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Transcript
Cell Structure and Function Chapter 4 Early Discoveries • Mid 1600s - Robert Hooke observed and described cells in cork • Late 1600s - Anton van Leeuwenhoek observed sperm, microorganisms • 1820s - Robert Brown observed and named nucleus in plant cells Developing Cell Theory • Matthias Schleiden • Theodor Schwann • Rudolf Virchow Schleiden and Schwann proposed the idea that all living things were composed of cells. Schleiden worked with plants and Schwann worked with animals. Virchow concluded that all cells come from cells. Cell Theory 1) Every organism is composed of one or more cells 2) The cell is smallest unit having properties of life 3) Continuity of life arises from growth and division of single cells (cells come from preexisting cells.) Cell • Smallest unit of life • Can survive on its own or has potential to do so • Is highly organized for metabolism • Senses and responds to environment • Has potential to reproduce Why Aren't All Cells Big? 1. Most cells are too small to be seen without a microscope. 2. The small size of cells permits efficient diffusion across the plasma membrane and within the cell. 3. As the surface area of a cell increases by the square of the diameter, the volume increases by the cube of the diameter. (Surface-to-volume ratio) -The bigger a cell is, the less surface area there is per unit volume. -Above a certain size, material cannot be moved in or out of cell fast enough Microscopes • Create detailed images of something that is otherwise too small to see • Light microscopes – Simple or compound • Electron microscopes – Transmission EM or Scanning EM Limitations of Light Microscopy • Wavelengths of light are 400-750 nm • If a structure is less than one-half of a wavelength long, it will not be visible • Light microscopes can resolve objects down to about 200 nm in size Red blood cells Plant cells Onion cells • Light Microscope Images Electron Microscopy • Uses streams of accelerated electrons rather than light • Electrons are focused by magnets rather than glass lenses • Can resolve structures down to 0.5 nm Blood Cells White Blood Cell infected with HIv Stem Cell from Human Bone Marrow • Scanning Electron Microscope – “scans” the surface of objects Plant Root Cell Liver Cell (color enhanced) Animal Cell (type?) • Transmission Electron Microscope – “looks” inside an object Structure of Cells All start out life with: Two types: – Plasma membrane – Prokaryotic – Region where DNA is stored (nucleus or nucleoid) – Eukaryotic – Cytoplasm Structural Organization of Cells 1. A plasma membrane separates each cell from the environment, permits the flow of molecules across the membrane, and contains receptors that can affect the cell’s activities. 2. A nucleus or nucleoid region localizes the hereditary material, which can be copied and read. 3. The cytoplasm contains membrane systems, particles (including ribosomes), filaments (the cytoskeleton), and is a semifluid substance. 4. There are basically two kinds of cells in nature: A. Eukaryotic cells contain distinctive arrays of organelles, including a membrane-bound nucleus. B. Prokaryotic cells (bacteria) have no nucleus. Lipid Bilayer • Main component of cell membranes • Gives the membrane its fluid properties • Two layers of phospholipids one layer of lipids one layer of lipids Figure 4.3 Page 56 Organization of Cell Membranes 1. The lipid bilayer of plasma membranes forms a boundary between inside and outside of the cell, subdivides the cytoplasm into compartments, and regulates the entry/exit of substances. 2. Proteins positioned in the plasma membrane serve as channels, pumps, or receptors. (The proteins do most of the “work”). Membrane Proteins Recognition protein Receptor protein extracellular environment lipid bilayer cytoplasm Protein pump across bilayer Protein channel across bilayer Protein pump Figure 4.4 Page 57 Prokaryotic Cells • Archaebacteria and Eubacteria • DNA is not enclosed in nucleus • Generally the smallest, simplest cells • No organelles Prokaryotic Structure pilus cytoplasm with ribosomes DNA flagellum capsule cell plasma wall membrane Prokaryotic Cells A. The term prokaryotic (“before the nucleus”) indicates existence of bacteria before evolution of cells with a nucleus; bacterial DNA is clustered in a distinct region of the cytoplasm (nucleoid). B. Bacteria are some of the smallest and simplest cells (structurally). 1. Bacterial flagella project from the membrane and permit rapid movement. a. not the same structure as eukaryotic flagella 2. A somewhat rigid cell wall supports the cell and surrounds the plasma membrane, which regulates transport into and out of the cell. a. also different than eukaryotic cells 3. Ribosomes, protein assembly sites, are dispersed throughout the cytoplasm. 