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Introduction

Cells are the fundamental unit of life. All living things are composed of cells. While there are several characteristics that are common to all cells, such as the presence of a cell membrane, cytoplasm, DNA and ribosomes, not all cells are the same.

Prokaryotic cells lack a nucleus and membrane-bound organelles. Eukaryotic cells are compartmentalized by membrane-bound organelles with specialized functions. In eukaryotes, DNA is located within the nucleus; whereas, prokaryotes lack a nucleus.

In some stained preparations the nucleus may contain one or more darker-stained bodies, the nucleoli (singular-nucleolus). Ribosomal RNA (rRNA) is transcribed within the nucleolus. The nucleus also contains a number of structures called chromosomes, which are composed of DNA and proteins. Chromosomes can be seen only after special staining procedures applied to dividing cells. When cells are not dividing, chromosomes are seen as chromatin. An exception is one inactivated and condensed chromosome in female mammals. This chromosome, which may be visible as a dark spot in the nucleus, is called a Barr body.

The nucleus in living cells is sometimes indistinguishable from the cytoplasm unless the cell is stained. When stained, the nucleus appears darker than the surrounding cytoplasm.

The cytoplasm is the region of the cell outside the nucleus. It contains fluid, ribosomes, the cytoskeleton, and, in eukaryotes, other membrane-bound organelles.

The minute “power-plants” in the cytoplasm of cells are called mitochondria. These organelles are roughly the size of many bacteria and can only be seen at higher magnifications in specially prepared slides.

Plastids are organelles found in plants and algae. Some organic compounds are produced and stored in plastids. The green pigment, chlorophyll, is located in special photosynthetic plastids called chloroplasts. Other kinds of plastids include chromoplasts, which contain pigments other than chlorophyll, and amyloplasts which store starch.

The central sap vacuole often occupies a large space within the cytoplasm of plant cells, but may be small or absent in other types of cells. Often, other organelles found in plant cells are located adjacent to the plasma membrane because the central vacuole takes up so much space within the cell. Under healthy conditions for plant cells, the central vacuole is large and produces turgor pressure against the cell wall, which is located outside the cell membrane. The cell wall keeps plant cells from bursting. Some other cells also have cell walls, but they are generally made of different materials. Plant cell walls are made of cellulose, while bacteria have cell walls made of peptidoglycan and fungi have cell walls made of chitin. Archaea and algae also have cell walls made of various compounds.

Part 1: Animal Cells

Animals are a group of eukaryotic, multicellular, heterotrophic organisms that ingest organic matter for sustenance. Many animals have cells that differentiate into specialized tissues including epithelial, connective, muscle, and nervous tissue.

Starfish egg cell

Because of the quantity of stored food, animal egg cells are among the largest known single cells.

Materials

• prepared slide of unfertilized starfish egg
• compound microscope

Procedure

1. Examine your slide of the starfish eggs and select a perfectly spherical cell to study. Move the slide so that the cell will be near the center of the field and turn to high power. Regulate the light for good contrast and focus carefully. Note the central spherical structure, the nucleus, and the surrounding cytoplasm. The cytoplasm is bounded by the cell membrane, which is very thin and pliable. Also note that the spherical nucleus is bounded by a well-defined nuclear envelope. Within the nucleus is the granular appearing chromatin and the nucleolus, a distinct, deeply stained, rounded body.
2. Draw a starfish egg with a diameter of approximately 2 cm. Label the cell membrane, chromatin, nucleolus, nuclear envelope, nucleus, and cytoplasm.

Cheek Epithelial Cells

Cells that cover a surface, whether outside the body or inside the body are called epithelial cells. Epithelial cells from inside your mouth are easily collected and examined under the microscope.

Materials

• slide
• cover slip
• toothpick
• dropping pipette
• methylene blue stain
• metric ruler
• compound microscope

Procedure

1. With a toothpick, gently scrape the inside lining of your cheek. Place the material collected into a drop of water on a slide. Add one drop of methylene blue stain and mix the two solutions. Add a cover slip and observe with the microscope. Use both low and high power.
2. Find the cell membrane, nucleus, nuclear envelope, and cytoplasm.
3. Draw three representative cells, each about 2 cm in diameter. Label one cell with structures listed above.
4. What purpose do epithelial cells serve?

