Generally, it is surrounded by a membrane, which defines the boundaries of the cell. Also, the fluid environment inside the cell, the cytoplasm, seems different from that outside the cell. We will discuss each of these items in more detail-- but only to refresh your memory on important points since you should have already learned most of this before. The above cartoon is clickable, so that you can go to any one particular cell region later and review it. On your first visit to this page, you may want to just continue reading below.

When looking at cells in lab, you won't see nearly as much in them as in this model cell; that is because some of the organelles depicted here are only visible with an electron microscope.

?Do you know which ones are visible with light microscopes like in lab?

Click here to find out!

This page proceeds in the following order:

The plasma membrane of the cell
What's inside the cell?
A. the nucleus
B. the cytoplasm
The cellular organelles:
A. organelles involved in protein synthesis
B. other organelles
The cytoskeleton

If you want to check out another resource for cell terminology, check out the Biology Hypertextbook online.

A really cool web page dedicated to the cell is The Virtual Cell. You should check it out after you review the material that follows.

1. The plasma membrane of the cell

Cells are surrounded by a membrane. This membrane has many functions, some of which are listed here:

	to maintain all the cell contents within it
	to protect the cell
	to allow for selective permeability
	to enable signal transduction

The first two items, maintaining and protecting are rather simple. You'll see in next week's web page that it can take on these roles because its primary component is lipid, and so everything that is soluble in water sees the membrane as a general barrier.

Selective permeability means that the membrane will let some things cross it, while not permitting other things to cross. This is important, because we need things like oxygen and glucose to be able to cross the membrane, but we don't want toxins crossing the membrane. In order to understand selective permeability, you need to understand passive and active transport, as well as endocytosis and exocytosis. These concepts can seem quite difficult, but they are rather straightforward. We will spend more time on them throughout the semester. But for a little more information on them now, link to the selective permeability page.

Signal transduction is recognizing and then responding to cues. Cells need to be able to do this. For example, cells of your digestive tract need to be able to recognize food on their surfaces and then respond to this food by taking it in and making digestive enzymes. Another example is that bone cells need to be able to recognize growth hormone as it circulates by, so that the bone cell can then respond to it by growing and dividing. What does a cell need for signal transduction? It needs receptors on the surface of its membrane to bind to the signaling molecules. Then it also needs enzymes within the membrane (or special channels will also work, as you'll see in our study of the nervous system) in order to start generating a chemical response to the signal. Remember, cells are the smallest living things, so when their parts have to respond, their parts are chemicals, and chemicals respond with chemical reactions-- thus, the need for enzymes in signal transduction.

Note that I did not mention cell shape or movement. These things have nothing to do with the membrane... they are functions of the cytoskeleton. Please keep that in mind.

Throughout the year in this course, we will spend more time on items and issues that relate to the cell membrane than any other part of the cell. Therefore, you will need to try to really understand the concepts that relate to the cell membrane; then you will be able to expand on these concepts later in the semester.

Finally, it will be important for you to understand the structure of a membrane. I'll go into this a bit more next week when we discuss the organic molecules.

2. What's inside the cell?

A. the nucleus

The nucleus is a membrane-bounded region that contains the cell's genetic information (genome) within it. The membrane that surrounds it is called the nuclear envelope because it is a double membrane with a thin space between the two membranes (like the thin space between the two papers of an envelope). There are numerous pores in the membrane that allow passage of material into and out of the nucleus, called nuclear pores. The nucleus also contains the nucleolus, which is just a particularly busy region of the cell's DNA, where ribosomes are being made. Since there is so much activity in the region of the DNA to make ribosomes, there are lots of proteins and RNA there; because of all the molecules in that tiny region of the genome, it looks like a dark spot when viewed in the microscope.

There is a lot of DNA in the genome, and it all has to fit into the tiny nucleus. Therefore, the DNA is packed up into chromatin. The DNA can also be packed into chromosomes, which is a much tighter kind of packing, but that is only done when the cell is dividing. All of the rest of the time the DNA is in the form of chromatin. The chromatin never leaves the nucleus.

B. the cytoplasm

The cytoplasm is the solution within the membrane, called the cytosol, plus all the organelles (except the nucleus) that are in it. That means that it is the cytosol, rER, sER, Golgi Apparatus, ribosomes, mitochondria, and lysosomes. There are other organelles, but we won't worry about them.

