Ch05 -Cells The Working Units of Life PDF

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Chapter 5: Cells: The Working Units of Life Introduction Cells are the fundamental units of life.

5.1 What Features Make Cells the Fundamental Units of Life? Cell theory was the first unifying theory of biology:  Cells are the fundamental units of life.  All organisms are composed of cells.  All cells come from preexisting cells. Implications of the cell theory:  Functions of all cells are similar.  Life is continuous.  Origin of life was the origin of cells. Cells are small (mostly). Exceptions: Bird eggs, some algae, and bacteria. Cells are small because a high surface area-to-volume ratio is essential. Volume determines the amount of chemical activity in the cell per unit time. Larger cells have more chemical activity. Cell surface area limits the amount of resources and waste products that can cross the cell boundary per unit time. Most cells are < 200 μm in size. To see them, we use microscopes: Magnification: increases apparent size. Resolution: clarity of magnified object—minimum distance two objects can be apart and still be seen as two objects. Two basic types of microscopes: Light microscopes: use glass lenses and light. Resolution = 0.2 μm Electron microscopes: electromagnets focus an electron beam. Resolution = 0.2 nm Pathology is a branch of medicine that uses microscopy to analyze cells and diagnose diseases. Many methods are used, including phase-contrast microscopy, staining the cells with general or selective dyes, and electron microscopy. The plasma membrane is the outer surface of every cell and has more or less the same structure in all cells. It is made of a phospholipid bilayer with embedded proteins and other molecules. The plasma membrane: © 2014 Sinauer Associates, Inc.

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is a selectively permeable barrier allows cells to maintain a constant internal environment is important in communication and receiving signals often has proteins for binding and adhering to adjacent cells

Two types of cells: Prokaryotic and eukaryotic. Bacteria and Archaea are prokaryotes. They have no membrane-enclosed internal compartments. The first cells were probably prokaryotic. Eukarya are eukaryotes—cells with membrane-enclosed compartments called organelles. The DNA is in a compartment called the nucleus. Specific chemical reactions occur in other organelles. This “division of labor” was important in the evolution of complex organisms.

5.2 What Features Characterize Prokaryotic Cells? Prokaryotic cells are very small. Individuals are single cells but often form chains or clusters. Prokaryotes are very successful; and there is a huge diversity of species in the Bacteria and Archaea domains. Characteristics of prokaryotic cells:  Enclosed by a plasma membrane.  DNA is contained in a region called the nucleoid.  Cytoplasm consists of cytosol (liquid component) plus filaments and particles.  

Cytosol: water with dissolved ions, small molecules, and soluble macromolecules. Ribosomes: RNA and protein complexes; sites of protein synthesis.

Most prokaryotes have a rigid cell wall outside the plasma membrane. Bacterial cell walls contain peptidoglycan. Some bacteria have an additional outer membrane. Some bacteria have a slimy capsule of polysaccharides. Photosynthetic bacteria have an internal membrane system that contains molecules necessary for photosynthesis. Others have internal membrane folds that are attached to the plasma membrane; they may function in cell division or in energy-releasing reactions. Some prokaryotes swim by means of flagella, made of the protein flagellin. Some bacteria have pili—hairlike structures projecting from the surface. They help bacteria adhere to other cells. Fimbriae are shorter than pili; they help cells adhere to surfaces such as animal cells.

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Cytoskeleton: system of protein filaments that maintain cell shape and play roles in cell division.

5.3 What Features Characterize Eukaryotic Cells? Eukaryotic cells are up to ten times larger than prokaryotes. Eukaryotic cells have membrane-enclosed compartments called organelles. Each organelle has a specific role in cell functioning. Compartmentalization allowed eukaryotic cells to specialize and form the tissues and organs of multicellular organisms. To determine the functions of organelles, they were first studied using light microscopy and then electron microscopy. Cell fractionation separates organelles by size or density for study by chemical methods. Ribosomes: sites of protein synthesis. Occur in both and cells and have similar structure. consist of ribosomal (rRNA) and more than 50 different molecules. In eukaryotes, ribosomes are , attached to the , or inside mitochondria and chloroplasts. In prokaryotic cells, ribosomes float freely in the cytoplasm. The nucleus is usually the largest organelle.  Contains the DNA  Site of DNA replication  Site where gene transcription is turned on or off  of ribosomes begins in a region called the nucleolus The nucleus is surrounded by the nuclear envelope. Many pores control the movement of molecules across the envelope. In the nucleus, DNA combines with proteins to form chromatin in long, thin threads called chromosomes. Before cell division, chromatin condenses, and individual chromosomes are visible in the light microscope. The chromatin is attached to a protein meshwork (the nuclear lamina), which maintains the shape of the nucleus. The outer membrane of the nuclear envelope folds outward into the cytoplasm and is continuous with the endoplasmic reticulum. The endomembrane system includes the plasma membrane, nuclear envelope, endoplasmic reticulum, Golgi apparatus, and lysosomes. Tiny, membrane-surrounded vesicles shuttle substances between the various components.