Part 2: Plant Cells

There are two fundamental cell types:

• Parenchyma: These cells have thin walls, allowing free transfer of materials between membranes of adjacent cells. Major functions include formation of boundary layers (epidermis), chemical synthesis (chlorenchyma), and food storage.
• Sclerenchyma: These cells are thick-walled and non living at maturity. Among sclerenchyma cells are those that produce hard parts such as nut shells (stone cells), those that provide strength for stems (fibers), and those that transport water and dissolved minerals up the plant stem (vessel elements).

Parenchyma: Onion Epidermis

The onion bulb is made up of specialized leaves or scales. Each leaf is covered by a single layer of epidermal cells.

Materials

• onion bulb
• slide
• cover slip
• compound microscope
• Iodine solution

Procedure

1. Peel a small portion of the delicate epidermis covering the inner surface of an onion scale and place it on a slide. Make a wet mount by covering the piece of epidermis with Iodine solution and a cover slip. Reduce the amount of light passing through the preparation.
2. Draw several cells. Make the individual cells 20 mm wide. Label the structures in one cell: nucleus, nucleoli, nuclear envelope, cytoplasm, and cell wall.
3. How many nucleoli are present in each nucleus?

Synthetic Parenchyma: Chlorenchyma of Elodea

Elodea is a water plant that grows abundantly in ponds around Spokane. The cells are favorable objects for the study of cell structure and an example of cells synthesizing food.

Materials

• Elodea
• slide
• cover slip
• dropping pipette
• compound microscope
• metric ruler

Procedure

1. Place a leaf from an Elodea on a slide with a drop of water and cover with a cover slip. Under low power move the slide about and observe the cells present. Change to high power and with the fine adjustment, focus at various depths of the leaf.
2. Distinguish the following parts of the cell: cell wall, cytoplasm, and chloroplasts. A center portion of the cell within the cytoplasm appears clear. This area contains water and materials in solution and is called the central sap vacuole or water vacuole. The membrane around this structure is too narrow to be distinguishable with the light microscope. The cell or plasma membrane is present but is not visible because it is thin and in direct contact with the cell wall. A nucleus is present but difficult to see because this preparation is not stained and the chlorophyll masks other structures in the cell.
3. Draw one Elodea cell. Make your drawing 75 mm in its longest dimension. Label the cell parts you can distinguish.
4. The movement of the cytoplasm is detected by the movement of the chloroplasts which are suspended in cytoplasm. The chloroplasts are carried along in the cytoplasm as if in a current. This movement is known as cyclosis or cytoplasmic streaming. With arrows, indicate the directions of streaming (cyclosis) on the drawing above.

Part 3: Prokaryotic Cells

Prokaryotic cells belonging to the archaea and bacteria generally lack membrane-bound organelles.

Bacteria

Materials

• prepared Gram stain slides
• lens paper
• compound microscope

Procedure

1. Using the compound microscope, getting the stained bacteria cells in focus at high power.
2. Draw a few of the bacteria cells.
3. Do you see any details in the cytoplasm of individual bacteria cells?
4. What is the difference in size between the prokaryotic cells and the eukaryotic cells? Which ones in general are bigger and which ones are smaller?

Summary

Which structures are found in the following cells?

Cell Models

The lab classroom has multiple examples of plant, animal, and bacterial cells. Take some time to view these models and make sure you can identify and know the function of the following structures:

• Nucleus
• Nucleolus
• Cytoplasm
• Plasma membrane
• Chloroplast
• Central Vacuole
• Cell Wall
• Flagella
• Lysosome
• Cytoskeleton
• Ribosome
• Smooth Endoplasmic Reticulum
• Rough Endoplasmic Reticulum
• Golgi Bodies
• Mitochondria
• Vesicle
• Nucleoid Region
• Pilli
• Centriole