Chapter 2 • Lesson 8Single-Celled OrganismsObjectives: 1.1,1,1.1.2,1.2.3Key Wordsunicellular • bacteria • archaea • protist • microscope • contractile vacuole • cilia • flagellum pseudopod • eyespot • chemotaxis • phototaxis

Getting the IdeaCells are living systems that serve as the basic unit of structure and function in all organisms. While many organisms you are familiar with are multicellular, or made up of many cells, others consist of only a single cell. In these organisms, this single cell carries out all the processes of life.

Unicellular OrganismsA unicellular organism consists of only one cell. This single cell carries out all the functions needed for the organism to survive and reproduce. All prokaryotes are unicellular. Recall from Lesson 1 that a prokaryote is an organism without a nucleus or membrane-bound organelles. Prokaryotes are classified as either bacteria or archaea.Bacteria (singular: bacterium) are prokaryotes whose cell walls are made up of peptidoglycan. Archaea are prokaryotes whose cell walls do not contain peptidoglycan. In addition to lacking peptidoglycan in their cell walls, archaea have plasma membranes composed of lipids that differ from those in the plasma membranes of bacteria or eukaryotes.Unlike prokaryotes, eukaryotes have cells that contain a nucleus and membrane-bound organelles. Eukaryotes include protists, fungi, plants, and animals. All plants and animals are multicellular. In contrast, the protist and fungi kingdoms include both multicellular and unicellular organisms. Protists are a diverse group of eukaryotes with characteristics that prevent them from being classified as fungi, plants, or animals.

Specialized Structures of Unicellular OrganismsAll organisms are adapted, or suited, to life in a particular environment. An adaptation is any structure or behavior that helps an organism survive or reproduce. Although they are made up of only a single cell, unicellular organisms are not as simple as they might seem at first. For example, many unicellular organisms have a variety of specialized structures that aid in their survival. As you read about these structures, keep in mind that most are too small to be seen without a microscope. A microscope is a device that enables its user to see enlarged images of tiny objects. You will read more about microscopes later in the lesson.

Contractile VacuolesMost protists live in water. Many of them have contractile vacuoles. A contractile vacuole is a membrane-bound organelle that helps a cell maintain its water balance. Excess water that enters the cell is temporarily stored in the contractile vacuole. The vacuole then contracts to pump the excess water out of the cell. As you read, locate the contractile vacuoles in the diagrams of the different protists discussed in this lesson.

CiliaStructures called cilia are present on some unicellular organisms. Cilia are short, hairlike projections on the outside of a cell that move the cell using coordinated strokes. The strokes alternate in different regions on the outside of the cell for controlled movement. A paramecium, shown below, is an animal-like protist that uses cilia to move through water. The paramecium also uses its cilia to sweep food toward its food passageway (oral groove and gullet).

FlagellaSome bacteria and protists have one or more flagella on the outside of their cells. A flagellum (plural: flagella) is a long, whiplike projection whose uniform rhythmic motion is used to move the cell. You can see a flagellum on the bacterium shown below. The euglena shown later in this lesson is an example of a protist that moves using a flagellum.

PseudopodsSome protists move using temporary projections of the cell, called pseudopods. The word pseudopod means "false foot." Amoebas are examples of these protists. As the diagram shows, an amoeba can extend a pseudopod outward from the center of the cell. The rest of the cell follows as its cytoplasm moves into the pseudopod. The amoeba then retracts the pseudopod.

Amoebas do not use pseudopods only for movement. Pseudopods are also used to capture food. As shown in the diagram, amoebas capture food with pseudopods by surrounding the food and then pulling it inward to form a food vacuole. Once the food is inside the cell, it is digested. Wastes that cannot be digested are released from the cell.

EyespoteSome unicellular organisms have eyespots. An eyespot is an organelle that contains a pigment that is sensitive to light. A euglena, like the one shown below, is an example of a protist with an eyespot. Notice that the euglena also has a flagellum and chloroplasts containing chlorophyll.

Recall from Lesson 1 that chloroplasts are organelles in which photosynthesis takes place. Chlorophyll is a green pigment that absorbs the energy of sunlight, which drives this process. When a euglena's eyespot detects light, the euglena can use its flagellum to move toward the light in order to make food by photosynthesis.Unlike most organisms that make food by photosynthesis, a euglena does not get all its energy this way. Euglenas also feed on other organisms. When its eyespot does not detect light, a euglena can move through the water to capture and feed on other organisms.

Unicellular Organisms Respond to Their EnvironmentAll organisms have adaptations that allow them to respond to changes in their surroundings. In many cases, these responses involve movement. A taxis (plural: taxes) is an organism's movement toward or away from a stimulus. Taxes are involuntary responses to chemicals, light, magnetism, electric currents, and other variables in an organism's environment. When the organism moves toward a stimulus, the taxis is described as positive. Movement away from a stimulus is described as negative.

