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- Cells become specialized through the process of differentiation or maturation.
- The eight specialized cellular functions are movement, conductivity, metabolic absorption, secretion, excretion, respiration, reproduction, and communication.
Structure and Function of Cellular Components
- The eukaryotic cell consists of three general components: the plasma membrane, the cytoplasm, and the intracellular organelles.
- The nucleus is the largest membrane-bound organelle and is found usually in the cell’s center. The chief functions of the nucleus are cell division and control of genetic information.
- Cytoplasm, or the cytoplasmic matrix, is an aqueous solution (cytosol) that fills the space between the nucleus and the plasma membrane.
- The organelles are suspended in the cytoplasm and are enclosed in biologic membranes.
- The endoplasmic reticulum is a network of tubular channels (cisternae) that extended throughout the outer nuclear membrane. It specializes in the synthesis and transport of protein and lipid components of most of the organelles.
- The Golgi complex is a network of smooth membranes and vesicles located near the nucleus. The Golgi complex is responsible for processing and packaging proteins into secretory vesicles that break away from the Golgi complex and migrate to a variety of intracellular and extracellular destinations, including the plasma membrane.
- Lysosomes are saclike structures that originate from the Golgi complex and contain digestive enzymes. These enzymes are responsible for digesting most cellular substances down to their basic form, such as amino acids, fatty acids, and sugars.
- Cellular injury leads to a release of the lysosomal enzymes, causing cellular self-digestion.
- Peroxisomes are similar to lysosomes but contain several enzymes that either produce or use hydrogen peroxide.
- Mitochondria contain the metabolic machinery necessary for cellular energy metabolism. The enzymes of the respiratory chain (electron-transport chain), found in the inner membrane of the mitochondria, generate most of the cell’s ATP.
- The cytoskeleton is the “bone and muscle” of the cell. The internal skeleton is composed of a network of protein filaments, including microtubules and actin filaments (microfilaments).
- The plasma membrane encloses the cell and, by controlling the movement of substances across it, exerts a powerful influence on metabolic pathways.
- Protein receptors (recognition units) on the plasma membrane enable the cell to interact with other cells and with extracellular substances.
- The plasma membrane is a bilayer of lipids (phospholipids, glycolipids) and cholesterol, which gives the membrane its structural integrity.
- Membrane functions are determined largely by proteins. These functions include recognition by protein receptors and transport of substances into and out of the cell.
- The fluid mosaic model accounts for the fluidity of the lipid bilayer and the flexibility, self-sealing properties, and selective impermeability of the plasma membrane.
- Cellular receptors are protein molecules on the plasma membrane, in the cytoplasm, or in the nucleus that is capable of recognizing and binding smaller molecules, called ligands.
- The dynamic mature of the fluid plasma membrane enables it to vary the number of receptors on its surface. The cell is therefore capable of “hiding” from injurious agents by altering receptor number and pattern.
- The ligand-receptor complex initiates a series of protein interactions, causing adenylate cyclase to catalyze the transformation of cellular ATP to messenger molecules that stimulate specific responses within the cell.
- Cell-to-cell adhesions are formed on plasma membranes, thereby allowing the formation of tissues and organs. Cells are held together by three different means: (a) the extracellular membrane, (b) cell adhesion molecules in the cell’s plasma membrane, and (c) specialized cell junctions.
- The extracellular matrix includes three groups of macromolecules: (1) fibrous structural proteins (collagen and elastin), (2) adhesive glycoproteins, and (3) proteoglycans and hyaluronic acid. The matrix helps regulate cell growth, movement, and differentiation.
- The three major types of cell junctions are desmosomes, tight junctions, and gap junctions.
Cellular Communication and Signal Transduction
- Cells communicate in three ways: (a) they form protein channels (gap junctions); (b) they display receptors that affect intracellular processes or other cells in direct physical contact; and (c) they secrete signals for long-distance communication.
- Primary modes of chemical signaling include hormonal, neurohormonal, neurotransmitters, paracrine and autocrine.
- Signal transduction involves signals or instructions from extracellular chemical messengers that are conveyed to the cell’s interior for execution.
- The chemical tasks of maintaining essential cellular functions are referred to as cellular metabolism. Anabolism is the energy-using process of metabolism, whereas catabolism is the energy-releasing process.
- Adenosine triphosphate (ATP) functions as an energy-transferring molecule. Energy is stored by molecules of carbohydrate, lipid, and protein, which, when catabolized, transfer energy to ATP.
- Oxidative phosphorylation occurs in the mitochondria and is the mechanism by which the energy produced from carbohydrates, fats, and proteins is transferred to ATP.
Membrane Transport: Cellular Intake and Output
- Water and small, electrically uncharged molecules move through pores in the plasma membrane’s lipid bilayer in the process called passive transport.
