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Transcript
Chapter 6 A Tour of the Cell I. The Cell Theory: A. All living organisms are made of one or more cells. B. The cell is the basic unit of structure and function of living things. C. All cells come from pre-existing cells. II. Tools for Studying Cells - Improvements in technology used to study cells have led to a better understanding of cells. A. Microscopy - Scientists use microscopes to visualize cells too small to see with the naked eye. - Light microscopes pass visible light through a specimen and magnify cellular structures with lenses. - Different types of microscopes can be used to visualize different sized cellular structures and use different methods for enhancing visualization of cellular structures. - Electron microscopes focus a beam of electrons through a specimen (TEM) or onto its surface (SEM). - The scanning electron microscope (SEM) provides for detailed study of the surface of a specimen. - The transmission electron microscope (TEM) provides for detailed study of the internal ultrastructure of cells. B. Isolating Organelles by Cell Fractionation - Cell fractionation takes cells apart and separates the major organelles from one another based on their density. - The centrifuge is used to fractionate cells into their component parts. III. Comparing Prokaryotic and Eukaryotic Cells - Two types of cells make up every organism: prokaryotic and eukaryotic. A. All cells have several basic features in common - bounded by a plasma membrane - contain a semifluid substance called the cytosol - contain chromosomes - have ribosomes B. Prokaryotic cells - do not contain a nucleus - have their DNA located in a region called the nucleoid C. Eukaryotic cells - contain a true nucleus, bound by a membranous nuclear envelope - usually quite a bit bigger than prokaryotic cells - have extensive and elaborately arranged internal membranes which form organelles - Plant and animal cells have most of the same organelles. D. Size Limits of Cells - The logistics of carrying out cellular metabolism sets limits on the size of cells. - A smaller cell has a higher surface to volume ratio, which facilitates the exchange of materials into and out of the cell. - The plasma membrane functions as a selective barrier allowing sufficient passage of nutrients and waste. IV. Nucleus and Ribosomes - involved in the genetic control of the cell A. The Nucleus: Genetic Library of the Cell - Contains most of the genes in the eukaryotic cell. - The nuclear envelope, a lipid bilayer, encloses the nucleus, separating its contents from the cytoplasm. - Chromosomes: structures in nucleus that contain the DNA - Nucleoli: structures in the nucleus that are involved in the production of ribosomes B. Ribosomes: Protein Factories in the Cell - particles made of ribosomal RNA and protein - carry out protein synthesis - free ribosomes located in the cytosol - bound ribosomes attached to endoplasmic reticulum or nuclear envelope V. Endomembrane System - involved in synthesis and transport of proteins and lipids, and detoxification of poisons - includes nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and plasma membrane A. The Endoplasmic Reticulum (ER): Biosynthetic Factory - accounts for more than half the total membrane in many eukaryotic cells - is continuous with the nuclear envelope - consists of the ER lumen (cavities) and cisternae (sacs) - two distinct regions of ER: Smooth ER, which lacks ribosomes and Rough ER, which contains ribosomes 1. Functions of Smooth ER - synthesizes lipids (such as steroids such sex hormones) - metabolizes carbohydrates - detoxifies poison (in liver cells) - stores calcium ions (in muscle cells) 2. Functions of Rough ER - has ribosomes attached - produces proteins (esp. secretory) and membranes, which are distributed by transport vesicles B. The Golgi Apparatus: Shipping and Receiving Center - center of manufacturing, storing, sorting, and shipping of cell proteins (esp. secretory proteins and hydrolytic enzymes) - receives many of the transport vesicles produced in the rough ER - consists of flattened membranous sacs called cisternae - functions of Golgi include: - modification of the products of the rough ER - manufacture of certain macromolecules C. Lysosomes: Digestive Compartments - membranous sac of hydrolytic enzymes - digest all kinds of macromolecules - carry out intracellular digestion by phagocytosis - recycle the cell’s own organic molecules by autophagy D. Vacuoles: Diverse Maintenance Compartments - A plant or fungal cell may have one or several vacuoles. - Food vacuoles are formed by phagocytosis - Contractile vacuoles pump excess water out of protist cells - Central vacuoles, found in plant cells, hold reserves of important organic compounds and water. VI. Mitochondria and Chloroplasts - change energy from one form to another - Mitochondria are the sites of cellular respiration. - Chloroplasts found only in plants, are the sites of photosynthesis. - Both contain their own DNA and ribosomes. A. Mitochondria: Cellular Respiration - found in nearly all eukaryotic cells - enclosed by two membranes: a smooth outer membrane and an inner membrane folded into cristae B. Chloroplasts: Photosynthesis - a specialized member of a family of closely related plant organelles called plastids - contains chlorophyll - found in leaves and other green organs of plants and in algae - Chloroplast structure includes thylakoids, membranous sacs, and stroma, the internal fluid. - Other plastids include chromoplasts (store colored pigments) and amyloplasts (store starch, amylase). C. Peroxisomes: Oxidation - produce hydrogen peroxide and convert it to water - breakdown fatty acids VII. Cytoskeleton - a network of fibers extending throughout the cytoplasm that organizes structures and activities in the cell - composed of 3 types of molecular structures: microtubules, microfilaments, and intermediate filaments A. Roles of the Cytoskeleton: Support, Motility, and Regulation - gives mechanical support to the cell - provides anchorage for organelles and some enzymes - involved in cell motility, which utilizes motor proteins B. Components of the Cytoskeleton - three main types of fibers that make up the cytoskeleton 1. Microtubules - shape and support the cell - guide movement of organelles - help separate the chromosome copies in dividing cells a. Centrosomes and Centrioles - The centrosome is considered to be a “microtubule-organizing center” and contains a pair of centrioles (composed of 9 triplets of microtubules). - Produce a type of microtubule that functions as compression resistant ‘girders’. b. Cilia and Flagella - are locomotor appendages of some cells - share a common ultrastructure - contain specialized arrangements of microtubules - Dynein is a motor protein responsible for the bending movement of cilia and flagella. 2. Microfilaments (Actin Filaments) - twisted, double chain of actin (a protein) molecules - tension-bearing structures - found in all eukaryotic cells - Microfilaments that function in cellular motility contain the protein myosin in addition to actin. - Amoeboid movement involves the contraction of actin and myosin filaments. - Cytoplasmic streaming is another form of locomotion created by microfilaments. 3. Intermediate Filaments - permanent, tension-bearing structures made of various protein such as keratin - support cell shape - fix organelles in place VIII. Extracellular Components and Intracellular Connections - help coordinate cellular activities A. Cell Walls of Plants - a rigid, protective, extracellular structure of plant, fungal, and bacterial cells - Plant cell walls are made of cellulose fibers embedded in other polysaccharides and protein. - may have multiple layers (middle lamella, and primary and secondary walls) B. The Extracellular Matrix (ECM) of Animal Cells - Animal cells, which lack cell walls, are covered by an elaborate matrix, the ECM. - made up of glycoproteins, such as collagen, and other macromolecules - functions include support, adhesion, movement, and regulation C. Intercellular Junctions - allow neighboring cells to adhere, interact, and communicate 1. Plants: Plasmodesmata - channels that perforate plant cell walls - allow water and small molecules to freely pass from cell to cell 2. Animals: Tight Junctions, Desmosomes, and Gap Junctions a. Tight junctions - form continuous seals around cells that prevent leakage between cells b. Desmosomes - act as rivets fastening cells together into strong sheets c. Gap junctions - provide cytoplasmic channels through which ions, sugars, amino acids, etc. can pass - allows intercellular communication