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What is the structure of cells?

Each cell is surrounded by a membrane and contains parts called cellular organelles. Each cellular organelle has a specific function. Some parts of a cell are involved in converting energy from nutrients in the food you eat into a form of energy that the cell can use. Other parts of the cell are involved in storing the genetic information that serves as the blueprint that makes you different from a gorilla or a hummingbird. Still other parts of a cell are responsible for building the proteins that enable the cell to do its many tasks.

The Cell Membrane

Every cell has a thin outer covering called the cell membrane, or plasma membrane. The cell membrane surrounds the contents of the cell and separates it from other cells and the environment. The cell membrane also controls what materials go into and out of the cell. For example, the cell membrane allows nutrients and oxygen to move into the cell, and carbon dioxide and waste materials to move out of the cell.

The cell membrane is made of molecules called proteins and lipids arranged in an orderly manner. You could think of the cell membrane as two sheets or layers of lipid molecules with different kinds of proteins positioned in the sheets. The lipids making up the bilayer membrane are arranged in a special way. Each lipid molecule has a head and a tail.

Figure 2.1 A typical lipid molecule found in the cell membrane.

The head ends of the lipid molecules are attracted by water, and the tail ends are repelled by water. In a watery environment, the lipid tail ends tend to associate with (face) each other. Since the head ends are attracted by water, they face the watery environment, some toward the inside of the membrane and some to the other side of the membrane. In a cell membrane, each lipid layer has its molecules lined up in the same way. The two layers are arranged so the tails of the lipid molecules face each other.

Figure 2.2 Bilayer sheet of lipid molecules, as in a cell membrane.

Figure 2.3 Scouts in two rows of sleeping bags.

A Drawing or Model of a Cell Membrane Draw your own picture or build a model to show the arrangement of lipids in a cell membrane.

Think of a troop of scouts all sleeping in their sleeping bags in a rectangular tent. They want to arrange themselves so no one's feet are in someone else's face. One arrangement would be two rows of sleeping scouts with feet pointing toward each other as shown in Figure 2.3. That arrangement is similar to the composition of the lipid bilayer of the cell membrane.

Did You Know?
A person coming out of the bath carries away a film of water about 1/50 of an inch thick. This water weighs about a pound. A mouse getting out of the same tub carries away water about equal to its own weight. A fly or honey bee coming out of a pool of water carries a film of water that weighs many times more than its own weight.

The organized lipid bilayer allows water and molecules that dissolve in water to come right up to the membrane, but not pass freely through the membrane. Because of the lipid bilayer, water molecules can only pass through protein channels in the membrane. Other molecules that dissolve in water must go through protein channels to move into or out of cells. The lipid bilayer helps control the environment inside a cell, and the proteins of the cell membrane determine which substances can move into and out of a cell through the cell membrane.

Figure 2.4 Movement of water through a protein channel in the cell membrane.

Soap Bubbles Have you ever noticed a swirl of subtle colors on a large soap bubble? This free movement is similar to the movements of molecules within the membrane of a cell. Try the following activities to help you imagine the direction of free movement or the fluidity of proteins and lipids in a cell membrane.

  1. Blow soap bubbles in the light and watch the movement of the colors on the surface of the bubbles. Cells allow movement somewhat like this.
  2. Straighten a paper clip. Dip the straightened paper clip into a bubble solution. Move the paper clip into a bubble and then across a bubble. What happens? Repeat using many different objects (or your finger).

Also try bubbles of different sizes and see which size bubbles last longest. Try to make elongated bubbles and see if those ever divide into two spherical bubbles. Why do you think that occurs?

Figure 2.5 Proteins and lipids make up the cell membrane. The proteins have different functions. Some might be receptors for hormone signals. Others might be channels.

Passage through a Cell Membrane

Assume that all factors influencing the movement of molecules are the same except for the molecular size of the substance. If we know the size of the pore opening of the membrane, we can estimate the size of the substances based on their ability to move through the membrane pores.

Think of a place or building that only allows certain people to enter or leave, such as a ride at an amusement park or a building with security guards at the doors. Describe how that place regulates who can come and go. What criteria do they use? How does this compare to a cell membrane?

Study the table in Figure 2.6. Then answer the following questions.

