CELLS 4
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HOW ARE PUT TOGETHER
Chapter Outline
IMPACTS, ISSUES: WHERE DID CELLS COME FROM? SO WHAT IS "A CELL?" Components of All Cells Why Aren't Cells Bigger? HOW DO WE "SEE" CELLS? The Cell Theory Some Modern Microscopes ALL LIVING CELLS HAVE MEMBRANES INTRODUCING PROKARYOTIC CELLS INTRODUCING EUKARYOTIC CELLS THE NUCLEUS THE ENDOMEMBRANE SYSTEM Endoplasmic Reticulum Golgi Bodies Membranous Sacs with Diverse Functions MITOCHONDRIA AND CHLOROPLASTS Mitochondria Chloroplasts VISUAL SUMMARY OF EUKARYOTIC CELL COMPONENTS THE CYTOSKELETON Moving Along with Motor Proteins Cilia, Flagella, and False Feet CELL SURFACE SPECIALIZATIONS Eukaryotic Cell Walls Matrixes Between Animal Cells Cell Junctions SUMMARY SELF-QUIZ CRITICAL THINKING
Objectives
1. 2. 3. 4. 5. Understand the basic tenets of the cell theory. Understand the essential structure and function of the cell membrane. Contrast the general features of prokaryotic and eukaryotic cells. Describe the nucleus of eukaryotes with respect to structure and function. Describe the organelles associated with the endomembrane system, and tell the general function of each. 6. Contrast the structure and function of mitochondria and chloroplasts. 7. Describe the cytoskeleton of eukaryotes and distinguish it from the endomembrane system. 8. List several surface structures of cells and tell how they help cells survive.
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Key Terms
cell plasma membrane nucleus nucleoid cytoplasm ribosomes prokaryotic cells eukaryotic cells surface-to-volume ratio cell theory wavelength compound light microscope transmission electron microscope scanning electron microscope phospholipid cell membrane lipid bilayer fluid mosaic model prokaryotes eubacteria archeae bacterial flagella pili organelles secretory pathway endocytic pathway enzymes nucleus chromosomes chromatin nuclear envelope nucleoplasm nucleolus endomembrane system endoplasmic reticulum rough ER ribosomes smooth ER Golgi bodies vesicles lysosome peroxisomes central vacuole ATP mitochondria chloroplasts stroma thylakoids cytoskeleton microtubules microfilaments cell cortex motor proteins intermediate filaments lamins actin myosin kinesins dyneins flagellum, flagella cilium, cilia centriole basal body pseudopods cell wall cellulose primary wall secondary wall lignin cell junctions plasmodesma tight junctions adhering junctions gap junctions
Lecture Outline
Impacts, Issues: Where Did Cells Come From? A. Cells appeared on the earth between 3.9 and 2.5 billion years ago. 1. The first cells were prokaryotes and did not use oxygen. 2. Later, eukaryotic cells evolved that used oxygen in energy pathways. B. Eukaryotes developed a wide variety of organelle compartments leading to greater cell specialization. 4.1 What Is "A Cell"? A. Components of All Cells 1. The cell is the smallest unit with the properties of life: metabolism, response to environment, growth, and reproduction. 2. All cells have three features in common: a. 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. b. A nucleus or nucleoid region localizes the hereditary material, which can be copied and read. c. The cytoplasm contains membrane systems, particles (including ribosomes), filaments (the cytoskeleton), and a semifluid substance. 3. There are basically two kinds of cells in nature. a. Prokaryotic cells (bacteria) have no nucleus. b. Eukaryotic cells contain distinctive arrays of organelles, including a membranebound nucleus. B. Why Aren't Cells Bigger? 1. Cell size is constrained by the surface-to-volume ratio. a. If a cell expands in diameter during growth, its volume will increase more rapidly than its surface area will. b. A cell that is too large will not be able to move materials into and out of the cell interior.