4. also; capsule, membrane and pili 5. 2 main types: archaebacteria and eubacteria Eukaryotic Cells • Have a nucleus and other organelles compartmentalized portions within the cytoplasm that allow reactions to be separated with respect to time (allowing proper sequencing) and space (allowing incompatible reactions to occur in close proximity). • Eukaryotic organisms – Plants – Animals – Protistans – Fungi Animal Cell Features • • • • • • • • Plasma membrane Nucleus Ribosomes Endoplasmic reticulum Golgi body Vesicles Mitochondria Cytoskeleton Figure 4.10b Page 61 Plant Cell Features • • • • • • • • Plasma membrane Nucleus Ribosomes Endoplasmic reticulum Golgi body Vesicles Mitochondria Cytoskeleton • Cell wall • Central vacuole • Chloroplast Figure 4.10a Page 61 Defining Features of Eukaryotic Cells A. Major Cellular Components 1. The nucleus controls access to DNA and permits easier packing of DNA during cell division. 2. The endoplasmic reticulum (ER) modifies/delivers proteins and is also involved with lipid synthesis. 3 Golgi bodies also modify proteins, sort and ship proteins, and play a role in the biology of lipids for secretion or internal use. Major Cellular Components, continued 4. Various vesicles transport, store, and digest various materials within the cell. 5. Mitochondria have enzymes responsible for ATP formation. 6. Ribosomes , either “free” or attached to membranes are the assembly sites of polypeptide chains. 7. The cytoskeleton determines cell shape and internal organization; it also provides for motility. Functions of Nucleus • Keeps the DNA molecules of eukaryotic cells separated from metabolic machinery of cytoplasm • Makes it easier to organize DNA and to copy it before parent cells divide into daughter cells Components of Nucleus nuclear envelope nucleoplasm nucleolus chromatin Figure 4.11b Page 62 The Nucleus A. The nucleus isolates DNA—which contains the code for protein assembly, from the sites—ribosomes in cytoplasm, where proteins will be assembled. 1. the fluid within the nucleus is called nucleoplasm B. Nuclear Envelope 1. The nuclear envelope consists of two lipid bilayers with pores. 2. The inner surface has attachment sites for protein filaments, which anchor the DNA molecules and keep them organized. 3. The outer surface is studded with ribosomes. Nuclear Envelope • Two outer membranes (lipid bilayers) – 4 layers • Innermost surface has DNA attachment sites Nuclear pore bilayer facing cytoplasm Nuclear envelope bilayer facing nucleoplasm Figure 4.12b Page 63 The Nucleus, continued C. Nucleolus 1. The nucleolus appears as a dense, globular mass of material within the nucleus. 2. It is a region where RNA subunits of ribosomes are pre-fabricated before shipment out of the nucleus. D. Chromosomes 1. Chromatin refers to the total collection of DNA and proteins. (uncoiled form) 2. Each chromosome is a single molecule of DNA and its associated proteins; it may take on different appearances depending on the events currently happening within the cell. Cytomembrane/Endomembrane System • Group of related organelles in which lipids are assembled and new polypeptide chains are modified • Products are sorted and shipped to various destinations Components of Cytomembrane/endomembrane System Endoplasmic reticulum Golgi bodies Vesicles Endoplasmic Reticulum • In animal cells, continuous with nuclear membrane • Extends throughout cytoplasm • Two regions - rough and smooth A. Endoplasmic Reticulum 1. The endoplasmic reticulum is a collection of interconnected tubes and flattened sacs that begins at the nucleus and winds its way through the cytoplasm. 2. Two kinds of ER may be found in a cell: a. Rough ER consists of stacked, flattened sacs with many ribosomes attached; oligosaccharide groups are attached to polypeptides as they pass through on their way to other organelles or to secretory vesicles. b. Smooth ER has no ribosomes; Lipid synthesis (including phospholipids and hormones). It is the area from which vesicles carrying proteins and lipids are budded; it also helps inactivate harmful chemicals. Golgi Body • Puts finishing touches on proteins and lipids that arrive from ER • Packages finished material for shipment to final destinations • Material arrives and leaves in vesicles budding vesicle Figure 4.15 Page 65 C. Golgi Bodies 1. A Golgi body consists of flattened sacs—resembling a stack of pancakes—whose edges break away as secretory vesicles. 