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What are some cell functions?

The life of a cell depends on its environment and on the activities that take place within it. This section explains some of the processes that take place in cells and some of the ways cells respond to their environment.

The functions of all cells depend on or require special molecules called enzymes. Enzymes are proteins that help chemical reactions take place. They help cells build products like proteins, make copies of DNA molecules, make energy available for cell work, and even break down certain molecules. Each enzyme is very specific in its action.

Without enzymes, most chemical reactions that take place in cells would proceed very slowly, if at all. Enzymes enable those reactions to happen faster.

Different enzymes exist in different parts of a cell. Enzymes on the surface of a cell help receptor proteins signal the cell when they detect certain molecules in the fluid environment. Enzymes in the cell cytoplasm allow the structural proteins of the cytoplasm to do their work, for instance, to contract, to change the cell shape, or to divide the cell. Enzymes in the nucleus of a cell allow the cell to copy its DNA. Enzymes in the mitochondria of a cell allow the cell to convert energy from food nutrients into ATP.

Figure 3.1 The action of an enzyme that breaks down a certain kind of molecule.

Activity 3-1: Catalysts and Enzymes in Your Life

Introduction

How do the many necessary reactions in the cells of your body occur? What helps these reactions maintain homeostasis in your body? How are catalysts and enzymes similar? How are they different? In this activity you investigate the action of the catalyst manganese dioxide (MnO_2) and the enzyme catalase, on hydrogen peroxide. This activity helps you learn more about how chemical reactions occur in your body and what happens when these “facilitators” are absent or do not function properly.

Materials

  • Test tubes (5 per team)
  • Test tube rack
  • Test tube holder
  • Marking pen/pencil for test tubes
  • Graduated cylinder
  • Forceps
  • Stirring rod
  • Hydrogen peroxide (H_2O_2-3\%)
  • Sand
  • Manganese dioxide (MnO_2)
  • Wooden splints
  • Paper towels
  • Goggles for each team member
  • Liver cell samples (cooked and uncooked)
  • Activity Report
  • Data Table

Procedure

Step 1 Design a data table that includes space to write and illustrate the procedure and record observations for test tubes #1-#5.

Part A

How Are a Catalyst and an Enzyme the Same and Different?

Step 2 Label three clean test tubes #1, #2, and #3.

Step 3 Measure and pour 5 \ ml (milliliters) of hydrogen peroxide into each test tube. Be sure to put the cap back on the bottle of hydrogen peroxide.

Step 4 Use a wooden splint to place a small amount of sand into test tube #1. Observe what happens and record your observations in your data table.

Step 5 Use a wooden splint to place a small amount of manganese dioxide MnO_2, into test tube #2. Observe what happens and record your observations in your data table.

Step 6 Obtain a sample of uncooked liver. Use the forceps to place the sample of uncooked liver into test tube #3. Observe what happens and record your observations in your data table. Save test tube #3 for use as your control for each test in Part B.

Step 7 Complete questions 1-5 on the Activity Report.

Part B

What Affects Enzyme Action?

Step 8 Label two clean test tubes #4 and #5.

Step 9 Measure and pour 5 \ ml of hydrogen peroxide into each test tube. Be sure to put the cap back on the bottle of hydrogen peroxide.

Step 10 Obtain a sample of cooked liver. Remember that cooking the liver causes the temperature of the liver to become very high for a period of time. Use the forceps to place the sample of cooked liver into test tube #4. Observe what happens and record your observations in your data table.

Step 11 Obtain a sample of liver soaked in vinegar. Remember that vinegar is an acid, although not as strong as the acid in the digestive juice in your stomach. Use the forceps to place the sample of liver soaked in vinegar into test tube #5. Observe what happens and record your observations in your data table.

Step 12 Complete the remaining items on the Activity Report.

Transport of Nutrients: Exploring Diffusion Answer the following questions in writing.

  • What do you think would happen if you filled a small beaker with tap water and added one drop of food coloring?
  • What would happen if you filled a small beaker with tap water and added a cube of sugar?
  • How is diffusion affected by temperature?
  • What happens when someone opens a container of perfume or uses a spray bottle of room deodorizer?

