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Chapter 6: A Tour of the Cell Overview: The Importance of Cells • The cell theory o The __________________________________ is the basic unit of living things o All organisms are made of cells o All cells come from pre-existing cells Cell structure is correlated to cellular function • To study cells, biologists use __________________________________ and the tools of __________________________________ • Though usually too small to be seen by the unaided eye, cells can be complex • Scientists use microscopes to visualize cells too small to see with the naked eye • __________________________________ microscope: visible light passes through a specimen and then through glass lenses, which magnify the image o The minimum resolution of an LM is about 200 nanometers (nm), the size of a small bacterium • Two basic types of electron microscopes (EMs) are used to study subcellular structures o __________________________________ electron microscopes (SEMs): focus a beam of electrons onto the surface of a specimen, providing images that look 3D o __________________________________ electron microscopes (TEMs): focus a beam of electrons through a specimen TEMs are used mainly to study the internal ultrastructure of cells Isolating Organelles by Cell Fractionation • Cell fractionation takes cells apart and separates the major organelles from one another • Ultracentrifuges fractionate cells into their component parts • Cell fractionation enables scientists to determine the functions of organelles Eukaryotic cells have internal membranes that compartmentalize their functions • The basic structural and functional unit of every organism is one of two types of cells: Prokaryotic • Only organisms of the domains Bacteria and Archaea consist of prokaryotic cells Eukaryotic • Protists, fungi, animals, and plants all consist of eukaryotic cells Comparing Prokaryotic and Eukaryotic Cells • Basic features of all cells: o Plasma membrane o Semifluid substance called the __________________________________ o __________________________________ (carry genes) o __________________________________ (make proteins) • Prokaryotic cells: o Have no __________________________________ o DNA is in an unbound region called the __________________________________ o Lack membrane-bound organelles • Eukaryotic cells: o Have DNA in a nucleus that is bounded by a membranous __________________________________ o Contain membrane-bound organelles • Eukaryotic cells are generally much larger than prokaryotic cells • The logistics of carrying out cellular metabolism sets limits on the size of cells A Panoramic View of the Eukaryotic Cell • Plant and animal cells have most of the same organelles The Nucleus: Genetic Library of the Cell • The nuclear envelope encloses the nucleus, separating it from the cytoplasm o The nuclear membrane is a double layered membrane perforated by __________________________________ o Pores are stabilized by encompassing __________________________________ • The nucleus contains most of the cell’s genes in either a chromatin or chromosome format o __________________________________: expanded, can still be transcribed o __________________________________: condensed, utilized for replication • The nucleus contains the __________________________________ • The nucleolus assembles __________________________________ Ribosomes: Protein Factories in the Cell • Ribosomes are assembled in the nucleolus o They are molecules composed of ribosomal RNA and protein o They are composed of one __________________________________ subunit and one __________________________________ subunit • Ribosomes carry out protein synthesis in two locations: o __________________________________ ribosomes: in the cytosol o __________________________________ ribosomes: on the outside of the endoplasmic reticulum (ER) or the nuclear envelope The endomembrane system regulates protein traffic and performs metabolic functions in the cell • Components of the endomembrane system: 1. Nuclear envelope 2. Endoplasmic reticulum 3. Golgi apparatus 4. Lysosomes 5. Vacuoles 6. Plasma membrane • These components are either continuous or connected via transfer by __________________________________ The Endoplasmic Reticulum: Biosynthetic Factory • The endoplasmic reticulum (ER) accounts for more than half of the total membrane in many eukaryotic cells • The ER membrane is continuous with the nuclear envelope o The ER membrane is convoluted, thee folds are known as __________________________________ o Cisternae are hollow, the hollow space is called the __________________________________ • There are two distinct regions of ER: o __________________________________ ER: lacks ribosomes o __________________________________ ER: studded with ribosomes Functions of Smooth ER • Smooth ER o Synthesizes __________________________________ for membranes o Metabolizes __________________________________ o Stores __________________________________ o __________________________________ poison Functions of Rough ER • Rough ER o Produces __________________________________ and membranes o Assembles __________________________________ The Golgi Apparatus: Shipping and Receiving Center • The Golgi apparatus consists of flattened membranous sacs called cisternae o The side which receives materials is the __________________________________ face o The side which transports materials is the __________________________________ face • Functions of the Golgi apparatus: 1. Modifies products of the ER 2. Manufactures certain macromolecules 3. Sorts and packages materials into transport __________________________________ Lysosomes: Digestive Compartments • A lysosome is a membranous sac of __________________________________ • Lysosomal enzymes can hydrolyze: o Proteins o Fats o Polysaccharides o Nucleic acids • Lysosomes also use enzymes to recycle organelles and macromolecules, a process called __________________________________ Phagocytosis and Autophagy • Phagocytosis 1. Cell encompasses usable material via __________________________________, forming a vesicle 2. Vesicle fuses with a lysosome, forming a __________________________________ 3. Enzymes digest material and either dispose or recycle it • Autophagy 1. Damaged organelle is enclosed in a membrane, forming a vesicle 2. Vacuole fuses with a lysosome, forming a phagolysosome 3. Enzymes digest material and either dispose or recycle it Vacuoles: Diverse Maintenance Compartments • Vesicles are membrane-bound sacs with varied functions • A plant cell or fungal cell may have one or several vacuoles o Food vacuoles: formed by __________________________________ o Contractile vacuoles: found in many freshwater protists, pump excess water out of cells o Central vacuoles: found in many plant cells, hold organic compounds and __________________________________ The Endomembrane