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Essentials of the Living World Second Edition George B. Johnson Jonathan B. Losos Chapter 5 Cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5.1 Cells • All cells are small most cells can only be observed under a microscope • Robert Hooke first described cells in 1665 • Cells are fundamentally important • Matthias Schleiden in 1838 recognized that cells were fundamental to plant composition • Theodor Schwann in 1839 reported that all animal tissues also are comprised of cells 5.1 Cells Figure 5.1 • Cells are not all the same size 5.1 Cells • Cell theory states the importance of cells to life 1. All organisms are composed of one or more cells 2. Cell are the smallest living things 3. Cells arise only by division of previously existing cells 5.1 Cells • Most cells are small because larger cells do not function as efficiently larger cells are more difficult to control because of the distances involved from the command center at the core to the peripheral regions organisms that are comprised of many, small cells are at an efficiency advantage over organisms comprised of few, larger cells 5.1 Cells • Cell size is also limited by the surface-tovolume ratio as cell size increases, the volume grows more rapidly than surface area • a cell’s surface provides the interior’s only opportunity to interact with the environment Figure 5.2 Surface-to-volume ratio Large cells have far less surface for each unit of volume than do small cells. 5.1 Cells • Some cells can be large because they have special modifications to increase surface area some cells, such as neurons, are very long and thin other cells, such as the ones lining the digestive tract, have microvilli—finger-like projections that make more surface area • Otherwise, in order for organisms to become larger, they must have many cells 5.1 Cells • Cellular structure is organized plasma membrane forms the boundary of the cell • also controls the permeability of the cell to water and dissolved substances cytoplasm fills the interior of the cell 5.1 Cells • Most cells are too small to be viewed by the naked eye, which has limited resolution • resolution refers to the minimum distance that two points can be apart and still be distinguished as two separated points • the limit of resolution of the human eye is about 100 micrometers • One way to increase resolution is to increase magnification, such as by using a microscope 5.1 Cells • Different types of microscopes are used to increase magnification for viewing compound light microscope uses sets of magnifying glass lenses to resolve structures that are separated by more than 200 nanometers electron microscopes have 1000 times the resolving power of light microscopes and can resolve objects as close as 0.2 nanometers apart Figure 5.3 A scale of visibility 5.1 Cells • Stains are used to analyze otherwise transparent or hidden cellular components histology is the analysis of tissues using microscopy • specific stains can be applied that bind to target molecules for studying structure and function Table 5.1 Types of Microscopes 5.2 The Plasma Membrane • The plasma membrane is conceptualized by the fluid mosaic model a sheet of lipids with embedded proteins • the lipid layer forms the foundation of the membrane • the fat molecules comprising the lipid layers are called phospholipids 5.2 The Plasma Membrane • A phospholipid has a polar head and two nonpolar tails • The polar region is comprised of a phosphate chemical group and is water-soluble • The non-polar region is comprised of fatty acids and is water-insoluble Figure 5.4(a) Phospholipid structure 5.2 The Plasma Membrane • A lipid bilayer forms spontaneously whenever a collection of phospholipids is placed in water Figure 5.5 The lipid bilayer. 5.2 The Plasma Membrane • The interior of the lipid bilayer is completely nonpolar no water-soluble molecules can cross through it cholesterol is also found in the interior • it affects the fluid nature of the membrane • its accumulation in the walls of blood vessels can cause plaques • plaques lead to cardiovascular disease 5.2 The Plasma Membrane • Another major component of the membrane is a collection of membrane proteins some proteins form channels that span the membrane • these are called transmembrane proteins other proteins are integrated into the structure of the membrane • for example, cell surface proteins are attached to the outer surface of the membrane and act as markers Figure 5.6 Proteins are embedded within the lipid bilayer 5.3 Prokaryotic Cells • There are two major types of cells prokaryotic • lacks a nucleus and does not have an extensive system of internal membranes • all bacteria and archae have this cell type eukaryotic • has a nucleus and has internal membrane-bound compartments • all organisms other than bacteria or archae have this cell type 5.3 Prokaryotic Cells • Prokaryotes are the simplest cellular organisms have a plasma membrane surrounding a cytoplasm without interior compartments • some bacteria have additional outer layers to the plasma membrane – cell wall comprised of carbohydrates to confer rigid structure – capsule may surround the cell wall 5.3 Prokaryotic Cells • The