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2. The smaller the cell, the more efficiently materials cross its surface and become distributed through the interior. 4.2 How Do We "See" Cells? A. The Cell Theory 1. Observations lead to the idea of cells. a. Galileo saw details of insect eyes with two crude lenses in a tube. b. Robert Hooke used simple lenses to observe cork in which he saw tiny compartments he called cellulae. c. van Leeuwenhoek was able to see protistans, sperm, and even bacteria. d. Schleiden and Schwann believed that animals as well as plants consist of cells. e. Virchow said all cells come from preexisting cells. 2. The cell theory has three generalizations: a. All organisms are composed of one or more cells; b. The cell is the smallest unit having the properties of life; and c. The continuity of life arises directly from the growth and division of single cells. B. Some Modern Microscopes 1. Most cells are too small to be seen without a microscope. 2. Light microscopes are useful to enlarge cells up to 2,000 times. 3. Transmission and scanning electron microscopes can achieve magnifications many thousands of times greater to see the fine detail in cell structure. All Living Cells Have Membranes A. Cell membranes form a continuous boundary layer around the cell. 1. Phospholipids are the most abundant components of the cell membrane. a. Each molecule has a phosphate-containing head and two fatty acids attached to a glycerol backbone. b. The head is hydrophilic and the tails are hydrophobic, causing phospholipids to spontaneously form a lipid bilayer when exposed to an aquatic environment. 2. The fluid mosaic model describes a cell membrane of mixed composition. a. The mosaic consists of phospholipids, glycolipids, sterols, and proteins. b. The fluid aspects of the model refer to the motions and interactions of its components, which allow the membrane to remain very flexible. B. Proteins interspersed in the lipid bilayer serve many functions. 1. Some are receptors for chemical signals. 2. Others transport solutes across the bilayer. 3. Still others participate in cell identification, defense, and communication. Introducing Prokaryotic Cells A. Prokaryotes are the smallest known cells. 1. The term prokaryotic ("before the nucleus") indicates the existence of bacteria before evolution of cells with a nucleus. a. A somewhat rigid cell wall supports the cell and surrounds the plasma membrane, regulating transport into and out of the cell. b. Sticky polysaccharides help cells attach to surfaces, such as teeth. c. Ribosomes, protein assembly sites, are dispersed throughout the cytoplasm. d. The bacterial chromosome is a single, circular DNA molecule; other smaller circles of DNA called plasmids are present in some bacteria. 2. Bacterial flagella project from the membrane and permit rapid movement; pili filaments aid in attachment to surfaces. B. Two major groups of prokaryotes exist: Archaea and Eubacteria.
4.3
4.4
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4.5
Introducing Eukaryotic Cells A. Eukaryotic cells are larger and generally more complex. 1. They usually have an internal framework of proteins called the cytoskeleton. 2. Organelles form compartmentalized portions within the cytoplasm allowing reactions to be separated with respect to time (allowing proper sequencing) and space (allowing incompatible reactions to occur in close proximity). B. Some substances move through the cytoplasm by a series organelles. of 1. In the secretory pathway, new polypeptides move from the ribosomes through the ER, on to the Golgi bodies, and eventually to the plasma membrane for export. 2. In the endocytic pathway, substances are moved into the cytoplasm enclosed in vesicles pinched off from the plasma membrane. The Nucleus A. Eukaryotic cells possess numerous linear DNA molecules. 1. A chromosome is an individual DNA molecule and its associated proteins (histones). 2. Chromatin refers to the cell's total collection of DNA and associated proteins. B. The nucleus isolates DNA, which contains the code for protein assembly, from the sites (ribosomes in cytoplasm) where proteins will be assembled. 1. Localization of the DNA makes it easier to sort out hereditary instructions when the time comes for a cell to divide. 2. The membranous boundary of the nucleus helps control the exchange of signals and substances between the nucleus and the cytoplasm. 3. DNA is duplicated and becomes more condensed before cell division occurs. B. A nuclear envelope (membrane) encloses the semifluid interior of the nucleus. 1. It consists of two lipid bilayers with pores. 2. Located within the nucleus, the nucleolus appears as a darker globular mass where subunits of ribosomes are prefabricated before shipment out of the nucleus. The Endomembrane System A. New polypeptide chains become folded into proteins. 