Structural Levels in the Body
Four basic structural levels are found within the body:
Cells—the basic units
Tissues—made up of cells
Organs—made up of tissues
Systems—made up of organs
Cells
The cell is the basic unit of structure and function in all living things. Each cell is alive and carries out specific activities. The smallest forms of life, such as bacteria, are composed of a single cell. The human body, on the other hand, is made up of trillions of cells. Although the human body has many different types of cells, all cells contain the same basic chemicals and similar structural features. Despite being the smallest living subunit of the human body, the cell functions as a member of a highly organized team.
FIGURE 15-6 · The nine regions of the abdomen.
Key Concept The cell is the basic unit of structure and function for all living things.
Special Properties
As team members, cells have become specialized in anatomic structure and physiologic function. Some cells have highly developed abilities for metabolism—the ability to process the chemicals found in foods in order to obtain energy and to create new products. The two phases of metabolism are anabolism and catabolism. Anabolism is the building up, assimilation, or conversion of ingested substances. Catabolism is the process of breaking down, disintegrating, or tearing substances into simpler substances. By virtue of the breakdown of substances, particularly food, energy is released.
Cells have other specialized properties:
Contractility: Muscle cells can stretch or contract. Conductivity: Nerve cells are specialized to send and receive impulses.
Irritability: Cells respond to stimuli.
Reproduction: Cells duplicate themselves.
The properties of metabolism, contractility, conductivity, irritability, and reproduction are present to some degree in all cells. However, an individual cell does not function independently. Rather, it develops specialties, interrelated with other cells. This teamwork permits the organism to have organization and adaptability, which is not possible in a single-cell microorganism.
Key Concept Cells have special abilities such as metabolism, contractility conductivity irritability and reproduction.
Cellular Structure and Function
Cytology is the science that investigates the formation, structure, and function of cells. The study of various sample cells teaches a great deal about the general condition of an entire organism.
Human cells are too small to be seen without a microscope, except for the egg cell (ovum), which is just barely visible to the naked eye. Human cells vary in shape, function, and size. Cell shape may be round, spherical, rectangular, or irregular. Some cells change shape as they move, but each category of cells retains its shape; for example, nerve cells will always look like nerve cells.
Although cells have similar abilities, their functions are specialized. Nerve cells have filaments on their ends that carry or receive impulses. Muscle cells are long, thin fibers that permit contraction and relaxation of the cell. A fat cell is large, with empty spaces suitable for storing lipids (fats).
Most cells vary in size from 1 to 100 micrometers (commonly referred to as a micron, abbreviated μ). A micrometer, or micron, is one millionth of a meter or one-thousandth of a millimeter (1 micron = 1/25,000 of an inch). Ten to 1,000 cells fit on the head of a pin, depending on the cell type.
Although minute in size, the complexity of cell structure is amazing, as shown in Figure 15-7. Collectively, all parts that make up a cell are called protoplasm. The cell parts can be divided into those found in the nucleus and those found in the cytoplasm (outside the nucleus). The cell’s nucleus is its control center, responsible for reproduction and coordination of other cellular activities. The nucleus is surrounded by the nuclear membrane and houses the chromosomes and the nucleolus. Nuclear pores provide a means for the nucleus to communicate with the cytoplasm and to transport substances. The cytoplasm is the gel-like fluid inside the cell that is not located in the nucleus. The structures located in the cytoplasm are distinct structures called organelles. These include mitochondria, the Golgi apparatus (or Golgi bodies), lysosomes, endoplasmic reticulum, ribosomes, and the centrosomes (which contain cen-trioles) (Table 15-3). Cytoplasm is the medium for chemical reactions, and all functions for cell reproduction occur here. To simplify the study of the cell and the functions of its parts, it is useful to consider separately the structures found in the nucleus and those found in the cytoplasm.
Inside the nucleus is a structure called the nucleolus (plural: nucleoli). The nucleolus is composed of protein and threads of chromatin containing deoxyribonucleic acid (DNA, a double-stranded molecule), as well as ribonucleic acid (RNA, a single-stranded molecule). The nucleolus’ function is not well understood, but it is involved in making ribosomes (minute granules that float free in the cytoplasm or are attached to the endoplasmic reticulum in cells that contain a high concentration of RNA). Ribosomes play an important role in protein synthesis.
FIGURE 15-7 · Diagram of a typical animal cell, showing the main organelles. The nucleus is the control center of the cell.The organelles (endoplasmic reticulum, mitochondria, Golgi bodies or Golgi apparatus, ribosomes, centrosomes/centrioles, and lysosomes) in the cytoplasm are the functional substances. The plasma membrane is made up of lipids (fats) and proteins. Channels in the membrane are of vital importance in the transport of materials across the plasma membrane.
The nucleus also contains chromosomes. Chromosomes are made up of DNA molecules that form genetic material called genes. Genes contain information about inherited characteristics. These genes are carried on the chromosomes in single file. The human cell has 46 chromosomes.
