Basic Cell Structure and Function

Unless otherwise noted* the artwork and photographs in this slide show are original and © by Burt Carter. Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin. Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities. Other copyrighted material is used under the fair use clause of the copyright law of the United States. *Scanned images are from course textbook: Hewitt et al., Integrated Science and are used under fair use clause of copyright law. (“Hewitt” on images.)

Most eukaryotic cells share many characteristics, though there are also some things that occur entirely or mostly in specific kingdoms of life. This slide (this presentation) discusses some of the similar and a couple of the dissimilar components of plant and animal cells.

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Both types of cells are enclosed in a cell membrane composed of a bilayer of lipid molecules. The structure of the cell, as well as some of the mechanical aspects of cell division, are controlled by the cytoskeleton. Plant cells also have a cell wall composed of cellulose that forces the cell into a specific shape. Materials from outside the cells (digested food, water, ions, etc.) cross the membranes and get into the cells in three ways. Very small things simply diffuse across the membrane, somewhat larger ones pass through channels – proteins that make a small pore in the membrane. Very large things are moved by active transport – proteins change their shape and an ion or molecule that begins on one side of the membrane ends up on the other, and is released.

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In addition to the structural features of cells shown in the previous slide the inside always contains a fluid (mostly water with lots of dissolved and suspended chemicals, both organic and inorganic) called the cytoplasm, some small clusters of molecules with specific functions, and a number of bodies enclosed within membranes like the cell membrane. These are called organelles. Prokaryotic cells (“bacteria”) have cytoplasm, and the functional molecules, of course, but they lack organelles. All the functions that we’ll look at for eukaryotic organelles are carried out in the cytoplasm of a prokaryotic cell.

The ribosomes are small clusters of RNA (ribosomal- or rRNA) that “read” RNA from the nucleus and synthesize the proteins that are coded there. Many are loose in the cytoplasm (cellular fluid) but many are also located on the rough endoplasmic reticulum. (See next slide.)

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The endoplasmic reticula come in two types and each has a separate function (below). Both are essentially highly folded and convoluted membranes, and are continuous with the outer main cell membrane. The rough endoplasmic reticulum has ribosomes densely studded on its outer surface and these, of course, synthesize proteins (previous slide). The smooth endoplasmic reticulum lacks the ribosomes. It is involved in metabolism of lipids and carbohydrates.

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Both types of cells are eukaryotic – meaning that the DNA is enclosed within an internal membrane in the cell. This is the nucleus. The DNA is, of course, the material responsible for directing the function of the cell and for heredity. It is replicated completely in mitosis and in half (with some mixing) in meiosis.

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The Golgi apparatus (or Golgi body) is an organelle that works like a packaging department. Here proteins synthesized by the ribosomes are enclosed in membranes and then directed to where they are to be utilized.

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The mitochondria are the organelles where various processes involved in converting energy from place to place occur. Of primary importance is the system that takes energy stored in carbohydrate molecules (like sugar) and synthesize or “recharge” ATP with it. Mitochondria have a separate set of DNA, independent of and dissimilar to the DNA in the nucleus. This DNA comes only from mothers – sperm cells have no mitochondria. The structure and genome of this DNA is similar to that of bacteria and this is taken as a good clue that eukaryotic cells arose from what were originally symbiotic cells of different species.

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Lysosomes are small organelles in animal cells that contain protein enzymes whose function is to break down organic molecules – carbohydrates, proteins, nucleic acids, anything at all. The components are then routed to other organelles to be put to use in the cell. Vacuoles are large membrane-enclosed cells in plant cells (and rarely in animal cells too) that simply contain substances that do not need to be in the cytoplasm because they might interfere with other functions. For example, the citric acid in citrus fruits is mostly inside of vacuoles.

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The chloroplasts of a plant cell (or algal cell, in which case there is only one) are where chlorophyll is located and where photosynthesis takes place. (Some other things occur here as well.)

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NEXT SLIDE

Cells make exact copies of themselves (unless accidents occur) by a process called mitosis. Cytokinesis is the final step in reproduction of a cell, when it actively divides into two separate cells. Mitosis simply assures that the two daughter cells will be exact duplicates of each other. In the “Gap 0” stage the cell is doing what its normal function is. “Gap 1” begins the “cell cycle”. The cell is first prepared in a couple of way for mitosis before it actually begins.

(23 paired strands of DNA are copied to make 46 paired strands or 92 individual strands.)

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Before we move on, remember that in an ordinary human cell there are 23 matched homologous pairs of DNA strands, for a total of 46 individual strands. (In other organisms the number is not necessarily 23.) Actually, in this figure the pair at bottom right are not considered homologous. The larger is called “X” and the smaller “Y” (even though it really looks like an X with one short set of legs more than a Y.) These chromosomes are from a male. A female would have a homologous pair of X chromosomes here. This means that when the DNA is replicated in the synthesis stage there are 2x46=92 DNA strands. There are now 2 sets of each homologous pair, 2 X’s and 2 Y’s – should we call these homologous quartets? We can’t really follow a series of diagrams with 92 chromosomes on it so schematic diagrams to show the processes of cell division typically only include 2 or 3. Keep in mind that all the chromosomes are doing the same thing! One further point. Remember that during the cell’s normal activity the DNA is not condensed like this. All photographs and drawings you see of chromosomes are done when the strands have condensed and are more easily visible and discernable. That happens after the strands have replicated, in what is called prophase of mitosis. So make sure that you understand that in this picture the 96 DNA strands are already present, even though it only looks like 46 objects.

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During Prophase!

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1 DNA molecule and it’s exact replicate are stuck together (after the parent is copied) at a point called a centromere. Each half, once it is separated, is called a chromatid.

The homologous pairs of chromosomes have the same shape, leg lengths, and so on. (Though obviously the legs might not really flop in exactly the same way, as happens when you cut and paste.) In diagrams of cell division they are usually drawn one in red and the other blue to help you keep up with them. Remember that there are 4 DNA molecules here. Identify all four, indicate which are homologous. Indicate which are exact replicates of each other.

1 2 3 4

Each chromatid is a DNA molecule. 1 and 2 are exact replicates, as are 3 and 4. 1 is homologous to 4 and 2 to 3. (Look carefully at the shape.) Alternately you can say that 1+2 is homologous to 3+4.

Now lets move on to how these are sorted into two daughter cells in mitosis.

(46 pairs)

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