System: A Review • The endomembrane system is a complex and dynamic player in the cell’s compartmental organization Mitochondria: Chemical Energy Conversion • Mitochondria are in nearly all eukaryotic cells o They are the site of cellular __________________________________ • They have a double membrane structure, with a convoluted inner membrane o The folds are known as __________________________________ Cristae increase the __________________________________, thus increasing the amount of area that cellular respiration can occur The inner membrane creates two compartments: intermembrane space and mitochondrial matrix Chloroplasts: Capture of Light Energy • Chloroplasts are found in leaves and other green organs of plants and in algae • Chloroplast structure includes: o __________________________________: membranous sacs o Thylakoids contain the green pigment __________________________________ Thylakoids are stacked into __________________________________ Stacking increase the __________________________________, thus increasing the amount of area that photosynthesis can occur • __________________________________: internal fluid Peroxisomes: Oxidation • Peroxisomes are specialized metabolic compartments bounded by a single membrane • Peroxisomes produce __________________________________ and convert it to __________________________________ The cytoskeleton is a network of fibers that organizes structures and activities in the cell • The cytoskeleton is a network of fibers extending throughout the cytoplasm o It organizes the cell’s structures and activities, anchoring many organelles • It is composed of three types of molecular structures: 1. __________________________________: thickest 2. Microfilaments: also called actin filaments, thinnest 3. Intermediate filaments: diameters in a middle range Roles of the Cytoskeleton: Support, Motility, and Regulation • The cytoskeleton: o Aids in cell shape maintenance o Interacts with motor proteins to produce motility Inside the cell, vesicles can travel along “monorails” provided by the cytoskeleton Centrosomes and Centrioles • Microtubules grow out from a __________________________________ near the nucleus • The centrosome is a “microtubule-organizing center” o In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring Microtubules control the beating of cilia and flagella, locomotor appendages of some cells Cilia and flagella differ in their beating patterns • Cilia and flagella share a common ultrastructure: o __________________________________: microtubules sheathed by a plasma membrane o Basal body: anchors the cilium or flagellum o Motor proteins: called __________________________________, drives the bending movements of a cilium or flagellum • How dynein “walking” moves flagella and cilia: o Dynein arms alternately grab, move, and release the outer microtubules o Protein cross-links limit sliding o Forces exerted by dynein arms cause doublets to curve, __________________________________ the cilium or flagellum Microfilaments (Actin Filaments) • Microfilaments are solid rods about 7 nm in diameter, built as a twisted double chain of actin subunits o Bear tension, resisting pulling forces within the cell o Form a 3D network just inside the plasma membrane to help support the cell’s shape • Example: o Bundles of microfilaments make up the core of microvilli of intestinal cells • Microfilaments that function in cellular motility contain the protein myosin in addition to actin o In muscle cells, thousands of actin filaments are arranged parallel to one another o Thicker filaments composed of myosin interdigitate with the thinner actin fibers • Amoeboid movement: localized contraction brought about by actin and myosin o __________________________________ (cellular extensions) extend and contract through the reversible assembly and contraction of actin subunits into microfilaments • Cytoplasmic streaming: circular flow of cytoplasm within cells o Speeds distribution of materials within the cell o In plant cells, actin-myosin interactions and sol-gel transformations drive cytoplasmic streaming Intermediate Filaments • Intermediate filaments range in diameter from 8–12 nanometers o Support cell shape and fix organelles in place o More permanent cytoskeleton fixtures than the other two classes Plasma Membrane • Plasma membrane: a selective barrier that allows sufficient passage of oxygen, nutrients, and waste to service the volume of the cell o The general structure of a biological membrane is a double layer of __________________________________ Extracellular components and connections between cells help coordinate cellular activities • Most cells synthesize and secrete materials that are external to the plasma membrane • These extracellular structures include: 1. __________________________________ of plants 2. __________________________________ (ECM) of animal cells 3. Intercellular junctions Cell Walls of Plants • The cell wall is an extracellular structure that distinguishes plant cells from animal cells • Functions include: 1. Protects the plant cell 2. Maintains its shape 3. Prevents excessive uptake of __________________________________ • Plant cell walls are made of __________________________________ fibers embedded in other polysaccharides and protein • Plant cell walls may have multiple layers: o Primary cell wall: relatively thin and flexible o Middle __________________________________: thin layer between primary walls of adjacent cells o Secondary cell wall (in some cells): between the plasma membrane and the primary cell wall • __________________________________ are channels between adjacent plant cells The Extracellular Matrix (ECM) of Animal Cells • Animal cells lack cell walls but are covered by an elaborate extracellular matrix (ECM) • Functions of the ECM include: 1. Support 2. Adhesion 3. Movement 4. Regulation The ECM is made up of glycoproteins and other macromolecules • The ECM is composed of: o __________________________________: provides tensile strength o __________________________________: used to bind to integral proteins o __________________________________: recognized in cell to cell communication Intercellular Junctions • Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact • Intercellular junctions facilitate this contact Plants: Plasmodesmata • Plasmodesmata are channels that perforate plant cell walls • Through plasmodesmata, water and small solutes (and sometimes proteins and RNA) can pass from cell to cell Animals: Tight Junctions, Desmosomes, and Gap Junctions • __________________________________ junctions: membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid • __________________________________: (anchoring junctions) fasten cells together into strong sheets • __________________________________ junctions: (communicating junctions) provide cytoplasmic channels between adjacent cells