interior of the prokaryotic cell shows simple organization cytoplasm is uniform with little or no internal support framework ribosomes (sites for protein synthesis) are scattered throughout the cytoplasm nucleoid region (an area of the cell where DNA is localized) • not membrane-bound, so not a true nucleus 5.3 Prokaryotic Cells • Other structures sometimes found in prokaryotes relate to locomotion, feeding, or genetic exchange flagellum (plural, flagellae) is a collection of protein fibers that extends from the cell surface • may be one or many • aids in locomotion and feeding pilus (plural, pili) is a short flagellum • aids in attaching to substrates and in exchanging genetic information between cells Figure 5.9 Organization of a prokaryotic cell 5.4 Eukaryotic Cells • Eukaryotic cells are larger and more complex than prokaryotic cells have a plasma membrane encasing a cytoplasm • internal membranes form compartments called organelles • the cytoplasm is semi-fluid and contains an network of protein fibers that form a scaffold called a cytoskeleton 5.4 Eukaryotic Cells • many organelles are immediately conspicuous under the microscope nucleus • a membrane-bound compartment for DNA that gives eukaryotes (literally, “true-nut”) their name endomembrane system • gives rise to the internal membranes found in the cell • each compartment can provide specific conditions favoring a particular process 5.4 Eukaryotic Cells • not all eukaryotic cells are alike the cells of plants, fungi, and many protists have a cell wall beyond the plasma membrane all plants and many protists contain organelles called chloroplasts plants contain a central vacuole only animal cells contain centrioles Animal versus Plant Cell Figure 5.10 Structure of animal cell Figure 5.11 Structure of plant cell 5.5 The Nucleus: The Cell’s Control Center • The nucleus is the command and control center of the cell it also stores hereditary information • The nuclear surface is bounded by a doublemembrane called the nuclear envelope groups of proteins form openings called nuclear pores that permit proteins and RNA to pass in and out of the nucleus 5.5 The Nucleus: The Cell’s Control Center • The DNA of eukaryotes is packaged into segments and associated with a protein this complex is called a chromosome • the proteins enable the DNA to be wound tightly so it appears condensed – the condensed or chromosome form of DNA occurs during cell division • the DNA is uncoiled into strands called chromatin that are no longer visible as segments – protein synthesis occurs when the DNA is in the chromatin form 5.5 The Nucleus: The Cell’s Control Center • The nucleus is the site for the subunits of the ribosome to be synthesized the nucleolus is a dark-staining region of the nucleus • it contains the genes that code for the rRNA (ribosomal RNA) that makes up the ribosomal subunits • the subunits leave the nucleus via the nuclear pores and the final ribosome is assembled in the cytoplasm Figure 5.12 The nucleus 5.6 The Endomembrane System • The endoplasmic reticulum (ER) is an extensive system of internal membranes some of the membranes form channels and interconnections other portions become isolated spaces enclosed by membranes • these spaces are known as vesicles 5.6 The Endomembrane System • The segment of the ER dedicated to protein synthesis is called the rough ER the surface of this region looks pebbly the rough spots are due to embedded ribosomes • The segment of the ER that aids in the manufacture of carbohydrates and lipids is called the smooth ER the surface of this region looks smooth because it contains no embedded ribosomes Figure 5.13 The endoplasmic reticulum (ER) 5.6 The Endomembrane System • After synthesis in the ER, the newly-made molecules are passed to the Golgi bodies Golgi bodies are flattened membranes that that form collective stacks called the Golgi complex their numbers vary depending on the cell their function is to collect, package, and distribute molecules manufactured in the cell Figure 5.14 Golgi complex 5.6 The Endomembrane System • The ER and Golgi complex function together as a transport system in the cell Figure 5.15 How the endomembrane system works 5.6 The Endomembrane System • The Golgi complex also gives rise to lysosomes these membrane-bound structures contain enzymes that the cell uses to break down macromolecules • worn-out cell parts are broken down and their components recycled to form new parts • particles that the cell has ingested are also digested 5.6 The Endomembrane System • Peroxisomes are different components of the endomembrane system that are also found in the cell the reactions that are confined to these organelles function to 1. detoxify harmful byproducts of metabolism 2. convert fats to carbohydrates in plants seeds for growth 5.7 Organelles That Contain DNA • Eukaryotic cells contain cell-like organelles that, besides the nucleus, also contain DNA these organelles appear to have been derived from ancient bacteria that were then assimilated by the eukaryotic cell they include the following organelles: mitochondria and chloroplasts 5.7 Organelles That Contain DNA • Mitochondria are cellular powerhouses • Sites for chemical reactions called oxidative