In what ways does your general health depend on the health of your cells?
Cells play many important roles in keeping your body in homeostasis, or internal balance. Cell function is crucial in keeping the levels of ions, proteins, and other factors at normal levels throughout the body. Cells depend on feedback, or information from their environment, in order to keep all the organs and, therefore, the body in working order.
The health of the cells of your body can be affected by many factors. Some factors that affect the function of cells come from within the body. For example, if a blood vessel that serves a portion of the brain becomes clogged or breaks, that region of the brain will not get oxygen or nutrients. Waste products will build up. The cells in that brain region will get sick and may die. This blockage of or break in a blood vessel is called a stroke. A stroke results in a loss of the functions of the part of the brain affected.
Other factors that affect the functions of cells come from the environment. For example, air pollution can affect the health of cells in your breathing system. Certain toxic chemicals can cause damage to the DNA in some cells in the body. The normal functions of these cells will be altered. In some cases, their growth may become uncontrolled producing the disease we call cancer. Some external agents that can affect cells and cause serious damage include tobacco smoke (lung cells), solar radiation (skin cells), certain pesticides (liver cells), certain drugs (brain and muscle cells), and organisms such as bacteria and viruses (infection of any cell in the body).
It seems straightforward to state that diseases like cancer and heart disease are the result of unhealthy or damaged cells, but what about some other diseases or disabilities, such as hearing loss? How are cells involved in those disorders?
If a person listens to extremely loud music regularly, the very delicate sensory cells of the inner ear may be permanently damaged. Once these sensory cells are damaged and lose their function, they cannot repair themselves. The hearing loss may be total if enough sensory cells are damaged. Rock musicians who perform concerts for a number of years can suffer a tremendous loss of hearing. Soldiers who have been exposed to horribly loud explosions of gunfire can suffer “artillery ear” deafness.
In this section you will examine several of the effects on the body when cells do not function normally. We will consider some genetic diseases and cancer.
A permanent change in the DNA of a cell is called a mutation. Mutations that occur in somatic cells can affect the life of an individual. But mutations that occur in the somatic cells of an individual are not inherited by his or her offspring. If changes in DNA happen in gametes (sperm or egg) or stem cells that produce gametes, the mutations may be transmitted to the next generation. Not all mutations are bad, however. Mutations that occurred long ago are the basis for the variety among people and differences between species. Without mutations, life on Earth would not have progressed past the original single-cell stage. There would be none of the current animals and people that we find so fascinating. So changes in DNA are an essential part of the history of life on our planet. In the short run, however, most new mutations are harmful.
Mutations can be caused by factors in the environment, such as solar radiation, and some chemicals. Again, if those factors cause a mutation in a somatic cell, it may affect that individual only. If those factors cause a mutation in the gamete cells, the offspring of that individual could be affected. We do not know the reason that most mutations happen. Most mutations are spontaneous and are not caused by factors in the environment. You learned earlier about DNA replication. The replication of DNA is incredibly accurate. Think about how the cell usually copies the estimated 80,000 to 100,000 genes with great accuracy during every division. It is not surprising that mutations do happen, but it is amazing that they do not happen more often.
Some cells cannot function properly because of a problem in the DNA that the individual inherited from his or her biological parents. The individual may have inherited DNA altered in a particular gene that is crucial for normal cell function. Such an alteration in the genetic code is called a mutation. If there has been a mutation in an important region of the DNA, then the cells of the body that use or express that region of DNA and their daughter cells will not be able to carry out their normal tasks. In some cases an entire organ may not function properly. The function of an organ requires that the great majority of cells that make up the organ function normally. For example, if certain cells of the nervous system don't function, then you might not be able to think well, see clearly, or move properly. If the cells of the lungs don't function, you can't breathe. If the cells of the heart muscle don't work, your heart cannot pump blood.
Genetic diseases are also called hereditary diseases. They are conditions inherited from either one or both parents that are caused by mutations in the DNA. Some genetic diseases are so severe that the embryo or offspring may not be able to survive.
Explain how a mutation in a DNA molecule of only one base could cause a genetic disease in which an essential protein is altered so that it does not function normally. Make sure you explain how each step of protein synthesis is affected.
An Example of a Genetic Disorder
Cystic fibrosis is a genetic disease in which the affected person receives a mutated gene from both the mother and the father. Children with cystic fibrosis have problems with recurring lung infections because certain cells in their lungs do not function properly. They have increasingly difficult health problems with these persistent infections including pneumonia.
