Main->Readings->5th Grade Readings->Simple Living Things->Part 4
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It is hard to believe that living organisms exist which are even smaller than protists, but they do. These creatures belong to the kingdom of monerans. As far as scientists can tell, the monerans have existed longer than any other kind of life. There are more of them than any other kind of life on Earth, and though we cannot see them without a microscope, they are important to us.
Monerans (often called " bacteria") are organisms which are one-celled and do not have a nucleus. Scientists have begun to think that the earliest kinds of life were all like this, with no nucleus. Later, two different organisms merged, and one became the nucleus of the other. This double cell became the ancestor of all the cells that have a nucleus. Before that, though, no organism had a nucleus.
However, even though the monerans are the oldest and simplest form of life, and even though they do not have a nucleus, this does not stop them from being very successful. Monerans are everywhere. You have them on your skin right now, and in your eyelashes, and in your mouth and your digestive system. Some of them are good for you. Others are bad. Still others have no effect on you at all.
The common name for monerans is bacteria. Bacteria are usually one of three shapes: rods, spheres, and spirals. Like the protists, they reproduce by dividing in half. Also like protists, bacteria can get their energy in different ways. Some contain chlorophyll and can make their own food, others must eat other living matter, and some can get energy from chemicals like ammonia and sulfur and can live where no other living thing can survive.
Monerans can be good for you. Like fungi, bacteria help decompose dead things and return them to the soil. Also, there are bacteria which live in your digestive system which help you with your digestion. Bacteria help make yogurt, and they are what make sourdough bread delicious. However, bacteria are better known for their bad side.
People often call bacteria "germs" and they can cause many diseases. For instance, strep throat is a disease caused by bacteria, and so is Lyme disease. Some of these diseases can be killed using antibiotics such as penicillin, which is made from a fungus. Others can be prevented by vaccination, a way of using killed bacteria to help the body protect itself against the live bacteria.
A common bacterium which often causes disease is salmonella. Salmonella bacteria are often found in water, soil, insects, chicken farms, eggs, and many other places. It can cause people who get it in their bodies to have a fever, feel very ill, get diarrhea, and vomit. This is often called "food poisoning." Usually, the person who gets food poisoning recovers quickly. Sometimes, when a very old or very young person or a person who is already ill gets a salmonella infection, he or she may die. Millions of people get salmonella every year, mostly from eggs and foods containing eggs.
Yet salmonella infection, like many other bacterial diseases, is easy to prevent. Bacteria do not divide quickly when they are cold, so refrigeration helps keep them from spreading. They can be killed with heat, too. Also, washing your hands thoroughly with soap and water reduces the amount of bacteria on your hands.
So if you wash your hands regularly, keep your food in a refrigerator, cook it thoroughly, and do not leave food out for very long after it is cooked, you are not likely to get salmonella. This is true for many other bacteria which cause food poisoning.
And you thought adults told you to wash your hands because they didn't like looking at dirt! It is not the dirt that bothers them, it is the bacteria you can't even see.
Is a cold caused by bacteria? No, the cause of a cold is a virus. A virus has DNA or similar materials in it, as cells do, and it is made out of the proteins which are the building blocks of cells, but it is not a cell. The virus does not belong to a kingdom of living things, because it is not clearly alive. It is a good example of the messiness of the natural world. Things do not always fit our explanations, which is why scientists keep looking for better explanations.
Adenovirus
Bacteriophage
Viruses, obviously, cannot make seeds or spores or eggs. They are not cells, so they cannot divide like protists and monerans. Yet they reproduce themselves. They do this in a fascinating way.
The virus attaches itself to a cell. It invades the cell and puts its DNA into the cell. DNA contains instructions which the cell must follow. The instructions are, very simply, to make more viruses just like the invader. The cell begins making viruses, and does not stop. Sometimes the new viruses fill the cell and make it explode. Other times the virus stays inside the cell for some time. Either way, a virus can make the organism it infects very ill.
Diseases caused by viruses include colds, but also the flu, measles, chicken pox, and warts. HIV, human immunodeficiency virus, is the cause of AIDS. Viruses can cause diseases in bacteria, fungi, plants, and animals.
Some viruses spread easily, like the common cold. The cold virus is very hardy and can move from person to person with a sneeze or a handshake. Others, like hepatitis and AIDS, do not spread easily and must move directly from one person's blood to another. This is why doctors, police officers, and ambulance workers wear protective gloves, so they will not expose themselves or others to blood.
A virus is so small it cannot be seen with an ordinary microscope. Most pictures of viruses are taken with electron microscopes, which can make images of things too small to be seen with regular light. Yet, like plants, animals, fungi, protists, and monerans, it can make more of itself.
So are viruses alive or not? Scientists keep discussing this. Like living things, they can reproduce. They are made of the same things as living things, like nucleic acids and proteins. They may have come from living things in the beginning.
Most living things can do more than just reproduce, however. For instance, they can eat. Though some living things eat other living things (they are "animal-like) while others make their own food using chlorophyll, water, and carbon dioxide (they are "plant-like"), they all need food for energy. Viruses do not eat.
Another thing all living things can do is grow, or get bigger. Viruses do not grow. Living things also respond, or react, to the world around them. Do viruses react? Some people might say they do. There are many other things which most or all living things do, such as eliminate wastes, breathe, and move.
So what do you think? Are viruses alive?
First assignment of the week: Answer the following questions in writing on a sheet of paper to be handed in.
Second assignment of the week: Article summary, using the article and notes you brought in on Monday.
Third assignment of the week: In your journal, definitions of the vocabulary words from the beginning of this section.
Over the weekend: Find a science-related article. In your journal, write down the author, title, publication, publication date, date accessed, main idea, supporting details, and your reaction.
Go on to Chapter Review
This page last modified on October 19, 2002