By Robert Hazen, George Mason University
Electron microscopy has demonstrated that cells have extremely complex internal structures which are isolated from their environment by the cell membrane. We see the cell membrane as being formed primarily from a lipid bilayer, and they form spontaneously from individual phospholipid molecules that have a water-loving and a water-repellent end.
Cell Membrane
Molecules self-organize themselves into that double layer, so that the outsides of the layer see water both on the inside of the cell and the outside of the cell. But the cell membrane can’t be a solid surface; it has to be studded with receptors—protein receptors that provide doorways for food to enter, and for waste to be excreted.
The membrane thus regulates the exchange in materials from the inside of the cell to the outside. You can imagine a protein as having a particular shape, as a receptor, and it’s sitting there waiting on the surface of a cell for the right kind of food to come along. When that food comes along, it fits into the active site of the protein, which then suddenly changes shape, allowing the material inside the cell.
As soon as the material is put into the inside of the cell, the protein changes shape again, ready to take in more food. This also provides a vulnerability because if a virus can look to the outside world like a piece of food, a protein may accidentally trap onto a virus, and let the virus inside the cell. The excreting waste follows the same process in reverse.
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Organelles in Eukaryotic Cells
Inside a eukaryotic cell, there are numerous separate structures called organelles. Each organelle is itself surrounded by a cell membrane, and each performs a specific function. There are lots of organelles; the nucleus being the largest of all the organelles.
The nucleus contains all the cell’s genetic information, the blueprint necessary to perform all those chemical operations, and the chemical instructions that make the cell function. We also see that cell division is controlled from the nucleus.
The storage and the transfer of biological information, which is keyed to the challenge of all cells to reproduce, is also carried out in that nucleus. As one might expect, there are several organelles that play a role in the assembling or manufacturing of the chemicals necessary for life; for example, distributing proteins and lipids and other cell materials. Some of these machines—the organelles inside a cell—include the endoplasmic reticulum, the Golgi complex, among others.
Lysosomes
Lysosomes are organelles that contain various digestive enzymes and help to break down food molecules. They’re like little digestive stomachs inside the cells. Also, lysosomes can be used to attack and kill invading bacteria, and other foreign objects, because they have these digestive enzymes that can break down the invader.
Lysosomes play fascinating roles in the development of embryos. For example, at a very early stage in the human embryo, you actually have webbed fingers and webbed toes; lysosomes break down the very delicate membrane in between the fingers, so that we have separated fingers rather than webbed fingers.
How Lysosomes Help in Development
In many cases in the development of organisms, lysosomes come into play, killing cells, breaking down structures to open pathways, and channels, and so forth, in the developing organism. It turns out that if you have severe oxygen deprivation, it can cause cells to become too acidic. When that happens, the lysosome walls actually break down, and cells are killed from the inside, because those digestive enzymes are released into the cell at large, and kill the cell quite quickly.
In humans, one of the first places this happens is the brain cells; which is why when you’re oxygen-deprived—when you’re suffocated—the brain can be damaged very quickly in that process, in just a matter of minutes, as the oxygen supply is cut off.
Cytoskeleton
Another part of the cell’s interior is called the cytoskeleton. It provides a supporting framework; it provides a kind of conveyor-belt system that moves materials from one part of the cell to the other.
The cytoskeleton employs a whole array of strong protein fibers that connects all parts of the cell with each other.
Mitochondria
Mitochondria are organelles that are present in every cell. They’re miniature power plants of all eukaryotic cells. They convert the energy-rich carbohydrates such as glucose—sugar—into compact, battery-like ATP molecules.
It needs to be remembered that these are the tiny molecules that literally plug into other cellular molecules to provide energy to do work. Mitochondria are, by far, the most abundant organelles in cells. They do mechanical work such as in muscles, where steady supplies of oxygen are needed.
Tracing Maternal Lineage in Mitochondria
Mitochondria have one property that has led to interesting speculations about their evolution. They have their own separate supplies of genetic material, of DNA. So, mitochondria divide, like individual little cells, on their own. In every cell in your body, the mitochondria themselves are constantly multiplying and dividing on their own, to produce more energy centers, more ATP-producing molecules.
This also leads to an interesting genetic curiosity. It turns out that the tail portion of the male sperm is discarded during fertilization, and that tail portion is the only part of the male sperm that contains mitochondria, because you’re trying to get the tail to move back and forth, to oscillate back and forth, as the sperm swims upstream.
As a result, when the tail breaks off, the male sperm has no more mitochondria. The fertilized egg, therefore, has mitochondria only from the female. So every mitochondrion in your body is identical to that of your mother, and her mother, and her mother, and her mother before that, in a maternal lineage. Mitochondrial DNA is an excellent way of tracing maternal lineage. There’s no paternal DNA in any of the mitochondria in your body.
Chloroplasts
Another organelle that’s in many plants is the chloroplast.
These are organelles that help convert the energy of sunlight into energy-rich molecules—sugar molecules, like glucose—which can then be burned by mitochondria to produce the ATP.
Common Questions about the Internal Structure of a Cell
The nucleus contains all the cell’s genetic information, the blueprint necessary to perform all those chemical operations, and the chemical instructions that make the cell function.
Lysosomes are organelles that contain various digestive enzymes and help to break down food molecules. They’re like little digestive stomachs inside the cells.
The chloroplast is an organelle that helps convert the energy of sunlight into energy-rich molecules—sugar molecules, like glucose—which can then be burned by mitochondria to produce the ATP.