The cytosol is important, because it contains many dissolved materials. Simple sugars, ions, amino acids, nucleotides (including ATP), and other molecules (like vitamins) are dissolved in the fluid cytosol. These items are all necessary for the cell. Students tend to forget that stuff is in the cytosol, since the stuff is dissolved and we can't see it. But if, for example, any of us got a cup of tea that had salt dissolved in it, we would quickly realize that even things that can't be seen are crucial! In the cell cartoon above, all of the green-colored interior is the cytosol.

Animal cells are eukaryotic cells, and they contain many functional areas within the cytoplasm in independent compartments, called organelles. Most organelles are membrane-bounded; the only exception is the ribosome. You should be able to see how this compartmentalization allows one organelle to do one thing while another organelle does something different. Therefore, having organelles allows for segregation of function within a cell.

3. The cellular organelles

Each organelle has a particular function within the cell. However, a few of the organelles are all involved in different aspects of making proteins. It takes quite a few organelles to accomplish the task of making proteins, since proteins come in many varieties and have many functions. The rest of the organelles have other functions, like destroying waste or making energy. For other pages that describe organelles, check out the MIT cell bio page.

A. organelles involved in protein synthesis

The four main organelles that are important in making proteins, in the order they are used, are: 1) nucleus; 2) ribosomes; 3) rough endoplasmic reticulum (also known as the rER); and 4) Golgi apparatus. Since we have already discussed the nucleus, we can pick up this description there.

The DNA in the nucleus holds the plans for all the proteins that a cell will ever need to make. But this DNA doesn't leave the nucleus, and the rest of the protein-synthesizing machinery is in the cytoplasm. So, within the nucleus, the DNA is used to make RNA. RNA can slip out the nuclear pores and enter the cytoplasm.

Ribosomes:

This is where the ribosomes come in. They are able to grab onto the RNA and translate the RNA information into proteins. It is on the ribosomes that proteins are made. But ribosomes are of two different types: 1) free ribosomes, loose ribosomes in the cytosol that are not associated with membranes; and 2) attached ribosomes, those that are attached to the endoplasmic reticulum (ER), causing the ER to be called roughER.

If the protein is made on a free ribosome, then the process is completed as the protein falls off the ribosome into the cytosol. The rER and Golgi apparatus are not needed.

If the protein is made on an attached ribosome, it is made there because it has to end up somewhere other than the cytosol. So, proteins destined for secretion and proteins destined to end up within the plasma membrane are made on rER... that way, as they are made they are already associated with membrane for release or insertion.

Rough Endoplasmic Reticulum:

The rER (see above, too) is thus the site where proteins are made (on ribosomes) for secretion or for insertion into membrane. Once made, the protein is then either inside the space (called the lumen) of the rER or in membrane of the rER. But those proteins made here always need a bit more work to get into their final form. So, they are sent on to the Golgi apparatus.

Golgi Apparatus:

The Golgi apparatus will make final modifications to the protein and will send it to where it has to go in the cell.

B. other organelles & items within the cell

1. Organelles found in all eukaryotic cells (in both animal and plant cells!)

sER: Not only do cells have rER, but they also have smooth endoplasmic reticulum (sER). This has absolutely nothing to do with protein synthesis (because it has no ribosomes attached). Instead, this organelle is involved with making lipids or with storing calcium ions. Any cells that have to make a lot of lipids (like cells that produce steroids such as testosterone or estrogen) will have a lot of sER. You'll also hear more about a calcium-storing sER when we talk about the sER of muscle cells, which is also called a sarcoplasmic reticulum (don't worry about that term now).

lysosomes: these organelles are the waste centers of the cell, and it is also absolutely necessary for cells to use lysosomes in order to access the material that they take in through endocytosis.

Lysosomes look just like vesicles. They are simply membrane-bounded, spherical organelles. But the thing that makes them special is what they have inside their lumen. They contain powerful, hydrolytic enzymes! Remember that hydrolytic enzymes are ones that facilitate hydrolysis reactions. We'll review hydrolysis reactions, which break large molecules down into smaller molecules, next week.

