The Role of Cells

The cell is the basic unit of all life. It is the simplest structure that shows all the characteristics of life, including organization, metabolism, responsiveness, homeostasis, growth, and reproduction. In fact, it is possible for a single cell to live independently of other cells. Examples of some free-living cells are microscopic organisms such as protozoa and bacteria, some of which produce disease. In a multicellular organism, cells make up all tissues. All the activities of the human body, which is composed of trillions of cells, result from the activities of individual cells. Cells produce all the materials manufactured within the body. The study of cells is cytology.

Figure 1 Cilia photographed under three different microscopes. (A) Cilia (hairlike projections) in cells lining the trachea under the highest magnification of a compound light microscope (1000 ) (B) Cilia in the bronchial lining viewed with a transmission electron microscope (TEM). Internal components are visible at this much higher magnification. (C) Cilia on cells lining an oviduct as seen with a scanning electron microscope.

Cell Structure

Just as people may look different but still have certain features in common two eyes, a nose, and a mouth, for example all cells share certain characteristics. Refer to Figure 2 as we describe some of the parts that are common to most animal cells. Table 1 summarizes information about the main cell parts.

Plasma Membrane
The outer limit of the cell is the plasma membrane, formerly called the cell membrane (Fig. 3). The plasma membrane not only encloses the cell contents but also participates in many cellular activities, such as growth, reproduction, and interactions between cells, and is especially important in regulating what can enter and leave the cell. The main substance of this membrane is a double layer of lipid molecules, described as a bilayer. Because these lipids contain the element phosphorus, they are called phospholipids. Some molecules of cholesterol, another type of lipid, are located between the phospholipids. Cholesterol strengthens the membrane. A variety of different proteins float within the lipid bilayer. Some of these proteins extend all the way through the membrane, and some are located near the inner or outer surfaces of the membrane. The importance of these proteins will be revealed in later chapters, but they are listed here along with their functions (Table 2): Channels-pores in the membrane that allow specific substances to enter or leave. Certain ions travel through channels in the membrane.

NAME DESCRIPTION FUNCTION

Transporters-shuttle substances from one side of the membrane to the other. Glucose, for example, is carried into cells using transporters.
    Receptors-points of attachment for materials coming to the cell in the blood or tissue fluid. Some hormones, for example, must attach to receptors on the cell surface before they can act upon the cell, as described in pages on the endocrine system.
    Enzymes-participate in reactions occurring at the plasma membrane.
    Linkers-give structure to the membrane and help attach cells to other cells.
    Cell identity markers-proteins unique to an individual’s cells. These are important in the immune system and are also a factor in transplantation of tissue from one person to another.
    Carbohydrates are present in small amounts in the plasma membrane, combined either with proteins (glycoproteins) or with lipids (glycolipids). These carbohydrates help cells to recognize each other and to stick together.

In some cells, the plasma membrane is folded out into multiple small projections called microvilli. Microvilli increase the surface area of the membrane, allowing for greater absorption of materials from the cell's environment, just as a sponge absorbs water. Microvilli are found on cells that line the small intestine, where they promote absorption of digested foods into the circulation. They are also found on kidney cells, where they reabsorb materials that have been filtered out of the blood.

Figure 2 A generalized animal cell, sectional view.

Figure 3 The plasma membrane. This drawing shows the current concept of its structure.

Cilia photographed under three different microscopes

The part of information is from the book "Memmler's The Human Body In Health And Disease" by Barbara Janson Cohen and Jason J. Taylor

Plasma membrane

Microvilli
Nucleus

Nucleolus

Cytoplasm

Cytosol
Endoplasmic
reticulum (ER)

Ribosomes

Mitochondria
Golgi apparatus

Saclike bodies

Lysosomes
Peroxisomes
Vesicles

Centrioles

Surface projec-
tions
Cilia
Flagellum

Outer layer of the cell; composed mainly of lipids
and proteins

Short extensions of the cell membrane
Large, dark-staining organelle near the center of
the cell, composed of DNA and proteins
Small body in the nucleus; composed of RNA,
DNA, and protein
Colloidal suspension that fills the cell from the
nuclear membrane to the plasma membrane
The fluid portion of the cytoplasm
Network of membranes within the cytoplasm.
Rough ER has ribosomes attached to it; smooth
ER does not.
Small bodies free in the cytoplasm or attached to
the ER; composed of RNA and protein
Large organelles with folded membranes inside
Layers of membranes

Small, membrane-enclosed bodies

Small sacs of digestive enzymes
Membrane-enclosed organelles containing enzymes
Small membrane-bound bubbles in the cytoplasm

Rod-shaped bodies (usually two) near the nucleus

Structures that extend from the cell
Short, hairlike projections from the cell
Long, whiplike extension from the cell

Encloses the cell contents; regulates what
enters and leaves the cell; participates in
many activities, such as growth, reproduc-
tion, and interactions between cells
Absorb materials into the cell
Contains the chromosomes, the hereditary
units that direct all cellular activities
Makes ribosomes

Site of many cellular activities, consists of
cytosol and organelles
Surrounds the organelles
Rough ER sorts proteins and forms them into
more complex compounds; smooth ER is
involved with lipid synthesis
Manufacture proteins

Convert energy from nutrients into ATP
Makes compounds containing proteins; sorts
and prepares these compounds for transport
to other parts of the cell or out of the cell
Store materials, transport materials through
the plasma membrane, or destroy waste
material
Digest substances within the cell
Break down harmful substances
Store materials and move materials into or out
of the cell in bulk
Help separate the chromosomes during cell
division
Move the cell or the fluids around the cell
Move the fluids around the cell
Moves the cell

TABLE 1 CELL PARTS