Cell Structure

You may know that you can isolate a cell from an organism like yourselves and under the proper conditions, it will continue to manifest the properties we associate with life. If however, one takes some cells and separates them into their biochemical constituents-- proteins, carbohydrates, lipids, nucleic acids-- the properties associated with life are lost.

Cells then, are the basic functional units of all living organisms. The organization of matter into something that we recognize as living simply does not exist in units smaller than a cell.

This cell concept has NOT been with us since the beginning. In fact, it developed gradually over a period of almost two centuries.

A series of observations by many individuals finally led a man named Schleiden to develop a series of hypotheses about the growth of cells in plants where the nucleus had a central role in the reproduction of cells. Turns out that the details of the hypotheses were completely wrong, but the influence of Schleiden on another researcher, Schwann, led to the realization of the universality of cells as the structural units of living matter.

Schwann developed two postulates that were to provide the basis of the cell theory:

1. that all living organisms are composed of one or more cells and
2. that the cell is the unit of function of living organisms
   

Equivalent observations were made in animal tissues and finally in 1855, Rudolf Virchow, after an exacting study of the growth of cells in human tissues, clearly stated that all cells arise only by the division of pre-existing cells. This gave us the cell theory pretty much as we know it today:

1. that all living organisms are composed of one or more cells and
2. that the cell is the unit of function of living organisms
3. cells arise only from pre-existing cells by division

Cells are wonderful structures and I want to show you some pictures of cells. Back when you last studied anything about cells, your teacher probably referred to something called a typical cell. Let's look a a couple of typical cells.

A very common "typical cell" comes from pancreatic tissue. Take a look at a transmission electronic micrograph of a cell derived from a thin section of rat pancreatic tissue.

This cell has a nice nucleus, nuclear pores, endoplasmic reticulum, chromatin, nucleolus, mitochondria and were we able to see its edges, a nice plasma membrane.

I must say something about how these micrographs are made to have you better understand what you are looking at. First, they were made with a transmission electron microscope which is designed to look at thin sections of tissue. Another kind of electron microscope, a scanning electron microscope looks at surfaces of cells.

Second, the thin sections are very thin indeed--on the order of 90 nanometers. The wavelength of green light is much greater. There is a great deal more cell behind and in front of this section. These thin sections are nearly two-dimensional in their information content. Cells are three-dimensional objects. What I am trying to get at here is that a single thin section of a cell conveys very little information about the over-all three-dimensional nature of the structures within. One has to study a series of 30-40 thin sections and re-construct the third dimension for this.

Third, these are images of dead cells. They have been chemically treated to stop their metabolic processes and their proteins have been cross-linked in an attempt to preserve their internal structure. Living cells are dynamic objects. Things move, shapes change. None of this dynamicism is ever seen when one uses an electron microscope.

To get an idea of this three dimensionality, take a look at a diagrammatic representation of a whole cell. You should really try to keep this sort of image in mind when you look at micrographs of cells.

We haven't spoken much about plants in this course and really won't but plants too have cells and I will present a few to you for the sake of comparisons. Plant and animal cells have their similarities and differences.

An electron micrograph of a typical plant cell is shown next. Perhaps the most striking differences are the presence of a cell wall (CW) and chloroplasts (C).

A diagram of a typical plant cell showing its three dimensionality is also available. Notice that plant cells have their plasma membranes pressed up tightly against their cell walls.

I should also mention that there are two fundamentally different classes of cells: cells are either eukaryotic or prokaryotic. Both types are bounded by a similar plasma membrane. Some may be surrounded by a cell wall. In all other respects, the two classes of cells are rather different.

Prokaryotes are thought to be more simple. They have rather less genetic information and this is found in a poorly delineated region sometimes referred to as a nucleoid. Prokaryotes have no true nucleus.

Prokaryotes do not have much in the way of organelles except ribosomes (and these are somewhat different from those of eukaryotes).

Eukaryotic cells are usually somewhat larger and are more structurally complex than prokaryotic cells. Eukaryotes possess a true nucleus (one with a nuclear envelope to delineate it) and their cytoplasm is filled with several different kinds of organelles that act to compartmentalize various cell functions.

You will recall that I made much of constructing a phospholipid molecule when I spoke of chemistry important to biology. I want to remind you that this because phospholipids are so important for membrane structure. Membranes can be thought of as a phospholipid bilayer with embedded proteins and glycoproteins.

Take a look at the next diagram--a typical membrane--maybe a plasma membrane.

Remember how I pointed out that phospholipids are often diagrammed as a "circle with two tails." You easily see that in the diagram. Notice the two layers or bilayer of phospholipids. Notice also that the fatty acid chains (which are "oily") are pointed towards each other. The charged phosphate "heads" are jutting out where there is water. Oily, fatty acid chains would rather associate with their own and really don't care much for water.