RNA is responsible for taking the genetic message from the DNA molecule and transporting this message to the ribosomes in the cytoplasm. The ribosomes then use this message to reproduce protein substances according to the specifications carried by the RNA. Therefore, DNA (Fig. 15-8) and RNA are key to the reproductive process. Cell reproduction will be discussed briefly later in this topic.
The cell membrane or plasma membrane is a double layer of phospholipid cells with proteins randomly embedded. Substances soluble in lipids (fats) easily pass through (e.g., alcohol). Water passes through the protein-lined pores. The cell membrane surrounds the cell’s outer boundary, provides shape, maintains the integrity of the cell, and is capable of selective permeability, meaning that it can regulate what enters and leaves the cell.Some cells have cilia: hair-like threads that sweep materials across the cell surface. Refer to Table 15-3 for functions of cellular parts and to Figure 15-7 for their locations.
Cell Reproduction
Mitosis. Through a complicated process called mitosis, cells divide into two parts to reproduce themselves. Each of the “daughter cells” is an exact genetic duplicate of the original or “mother” cell. The body can be thought of as a group of cells, and mitosis is responsible for the body’s growth, repair, and replacement of injured and dead tissues.
The amazing process of mitosis occurs as a result of a rearrangement of particles in the nucleus (Fig. 15-9). Briefly, two centrosomes (clusters of cytoplasm located near the nucleus) separate and are drawn toward opposite ends of the cell (metaphase).
TABLE 15-3. Cellular Parts and Their Functions
|
PART |
FUNCTION |
|
Parts Found in the Nucleus Chromosome • Gene |
Carries genetic factors Contains hereditary information found on the chromosome |
|
Nucleoli • DNA • RNA |
Globules that contain RNA and DNA Stores and transfers genetic information Chemical messenger that facilitates the duplication of genes by DNA |
|
Parts Found in the Cytoplasm Mitochondria (the source of aerobic respiration) |
Powerhouse of the cell—a double membrane; the matrix contains enzymes of the Krebs’ cycle to make energy (ATP); the cristae cutis (crista = sing) on the membrane’s surface contain enzymes of the electron transport system |
|
Golgi apparatus (Golgi body) |
Synthesizes carbohydrates; modifies, concentrates, and transports proteins out of cell within round vesicles |
|
Lysosomes |
Sacs that contain digestive enzymes to break down all biological materials, including DNA, RNA, proteins, and some fats, as well as bacteria, damaged cells and worn-out organelles |
|
Endoplasmic reticulum (ER) |
Extensive tubule network—rough ER synthesizes/transports proteins (mostly in liver and pancreas); smooth ER synthesizes and transports lipids and steroids |
|
Ribosomes |
Site of protein synthesis: some are found in endoplasmic reticulum and some are free floating in cytoplasm |
|
Centrosome |
Plays role in cellular reproduction (mitosis) (The centrioles are contained within the centrosome.) |
|
Cell membrane |
Maintains cell shape, protects cell, and regulates what enters and leaves the cell |
ATP adenosine triphosphate.
FIGURE 15-8 · A DNA molecule (simplified), showing the double strands. This is called a double helix.
FIGURE 15-9 · A simplified version of mitosis: The centrioles within the centrosome divide, the chromatin material of the nucleus changes into rod-shaped chromosomes, and two daughter cells form within the cell membrane. The daughter cells then split into two distinct cells that are identical to the original cell. The cell shown is for illustration purposes only. (It is not a human cell, which has 46 chromosomes—23 pairs).
The nuclear membrane then disappears. The chromosomes split, and half of each chromosome moves toward each centrosome. The cell then begins to elongate, thinning in the middle with the plasma (cell) membrane following the same shape (anaphase). The cell finally splits into two parts, with half of the cytoplasm, nuclear material, and cell membrane in each new cell. Because of the genes, each new cell is identical to the original from which it was formed.
Mitosis is essential for the following:
• Growth of a single fertilized egg; after conception, the fertilized egg grows into trillions of cells and becomes an individual person or animal
• Repair of wounds by replacing damaged or dead cells
• Tumor formation in which abnormal cells, dividing by mitosis, result in more abnormal cells
Certain cells in the body are unable to reproduce in an adult. Muscle cells or neurons (nerve cells) that die lose their functions. Loss of muscle cells in the heart due to a heart attack may damage the heart so severely that it loses the ability to contract effectively. Destroyed spinal cord neurons cannot reproduce, often causing paralysis and loss of sensation below the level of the injury.
Meiosis. Human sperm cells and ova reproduce by meiosis, a more complex process of cell division. In meiosis, cell division produces eggs or sperm that contain half the total number (23) of chromosomes. Upon fertilization, the nuclei of an egg and a sperm cell fuse, forming a new organism that has the full complement of chromosomes (46 chromosomes).