1. Some of the polypeptide chains assembled on the ribosomes are stockpiled in the cytoplasm. 2. Others pass through the endomembrane system, where they take on their final form and become packaged in vesicles for use within the cell or for export. B. Endoplasmic Reticulum 1. The endoplasmic reticulum is a collection of interconnected tubes and flattened sacs that begin at the nucleus and ramble through the cytoplasm. 2. There are two types distinguished by the presence or absence of ribosomes. 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; it is the area from which vesicles carrying proteins and lipids are budded; and it also inactivates harmful chemicals. C. Golgi Bodies 1. In the Golgi bodies, proteins and lipids undergo final processing, sorting, and packaging. 2. The membranes of the Golgi are arranged in stacks of flattened sacs whose edges break away as vesicles. D. Membranous Sacs with Diverse Functions 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 vesicles containing enzymes that break down fatty acids and amino acids; the hydrogen peroxide released is degraded by another enzyme. 3. The central vacuole of mature plant cells accumulates a watery solution of ions, amino acids, sugars, and toxic substances.
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4.6
4.7
4.8
Mitochondria and Chloroplasts A. Mitochondria 1. These are the primary organelles for transferring the energy in carbohydrates to ATP under oxygen-plentiful conditions. 2. Each mitochondrion has two membranes, an inner folded membrane (cristae) surrounded by a smooth outer membrane. 3. Inner and outer compartments formed by the membranes are important in energy transformations. B. Chloroplasts 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 (thylakoids), pigments and enzymes trap sunlight energy to form ATP and NADPH. b. Sugars are formed in the fluid substance (stroma) surrounding the stacks. c. Pigments such as chlorophyll (green) confer distinctive colors to the chloroplasts. 2. Mitochondria and chloroplasts have their own DNA and some ribosomes, which points to the possibility that they were once independent entities. Visual Summary of Eukaryotic Cell Components [Diagrams of typical organelles of plant and animal cells]
4.9
4.10 The Cytoskeleton A. The cytoskeleton is an interconnected system of fibers, threads, and lattices that extends between the nucleus and the plasma membrane. 1. It gives cells their internal organization, overall shape, and capacity to move. 2. 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. 3. The main components are microtubules, microfilaments, and intermediate filaments-- all assembled from protein subunits. a. Microtubules are long, hollow cylinders of tubulin; they are responsible for the movement of chromosomes during cell division. b. Microfilaments consist of two coiled-up polypeptide chains of actin; they participate in muscle contraction and pinching animal cells in two after mitosis. c. Intermediate filaments, the most stable of the cytoskeleton elements, occur only in animal cells of specific tissues. B. Moving Along with Motor Proteins 1. Microtubules and microfilaments function as tracks while kinesins, dyneins, and myosins are the freight engines. 2. Myosins and actins interact to slide the filaments past each other to allow muscles to contract. C. Cilia, Flagella, and False Feet 1. Microtubular extensions of the plasma membrane have a 9 + 2 cross-sectional array that arises from a centriole and are useful in propulsion. a. Flagella are quite long, not usually numerous, and are found on one-celled protistans and animal sperm cells. b. 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. 2. Pseudopods are temporary lobes that project from the cell, used in locomotion and food capture. 4.11 Cell Surface Specializations A. 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.
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a. In growing plant parts, bundles of cellulose strands form a primary cell wall that is pliable enough to allow enlargement under pressure. b. Later, more layers are deposited on the inside of the primary wall to form the secondary wall. 2. 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. B. Matrixes between Animal Cells 1. The matrix between animal cells includes cell secretions and materials drawn from the surroundings between cells. 2. For example, cartilage consists of scattered cells and collagen embedded in a "ground substance" of modified polysaccharides; bone is similarly constructed. C. 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.
How Cells Are Put Together
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