Design your own activity to explore one of these questions. Then, answer this question.

How is the diffusion described here like the diffusion of nutrients and wastes in your cells?

Cell Transport

You learned earlier that the cell membrane controls what goes into and out of the cell. Some of the proteins in the membrane act as channels that allow certain molecules to move across the membrane. Some of the protein channels can be open or closed at different times. Other proteins in the membrane act as “pumps” that transport molecules across the membrane. Still other proteins act as “receptors” that signal to the cell when certain types of chemicals are present in the fluid outside the cell. Because the cell membrane is permeable to some molecules and not to others, it is called a semipermeable, or selectively permeable, membrane.

Diffusion is the random movement of molecules from a region of higher concentration to a region of lower concentration. Diffusion will continue as long as there is a difference in the concentration of molecules. When diffusion stops, a state of equilibrium is reached and the concentration of the molecules is equal throughout the given system.

Osmosis

The semipermeable membranes of most cells allow water to pass through, but not many of the molecules dissolved in water, which are called solutes. When a membrane is permeable to water but not to the materials inside a cell, water molecules will move from where there is a lower solute concentration to where there is a higher solute concentration. Let's see how this movement of water affects cells in your body. Think about your red blood cells. They are round discs that look like doughnuts without holes. If you put some of your red blood cells in fresh water from the faucet, the red blood cells will swell up and form little balls. Some or all may burst. This happens because the solute concentration inside the cells with all the dissolved proteins and other substances in the cytoplasm is greater than the solute concentration in the tap water. The process whereby water moves across a semipermeable membrane (such as the cell membrane) because of differences in solute concentration on the two sides is called osmosis. The red blood cells will swell and burst because water from the faucet has fewer materials dissolved in it than the contents of the cell. In contrast, seawater has more materials (such as salts) dissolved in it than the cell contents. If you put red blood cells in seawater they lose water and shrink and shrivel.

What Do You Think?

Considering what happens to red blood cells in different environments, describe what you think would happen if you moved a single-celled organism from its normal freshwater pond to a saltwater environment. What about moving a saltwater single-celled organism to fresh water? Explain.

Passive and Active Transport

Diffusion and osmosis are examples of passive transport. Passive cell transport is the result of differences in concentration and the random motion of the molecules. Passive transport does not require the cell to use any energy to move molecules.

In active transport the materials move from a region where they are in a lower concentration to a region where they are in a higher concentration. This is an “uphill” process that requires the cell to do work. The cell must provide energy to do this work. The energy comes directly from adenosine triphosphate (ATP). A good example of active transport is what happens at the cell membrane of a nerve cell. The concentration of charged potassium ions (K+) is maintained in a higher concentration on the inside of the nerve cell membrane. The concentration of charged sodium ions (Na+) is maintained in a higher concentration outside the nerve cell membrane. Because of these differences in concentration, potassium tends to leak out of nerve cells and sodium tends to leak in. However, ion pumps in the cell membrane are always using ATP to power their pumping of potassium (K+) into the cell and not out of the cell. This active transport is very important in order for a nerve cell to send an electrical nerve impulse.

You and your best friend are in a canoe on a river. Describe the effort you put in to get the boat to move with the current. Suddenly you realize you forgot something on the shore, so you tum around and paddle upstream. Describe your efforts to move against the current. How is this scenario like active and passive transport? Explain.

Cellular Respiration

In the previous section you learned that the mitochondria are factories and storage centers for ATP. An active cell requires millions of molecules of ATP every second to carry out its activities. Cells make the ATP they need using oxygen and glucose in a process called cellular respiration. Glucose is broken down through a series of enzymatic reactions. Cellular respiration produces ATP, as well as carbon dioxide and water.

The overall equation for cellular respiration is summarized below:

&C_6H_{12}O_6 \quad + \quad 6 \ O_2 \ \xrightarrow {\text{enzymes}} \ 36 \ ATP \quad + \quad 6 \ CO_2 \quad + \quad 6 \ H_2O\\&\text{Glucose} \qquad \quad \text{Oxygen} \qquad \quad \text{Adenosine} \qquad \quad \text{Carbon} \qquad \quad \text{Water}\\&\qquad \qquad \qquad \qquad \qquad \qquad \text{Triphosphate} \qquad \ \text{Dioxide}

The cell cannot use glucose as a direct source of energy. An analogy might be the gasoline in the gas tank of a car. If all the gasoline were burned at once, the car would explode. Instead only a small amount is supplied to the engine where controlled burning takes place. The same is true for cells. The energy from glucose is used in small amounts to synthesize or make molecules of ATP, as shown above.