Cystic fibrosis patients have a mutation in a gene that makes a protein. In this case it's a membrane protein. This membrane protein allows ions (like chloride) to move across a cell membrane. Because the protein in these cells is altered, the lungs cannot function normally. Figure 5.1 shows how this gene alteration affects the lungs of cystic fibrosis patients.
Without good medical care, children with cystic fibrosis may die at a young age. However, with good medical care and a good program of physical therapy, cystic fibrosis patients can live relatively normal lives. There are very encouraging treatments under development for people with cystic fibrosis, now that scientists have greater knowledge of how cells work. These treatments potentially will save many lives and make life easier for cystic fibrosis patients.
What is cancer? What does cancer have to do with cells? Cancer is a change in the way cells are able to control their own reproduction. When cancer occurs, the cells lose their normal controls on cell division. Cancer cells continue to divide and produce abnormal tissues called tumors.
How Cancer Affects Cells
A cancer cell is different from a normal cell in the way it moves, in its shape, and in the way it responds to chemicals and to nutrients.
Figure 5.1 Gene alteration affects the lung cells of cystic fibrosis patients.
To learn about cancer, you need to learn more about how normal cells respect each other's space, how they form associations or groups of cells, and how cells recognize each other.
Normal cells don't just sit still. They are active.
- Cells can recognize their neighbors.
- Cells can tell the difference between cells that are like themselves and other cells. For instance, a lung cell can tell the difference between other lung cells and liver cells.
- Cells can stick close to their neighbors or they can keep their distance.
- Cells can send messages. Cells can send each other signals. Some of these signals are specific and can only be read by certain cells.
- Cells can receive and decode messages. They can make special protein products. These products can change the way a person grows and develops.
- Cells can reproduce. They can make more cells.
Cancer cells are abnormal in that they do not recognize and respect the boundaries of cells of different tissues. Cancer cells move and grow where they do not belong. For example, tumor cells that start in the lungs can travel to other tissues and other organs. Tumor cells make associations with these other cells that normal cells would not make. Tumor cells can take over other regions of the body, partly because they do not have the same “respect” for boundaries, for territories, or for “their place” that normal cells have.
Predicting Cancer Risks
How can scientists determine the risk of someone getting cancer? Let's look at the relative risk that a person of a certain age will get colon cancer. In this example, cells of the colon become cancerous. If not detected early, this is a very serious type of cancer that is often fatal. It is extremely rare in young people, yet the risks of getting colon cancer increase dramatically with age. Why is this so?
Let's make some educated guesses about the risks of colon cancer for a child and an older person. First, consider the simplest guess. If the transformation of a normal cell to a cancer cell depends on just one event, then a 70-year-old should have a seven-times greater risk of developing colon cancer than a 10-year-old child does. A single cause (one event) would show a relationship like the one in Figure 5.2.
Figure 5.2 Relationship of a single cause.
Now suppose that there is more than one step involved in causing a normal cell in the colon to become cancerous. Suppose there are two events (for example, X-ray exposure and exposure to a chemical). Then you would calculate the risk of cancer over the time period by multiplying the chance of exposure to one event by the chance of exposure to the second event. That would mean that a 70-year-old person might have a 7x7 or 49-times greater risk of developing colon cancer than a 10-year-old child would. This could be graphed as:
Figure 5.3 Risk for colon cancer of a 70-year-old person compared to that of a 10-year-old person as a result of two events.
Suppose five events are involved in causing a normal colon cell to become cancerous. That would mean that a 70-year-old might have a 7 x 7 x 7 x 7 x 7 greater risk of developing colon cancer than a 10-year-old would. The graph of this is shown in Figure 5.4.
Figure 5.4 Risk for colon cancer of a 70-year-old person compared to that of a 10-year-old person as a result of five events.
How does this series of guesses fit what really happens in a population of people? Let's look at the graphs of the actual number of cases of colon cancer to find out:
Figure 5.5 The actual annual U.S. death rate from colon cancer in relation to age.
Note that the risk of colon cancer increases as predicted by our third guess-there are five cancer-causing events that occur over a person's life span. The question then becomes, how do you decrease the chance that you, your friend, or a family member will get colon cancer?
