McKenna's Pharmacology for Nursing, 2e

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C H A P T E R 7  Introduction to cell physiology

Cytoplasm The cell cytoplasm lies within the cell membrane and outside the nucleus and is the site of activities of cellular metabolism and special cellular functions. The cyto- plasm contains many organelles , which are structures with specific functions such as producing proteins and energy. The organelles within the cytoplasm include the mitochondria, the endoplasmic reticulum, free ribo- somes, the Golgi apparatus and the lysosomes. Mitochondria Mitochondria are rod-shaped “power plants” within each cell that produce energy in the form of ATP, which allows the cell to function. Mitochondria are plentiful in very active cells such as muscle cells and are relatively scarce in inactive cells such as bone cells. Mitochon- dria, which can reproduce when a cell is very active, are always very abundant in cells that consume energy. For example, cardiac muscle cells, which must work con- tinually to keep the heart contracting, contain a great number of mitochondria. Milk-producing cells in breast tissue, which are normally quite dormant, contain very few mitochondria. If a woman is breastfeeding, however, the mitochondria become more abundant to meet the demands of the milk-producing cells. The mito- chondria can take carbohydrates, fats and proteins from the cytoplasm and make ATP via the Krebs cycle, which depends on oxygen. Cells use the ATP to maintain homeostasis, produce proteins and carry out specific functions. If oxygen is not available, lactic acid builds up as a by-product of cellular respiration. Lactic acid leaves the cell and is transported to the liver for conver- sion to glycogen and carbon dioxide. Endoplasmic reticulum Much of the cytoplasm of a cell is made up of a fine network of interconnected channels known as cisternae, which form the endoplasmic reticulum . The undulating surface of the endoplasmic reticulum provides a large surface for chemical reactions within the cell. Many granules that contain enzymes and ribosomes, which produce protein, are scattered over the surface of the rough endoplasmic reticulum. Production of proteins, phospholipids and cholesterol takes place in the rough endoplasmic reticulum. The smooth endoplasmic reticu- lum is the site of further lipid and cholesterol production and the production of cell products, such as hormones. The breakdown of many toxic substances may also occur here in particular cells. Free ribosomes Other ribosomes that are not bound to the surface of the endoplasmic reticulum exist throughout the cytoplasm. These free-floating ribosomes produce proteins that are important to the structure of the cell and some of the enzymes that are necessary for cellular activity.

Golgi apparatus The Golgi apparatus is a series of flattened sacs that may be part of the endoplasmic reticulum. These structures prepare hormones or other substances for secretion by processing them and packaging them in vesicles to be moved to the cell membrane for excretion from the cell. In addition, the Golgi apparatus may produce lysosomes and store other synthesised proteins and enzymes until they are needed. Lysosomes Lysosomes are membrane-covered organelles that contain specific digestive enzymes that can break down proteins, nucleic acids, carbohydrates and lipids and are responsible for digesting worn or damaged sections of a cell when the membrane ruptures and the cell dies. Lysosomes form a membrane around any substance that needs to be digested and secrete the digestive enzymes directly into the isolated area, protecting the rest of the cytoplasm from injury. This phenomenon can be seen with old lettuce in the refrigerator. The side of the lettuce head that has been “lying down” for a prolonged period becomes brown and wet as the lettuce cells die and self-digest when their lysosomes are released. If the lettuce is not used, the released lysosomes begin to digest any healthy lettuce that remains, with eventual destruction of the entire head. Lysosomes are important in ecology. Dead trees, animals and other organisms self-digest. ■■ The cytoplasm of the cell contains various organelles that are important for cellular function. ■■ The mitochondria produce energy for the cell; the endoplasmic reticulum contains ribosomes that produce proteins; the Golgi apparatus packages proteins; and lysosomes contain protein-dissolving enzymes that are important for digestion and the recycling of organisms in nature. CELL PROPERTIES Cells have certain properties that allow them to survive. Endocytosis involves incorporation of material into the cell by extending the cell membrane around the sub- stance. Pinocytosis , a form of endocytosis, refers to the engulfing of specific substances that have reacted with a receptor site on the cell membrane. This process allows cells to absorb nutrients, enzymes and other materials. Phagocytosis is a similar process; it allows the cell (white blood cell), usually a neutrophil or mac- rophage, to engulf a bacterium or a foreign protein and destroy it within the cell by secreting digestive enzymes KEY POINTS KEY POINTS