What Do You Think?

Why do you think it is so important for living things to reproduce and pass on their genetic material? Why is this “the purpose of life”?

The Cell Cycle

Of all the important things that a cell can do, the most important is to reproduce itself by dividing. Reproduction is an essential function of a living system. Each cell, each animal, and each plant will die and its genetic information will cease to exist if it does not copy (replicate) itself. Cell reproduction occurs when a single cell divides into two daughter cells.

The term daughter cell may seem unusual to you because human daughters are not true copies of their mothers. Daughter cells is a term that scientists use when they write about the process of cell division or mitosis. The reason that human daughters aren't the same as their mothers has to do with the other kind of cell division, meiosis, the kind that only occurs in the reproductive organs of males and females.

When a cell prepares to divide and actually goes through division, we say that it is cycling or going through the cell cycle. Most of the cells in our bodies can divide to replace themselves or, if necessary, divide to replace neighboring cells of the same type that are dying. Many cells only live for days or weeks, so certain cell types in our bodies are always dividing to replace those that are being lost. At any one time, some cells (such as those of our bone marrow, hair, skin, and intestine) are dividing more frequently than other cells (such as those in our muscles and our brain).

Figure 3.2 The cell cycle.

When animals grow, their cells must divide rather than just get bigger. Cells divide because there is a size beyond which they cannot work efficiently. Remember the relationship between the surface area and volume of a cell. Necessary materials must move into cells to provide energy, and wastes must be carried out of cells. When the cell grows beyond a critical size, this efficiency is decreased. As a result, the cell divides into two smaller daughter cells that are more efficient in carrying out cellular processes. So tissues and organs grow by adding new cells through cell divisions.

The cell cycle is a repeating cycle of growth and division that consists of four main phases. These phases are called G_1, S, G_2, and M. Collectively the G_1, S, and G_2 phases are known as the interphase stage of the cell cycle.

During the G_1 phase, a cell grows and carries out its basic functions. This phase makes up a major portion of the cell cycle.

During the S phase, the cell's DNA is copied or replicated.

During the G_2 phase, the cell prepares for the division of its nucleus. Proteins and membranes are made.

Finally, during the M phase, mitosis, or cell division takes place. Two nuclei are created, each with the same number of chromosomes.

At any given time, most of our cells are not actively dividing or cycling. Instead they are resting. We call the part of the cell cycle where cells are resting the G_0 phase. Most of your cells are resting in G_0 right now. Some of your cells, stem cells, and many cells of your intestine, bone marrow, and hair follicles may be cycling. There are different signals a cell may receive that make it cycle. The cell may get a signal from outside its membrane or from inside the cell itself. Then it will enter the G_1 phase. Once a cell enters G_1 and begins to cycle, it takes about 18 to 24 hours to complete the cycle and form two daughter cells. The relative duration of the different phases of the cell cycle depends on the species of the organism and the type of cell.

Let's look more closely at how cells divide. How does one cell become two?

Mitosis-Cell Division for Somatic Cells

There are two main categories of cells. The most common cells are the somatic cells. These include almost all the cells in your body-the cells that make up the structure of your body and all your organs, such as the brain, heart, muscles, intestine, and liver. The other cells are the gamete cells, which are either sperm in males or eggs in females. You can see that almost all cells belong in Category 1-Somatic Cells. Only a few of the many cells in Category 1 are listed.

More than 100 kinds of cells Category 1-Somatic Cells Only two kinds of cells Category 2-Gamete Cells
Lens cells sperm
Iris cells eggs
Skeletal muscle cells
Heart muscle cells
Pancreatic cells
Lung cells
Immune system cells
Blood cells
Nerve cells
Skin cells
Liver cells
SOMATIC CELLS DIVIDE ONLY THROUGH MITOSIS. GAMETE CELLS DIVIDE THROUGH MEIOSIS.

