Chapter 4.3-4.5 Compartmentalism is the key to eukaryotic cell function Organelles- membranous compartments that lack membranes but possess distinctive shapes and functions Each has a specific function/role The nucleus contains the DNA. Replication and decoding of DNA take place here. Mitochondria- the power plant where energy stores in the bonds of carbs and fatty acids is converted to ATP ER and Golgi Apparatus- compartments in which some proteins synthesized by the ribosomes are packaged and sent Lysosomes and vacuoles are cellular digestive systems in which large molecules are hydrolyzed into usable monomers Chloroplasts perform photosynthesis Organelles can be studied by microscopy or isolated for chemical analysis Cell fractionation begins with the destruction of the plasma membrane, which allows the cytoplasmic components to flow out into a test tube. The various organelles can then be separated by size or density Some organelles process information Nucleus Nucleus is the largest organelle in the cell It?s the site of DNA replication It?s the site of genetic control of cells activities A region within the nucleus, the nucleolus, begins the assembly of ribosomes from RNA and specific proteins The nucleus is surrounded by two membranes which together form the nuclear envelope Nuclear pores on membrane are composed of over 100 different proteins, interacting hydrophobically We know the sequence of the nuclear localization signal because of several things: The signal sequence occurs in most nuclear proteins, but not in proteins that remain in cytoplasm If the signal sequence is added to a protein that normally stays in the cytoplasm, that protein moves to the nucleus If the signal sequence is removed from a protein, it stays in the cytoplasm Some viruses have a signal sequence that allows them to enter the nucleus; viruses without the signal sequence did not enter the nucleus as virus particles Inside the nucleus, DNA combines with proteins to form a fibrous complex called chromatin Prior to cell division, chromatin aggregates to form discrete readily visible structures called chromosomes nucleoplasm is water and dissolves substances that surround the chromatin within the nucleoplasm a network of apparently structural proteins called the nuclear matrix which organizes the chromatin at the periphery of the nucleus, the chromatin is attached to a protein meshwork called the nuclear lamina which is formed by the polymerization of proteins called lamins into filaments Ribosomes Float freely in cytoplasm in prokaryotes In eukaryotes, they are found in two place, in the cytoplasm or in the mitochondria and chloroplasts The ribosomes are the sites where proteins are synthesized under the direction of nucleic acids Contain ribosomal RNA The Endomembrane system is a group of interrelated organelles Endomembrane system contains the ER and golgi apparatus Tiny, membrane surrounded droplets called vesicles appear to shuttle between the components of endomembrane system Endoplasmic Reticulum (ER) A network of interconnected membranes branching throughout the cytoplasm of a eukaryotic cell forming tubes and flattened sacs Interior compartment of ER is the lumen Rough endoplasmic reticulum (RER)- the ER that is studded with ribosomes Segregates certain newly synthesized proteins away from the cytoplasm and transports them to other locations in the cell Proteins can be chemically modified so as to alter their function and eventual destination Once in the lumen of the RER, these proteins undergo several changes including the formation of disulfide bridges and folding into tertiary structures Some proteins gain carb groups in the ER, thus becoming glycoproteins Smooth endoplasmic reticulum (SER) More tubular and lacks ribosomes It is the site of hydrolysis of glycogen in animal cells Responsible for chemically modifying small molecules taken in by cells, especially drugs and pesticides The site of synthesis for lipids and steroids Golgi apparatus Consists of flattened membranous sacs called cisternae Receives proteins from the ER and may further modify them Concentrates, packages and sorts proteins before they are sent to their cellular or extracellular destinations Where some polysaccharides for the plant cell wall are synthesized Lysosomes Contain digestive enzymes and are the site where macromolecules are hydrolyzed into their monomers Originate from the golgi apparatus Breakdown food and foreign objects taken up by the cell in phagocytosis Cells digest their own material by autophagy in which organelles are engulfed and hydrolyzed Some organelles transform energy Mitochondria Primary function is to convert potential chemical energy to fuel molecules into the energy form ATP Production of ATP in mitochondria is cellular respiration Mitochondria have 2 membranes, outer and inner Space enclosed in inner membrane is mitochondrial matrix Plastids Produced only in the cells of plants and protists Several types of plastids Chloroplasts- contain chlorophyll and are the site of photosynthesis Provide food for photosynthetic organisms Thykaloids- flat, closely packed circular compartments that make up