Each Cell in Your Body Is a Walled City Besieged by Enemies
Just as a brick is the basic building block of a wall, the human cell is the basic functioning unit of the human body. Our body has about a hundred trillion of them. And just as with a brick wall, the requirement that it not collapse means being sturdy enough to stand up to the forces of nature, our cells likewise need to stand up to nature. For this reason, and others, the two hundred different types of cells in the body have common features that allow them to follow the rules to live, grow, and work properly.
In Darwin's day, a cell was considered to be just a bag of chemicals containing within it various structures of unknown function. During the last century it has been shown that the cell is a huge software-driven micro-sized city containing many different nano-sized buildings with programmed pico-sized machines that are able to use energy to build the structures and perform the functions necessary for life. Here is a brief summary of some of the aspects of the human cell which must first be understood to appreciate why it must take control to survive in the world.
A very thin wall, called the plasma membrane, surrounds the cell. The plasma membrane defines the limits of the cell and separates it from other cells and from the outside world. It serves to keep what is needed inside the cell and what is not needed outside the cell. The important chemicals and vital structures of the cell would not be very useful if they were not kept in one place.
The main substance of the cell, which fills up the space within the plasma membrane, is a fluid called the cytosol. The cytosol consists of water with different chemicals dissolved within it. The amount of water inside the cell is its volume and the total number of chemical particles dissolved within each unit volume of water is its concentration. The cytosol is said to be more concentrated when there are more chemical particles per unit volume of water and less concentrated when there are fewer chemical particles per unit volume of water. Also, for a given number of chemical particles in the cytosol, an increase in volume results in a decrease in concentration and a decrease in volume results in an increase in concentration.
Each cell not only consists of water, but is also surrounded by water. The water inside the cell has a high concentration of potassium and protein and a low concentration of sodium. The water outside the cell has a high concentration of sodium and a low concentration of potassium and protein. In other words, the chemical make-up of the water inside the cell is exactly the opposite of the water outside. The plasma membrane serves to separate the two different solutions from each other.
Since the water in the cell takes up space, it applies a certain amount of pressure against the plasma membrane. Think of a bicycle tire. The more it is pumped up, the more air pressure is applied against the tire wall. Since the plasma membrane is made up of matter with a specific structure, like the bicycle tire, it too has physical limits when it comes to remaining intact and functional under pressure.
Suspended within the cell are structures, called organelles, and important proteins which together perform functions that allow for life. These include the nucleus, which contains the genetic information the cell needs to live and reproduce, the mitochondria, where the energy for cell function is obtained, the rough endoplasmic reticulum and the golgi apparatus, which are the factories that produce proteins, the lysosomes, which are the recycling plants where used cellular material is broken down, and the microtubules and microfilaments, which are the supportive cytoskeleton that allows the cell to alter its shape in response to changes in its environment.
Now consider what some of the laws of nature demand for the cell to survive in the world. Real numbers have real consequences. If the cell can't take control to follow the rules, then life will quickly turn into death.
Whether it's a mountain, a molehill, or a molecule, all material objects have mass and so energy is needed to change them. Therefore, to produce, move, or control anything requires that the cell have enough energy. Like a light bulb short on electricity or a car short on gas, without enough energy the cell is as good as dead.
The chemical content in the cell must be kept relatively constant for it to live and work properly. This means that the fluid inside the cell must maintain its high level of potassium and protein and its low level of sodium. If the chemical content of the cell isn't in the right range, then the cell dies a quick death.
Finally, as noted above, the plasma membrane surrounding the cell has definite physical limitations and is therefore sensitive to changes in pressure. Think of blowing up a balloon. There is only so much air pressure the wall of the balloon can handle before it explodes. So too the volume of the cell must be kept within certain limits. If the water pressure against the plasma membrane rises too high, then, as with a balloon, cell death will take place, literally by explosion.
Note, too, that the cell is not self-sufficient. To survive it needs to constantly receive new supplies of chemicals, like glucose, for energy. It must also constantly rid itself of toxic chemicals, like carbon dioxide from the breakdown of glucose. However, to survive, the cell faces a major dilemma. In letting these chemicals pass through its plasma membrane, the cell is exposed to the chemical content of the water just outside its doorstep. And remember, the chemical content of the water outside is totally different from that of the water inside the cell. The cell, remember, must control its chemical content and volume to stay alive.
Think of a walled city besieged by enemies. The residents of the city are slowly running out of food and water and are in desperate need of new supplies to stay alive. They must somehow be able to open the gates wide enough to bring in what they need without at the same time being overrun by the enemy.
In allowing these chemicals to pass through its plasma membrane the cell comes up against a dilemma, a result of the laws of nature that govern chemical and fluid movement. In letting down its guard to allow some chemicals to come in and go out, the cell runs the risk of losing control of its chemical content and volume. If that happens, the cell will perish.
Which laws of nature are involved in the cell's dilemma and, if not resisted by some ingenious design, how do they bring about the catastrophe that is cell death? Come back next time and we'll find out.
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