Lysosomes and Peroxisomes

Two organelles that play a vital role in cellular disposal and recycling are lysosomes and peroxisomes. Lysosomes contain enzymes that break down carbohydrates, lipids, proteins, and nucleic acids. These powerful enzymes must be kept within the lysosome because they would digest the cell if they escaped. In a process called autophagy, the cell uses lysosomes to safely recycle cellular structures, fusing with and digesting worn out organelles. The digested components then return to the cytoplasm for reuse. Lysosomes also break down foreign material, as when cells known as phagocytes engulf bacteria and then use lysosomes to destroy them. The cell may also use lysosomes to digest itself during autolysis, a normal part of development. Cells that are no longer needed “self-destruct” by releasing lysosomal enzymes into their own cytoplasm. Peroxisomes are small membranous sacs that resemble lysosomes but contain different kinds of enzymes. They break down toxic substances that may enter the cell, such as drugs and alcohol, but their most important function is to break down free radicals. These substances are byproducts of normal metabolic reactions but can kill the cell if not neutralized by peroxisomes. Disease may result if either lysosomes or peroxisomes are unable to function. In Tay-Sachs disease, nerve cells’ lysosomes lack an enzyme that breaks down certain kinds of lipids. These lipids build up inside the cells, causing malfunction that leads to brain injury, blindness, and death. Disease may also result if lysosomes or peroxisomes function when they should not. Some investigators believe this is the case in autoimmune diseases, in which the body develops an immune response to its own cells. Phagocytes engulf the cells and lysosomes destroy them. In addition, body cells themselves may self-destruct through autolysis. The joint disease rheumatoid arthritis is one such example.

Centrioles are rod-shaped bodies near the nucleus that function in cell division. They help to organize the cell and divide the cell contents during this process.

Surface Organelles
Some cells have structures projecting from their surface that are used for motion. Cilia are small, hairlike projections that wave, creating movement of the fluids around the cell. For example, cells that line the passageways of the respiratory tract have cilia that move impurities out of the system. Ciliated cells in the female reproductive tract move the egg cell along the oviduct toward the uterus. A long, whiplike extension from the cell is a flagellum. The only type of cell in the human body that has a flagellum is the sperm cell of the male. Each human sperm cell has a flagellum that is used to propel the sperm cell toward the egg in the female reproductive tract.

Cellular Diversity
Although all body cells have some fundamental similarities, individual cells may vary widely in size, shape, and composition according to the function of each. The average cell size is 10 to 15 nm, but cells may range in size from the 7 nm of a red blood cell to the 200 nm or more in the length of a muscle cell. Cell shape is related to cell function (Fig. 4). A neuron (nerve cell) has long fibers that transmit electrical energy from place to place in the nervous system. Cells in surface layers have a modified shape that covers and protects the tissue beneath. Red blood cells are small and round, which lets them slide through tiny blood vessels. They also have a thin outer membrane to allow for passage of gases into and out of the cell. As red blood cells mature, they lose the nucleus and most of the other organelles, making the greatest possible amount of space available to carry oxygen. Aside from cilia and flagella, most human cells have all the organelles described above. These may vary in number, however. For example, cells producing lipids have lots of smooth ER. Cells that secrete proteins have lots of ribosomes and a prominent Golgi apparatus. All active cells have lots of mitochondria to manufacture the ATP needed for energy.

Figure 4 Cellular diversity. Cells vary in structure according to their functions. (A) A neuron has long extensions that pick up and transmit electrical impulses. (B) Epithelial cells cover and protect underlying tissue. (C) Muscle cells have fibers that produce contraction. (D) Red blood cells lose most organelles, which maximizes their oxygen-carrying capacity, and have a small, round shape that lets them slide through blood vessels. (E) A sperm cell is small and light and swims with a flagellum.

Protein Synthesis
Because protein molecules play an indispensable part in the structure and function of the body, we need to identify the cellular substances that direct the production of proteins. As noted, the hereditary units that govern the cell are the chromosomes in the nucleus. Each chromosome in turn is divided into multiple subunits, called genes (Fig. 5). It is the genes that carry the messages for the development of particular inherited characteristics, such as brown eyes, curly hair, or blood type, and they do so by directing the manufacture of proteins in the cell.

Figure 5 Subdivisions of a chromosome. A gene is a distinct region of a chromosome. The entire chromosome is made of DNA. Nucleotides are the building blocks of DNA.

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