Since lysosomes contain powerful, hydrolytic enzymes, anything that the lysosome takes inside of it will get broken down. This can be handy if the cell needs to destroy old organelles or digest large food particles. The old organelle or the endocytotic vesicle merely has to fuse with a lysosome and its contents will encounter the hydrolytic enzymes.

mitochondria: these organelles are responsible for generating almost all of the cellular energy (called ATP) that the cell needs. The chemical reactions that lead to the production of energy by the mitochondria are collectively called cellular respiration. Why? When you think of respiration, you think of breathing, right? Well, why do you breathe? Of course! To take in oxygen gas from the environment. Did you ever stop and ask yourself why you needed to get that oxygen gas inside of you? It turns out that the oxygen is used within each cell of your body, within the mitochondria of the cells, in order to make ATP. So, cellular respiration is just the term that is used for those chemical reactions whereby your cells use oxygen gas to make ATP.

You know that you eat sugars (carbohydrates, we'll do it next week) in order to get energy. And now you also know that you breathe in order to get energy. So, if you have sugar monomers and oxygen gas, you should be able to generate ATP! Right? Well, that's actually how it works. I have given you this illustration of the process in a very simplified form. It may be even easier to understand after we do our chemistry material next week.

You certainly don't have to memorize this diagram... yet. Just try to understand it. When we talk about muscle, we will be coming back to this notion of making ATP, and spending a little time thinking about it now will help you later. By the way, I hope you noticed that although oxygen is used to make ATP, we end up making the gaseous waste product carbon dioxide (which we then have to exhale out of our bodies).

What does a mitochondrion look like? (note that mitochondrion is for one of these organelles, and that mitochondria is for more than one mitochondrion). It is unusual because it is surrounded by two independent membranes, like a double barrier. The outer membrane is pretty smooth. The inner membrane is all folded up inside to make membranous ridges called cristae. In the lumen of the inner membrane we find something really unusual: the mitochondrion has its own DNA, its own ribosomes, and makes its own proteins! For a great drawing of a mitochondrion, visit this mitochondrion page.

Vesicles: How do proteins and other materials shuttle around in the cell? They move around within vesicles. Vesicles are just little spheres of membrane with stuff inside, or you can think of them as membrane packets. Vesicles are formed when membrane pinches off organelles. For example, vesicles form off the rER as a bit of membrane pinches off with protein inside it; that vesicle can then deliver its protein to the Golgi apparatus by fusing with the Golgi's membrane. This pinching and fusing of membrane is the same thing that happens with endocytosis and exocytosis. Vesicles are not really considered organelles, but they are important for how membranous organelles function.

4. The cytoskeleton

What does the cytoskeleton do?

The cytoskeleton provides shape or support for the cell.
The cytoskeleton allows for cells to move. There are certain ways that cells can move:

1) amoeboid movement, which is kind of a creeping along of a cell (if you want to see a cell move by amoeboid movement, check out the very last listing on this web page for the movie "Time lapse of a locomoting human anaplastic astrocytoma cell"... but only do this when you have time!)
2) by a general contraction of the cell, as happens in muscles
3) by moving cilia or flagella, which are like tiny appendages on cells.

The cytoskeleton provides a highway system within a cell for the movement of material from one area of the cell to another area of the same cell. For example, when a vesicle has to move from the rER to the Golgi Complex, it gets directed along cytoskeletal elements within the cell-- otherwise it might never make it to the Golgi.

The cytoskeleton is made from three main components, each of which is build from long filaments of protein. The three cytoskeletal components are:

Microtubules. These are made of the protein called tubulin. Tubulin molecules join together into long chains that spiral around together to form long, strong, hollow tubes. It is these tubes that are called microtubules. These are what cause cilia and flagella to move.

Intermediate filaments. These filaments are not quite as strong individually as microtubules. However, they are rarely lying independently. Instead, they usually lie all twisted up together into thick bundles-- in this way, as a bundle, they end up being the strongest cytoskeletal components. It is because of this strength that they end up being supporting units for the cell (but they are not usually involved in cell movements). We won't really spend any more time on these filaments this year.

Microfilaments. These filaments may be made of actin or myosin. These filaments are the thinnest and the weakest, but they are used for quick and dynamic movements. These are important for amoeboid locomotion and for muscle contraction. We will be spending a lot of time on these cytoskeletal components this semester.

Actin is a long filament that is composed of two intertwined chains. Myosin also has intertwined chains, but its chains have round "head groups" on them. In the image of myosin filaments below you can probably make out the paired head groups on each myosin filament.

These microfilaments are found in most cells, but are really abundant in muscle cells. Their presence in muscle cells is what gives muscles the ability to contract. It is also the reason that when you eat meat (muscle), you are eating protein-- microfilaments are made of protein!

� 2006 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.