Essentials of Human Anatomy & Physiology: Chapter 3 - Cells and Tissues
The Cellular Basis of Life
Cells are the structural and functional units of all living things. The human body is composed of approximately to trillion cells. According to the Cell Theory, the cell is the basic structural and functional unit of living organisms, and the activity of an organism depends on the collective activities of its cells. The principle of complementarity states that the biochemical activities of cells are dictated by their specific structure, or anatomy, which determines their physiology or function. Furthermore, the continuity of life has a cellular basis.
Chemically, most cells are composed of four primary elements: Carbon, Hydrogen, Oxygen, and Nitrogen. On average, cells are composed of approximately water. A generalized human cell contains three main regions: the nucleus, the cytoplasm, and the plasma membrane.
The Nucleus and Genetic Material
The nucleus serves as the control center of the cell and contains genetic material known as deoxyribonucleic acid, or DNA. DNA is necessary for cell reproduction and provides the instructions for building proteins. The nucleus consists of three distinct regions: the nuclear envelope, the nucleolus, and chromatin.
The nuclear envelope is a double membrane that bounds the nucleus and contains nuclear pores. These pores allow for the exchange of material between the nucleus and the rest of the cell. The envelope encloses a jellylike fluid called the nucleoplasm. Within the nucleus, one or more dark-staining bodies called nucleoli are found. The nucleoli are the sites of ribosome assembly. Once assembled, ribosomes migrate through the nuclear pores into the cytoplasm to serve as sites for protein synthesis.
Chromatin is composed of DNA wound around proteins called histones. It is scattered throughout the nucleus when the cell is not dividing. During cell division, chromatin condenses to form dense, rodlike bodies known as chromosomes. DNA is a double helix molecule resembling a spiral staircase. Its backbone consists of alternating phosphate and sugar units, and its rungs are made of pairs of nitrogen-containing bases. In DNA replication, Adenine (A) always bonds with Thymine (T), and Guanine (G) always bonds with Cytosine (C).
The Plasma Membrane and Cell Junctions
The plasma membrane is a transparent, selectively permeable barrier that contains the cell contents and separates them from the surrounding environment. It is described by the fluid mosaic model, which consists of two layers of phospholipids arranged tail-to-tail, with cholesterol and various proteins scattered throughout. Sugar groups may be attached to the phospholipids, forming glycolipids.
Phospholipids are polar molecules. The hydrophilic (‘water-loving’) polar heads are oriented on the inner and outer surfaces of the membrane, while the hydrophobic (‘water-fearing’) nonpolar tails form the interior of the membrane. This hydrophobic center makes the membrane relatively impermeable to most water-soluble molecules. Specialized proteins in the membrane serve as enzymes, receptors for hormones, or transport channels and carriers. Glycoproteins are branched sugars attached to proteins; the fuzzy, sticky, sugar-rich area on the cell surface is called the glycocalyx.
Cells are bound together in three ways: through the adhesive action of glycoproteins in the glycocalyx, through the mechanical fit of wavy membrane contours, and through specialized cell junctions. Tight junctions are impermeable junctions that bind cells into leakproof sheets, where plasma membranes fuse together like a zipper. Desmosomes are anchoring junctions that act like rivets to prevent cells from being pulled apart under mechanical stress. Gap junctions, or communicating junctions, allow for direct passage of molecules between cells through hollow cylinders of proteins called connexons.
The Cytoplasm and Organelles
The cytoplasm is the cellular material located outside the nucleus and inside the plasma membrane. It is the site of most cellular activities and consists of three major components: cytosol, inclusions, and organelles. Cytosol is the fluid that suspends other elements and contains nutrients and electrolytes. Inclusions are chemical substances like stored nutrients or cell products that float in the cytosol. Organelles are the metabolic machinery of the cell.
Mitochondria are the ‘powerhouses’ of the cell, possessing a double membrane with an inner membrane folded into cristae. They carry out reactions where oxygen is used to break down food into ATP molecules. Ribosomes are made of protein and ribosomal RNA (rRNA) and are the sites of protein synthesis. They are found free in the cytoplasm or attached to the rough endoplasmic reticulum (ER).
The endoplasmic reticulum is a system of fluid-filled tunnels or canals continuous with the nuclear membrane. Rough ER is studded with ribosomes and is responsible for protein synthesis and transport via vesicles. Smooth ER lacks ribosomes and functions in lipid metabolism and the detoxification of drugs and pesticides. The Golgi apparatus appears as a stack of flattened membranes; it modifies and packages proteins from the rough ER into secretory vesicles, in-house protein/lipid packages, or lysosomes.
Lysosomes are membranous bags containing acid hydrolases, which are powerful digestive enzymes used to break down worn-out cell structures and foreign substances. Peroxisomes are sacs containing oxidase and catalase enzymes that detoxify harmful substances like alcohol and formaldehyde and break down free radicals. Free radicals are converted to hydrogen peroxide and then to water. Cytoskeletal elements include microfilaments (), intermediate filaments (), and microtubules (), which provide structural support and machinery for transport. Centrioles are rod-shaped bodies made of nine triplets of microtubules that direct the formation of the mitotic spindle during cell division.