metabolism • The organelle is surrounded by two membranes Figure 5.16(a) Mitochondria 5.7 Organelles That Contain DNA • Chloroplasts are the location for photosynthesis • The organelle is also surrounded by two membranes Figure 5.17 A chloroplast 5.7 Organelles That Contain DNA • Both mitochondria and chloroplasts possess circular DNA that is not found elsewhere in the cell • They cannot be grown free of the cell they are totally dependent on the cells within which they occur 5.7 Organelles That Contain DNA • The theory of endosymbiosis states that some organelles evolved from a symbiosis in which one cell of a prokaryotic species was engulfed by and lived inside of a cell of another species of prokaryote the engulfed species provided their hosts with advantages because of special metabolic activities the modern organelles of mitochondria and chloroplasts are believed to be found in the eukaryotic descendants of these endosymbiotic prokaryotes Figure 5.18 Endosymbiosis 5.7 Organelles That Contain DNA • In addition to the double membranes and circular DNA found in mitochondria and chloroplasts, there is a lot of other evidence supporting endosymbiotic theory mitochondria are about the same size as modern bacteria the cristae in mitochondria resemble folded membranes in modern bacteria mitochondrial ribosomes are similar to modern, bacterial ribosomes in size and structure mitochondria divide by fission, just like modern bacteria 5.8 The Cytoskeleton: Interior Framework of the Cell • The cytoskeleton is comprised of an internal framework of protein fibers that anchor organelles to fixed locations support the shape of the cell helps organize ribosomes and enzymes needed for synthesis activities • The cytoskeleton is dynamic and its components are continually being rearranged 5.8 The Cytoskeleton: Interior Framework of the Cell • Three different types of protein fibers comprise the cytoskeleton intermediate filaments • thick ropes of intertwined protein microtubules • hollow tubes made up of the protein tubulin microfilaments • long, slender microfilaments made up of the protein actin Figure 5.19 The three protein fibers of the cytoskeleton 5.8 The Cytoskeleton: Interior Framework of the Cell • Centrioles are complex structures that assemble microtubules in animal cells and the cells of most protists they anchor locomotory structures, such as flagella or cilia they assemble microtubules near the nuclear envelope they might also have an endosymbiotic origin 5.8 The Cytoskeleton: Interior Framework of the Cell • Cellular motion is associated with the movement of actin microfilaments and/or microtubules some cells “crawl” by coordinating the rearrangement of actin microfilaments some cells swim by coordinating the beating of microtubules grouped together to form flagella or cilia Figure 5.21 Flagella and cilia 5.8 The Cytoskeleton: Interior Framework of the Cell • Microtubules provide a means to transport material inside the cell efficiently over long distances Figure 5.22 Molecular motors 5.8 The Cytoskeleton: Interior Framework of the Cell • The cytoskeleton also anchors storage compartments vacuoles are membrane-bound storage centers • central vacuole appears to be a large empty space inside a plant cell but is actually filled with water and dissolved substances • contractile vacuole is found near the cell surface of some protists and accumulates excess water from inside the cell that it then pumps out 5.9 Outside the Plasma Membrane Cell walls • found in plants, fungi, and many protists • comprised of different components than prokaryotic cell walls • function in providing protection, maintaining cell shape, and preventing excessive water loss/uptake Figure 2.24 Cell walls in plants 5.9 Outside the Plasma Membrane Extracellular matrix (ECM) • takes the place of the cell wall in animal cells and is comprised by a mixture of proteins secreted by the cell • collagen and elastin proteins form a protective layer over the cell surface • fibronectin protein connects the ECM to the plasma membrane • the fibronectin molecules also connect to integrins, proteins that extend into the cytoplasm of the cell – this extracellular-intracellular connection allows the ECM to influence cellular behavior and to coordinate groups of cells functioning as tissues Figure 5.25 The extracellular matrix 5.10 Diffusion and Osmosis • Movement of water and nutrients into a cell or elimination of wastes out of cell is is essential for survival • This movement occurs across a biological membrane in one of three ways • diffusion • membrane folding • protein transport 5.10 Diffusion and Osmosis • Molecules move in a random fashion but there is a tendency to produce uniform mixtures • The net movement of molecules from an area of higher concentration to an area of lower concentration is termed diffusion • Molecules diffuse down a concentration gradient from higher to lower concentrations diffusion ends when equilibrium is reached Figure 5.26 How diffusion works 5.10 Diffusion and Osmosis • Only certain substances undergo diffusion across the plasma membrane molecules like oxygen, carbon dioxide, and nonpolar lipids ions and polar molecules cannot cross the interior of the membrane • Water, although polar, is able to diffuse freely across the plasma membrane aquaporins are selective channels that permit water to cross 5.10 Diffusion and Osmosis • Water moves down its concentration gradient in moving into or out of a cell through a process called osmosis the movement of water is dependent on the concentration of other substances in a solution the greater the amount of solutes that are dissolved in a solution, then the lesser the amount of water molecules that are free to move 5.10 Diffusion and Osmosis • The concentration of all molecules dissolved in a solution is called the osmotic concentration of the solution • Osmotic concentrations of different solutions can be compared relative to each other 5.10 Diffusion and Osmosis • Consider two solutions with unequal osmotic concentrations • the solution with the higher concentration is called hypertonic • The solution with the lower concentration is called hypotonic Figure 5.27(2) 5.10 Diffusion and Osmosis • Consider two solutions with equal osmotic concentrations • The solutions are each called isotonic Figure 5.27(1) 5.10 Diffusion and Osmosis • Movement of water by osmosis into a cell causes pressure called osmotic pressure enough pressure may cause a cell to swell and burst osmotic pressure explains why so many cell types are reinforced by cell walls 5.11 Bulk Passage into and out of Cells • Bulky substances are contained within vesicles as they are moved into and out of a cell endocytosis is the engulfing of substances outside of the cell in order to form a vesicle that is brought inside the cell exocytosis is the discharge of substances from vesicles at the inner surface of the cell Forms of Endosymbiosis • Phagocytosis is endocytosis of particulate (solid) matter Figure 5.28(a) endocytosis • Pinocytosis is endocytosis of liquid matter Figure 5.28(b) endocytosis Figure 5.29(a) Exocytosis 5.11 Passage into and out of Cells • Transport of specific molecules into the cell involves receptor-mediated endocytosis molecules to be transported must first bind to specific receptors in the plasma membrane a portion of the receptor extends into the membrane in an indented pit coated with the protein clathrin when a molecule binds to its specific receptor, the cell reacts immediately by initiating endocytosis of a now clathrin-coated vesicle Figure 5.30 Receptor-mediated endocytosis 5.12 Selective Permeability • Selective permeability allows cells to control specifically what enters and leaves involves using proteins in the membrane for transporting substances across transport can be down a concentration gradient (i.e., diffusion) or against a concentration gradient (i.e., active transport) 5.12 Selective Permeability Selective diffusion • proteins act as open channels for whatever is small enough to fit inside the channel • this form of diffusion is common in ion transport Facilitated diffusion • proteins act as carriers that can bind only to specific molecules to transport • transport is limited by the availability of carriers • if there are not enough carriers, then the transport is saturated 5.12 Selective Permeability • Active transport utilizes protein channels that open only when energy is supplied energy is used to pump substances against or up their concentration gradients allows cells to maintain high or low concentration of certain molecules • recall that diffusion always ends in equilibrium • There are two kinds of channels that perform active transport in cells sodium-potassium pump proton pump 5.12 Selective Permeability • Sodium-potassium (Na+-K+) pump uses energy, in the form of ATP, to pump three Na+ out of the cell and to pump two K+ into the cell nearly 1/3 of the energy expended by the body’s cells is given over to driving these pumps Figure 5.32 How the sodiumpotassium pump works 5.12 Selective Permeability • The result of the Na+-K+ pump is to generate a concentration gradient with more Na+ outside of the cell than inside • Cells exploit this gradient in key ways for the conduction of signals along nerve cells for the transportation of important molecules into the cell against their concentration gradient 5.12 Selective Permeability • the cell membrane has many facilitated diffusion channels for Na+ but it is only transported if partnered with another substance this is called coupled transport • The concentration gradient favoring the entry of Na+ into the cell is so strong that a coupled substance will be transported even if it is against the concentration gradient coupled transport is a common way for cells to accumulate sugars and amino acids Figure 5.33 A coupled channel 5.12 Selective Permeability • Proton pump expends energy to pump protons across membranes the result is a concentration gradient favoring the re-entry of protons back into the cell the only way that protons can cross back into the cell is through channels that generate ATP • this process, known as chemiosmosis, is essential to energy metabolism Figure 5.34 How the proton pump works Inquiry & Analysis • Why does the rate of glucose uptake by red blood cells in the experiment never exceed 500 mM/ml/hr? • At what glucose concentration are half of the transport channels occupied? Figure p. 106 graph