Decreasing the Risk of Cancer
If you look at a huge population of people, the chance of death from colon cancer at the various ages fits the estimate that there are five events that take place in the colon cells to cause colon cancer. These events do not need to happen in a particular order. This knowledge is important in planning to protect yourself from colon cancer and from cancer in other tissues and organs of your body. Suppose, for example, that a person smokes as a teenager. Because tobacco contains dozens of chemicals that cause cancer, that exposure could be sufficient to cause cancer over time. By the time that teenage smoker is in her forties she could develop lung cancer. We live in a world where we cannot avoid all dangerous chemicals, but it is clearly smart to avoid the ones that we can. We also can actively participate in working to clean up our environment, so we are exposed to fewer cancer-causing agents.
Figure 5.6 The normal lung cells (top) that line the human bronchus have cilia used for defense against debris and microorganisms. The cancerous lung cells (bottom right) are not ciliated and unable to provide defense.
There is now more evidence than ever before suggesting that what you eat can play a big role in your chances of developing certain types of cancer. Eating foods rich in fat causes an increased risk of colon cancer, breast cancer, and cancer of the prostate gland. The typical diet of an American is high in fat. Changing our diets and eating more grains, fruits, and vegetables may help reduce the risk of cancer. Intake of alcohol increases the chance of cancer of the mouth, esophagus, stomach, and possibly even breast cancer, so avoiding or limiting alcohol consumption may also decrease your chance of getting cancer.
What do think happens to the cell cycle in a tumor cell (cancer cell)? You know that tumors can grow very fast. Is that because of changes in the cell cycle of an individual cancer cell or changes in the whole group of cancer cells? Do you think that a cancer cell goes through its cell cycle faster, slower, or at the same rate as a normal cell?
Activity 5-1: Cells Gone Awry
What happens when the normal structure of a cell changes? How do you think changes in cells affect the function of the tissue they make up? Do you think the appearance of a cell could be used to explain why it does not function normally? Some cell biologists study differences in cell appearance to learn more about why cells do not work normally.
In this activity you play the role of a pathologist, a scientist who investigates normal and diseased cells. You examine and compare different kinds of tissues to identify changes in their cells that could explain why they may not function correctly.
- Compound light microscope
- Prepared laboratory slides from blood, lung, or liver tissue
- Activity Report
Step 1 Obtain microscope slides of both normal and diseased tissue from the same organ.
Step 2 Examine the normal tissue. On your Activity Report, draw and label each example.
Step 3 Observe the diseased tissue from the same organ. On your activity report, draw and label each example.
Step 4 Select one tissue sample, and compare the similarities between the normal and diseased tissue. On your Activity Report, answer question 3.
Step 5 Clean up. Carefully put away microscope and slides.
Step 6 Complete Activity Report.
Cells, as the building blocks of life, carry out a range of remarkable functions. The diversity of human cell types is astounding: from contracting heart cells, to signaling nerve cells, to swimming sperm cells, to oxygen-carrying red blood cells, to fibroblastic support cells, to light-transmitting lens cells, to natural killer cells of the immune system.
Even more remarkable than cell diversity is the wide spectrum of activities that cells carry out. Networks of communicating compartments within cells build the dynamic cell membranes crucial to cell communication and other functions. Each of the organelles of the cell has a vital function in the life and health of the cell.
Yet, of all the organelles, clearly the control center is the nucleus. Each somatic cell nucleus in a human contains two sets of the 22 autosomes and a pair of sex chromosomes. The gametes (sperm and eggs) contain 1 set of each of the 22 autosomes and only a single sex chromosome. The differences between the cell types of the human body is caused not by differences in DNA content, but by differences in the use or expression of the genes encoded in the DNA. The defining characteristics that distinguish one cell type from another include the content of specific types of protein produced by the different types of cells. In the DNA of the chromosomes, the cell contains the instructions, or blueprint, for that cell's products; the most noteworthy of which are the proteins.
The capacities of a cell for accurate replication of its DNA, for transcription of the DNA into mRNA, and for subsequent translation of the messenger RNA into protein are basic functions of the cell. Without these abilities, life, as it exists on our planet, would be unknown. The ability of cells to carry out behaviors (such as cell recognition and locomotion), to develop into the myriad of diverse cell types, and to make daughter cells that are exact copies are just a few of the many amazing abilities that cells have. Many of the most fascinating mysteries about cell function await the probing questions of your generation. The wide ranging frontier of the lives of cells is not yet fully mapped.
- What are mutations? Can they be harmful? Explain.
- How can mutations be beneficial? Explain.
- What can you do to reduce your risk of developing cancer? Explain.