the grana The fluid of the grana are suspended in the stroma which consists of ribosomes and DNA Chromoplasts- contain red, orange or yellow pigments and give color to plant organs such as flowers Have no chemical function in the cell Leucoplasts- storage depots for starches and fats Several other organelles are surrounded by a membrane Peroxisomes- collect the toxic peroxides that are unavoidable by-products of cellular chemical reactions Has single membrane and a granular interior containing specialized enzymes Glyoxysome- sites where stored lipids are converted into carbs for transport to growing cells Found only in plants Plants and protists contain membrane enclosed vacuoles filled with aqueous solutions containing dissolved substances Vacuoles have several functions: Storage- for toxic by-products and waste products stored within the cell Structure- provides turgor of stiffness that helps support the plants Reproduction- contain anthocyanins which are visual cues to attract animals that assist pollination Digestion- contain digestive enzymes Food vacuoles are in simple organisms with no digestive system Contractile vacuoles- found in water protists to help balance water coming in and out The cytoskeleton is important in cell structure Cytoskeleton- a set of long, thin fibers in the cytoplasm Has several important roles: Supports cell and maintains its shape Provides various types of cellular movement Position organelles within the cell Some of its fibers act as tracks or supports for motor proteins, which move organelles within the cell Interacts with extracellular structures, helping anchor cell Inhibition Use a drug that inhibits A and see if B still works, then A is causative factor for B Mutation Look at a cell that lacks genes for A and see if B still occurs Microfilaments can exist in single filaments or bundles Two major roles: They help the entire cell or parts of cell move Determine and stabilize cell shape Assembled from actin and interact with other strands of proteins Change cell shape and drive cellular motion called cytoplasmic streaming Intermediate filaments Made up of fibrous proteins organized into tough ropelike assemblages that help structure Help hold neighboring cells together Some make up nuclear lamina Microtubules- long, hollow unbranched cylinders Form rigid internal skeleton for some cells Act as framework along which motor proteins can move Assembled from the protein tubulin Motor proteins- specialized molecules that use energy to change shape and move Cilia and flagella Help cell move Cilia are shorter than flagella and occur in great numbers Flagella are longer and usually found single or paired Basal body- the base of cilia or flagella Centrioles are almost identical to basal bodies but found in microtubule organizing center, used in formation of mitotic spindle Motor proteins Work by undergoing reversible shape changes powered by ATP energy Help move microtubule doublets and have positive and negative ends The plant cell wall is an extracellular structure Cell wall of plant cells is a semi rigid structure outside the plasma membrane Consists of cellulose fibers embedded in other complex polysaccharides and proteins Three major roles: It provides support for the cell and limits its volume by remaining rigid Act as a barrier to infection by fungi and other organisms that can cause plant diseases Contributes to plant form by growing as plant cells expand Plasmodesmata- numerous plasma membrane lined channels that connect adjacent plant cells Permits diffusion of water, ions, small molecules and RNA and proteins between connected cells, ensuring uniform concentration The extracellular matrix supports tissue function in animals Extracellular matrix- surrounds cells and performs many functions such as adhesion of cells matrix is composed of fibrous proteins such as collagen, a matrix of glycoproteins termed proteoglycans Functions: Holds cells together in tissue Contributes to the physical properties of cartilage, skin and other tissues Helps filter material passing between different tissues Helps orient cell movements during embryonic development and during tissue repair Plays a role in chemical signaling from one cell to another The Endosymbiosis theory suggests how eukaryotes evolved Endosymbiosis- two species living together within one living body (or cell) of the other Endosymbiotic theory- the theory that the eukaryotic cell evolved via the engulfing of one prokaryotic cell by another, developed by Lynn Margulis Evidenced by mitochondria and chloroplasts which contain own DNA and ribosomes Also evidenced by: Movement of DNA between organelles in eukaryotic cell over time Many biochemical similarities between chloroplasts and photosynthetic bacteria DNA sequencing shows strong similarities between modern chloroplast DNA and that of a photosynthetic prokaryote Both prokaryotes and eukaryotes continue to evolve Prokaryotes and eukaryotes both: Use nucleic acids as genetic material Use the same 20 amino acids in proteins Use D sugars and L amino acids Prokaryotes have genetic material in nucleoid
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