- Passive transport does not require the expenditure of energy; rather, it is driven by the physical effects of osmosis, hydrostatic pressure, and diffusion.
- Larger molecules and molecular complexes (e.g., ligand-receptor complexes) are moved into the cell by active transport, which requires the cell to expend energy (by means of ATP).
- The largest molecules (macromolecules) and fluids are transported by the processes of endocytosis (ingestion) and exocytosis (expulsion).
- Two types of solutes exist in body fluids: electrolytes and non electrolytes. Electrolytes are electrically charged and dissociate into constituent ions when placed in solution. Nonelectrolytes do no dissociate when placed in solution.
- Diffusion is the passive movement of a solute from an area of higher solute concentration to an area of lower solute concentration.
- Filtration is the measurement of water and solutes through a membrane because of a greater pushing pressure.
- Hydrostatic pressure is the mechanical force of water pushing against cellular membranes.
- Osmosis is the movement of water across a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration.
- The amount of hydrostatic pressure required to oppose the osmotic movement of water is called the osmotic pressure of the solution.
- The overall osmotic effect of colloids, such as plasma proteins, is called the oncotic pressure or colloid osmotic pressure.
- Mediated transport can be passive or active. Mediated transport includes the movement of two molecules simultaneously in one direction (symport) or in opposite directions (antiport) or the movement of a single molecule in one direction (uniport).
- Passive mediated transport is also called facilitated diffusion. It does not require the expenditure of metabolic energy.
- Active mediated transport requires metabolic energy (ATP) to move molecules against the concentration gradient.
- Active transport occurs also by endocytosis, or vesicle formation, in which the substance to be transported is engulfed by a segment of the plasma membrane, forming a vesicle that moves into the cell.
- Pinocytosis is a type of endocytosis in which fluids and solute molecules are ingested through formation of small vesicles.
- Phagocytosis is a type of endocytosis in which large particles, such as bacteria, are ingested through formation of large vesicles, called vacuoles.
- In receptor-mediated endocytosis, the plasma membrane receptors are clustered, along with bristle-like structures, in specialized areas called coated pits.
- Endocytosis occurs when coated pits invaginate, internalizing ligand-receptor complexes in coated vesicles.
- Inside the cell, lysosomal enzymes process and digest material ingested by endocytosis.
- Caveolae are cavelike pits, and uptake through their opening and closing is called potocytosis.
- All body cells are electrically polarized, with the inside of the cell more negatively charged than the outside. The difference in voltage across the plasma membrane is the resting membrane potential.
- When an excitable (nerve or muscle) cell receives an electrochemical stimulus, cations enter the cell, causing a rapid change in the resting membrane potential known as the action potential. The action potential “moves” along the cell’s plasma membrane and is transmitted to an adjacent cell. This is how electro-chemical signals convey information from cell to cell.
Cellular Reproduction The Cell Cycle
- Cellular production in body tissues involves mitosis (nuclear division) and cytokinesis (cytoplasmic division).
- Only mature cells are capable of division. Maturation occurs during a stage of cellular life called interphase (growth phase).
- The cell cycle is the reproductive process that begins after interphase in all tissues with cellular turnover. There are four phases of the cell cycle: (1) the S phase, during which DNA synthesis takes place in the cell nucleus; (2) the G2 phase, the period between the completion of DNA synthesis and the next phase (M); (3) the M phase, which involves both nuclear (mitotic) and cytoplasmic (cytokinetic) division; and (4) the G1 phase (growth phase), after which the cycle begins again.
- The M phase (mitosis) involves four stages: prophase, metaphase, anaphase, and telophase.
- The mechanisms that control cell division depend on “social control genes” and protein growth factors.
- Cells of one or more types are organized into tissues, and different types of tissues compose organs. Organs are organized to function as tracts or systems.
- Three key factors that maintain the cellular organization of tissues are (a) recognition and cell communication, (b) selective cell-to-cell adhesion, and (c) memory.
- Tissue cells are linked at cell junctions, which are specialized regions on their plasma membranes called desmosomes, tight junctions, and gap junctions. Cell junctions attach adjacent cells and allow small molecules to pass between them.
- The four basic types of tissues are epithelial, muscle, nerve, and connective tissues.
- Neural tissue is composed of highly specialized cells called neurons that receive and transmit electrical impulses rapidly across junctions called synapses.
- Epithelial tissue covers most internal and
- external surfaces of the body. The functions of epithelial tissue include protection, absorption, secretion, and excretion.
- Connective tissue binds carious tissues and organs together, supporting them in their locations and serving as storage sites for excess nutrients.
- Muscle tissue is composed of long, thin, highly contractile cells or fibers called myocytes. Muscle tissue that is attached to bones enables voluntary movement. Muscle tissue in internal organs enables involuntary movement, such as the heartbeat.