2. Here proteins and lipids undergo final processing, sorting, and packaging. D. A Variety of Vesicles – tiny membranous sacs that are used for exocytosis, endocytosis, storage, membrane building and digestion 1. Lysosomes are vesicles that bud from Golgi bodies; they carry powerful enzymes that can digest the contents of other vesicles, worn-out cell parts, or bacteria and foreign particles. 2. Peroxisomes are small vesicles that contain enzymes using oxygen to degrade fatty acids and amino acids, forming a harmful byproduct, hydrogen peroxide, which is then converted to water. Vesicles • Membranous sacs that move through cytoplasm • Lysosomes • Peroxisomes What Happens to the Proteins Specified by DNA? 1. Within the cytoplasm, newly formed polypeptide chains may be stockpiled in solution or may enter the endomembrane system (ER, Golgi bodies, and vesicles). 2. Some of the proteins will be used within the cell in which they were made, other will be exported for use elsewhere. Mitochondria • ATP-producing powerhouses • Membranes form two distinct compartments • ATP-making machinery embedded in inner mitochondrial membrane Mitochondria A. Mitochondria are the primary organelles for transferring the energy in carbohydrates to ATP under oxygen-plentiful conditions. (powerhouse) B. Each mitochondrion has an outer membrane and an inner folded membrane (cristae). 1. Two compartments are formed by the membranes. 2. Hydrogen ions and electrons move between the compartments during ATP formation. C. Mitochondria have their own DNA and ribosomes, a fact which points to their origination from ancient bacteria engulfed by predatory cells. Mitochondrial Origins • Mitochondria resemble bacteria – Have own DNA, ribosomes – Divide on their own • May have evolved from ancient bacteria that were engulfed but not digested Specialized Plant Organelles • Plastids • Central Vacuole Chloroplasts Convert sunlight energy to ATP and carbohydrates through photosynthesis Chloroplasts and Other Plastids (organelles specialize in photosynthesis and storage) 1. Chloroplasts are oval or disk shaped, bounded by a double membrane, and are critical to the process of photosynthesis. a. In the stacked disks (grana), pigments and enzymes trap sunlight energy to form ATP. -the inner membrane, that makes-up the grana, is called the thylakoid membrane b. Sugars are formed in the fluid substance (stroma) surrounding the stacks. c. Pigments such as chlorophyll (green) confer distinctive colors to the chloroplasts. -also carotenoids – yellow, red, orange 2. Chromoplasts store red and brown pigments that give color to petals, fruits, and roots. No chlorophyll 3. Colorless amyloplasts store starch granules. Other Plastids • Chromoplasts – No chlorophyll – Abundance of carotenoids – Color fruits and flowers red to yellow • Amyloplasts – No pigments – Store starch Central Vacuole 1. In a mature plant, the central vacuole may occupy 50 to 90 percent of the cell interior. A. Central vacuoles store amino acids, sugars, ions, and wastes. B. The vacuole enlarges during growth and greatly increases the cell’s outer surface area. 2. The enlarged cell, with more surface area, has an enhanced ability to absorb nutrients. Cytoskeleton • Present in all eukaryotic cells • Basis for cell shape and internal organization • Allows organelle movement within cells and, in some cases, cell motility The cytoskeleton gives cells their internal organization, shape, and capacity to move. 1. It forms an interconnected system of bundled fibers, slender threads, and lattices that extends from the nucleus to the plasma membrane. 2. The main components are microtubules, microfilaments, and intermediate filaments—all assembled from protein subunits. 3. Some portions are transient, such as the “spindle” microtubules used in chromosome movement during cell division; others are permanent, such as filaments operational in muscle contraction. Cytoskeletal Elements intermediate filament microtubule microfilament tubulin subunit Microtubules • Largest elements • Composed of tubulin • Arise from microtubule organizing centers (MTOCs) • Involved in shape, motility, cell division Figure 4.21 Page 71 Microtubules—The Big Ones 1. Microtubules, the largest structural elements in the cytoskeleton, are composed of tubulin subunits which compose a cylinder. 2. Microtubule organizing centers (MTOCs) are small masses of proteins in the cytoplasm that give rise to microtubules. 3. Microtubules govern the division of cells and some aspects of their shape as well as many cell movements. 