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Endoplasmic reticulum (ER): network of interconnected membranes in the cytoplasm; has large surface area. Rough endoplasmic reticulum (RER): ribosomes are attached. Newly made proteins enter the RER lumen where they are modified, folded, and transported to other regions. Smooth endoplasmic reticulum (SER): more tubular, no ribosomes.  Chemically modifies small molecules such as drugs and pesticides  Site of glycogen degradation in animal cells  Synthesis of lipids and steroids The Golgi apparatus is composed of flattened sacs (cisternae) and small membraneenclosed vesicles.  Receives proteins from the RER—can further modify them  Concentrates, packages, sorts proteins  In plant cells, polysaccharides for cell walls are synthesized here  

The cis region receives vesicles (a piece of the ER that “buds” off) from the ER. At the trans region, vesicles bud off from the Golgi apparatus and are moved to the plasma membrane or other organelles.

Primary lysosomes originate from the Golgi apparatus. They contain digestive enzymes, which hydrolyze macromolecules into monomers. Food molecules enter the cell by phagocytosis—a phagosome is formed. Phagosomes fuse with primary lysosomes to form secondary lysosomes. Enzymes in the secondary lysosome hydrolyze the food molecules. Lysosomes also digest cell materials (autophagy). Cell components are frequently destroyed and replaced by new ones. In the mitochondria, energy in fuel molecules is transformed to the bonds of energy-rich ATP (cellular respiration). Cells that require a lot of energy have many mitochondria. Mitochondria have two membranes. The inner membrane folds inward to form cristae. This creates a large surface area for the proteins involved in cellular respiration reactions. The mitochondrial matrix contains enzymes, DNA, and ribosomes. Plastids occur only in plants and some protists. Peroxisomes: collect and break down toxic byproducts of metabolism such as H2O2, using specialized enzymes. Glyoxysomes: only in plants—lipids are converted to carbohydrates for growth.

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The cytoskeleton:  Supports and maintains cell shape  Holds organelles in position  Moves organelles  Involved in cytoplasmic streaming  Interacts with extracellular structures to hold cell in place The cytoskeleton has three components:  Microfilaments  Intermediate filaments  Microtubules Microfilaments:  Help a cell or parts of a cell to move  Determine cell shape  Made from the protein actin  Actin has + and – ends and polymerizes to form long helical chains (reversible) In muscle cells, actin filaments are associated with the “motor protein” myosin; interactions between the two result in muscle contraction. Microfilaments are also involved in the formation of pseudopodia (pseudo, “false”; podia, “feet”). In some cells, microfilaments form a meshwork just inside the plasma membrane. This provides structure, for example in the microvilli that line the human intestine. Intermediate filaments:  50 different kinds in six molecular classes  Tough, ropelike protein assemblages  Anchor cell structures in place  Resist tension Microtubules:  Long, hollow cylinders  Form rigid internal skeleton in some cells  Act as a framework for motor proteins  Made from the protein tubulin—a dimer  Have + and – ends  Can change length rapidly by adding or losing dimers Cilia and eukaryotic flagella are made of microtubules in “9 + 2” array. Cilia—short, usually many present, move with stiff power stroke and flexible recovery stroke. Flagella—longer, usually one or two present, movement is snakelike.

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The motion of cilia and flagella results from the sliding of the microtubule doublets past one another. Dynein binds to microtubule doublets and allows them to slide past each other. Nexin can cross-link the doublets and prevent them from sliding, and the cilium bends. The motor protein kinesin moves vesicles or organelles from one part of a cell to another. It binds to a vesicle and “walks” it along by changing shape. Experiments to determine the function of cellular components fall into two categories:  Inhibition: A drug that inhibits a structure or process—does the function still occur?  Mutation: Examine a cell that lacks the gene for the structure or process.

5.4 What Are the Roles of Extracellular Structures? Extracellular structures are secreted to the outside of the plasma membrane. Example: The peptidoglycan cell wall of bacteria. In eukaryotes, extracellular structures have a prominent fibrous macromolecule in a gellike medium. Many animal cells are surrounded by an extracellular matrix, composed of fibrous proteins such as collagen, gel-like proteoglycans (glycoproteins), and other proteins. The extracellular matrix:  Holds cells together in tissues  Contributes to properties of bone, cartilage, skin, etc.  Filters materials passing between different tissues  Orients cell movements in development and tissue repair  Plays a role in chemical signaling

5.5 How Did Eukaryotic Cells Originate? Eukaryotic cells first appeared about 1.5 billion years ago. The advent of compartmentalization was a major event in the history of life. How did compartmentalization arise? The endomembrane system and cell nucleus may have originated from the inward folds of plasma membrane of prokaryotes. Enclosed compartments would have allowed chemicals to be concentrated and chemical reactions to proceed more efficiently. Some organelles may have arisen by symbiosis (“living together”). The endosymbiosis theory proposes that mitochondria and plastids arose when one cell engulfed another cell.

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Support for the endosymbiosis theory:  The discovery of a single-celled eukaryote, Hatena, that ingests a green alga, Nephroselmis.  The green alga loses most of its structures and acts as a chloroplast.

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