Somatic cells all go through the same kind of cell division. This process is mitosis, which is the M phase of the cell cycle. The parent cell reproduces into two daughter cells as a result of division of the chromosomes and division of the cytoplasm. All our cells, except our gamete cells, are produced only through the process of mitosis. The gamete cells are produced through a process called meiosis.

Figure 3.3 A parent cell divides into two daughter cells.

Mitosis involves making exact copies of the parent cell. If the daughter cells are exact copies of the parent cell, what does this mean in terms of chromosomes? Let us look first at the overall process. How many chromosomes does the parent cell have? Remember each somatic cell is diploid which means double-it has two sets of each of the 22 autosomes and a pair of sex chromosomes. If the daughter cells are each exact copies of the parent cell, then how many chromosomes must they each have? The answer is, “The daughter cells each have the same as the parent-two sets of each of the chromosomes, or 46 chromosomes in each diploid cell.”

How can the parent cell become two daughter cells with each of these cells having the same identical number of chromosomes? If you guessed that the parent cell must duplicate its chromosomes, you guessed correctly. To duplicate each of its chromosomes, the parent cell must go through DNA synthesis or the S phase of the cell cycle.

Each of the new daughter cells is just like the parent cell. The process of mitosis proceeds with remarkable accuracy. In the end, the number of chromosomes per cell remains the same. The parent cell was diploid and each daughter cell is diploid, as well.

Figure 3.4 Diagram of a chromosome before it is duplicated.

Figure 3.5 Diagram of a duplicated chromosome.

In Activity 3-2, you will investigate the process of mitosis. Figure 3.7 below shows the steps in this process.

Figure 3.6 Diagram of the separated chromosome from Figure 3.5. The two daughter chromosomes become separated (at the centromere) during cell division. Each daughter cell gets two sets of 23 chromosomes. Each of these is just one of those 46 chromosomes.

Figure 3.7 The process of mitosis involves division of both nucleus and the cytoplasm. Mitosis can be divided into 4 stages-prophase, metaphase, anaphase, and telophase. When the cell is not in the process of cell division, it is in interphase.

  1. DNA replicates.
  2. Chromosomes become compact. Aster and spindles form.
  3. Nuclear membrane breaks down. Chromosomes line up.
  4. Centromeres divide. Duplicated chromosomes move to opposite sides.
  5. Cytoplasm is divided. Nuclear membrane reforms.
  6. Cell divides. Chromosomes become less compact.

Activity 3-2: Cell Division-Double or Nothing

Introduction

Many-celled organisms, including yourself, begin as a single cell. What is the process of producing millions of cells from a single cell? In this activity you explore how a cell reproduces (divides) to form two new cells. Mitosis is a continuous and orderly process that occurs in your body's somatic cells. In this activity you model each stage of mitosis using pipe cleaners to represent chromosomes.

Materials

  • Resources 1 and 2
  • Activity Report
  • Colored crayons or colored pens or pencils (same colors as pipe cleaners if possible)
  • Large paper plates (2)
  • Eight pipe cleaners, (2 long of color A, 2 long of color B, 2 short of color A, and 2 short of color B)

Figure 3.8 One long and one short pair, Color A. One long and one short pair, Color B.

Procedure

Step 1 Gather four pairs of pipe cleaners to represent the chromosomes. Remember that your somatic cells have 23 pairs of chromosomes. Here we follow two pairs of chromosomes through the process of mitosis.

Step 2 Arrange your pipe cleaners so each pipe cleaner in the pair is of the same length and color. Twist each pair together by one turn at the midpoint. Each pair of pipe cleaners represents a duplicated chromosome. The two different colors indicate that one chromosome came from the father and one came from the mother. Duplicate chromosomes are formed through the process of DNA replication, which occurs before mitosis begins.

Step 3 Take two plates and place one plate on top of the other. The plates represent a cell. Put your chromosomes on the top plate. Using the crayons, draw a picture of the chromosomes on your Activity Report.

This phase (prophase) can be recognized when the double chromosomes are visible and can be observed under the microscope as distinct bodies.