  1. Substance W is smaller than ? nanometers (nm).
  2. Substance X is larger than ? nm but smaller than ? nm.
  3. Substance Y is larger than ? nm but smaller than ? nm.
  4. Substance Z is larger than ? nm.
  5. If membranes with different pore sizes were available how could you determine the exact size of substance X?
Membrane Pore Size Can substance pass through pore?
W X Y Z
A 1nm Yes No No No
B 3nm Yes Yes No No
C 5nm Yes Yes Yes No

Figure 2.6 This figure shows the abilities of certain substances to pass through pores of different sizes.

Did You Know?

1 meter = 100 centimeters

1 centimeter = 10 millimeters

1 millimeter = 1,000 micrometers

1 micrometer = 1,000 nanometers

Cytoplasm and Organelles

The cytoplasm and specialized working parts called organelles are inside the cell membrane. The cytoplasm is a sea-like fluid that contains water and other chemicals. Some of these chemicals are found throughout the cytoplasm. Some of the chemicals are found just near the cell membrane, around the edges of the cytoplasm. Because the cytoplasm is fluid, the cell can change shape, much like a bag of water changes shape.

The parts of the cell that are organized for specific functions are called organelles. The organelles include such structures as the nucleus and the mitochondria. The nucleus contains the genetic material and the mitochondria convert energy. Figure 2.7 shows some of the major organelles of a typical animal cell.

Each of these organelles has a special role to play in the way the cell works. The role of control center goes to the nucleus.

What Does Each Part of a Cell Do? Different parts of the cell perform different functions. The cell releases energy or cells make more cells. While you proceed through this section of the text, keep a record of the function of each cell part, or cellular organelle.

Did You Know?
Human cells have nuclei bounded by nuclear membranes and are called eukaryotes. (The term eukaryote means “true nucleus.”) The cells of plants and animals are eukaryotes. Each of their cells has a true nucleus. Some very simple unicellular organisms, such as bacteria and blue-green algae, do not have nuclei. These organisms are called prokaryotes instead of eukaryotes. In this unit, we are studying only eukaryotic cells since those are the ones found in your body.

The Nucleus

The nucleus is the most important part of a cell. It is the “information” headquarters and is in charge of the cellular activities. It contains the information that will tell the cell what to do, what to make, and when to divide. A nucleus is a small and very powerful part of a cell.

What Do You Think?
Why do you think it would be an advantage for a cell to have a membrane-bound nucleus?

Figure 2.8 The nucleus.

All cells contain genetic material in units called genes. Genes control what a cell looks like, what it can do, and how it functions. In the cells of higher organisms, including humans, the genetic material is found inside the nucleus. A nucleus is a region within the cell that is surrounded by a membrane, called the nuclear membrane. The nuclear membrane controls which molecules enter and leave the nucleus.

Chromosomes

If you looked at one of your cells under a microscope, you would see that under certain conditions, structures called chromosomes appear in the nucleus of the cell.

There are 23 pairs of chromosomes in the nucleus of human cells. Of these 23 pairs there are two main kinds-22 pairs of matched chromosomes (autosomes) and a pair of sex chromosomes. Each chromosome is a particular length and has a short arm and a long arm. Besides size and shape, the banding pattern of a chromosome is also distinctive. The chromosomes are not found in any particular order within a nucleus. However, in the laboratory, scientists can sort them into pairs and identify each chromosome, as shown in the diagram.

Each chromosome is made up of two parts-DNA (deoxyribonucleic acid) and proteins. The DNA is the genetic material that codes for the characteristics of a person. A chromosome contains one long molecule of DNA.

Figure 2.9 Chromosome count of a typical human female. Every cell in a healthy human female has these chromosomes in the nucleus.

Mitochondria

Cells require a continuous supply of energy. Mitochondria have the job of converting energy in the food you eat to a form that is usable by cells. Mitochondria are fairly large organelles that look like this:

Figure 2.10 Mitochondrion.

Mitochondria are present in the cytoplasm of all cells. In a typical cell the mitochondria might be located randomly throughout the cytoplasm.

Figure 2.11 Mitochondria within a cell.

Mitochondria make a compound called adenosine triphosphate (ATP) from other molecules. When the chemical bonds of food molecules, such as sugar, are broken, energy in the form of ATP is produced. The ATP is then stored in the mitochondria until it is needed elsewhere in the cell.

ATP contains the energy that is used by the cell to make all of its products and to carry out its functions. ATP is like cash. It can be used to make goods and services. There are other forms of energy around the cell, but the cell must convert this energy into ATP to make products that help the cell to live and to function normally.