Cell Extensions and Diversity
Cells may have surface extensions such as cilia, flagella, and microvilli. Cilia move materials across the cell surface, such as mucus in the respiratory system. Flagella, found only on sperm cells in humans, propel the cell itself. Microvilli are fingerlike extensions of the plasma membrane that increase surface area for absorption.
There are over different cell types in the human body, varying in size from of an inch to over yard in length, as seen in nerve cells. Cell shape reflects function. Fibroblasts and erythrocytes (red blood cells) connect body parts; erythrocytes lack organelles to maximize oxygen transport. Epithelial cells pack together in sheets to cover and line organs. Skeletal and smooth muscle cells contain contractile filaments for movement. Fat cells store nutrients in large lipid droplets. Macrophages are phagocytic white blood cells that fight disease. Nerve cells (neurons) gather information and control body functions via long processes. The oocyte is the largest cell in the body, while the sperm is built for swimming.
Membrane Transport
Membrane transport involves the movement of substances across the plasma membrane. Solutions are homogeneous mixtures of a solvent (the dissolving medium, usually water) and solutes (components in smaller quantities). Intracellular fluid comprises nucleoplasm and cytosol. Extracellular or interstitial fluid is found outside the cells. The plasma membrane is selectively permeable.
Passive processes do not require energy (ATP). Diffusion is the movement of molecules from high to low concentration down a concentration gradient. Simple diffusion involves lipid-soluble or small particles. Osmosis is the diffusion of water through specific channel proteins called aquaporins. In isotonic solutions, cells maintain their size; in hypertonic solutions, cells shrink; in hypotonic solutions, cells plump or swell. Facilitated diffusion uses protein membrane channels or carriers for lipid-insoluble or large substances like glucose. Filtration involves water and solutes being forced through a membrane by hydrostatic pressure, as seen in the kidneys.
Active processes require ATP to move substances, often against a concentration gradient. Active transport uses solute pumps to move amino acids, sugars, and ions. The sodium-potassium pump moves three Sodium () ions out of the cell and two Potassium () ions into the cell, which is essential for nerve impulses. Vesicular transport moves substances in bulk. Exocytosis utilizes docking proteins to secrete hormones or mucus. Endocytosis engulfs extracellular substances into vesicles. Types of endocytosis include phagocytosis (‘cell eating’), pinocytosis (‘cell drinking’), and receptor-mediated endocytosis, which is highly selective for substances like cholesterol and iron.
Cell Division and Protein Synthesis
The cell life cycle includes interphase, where the cell grows and performs metabolic processes, and cell division. DNA replication occurs at the end of interphase. Cell division involves mitosis (nuclear division) and cytokinesis (cytoplasmic division). Mitosis has four stages: Prophase (chromatin coils into chromosomes, spindle forms), Metaphase (chromosomes align at the metaphase plate), Anaphase (centromeres split, chromatids move to opposite poles), and Telophase (chromosomes uncoil, nuclear envelopes re-form).
Protein synthesis involves transcription and translation. A gene is a DNA segment carrying the blueprint for one protein. DNA triplets are transcribed into mRNA codons in the nucleus. RNA is single-stranded, contains ribose, and uses Uracil (U) instead of Thymine. Messenger RNA (mRNA) carries instructions to the ribosomes. Transfer RNA (tRNA) brings amino acids to the ribosome, where its anticodon binds to the mRNA codon. Ribosomal RNA (rRNA) helps form the ribosome structure. During translation, amino acids are joined by peptide bonds through dehydration synthesis reactions to form polypeptide chains.
Body Tissues and Repair
Tissues are groups of cells with similar structure and function, categorized into four types: epithelial, connective, muscle, and nervous. Epithelial tissue covers and lines body surfaces, is avascular, and can be simple (one layer) or stratified (multiple layers). Shapes include squamous, cuboidal, and columnar. Connective tissue is the most abundant, characterized by an extracellular matrix of ground substance and fibers (collagen, elastic, reticular). Types include bone (osseous tissue), cartilage (hyaline, elastic, fibrocartilage), dense connective tissue (tendons and ligaments), loose connective tissue (areolar, adipose, reticular), and blood.
Muscle tissue contracts to produce movement and includes skeletal (voluntary, striated, multinucleate), cardiac (involuntary, striated, intercalated discs), and smooth (involuntary, non-striated, spindle-shaped). Nervous tissue consists of neurons and supporting neuroglia, functioning in irritability and conductivity.
Tissue repair occurs via regeneration (replacement by the same cell type) or fibrosis (replacement by scar tissue). Repair involves inflammation, the formation of granulation tissue (new capillaries and fibroblasts), and finally regeneration and fibrosis. Epithelium and bone regenerate easily, whereas cardiac muscle and nervous tissue in the central nervous system are largely replaced by scar tissue. Aging may be caused by chemical insults or genetic programming. Neoplasms represent abnormal cell masses. Hyperplasia is an increase in tissue size due to irritation, while atrophy is a decrease in size due to lack of stimulation.