Patho study Guide WK 1

All body functions depend on the integrity of cells. Therefore, an understanding of cellular biology is intrinsically necessary for an understanding of disease. An overwhelming amount of information is revealing how cells behave as a multicellular “social” organism. At the heart of cellular biology is cellular communication (“cellular crosstalk”)—how messages originate and are transmitted, received, interpreted, and used by the cell. This streamlined conversation between, among, and within cells maintains cellular function and specialization. Intercellular signals allow each cell to determine its position and specialized role. Cells must demonstrate a “chemical fondness” for other cells and their surrounding environment to maintain the integrity of the entire organism. When they no longer tolerate this fondness, the conversation breaks down and cells either adapt (sometimes altering function) or become vulnerable to isolation, injury, disease, or even death.

Prokaryotes and Eukaryotes

Living cells generally are divided into two major classes—eukaryotes and prokaryotes. The cells of higher animals and plants are eukaryotes, as are the single-celled organisms fungi, protozoa, and most algae. Prokaryotes include cyanobacteria (blue-green algae), bacteria, and rickettsiae. Prokaryotes traditionally were studied as core subjects of molecular biology. Current emphasis is on the eukaryotic cell; much of its structure and function has no counterpart in bacterial cells.

Eukaryotes (eu = good; karyon = nucleus; also spelled “eucaryotes”) are larger and have more extensive intracellular anatomy and organization than do prokaryotes. Eukaryotic cells have a characteristic set of membrane-bound intracellular compartments, called organelles, that includes a well-defined nucleus. Prokaryotes contain no organelles, and their nuclear material is not encased by a nuclear membrane. Prokaryotic cells are characterized by lack of a distinct nucleus.

Besides having structural differences, prokaryotic and eukaryotic cells differ in chemical composition and biochemical activity. The nuclei of prokaryotic cells carry genetic information in a single circular chromosome, and they lack a class of proteins called histones, which in eukaryotic cells bind with deoxyribonucleic acid (DNA) and are involved in the supercoiling of DNA (see Fig. 1.2). We now understand that the loops and coiling of DNA are important for many diseases. Eukaryotic cells have several chromosomes. Protein production, or synthesis, in the two classes of cells also differs because of major structural differences in ribonucleic acid (RNA)–protein complexes. Other distinctions include differences in mechanisms of transport across the outer cellular membrane and differences in enzyme content.

Cellular Functions

Cells become specialized through the process of differentiation, or maturation, so that some cells eventually perform one kind of function and other cells perform other functions. Cells with a highly developed function, such as movement, often lack some other property, such as hormone production, which is more highly developed in some other type of specialized cell.

The eight chief cellular functions follow:

• Movement. Muscle cells can generate forces that produce motion. Muscles that are attached to bones produce limb movements, whereas those that enclose hollow tubes or cavities move or empty contents when they contract. For example, the contraction of smooth muscle cells surrounding blood vessels changes the diameter of the vessels; the contraction of muscles in walls of the urinary bladder expels urine.

• Conductivity. Conduction as a response to a stimulus is manifested by a wave of excitation, an electrical potential that passes along the surface of the cell to reach its other parts. Conductivity is the chief function of nerve cells.

• Metabolic absorption. All cells take in and use nutrients and other substances from their surroundings. Cells of the intestine and the kidney are specialized to carry out absorption. Cells of the kidney tubules reabsorb fluids and synthesize proteins. Intestinal epithelial cells reabsorb fluids and synthesize protein enzymes.

• Secretion. Certain cells, such as mucous gland cells, can synthesize new substances from substances they absorb and then secrete the new substances to serve as needed elsewhere. Cells of the adrenal gland, testis, and ovary can secrete hormonal steroids.

• Excretion. All cells can rid themselves of waste products resulting from the metabolic breakdown of nutrients. Membrane-bound sacs (lysosomes) within cells contain enzymes that break down, or digest, large molecules, turning them into waste products that are released from the cell.

• Respiration. Cells absorb oxygen, which is used to transform nutrients into energy in the form of adenosine triphosphate (ATP). Cellular respiration, or oxidation, occurs in organelles called mitochondria.

• Reproduction. Tissue growth occurs as cells enlarge and reproduce themselves. Even without growth, tissue maintenance requires that new cells be produced to replace cells that are lost normally through cellular death. Not all cells are capable of continuous division.

• Communication. Communication is vital for cells to survive as a society of cells. Pancreatic cells, for instance, secrete and release insulin necessary to signal muscle cells to absorb sugar from the blood for energy. Constant communication allows the maintenance of a dynamic steady state.

Structure and Function of Cellular Components

Fig. 1.1 shows a “typical” eukaryotic cell. It consists of three components: an outer membrane called the plasma membrane, or plasmalemma; a fluid filling called cytoplasm; and the intracellular “organs,” or organelles, which are membrane bound and include the nucleus. Researchers are astounded by the advances in microscopy and computer software that allow resolution to the nanoscale—cells seem to come “alive” with the molecular world more visible. Understanding structure and function will reveal, for example, how the cell responds to mechanical forces or emerges from different gene expression patterns. The overall impact on biology is huge