One type of taxis in microorganisms is chemotaxis, movement toward or away from chemicals in the environment. Many unicellular organisms use chemotaxis to find food. For example, an organism that senses glucose may move toward the source of the glucose. By contrast, an organism may move away from an area in which it detects potentially harmful chemicals, such as acids, bases, or toxins.

Phototaxis is the movement of an organism toward or away from light. For example, if a euglena detects light, the euglena may move toward the light so it can use the energy for photosynthesis. Cyanobacteria, a group of bacteria that carry out photosynthesis, also move toward light so they can use its energy to make food.

MicroscopesMicroscopes are important tools for studying living things. Most individual cells, including almost all unicellular organisms, are too small to see with the unaided eye. A microscope can produce an enlarged image of these cells.

The microscopes most often used in biology classrooms are light microscopes. As their name suggests, light microscopes form images by gathering and focusing light. A typical high-power light microscope magnifies objects up to about 1000 times their actual size (1000x). Light microscopes include the hand lens, a magnifier held in the hand, and the compound microscope.

A compound microscope uses two or more lenses to form an enlarged and focused image of an object. A microscope slide is a piece of glass that is used to hold a specimen that will be observed using a compound microscope. Most microscope slides are flat, but some have a depression called a well. A well slide is useful for viewing specimens that are too large or bulky to fit under a coverslip. A coverslip is a thin square of glass that is placed on top of a specimen on a slide. The slide is then secured on the stage of the microscope for viewing.

A compound microscope is more powerful than a simple microscope. For this reason, a compound microscope is the tool most often used in the science classroom to study cells and their structures. In your study of biology, you may prepare slides that can be used to identify cells based on visible traits, such as cell walls, nuclei, chloroplasts, and flagella. You may also view prepared slides of plant and animal cells. The amount of detail you will see in such cells will depend upon the magnifying power of the microscope you use.

Most compound microscopes have two or three objective lenses. The magnification of each objective lens is marked on the microscope. To calculate the magnifying power of a compound microscope, the magnification of the ocular (eyepiece) lens is multiplied by the magnification of the objective lens being used. Consider an eyepiece lens with a magnification power of 5x and a low-power objective lens that has a magnification power of 10x. The total magnification of a specimen viewed with this combination of lenses will be 50x (5 x 10).

With a light microscope, it is possible to observe live organisms and their movements. For example, you can observe microorganisms living in a drop of pond water.Another type of microscope is the electron microscope. Modern electron microscopes can produce images that are hundreds of thousands or even millions of times larger than the original objects. Unlike light microscopes, electron microscopes cannot be used to view live organisms because the specimen must be observed in a vacuum. These microscopes are very costly, so they are used mostly in professional labs.

Although you are unlikely to use an electron microscope, you may see images produced using one, in books or online. The two most common types of electron microscopes are the transmission electron microscope (TEM) and the scanning electron microscope (SEM). The TEM produces a two-dimensional image that shows more details than are visible with a compound microscope. An SEM is useful for studying surfaces and cell structures because it can produce three-dimensional images.

Focus on Inquiry You have used microscopes and hand lenses to study cells and other small objects. To study cells under a microscope, scientists and students first prepare slides. To examine a liquid, such as milk or pond water, you can use a dropper to place a drop of the liquid on a glass slide. Then carefully lower a coverslip on top of the liquid.

You can also make slides of solid objects. To make a slide of plant material, such as cork or celery, scientists cut thin sections of the material. If you are making your own slides, your teacher may instruct you to use a scalpel or razor blade. Be sure to work carefully, and always cut away from your body. After cutting a thin slice of the plant material, cut a small piece that will fit on a slide. Place a drop of water on a slide, then carefully put the slice on the drop of water. Lower a coverslip onto the sample.

You will also work with prepared slides. In the lab, your teacher may give you slides of plant, fungus, protist, or animal cells. For this activity, your teacher will give you either prepared slides of various unicellular organisms, a sample of pond water that contains such organisms, or both. You will examine these specimens and compare them with the organisms described in this lesson.

Prepare a slide from the sample of pond water, or gather the prepared slides your teacher provides. Place the slide with the specimen on the stage of a light microscope. Observe the cells under the low-power objective and make a sketch of what you see in your lab notebook. Calculate the magnification of the low-power objective and record this magnification beside your sketch. Then look at the cells with the high-power objective. Sketch what you see using this objective and record the total magnification beside the sketch.

Compare your sketches with the diagrams in this lesson. Label any organisms you recognize as well as the cell parts that you can identify in each of your sketches.

What unicellular organisms did you observe?

What cell structures did you observe in each organism?

What cell structures of each organism are not visible?