An amino acid chain is really protein and you will remember that proteins have "conformations" or are folded in a particular manner. Some are only associated with one or the other lipid layer; others poke through both lipid layers. Some proteins have carbohydrates (sugars) associated with them and are referred to as glycoproteins.

From such a diagram you can begin to better appreciate how a protein in the membrane can perhaps change its shape and allow the transport of an ion or molecule across the membrane.

This idea of a membrane being a largely a lipid bilayer was not always with us. How do you think such an idea came to be?

Well, with the advent of light microscopes, people that looked were pretty certain there was a boundary separating cells from their environment--a cell membrane or plasma membrane. But they were very uncertain as to its nature.

If you were the researcher and needed some material to investigate the nature of such a boundary membrane, where would you look for a good supply of fresh cells? Why your assistant, of course! He has fine veins and blood has a lot of cells in it. The cells are already separated and fairly easily to get at. All one needs is a cooperative assistant.

So, before electron microscopes or x-ray diffraction machines or any fancy things like that, researchers were trying to figure out the nature of this boundary membrane.

Here is what they did. First, they counted the number of cells in a cubic milliliter volume of blood. Then, they placed some cells on a glass slide and using a light microscope, measured the surface area of an average red blood cell. Click here to see a scanning electron microscopic picture of red blood cells. Knowing the surface area of a single red blood cell, they were able to calculate the surface area of the blood cells in a cubic milliliter of blood. Human red blood cells are also nice in that they have little in the way of internal membranes and no nucleus at maturity. The extra membranes associated with the nucleus and other organelles would have confused the issue.

Back to the assistant now for some more blood. The researchers then took a milliliter of blood and extracted it with a chemical called acetone. Acetone dissolves lipids and so it dissolved the plasma membranes of the red blood cells.

The researchers then centrifuged the cell debris to the bottom of a test tube and took the acetone plus dissolved plasma membrane lipids to a rectangular trough of water. They spilled the acetone extract onto the surface of the water. The lipids did just what you see in the kitchen sink when you put oily things in water. They formed a monolayer film on the surface of the water.

The researchers then measured the area of the lipid film. They found that it was twice that of the total area of the red blood cells in their milliliter of blood. So, if they got twice as much lipid as would coat all of the red blood cells, the lipid must be arranged as a bilayer. Pretty neat experiment!

Later tools and experiments added protein and carbohydrate to this bilayer.

The 50s and 60s were the golden years for the study of cell ultrastructure. Electron microscopes became widely available after World War II and researchers all over the world studied cell structure and found a wonderful array of organelles to which they ultimately associated functions. What we found is just fascinating.

Below is a table that roughly describes several (but not all) organelles that are found in cells. You can click on any of the underlined table entries for an electron micrograph or diagram. You should take a look at them and generally know the function of each organelle. I will discuss the function of mitochondria and chloroplasts in much greater detail at a later time.

Finally, I have included several electron micrographs of cells from diverse tissue sources. While the individual cells are quite different in appearance (diversity), they all have many common organelle components (unity).

Liver parenchyma cell

Liver parenchyma cells are often used as examples of typical animal cells. One can easily spot the organelles typical of a eukaryotic cell (nucleus, endoplasmic reticulum, ribosomes, Golgi, etc.)

Olfactory epithelial cell

Some of the epithelial cells within your nose. These are ciliated cells--smoking knocks the cilia off of these cells (and those found in your lung tissue) and leads to less efficient "cleansing" of these areas. See if you can't pick out various organelles. The material marked Md = mucus droplets.

Sperm cell

Half of the humans in the world have these kinds of cells. They are flagellated and have an odd mitochondrion wrapped around the neck of the cell. The tip of the head is known as the acrosome and is important in egg penetration and the "closure" of the egg to additional sperm.

Plant sclerenchyma cell

You have likely eaten thousands of these cells. This is a picture of a "stone cell" from pear fruit. The gritty feeling that you might have noticed in your mouth while eating a pear is due to these cells with their huge cell walls. But note that there is some living cytoplasm within.

Osteocyte and bone matrix

We have given you an interesting story about bone formation. Here you can see an osteocyte surrounded by the very dark colored bone matrix material (marked X). Notice that the osteocyte has many of the organelles one associates with eukaryotic cells.

Retinal rod cell

The folded membranes contain a light-sensitive pigment. They transduce light energy into nerve impulses. Another membranous structure that transduces light energy is found within chloroplasts. And, another membrane system that transduces energy (chemical energy) is found within mitochondria.

Secondary xylem from a plant

These cells make up a tissue that you know as wood. They are dead at maturity and all that remains are their cell walls.

You may take a quiz on the material in this module. No record of the quiz is made. You decide after the quiz if you really know this material.

Return to the Syllabus page.