Porth's Essentials of Pathophysiology, 4e

8

Cell and Tissue Function

U N I T 1

Actin Microfilaments and Intermediate Filaments In addition to microtubules, two other cytoskeletal structures exist: microfilaments and intermediate fila- ments. Microfilaments are composed of actin, whereas intermediate filaments are a heterogeneous group of filaments with diameter sizes between those of microtu- bules and actin filaments. Actin, which can exist in globular and filamentous forms, is of central importance to cellular biology. It contributes to cell motility, positioning of organelles in the cell, and cell shape and polarity. Many functions of actin filaments are performed in association with myo- sin motor proteins. Contractile activities involving actin microfilaments and associated thick myosin filaments contribute to muscle contraction. The intermediate filaments include the cytokeratins, vimentin, and neurofilaments. They have structural and maintenance functions that are important in tissue, cellular, developmental, and differentiation processes. They are also very responsive to cellular stresses, such as heat, radiation, toxins, pathogens and oxidation. Neurofibrillary tangles found in the brain in Alzheimer disease contain microtubule-associated proteins and neurofilaments, evidence of a disrupted neuronal cytoskeleton. ■■ Cells are the smallest functional unit of the body.They are autonomous units that contain structures that are strikingly similar to those needed to maintain total body function. ■■ The cell membrane is a protein-studded lipid bilayer that surrounds the cell and separates it from its surrounding external environment. It contains receptors for hormones and other biologically active substances, participates in the electrical events that occur in nerve and muscle cells, and aids in the regulation of cell growth and proliferation.The cell surface is surrounded by a fuzzy-looking layer called the cell coat or glycocalyx.The cell coat participates in cell-to-cell recognition and adhesion, and it contains tissue transplant antigens. ■■ The nucleus is the control center for the cell. It contains deoxyribonucleic acid (DNA), which provides the information necessary for the synthesis of the various proteins that the cell must produce to stay alive and to transmit information from one generation to another. ■■ The cytoplasm contains the cell’s organelles. Ribosomes serve as sites for protein synthesis SUMMARY CONCEPTS

Cell Metabolism and Energy Storage Energy metabolism refers to the chemical processes involved in converting carbohydrates, fats, and pro- teins from the foods we eat into the energy needed for cell functions. Cells use oxygen to transform the breakdown products of the foods we eat into the energy needed for muscle contraction; the transport of ions and other molecules across cell membranes; and the synthesis of enzymes, hormones, and other macromolecules. The special “unit of currency” for transferring energy in living cells is ATP. Adenosine triphosphate molecules consist of adenosine, a nitrogenous base; ribose, a five- carbon sugar; and three phosphate groups (Fig. 1-8). The last two phosphate groups are attached to the remainder of the molecule by two high-energy bonds. Each bond releases a large amount of energy when hydrolyzed. Adenosine triphosphate is hydrolyzed to form adenosine in the cell.The endoplasmic reticulum (ER) functions as a tubular communication system through which substances can be transported from one part of the cell to another and as the site of protein (rough ER), carbohydrate, and lipid (smooth ER) synthesis.The Golgi apparatus modifies materials synthesized in the ER and packages them into secretory granules for transport within the cell or export from the cell. Lysosomes, which can be viewed as the digestive system of the cell, contain hydrolytic enzymes that digest worn-out cell parts and foreign materials.The mitochondria serve as power plants for the cell because they transform food energy into ATP, which is used to power cell activities. They contain their own extrachromosomal DNA, which is used in the synthesis of certain proteins required for mitochondrial function. ■■ In addition to its organelles, the cytoplasm also contains a network of microtubules, actin microfilaments, and intermediate filaments called the cytoskeleton. Microtubules are slender, stiff tubular structures that influence cell shape, provide a means of moving organelles through the cytoplasm, and form cilia, flagella, and centrioles. Actin microfilaments are dynamic, thin, threadlike cytoplasmic structures that are important in cell movement and organelle positioning. Many types of intermediate filaments function in supporting and maintaining the shape of cells and participate in numerous cellular processes.