CELLS CHAPTER 3 Introduction • Cells are the basic structural and functional unit of all living organisms. • Study of cells: cytology • Cell theory: – All organisms made of cells – Cell is the basic unit of life – New cells arise from preexisting cells Parts of a cell • The principal parts of a cell are: – 1. Plasma membrane: the outer boundary – 2. Cytosol: the fluid material in the interior. Contains mostly water, ions, sugar,amino acids, lipids, ATP, wastes. Site for chemical reactions. – 3. Organelles: structures found within that perform a specific function. Some organelles are membranous and others are non membranous. – 4. Nucleus: the largest organelle that controls all cellular functions. Cellular Organelles • Cytoskeleton: a network of protein filaments. Microfilaments, intermediate filaments, microtubules. Provides structural framework and responsible for cell movements. • Centrosome: consists of centrioles. Responsible for cell division process: mitosis and meiosis. • Cilia and flagella: hair-like projections. Move fluid along cell surface or entire cell. • Ribosomes: made of RNA and protein. Small and large subunits. Sites for protein synthesis. Cellular Organelles • Endoplasmic reticulum (ER): network of membranes that form sacs or tubules called cisterns. Rough ER: covered by ribosomes and function is protein synthesis. Also forms glycoproteins. Smooth ER: lacks ribosomes. Synthesizes steroids, fats phospholipids. Inactivates or detoxifies harmful drugs. • Golgi Complex: contains flattened sacs. It receives synthesized products from ER and then sorts, modifies, packages and transports or secretes them to various destinations. Cellular Organelles • Lysosomes: membrane bound organelles that contain digestive enzymes. Function in digestion of worn-out organelles, digestion of foreign invaders, and extra cellular digestion. • Peroxisomes: similar to lysosomes but smaller. Contain catalase that degrades hydrogen peroxide. • Mitochondria: double-membrane structures. Inner membrane has folds called “cristae” and a fluid- filled cavity called the “ matrix”. It is the powerhouse of the cell as most of the ATP is generated here by cellular respiration. Nucleus • Largest organelle of the cell. • Consists of a double nuclear envelope. • Nuclear pores: allow for the exchange of material between the nucleus and cytoplasm. • Fluid of nucleus: nucleoplasm. • Chromatin: genetic material. Made of DNA and protein. Chromosomes. • Nucleolus: pre-assembly site for ribosomes. • Genes (found on chromosomes) control cellular structure and most cellular functions. Plasma Membrane • Surrounds and contains of the cytoplasm of the cell. • It contains a 50:50 mix of protein and lipids. • The fluid-mosaic model: the membrane is a mosaic of proteins floating like icebergs in a sea of lipid bilayer. • The major lipid is phospholipid (bilayer), also present is cholesterol and glycolipids. • Proteins: integral or peripheral. Also glycoproteins. Plasma membrane • Membrane proteins serve various functions. Channels or transporters: help specific solutes to enter. Receptors: recognize specific molecules. Other proteins function as enzymes or cell-identity markers. • Membrane is selectively permeable. The lipid bilayer permits water, gases, nonpolar substances. Proteins allow for ions and charged molecules to pass through. • Membrane is a fluid structure. Fluidity increased by unsaturated fatty acids. The bilayer can self- seal when torn or punctured. Transport processes • Passive and Active mechanisms. In passive transport molecules move down the concentration gradient. In active transport molecules move uphill against concentration gradient. • In vesicular transport, tiny vesicles either detach from or merge with the plasma membrane to move materials across the membrane. Passive transport • In net diffusion molecules move from a high concentration area to a low concentration area until an equilibrium is reached. The rate of diffusion depends on steepness of concentration gradient, temperature, size or mass of diffusing substance. Eg. Oxygen, carbon dioxide, alcohols, steroids, fat soluble vitamins. • Ion channels allow diffusion across the membrane of small inorganic ions of sodium, potassium, calcium, chloride. • In facilitated diffusion, a solute such as glucose binds to a specific transporter protein and is Osmosis • Net movement of water across a selectively permeable membrane from an area of high water concentration to an area of low water concentration. • Isotonic solution • Hypotonic solution • Hypertonic solution Active Transport • Movement of molecules from a low concentration area to a high concentration area with the expenditure of ATP energy. Also requires a transport protein. Eg. Sodium, potassium, hydrogen, chloride, iodide, calcium ions. • The most prevalent primary active pump is the Na/K pump. • Secondary active transport: symporters and antiporters. Vesicular transport • Endocytosis: phagocytosis and pinocytosis. Phagocytosis is the ingestion of solid particles. Pinocytosis is the ingestion of extracellular fluid. • Receptor-mediated endocytosis: is the selective uptake of large particles and molecules called ligands. • Exocytosis: involves movement of secretory or waste products out of a cell by fusion of vesicles with the plasma membrane. Cell Division • Cell division is the process by which cells reproduce themselves. It consists of nuclear division(mitosis) and cytoplasmic division (cytokinesis). • Cell division that results in an increase in the number of cells involves mitosis. • Cell division that involves production of sperm or ova involves meiosis. Cell cycle • It is divided into interphase, mitosis and cytokinesis. • Interphase is further divided into G1, S and G2 phases. – G1 phase: cell duplicates its organelles and cytosolic contents. – S phase: during this phase DNA replication occurs. – G2 phase: enzymes and other proteins are syntyhesized. Mitosis • Mitosis is divided into prophase, metaphase, anaphase and telophase. – Prophase: nuclear envelope disintegrates, nucleolus disappears, centrioles start moving to opposite poles, spindle apparatus is formed, chromosomes are visible. – Metaphase: chromosomes are aligned along the equator of the cell. – Anaphase: the chromatids are pulled apart to opposite poles. Mitosis and Cytokinesis • Telophase: reappearance of nuclear envelope, chromatin, nucleolus. Cleavage furrow. • Cytokinesis: cytoplasmic division to form two identical daughter cells. • Certain genes regulate both cell division and apoptosis (programmed cell death). Cancer • Uncontrolled cell division. • Tumor or neoplasm: excess tissue in a part of the body. • Cancerous tumor is malignant tumor. • Metastasis: ability to spread to other parts. • Benign tumor: noncancerous tumor.