Figure 2.12 ATP structure.

Ribosomes

Proteins are important molecules produced by cells. Ribosomes are the organelles where proteins are produced or synthesized. Ribosomes are themselves made up of proteins. The ribosomes are scattered throughout the cytoplasm or are attached to part of another organelle called the endoplasmic reticulum.

Endoplasmic Reticulum

The endoplasmic reticulum is a series of folded membranes that is used to move materials around the cell. All cells have endoplasmic reticulum (or ER). The ER also is involved in helping to make different kinds of cell membranes, including the outer (plasma) cell membrane, mitochondrial membrane, and nuclear membrane.

There are two main kinds of ER-rough and smooth. Rough ER has ribosomes attached to it and makes proteins, including many important proteins for the plasma membrane. Smooth ER does not have ribosomes but is involved in modifying proteins synthesized by the rough ER.

Figure 2.13 Endoplasmic reticulum with ribosomes attached (left). Smooth endoplasmic reticulum without ribosomes (right).

Golgi Apparatus

The Golgi apparatus is a series of flattened sacs. When you observe the Golgi apparatus through a powerful microscope, the sacs appear to lie on top of one another in a stack. The bottom of the stack lies near the nucleus, or part of the rough ER. The top of the stack lies closer to the outer cell membrane. The Golgi apparatus helps sort the proteins synthesized on the rough ER. The proteins are transported to the Golgi apparatus where they may be stored or chemically modified and then packaged for delivery to the cell membrane for export outside the cell or to other places inside the cell.

Figure 2.14 Golgi apparatus.

Summary of the Cell Parts

As you look back through this section of the unit, you can revisit the different organelles in the cytoplasm and their functions. Here's a quick review of those organelles.

The cell membrane, or plasma membrane, forms a barrier to create an environment inside the cell that is separate from the environment outside the cell. The membrane is a vital border. It controls what enters and what leaves the cell.

The nucleus is the control center of the cell that contains the chromosomes with their genetic material, DNA. The nucleus controls all cellular functions.

Chromosomes are large molecules in the nucleus made up of DNA and protein.

Mitochondria are the factory and storage center for ATP, which is used as energy by the cell in making cellular products and carrying out the functions of the cell.

Ribosomes are the sites for protein synthesis.

The endoplasmic reticulum (ER) is an elaborate membrane system throughout the cytoplasm. Portions of ER contain ribosomes on the membrane surface.

The Golgi apparatus are the flattened sacs that help sort the proteins synthesized by the rough ER and ribosomes.

For each of the cell organelles or parts listed above, explain how each one contributes to the overall internal balance (homeostasis) of the cell to keep it functioning smoothly.

Activity 2-1: Making a Cell Model

Introduction

What do cells in your body look like? What makes up a cell in your body? In this activity you create a model of a cell in your body.

Student Materials

  • To be determined by each team, with teacher approval
  • Activity Report

Procedure

Step 1 Imagine a cell in your body. What does this cell look like? Complete question 1 on the Activity Report. Draw a picture and label the parts of this cell.

Step 2 What materials would you need to construct a model of this cell? Brainstorm to make a list of materials necessary for making your body cell. Be sure to select materials that will not spoil. After teacher approval of the materials you selected, complete question 2 on the Activity Report.

Step 3 Create your model of a cell using your drawing and approved list of materials.

Step 4 Name and label each of the following parts of your cell model and any other important parts of the cell.

Cell membrane Mitochondria

Cytoplasm Ribosomes

Nucleus Endoplasmic reticulum

Chromosomes Golgi apparatus

For labels, you can use toothpicks and paper flags. Include the name and function on the flag, or simply label the parts of the cell and include the functions of these parts in an accompanying key. Record your name on your model.

Complete question 3 on the Activity Report.

Step 5 Share the model of a cell with your classmates. Note how these models are different from one another. Which model impressed you the most? Why?

Complete your Activity Report.

Review Questions

  1. What is the primary function of the cell membrane? Describe three structural characteristics of the cell membrane.
  2. What is the difference between a eukaryotic and a prokaryotic cell? Include examples of eukaryotes and prokaryotes.
  3. What is the functional relationship among mitochondria, sugar, and ATP?
  4. How are ribosomes, the endoplasmic reticulum, and the Golgi apparatus involved in the production and transport of proteins?

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