Step 4 Line up the double chromosomes along a line that divides the top plate into two halves. Using crayons or colored pencils, draw a picture of the chromosomes on your Activity Report.

During this phase (metaphase) the chromosomes line up in the middle of the cell.

Step 5 Now separate each double chromosome by untwisting them. Leave them side by side on the midline through the center of the plate. Next move one single chromosome of each pair to the left of the plate and one to the right. Each chromosome is now a single chromosome, and each side of the plate should have two long and two short single chromosomes.

This phase of mitosis (anaphase) occurs when double chromosomes separate into two single chromosomes that move to opposite sides of the cell.

Step 6 Now it is time for the cell to divide into two daughter cells. You represent this step by bringing out the second plate and moving one set of single chromosomes to it. Each newly formed daughter cell has an identical set of chromosomes-two short and two long chromosomes.

In this phase of mitosis (telophase), the cytoplasm divides, resulting in two daughter cells. Each newly formed daughter cell has a nucleus containing a complete set of chromosomes, two sets of the 23 different chromosomes found in the cell. On your Activity Report record a colored drawing of the chromosomes placed on each plate.

Step 7 In the next phase of mitosis (interphase) the chromosomes lose their compact appearance. The chromosomes replicate their DNA so that each single chromosome becomes a duplicated chromosome as you saw in Steps 1 and 2 of the activity.

Step 8 Discuss with your partner the questions on the Activity Report and then record your responses.

Mitosis in Action Summarize what you have learned about mitosis by completing one of the following creative activities to share with your classmates.

  • Cartoon story
  • Poster
  • Story book, with drawings
  • Poem
  • Song
  • Dance
  • Build models

Meiosis-Cell Division to Produce Egg and Sperm

Meiosis is a special kind of cell division for the two types of cells that unite to make a zygote, which is a fertilized egg. As you read earlier, these two kinds of cells are called gametes. The gamete cells are the sperm in males and eggs in females. The two kinds of gamete cells must fuse at fertilization to form a fertilized egg. The fertilized egg has two sets of chromosomes and is diploid, so each of the gamete cells had only one set of chromosomes instead of two. The sperm brings in one set of chromosomes, and the egg brings in the other set of chromosomes. Meiosis is necessary for gamete cells to be produced, and then unite to form a fertilized egg that will develop into a new individual. How does meiosis happen in humans?

Typically, each of the somatic (body) cells in a human has 46 chromosomes in the nucleus. Remember that this number, 46, represents two full sets of our 23 chromosomes and is called diploid. The only exception to this rule is the gamete cells, which have 23 chromosomes and are called haploid for half. The gamete cells are produced through two cell divisions, which result in four daughter cells that are each haploid in chromosome number. Figure 3.9 on the next page shows you the process of meiosis by following one pair of homologous chromosomes. You can follow the changes in chromosome number to see how one diploid cell becomes four haploid gamete cells.

What Do You Think?

Some single-celled organisms such as an amoeba can reproduce through mitosis only. Why might this process be a good way to pass on their genetic material to their offspring? What might be some problems with this method of reproduction?

Once in a while there are mistakes in meiosis and one gamete cell ends up with an extra chromosome, or another gamete cell could be missing a chromosome. If either one of these abnormal gamete cells (such as the egg) were to unite with a sperm to form a fertilized egg, the developing fetus may not be able to survive, or the individual born could be affected by a genetic disorder. The outcome depends on which chromosome was involved and whether or not there is an extra copy of it, or a missing copy. For example, people born with Down syndrome have an extra copy of chromosome 21 in their cells. That happens because one of the gamete cells (the sperm or egg) that forms these individuals carries two copies of chromosome 21, instead of one copy. People affected with Down syndrome have three copies of chromosome 21 in their cells. Despite the errors that occur, meiosis overall is amazingly accurate when you think about how complicated the process is.

Figure 3.9 Meiosis. One pair of homologous chromosomes is shown inside the cell. The changes in the chromosome number is shown to the right for each stage of meiosis.

Review Questions

  1. What are enzymes and why are they important to the cell?
  2. What are the roles of osmosis and diffusion in the cell?
  3. Why do cells divide?
  4. How are mitosis and meiosis similar? How are they different?

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