4. involved in sustained directional movements of organelles 5. straight hollow cylinders - figure 4.21, p71 Microfilaments • Thinnest elements • consist of two helically twisted polypeptide chains assembled from actin monomers. • Take part in movement, formation, and maintenance of cell shape – Microfilaments are particularly important in movements that take place at the cell surface; they also contribute to the shapes of animal cells and actin subunit cytoplasmic streaming. Figure 4.21 Page 71 Myosin and Other Accessory Proteins 1. Extending from the microfilaments of muscle cells, myosin plays a vital role in contraction. 2. Other proteins attach microfilaments to the inner surface of the plasma membrane (spectrin) or span the plasma membrane to connect microfilaments to outside proteins (integrins). Intermediate Filaments • Only in animal cells of certain tissues • Most stable cytoskeletal elements • Six known groups • Examples include desmins and vimentins (support machinery by which muscle cells contract) and lamins (form a scaffold that reinforces the nucleus). • one polypeptide chain help strengthen and maintain cell shape Figure 4.21 Page 71 How Do Cells Move? A. Chugging Along With Motor Proteins 1. Through controlled assembly and disassembly of their subunits, microtubules; and microfilaments grow or diminish in length, thereby the structures attached to them are thereby pushed or dragged through the cytoplasm. 2. Parallel arrays of microfilaments or microtubules actively slide past one another to bring about contraction, as in muscle. 3. Microtubules or microfilaments shunt organelles from one location to another as in cytoplasmic streaming. 4. kinesins and dyneins with microtubules, myosin with microfilaments a. kinesin “walking” along a microtubule with its other end attached to an organelle Motor Proteins • Kinesins and dyneins move along microtubules • Myosins move along microfilaments kinesin microtubule Figure 4.24b, Page 72 B. Cilia, Flagella, and False Feet 1. Microtubular extensions of the plasma membrane have a 9 + 2 cross-sectional array that arises from a centriole (a type of MTOC) and are useful in propulsion. 2. Flagella are quite long, not usually numerous, and found on one-celled protistans and animal sperm cells. 3. Cilia are shorter and more numerous and can provide locomotion for free-living cells or may move surrounding water and particles if the ciliated cell is anchored. 4. Cilia and Flagella move by sliding mechanism 5. Pseudopods are temporary lobes that project from the cell, used in locomotion and food capture. a. microfilaments rapidly elongate Flagella and Cilia microtubule • Structures for cell motility • 9 + 2 internal structure Figure 4.25 Page 73 dynein Eukaryotic Cell Walls 1. Cell walls are carbohydrate frameworks for mechanical support in bacteria, protistans, fungi, and plants; cell walls are not found in animals. 2. In growing plant parts, bundles of cellulose strands form a primary cell wall that is pliable enough to allow enlargement under pressure. 3. Later, more layers are deposited on the inside of the primary wall to form the secondary wall. 4. Lignin composes up to 25 percent of the secondary wall in woody plants; it makes plant parts stronger, more waterproof, and less inviting to insects. 5. the cells themselves secrete the wall forming materials Plant Cell Walls Secondary cell wall (3 layers) Primary cell wall Plant Cuticle • Cell secretions and waxes accumulate at plant cell surface • Semitransparent • Restricts water loss Cell Junctions 1. In plants tiny channels called plasmodesmata cross the adjacent primary walls and connect the cytoplasm 2. Animal cells display three types of junctions: a. Tight junctions occur between cells of epithelial tissues in which cytoskeletal strands of one cell fuse with strands of neighboring cells causing an effective seal. b. Adhering junctions are like spot welds at the plasma membranes of two adjacent cells that need to be held together during stretching as in the skin and heart. c. Gap junctions are small, open channels that directly link the cytoplasm of adjacent cells. Cell-to-Cell Junctions • Plants – Plasmodesmata • Animals – Tight junctions – Adhering junctions – Gap junctions plasmodesma Animal Cell Junctions tight junctions adhering junction gap junction Matrixes between Animal Cells • Animal cells have no cell walls • Some are surrounded by a matrix of cell secretions and other material • The matrix between animal cells includes cell secretions and materials drawn from the surroundings between cells. • For example, cartilage consists of scattered cells and collagen embedded in a "ground substance" of modified polysaccharides; bone is similarly constructed Cell Communication 1. Signals and receptors allow cells to change their activities. 2. Hormones are well known stimulators of cell activity. We will look at cell communication more in depth on another powerpoint!