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Chapter 6 Cells: The Working Units of Life PART I AP Biology Cells: The Working Units of Life • Key Concepts • Cells Provide Compartments for Biochemical Reactions • Prokaryotic Cells Do Not Have a Nucleus • Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • The Cytoskeleton Provides Strength and Movement • Extracellular Structures Provide Support and Protection For Cells and Tissues Overview: The Fundamental Units of Life • • • • All organisms are made of cells Cell: simplest collection of matter that can live Cell structure is DIRECTLY related cellular function All cells related by their descent from earlier cells Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Cell theory was the first unifying theory of biology: 1. Cells are the fundamental units of life. 2. All organisms are composed of cells. 3. All cells come from preexisting cells. Cells Provide Compartments for Biochemical Reactions • Most cells are tiny, in order to maintain a good surface area-to-volume ratio. • The volume of a cell determines its metabolic activity per unit of time. • The surface area of a cell determines the amount of substances that can enter or leave the cell. Why Cells Are Small volume increases more rapidly than its surface area! Cells must maintain a large surface area-to-volume ratio in order to function! Some large cells increase surface area by folds in the cell membrane. Concept 6.1: To study cells, biologists use microscopes and the tools of biochemistry • Cells are small but can be complex o Scientists use microscopes to see cells too small to see with the naked eye • Light microscope (LM): visible light passes through specimen, then through glass lenses that magnifies image o can magnify effectively to about 1,000 times the size of the actual specimen o Most subcellular structures, including organelles (membraneenclosed compartments), are too small to be resolved by an LM • Quality of an image depends on: o Magnification: ratio of an object’s image size to its real size o Resolution: measure of the clarity of the image the minimum distance of two distinguishable points o Contrast: visible differences in parts of the sample • Various techniques enhance contrast and enable cell components to be stained or labeled The Scale of Life Light Microscope Electron Microscope Fig. 6-3ab TECHNIQUE RESULTS (a) Brightfield (unstained specimen) 50 µm (b) Brightfield (stained specimen) Fig. 6-3cd TECHNIQUE (c) Phase-contrast (d) Differential-interferencecontrast (Nomarski) RESULTS Fig. 6-3e TECHNIQUE RESULTS (e) Fluorescence 50 µm Fig. 6-3f TECHNIQUE RESULTS (f) Confocal 50 µm Electron Microscopes • Electron microscopes (EMs) used to study subcellular structures • Types: o Scanning electron microscopes (SEMs): focus beam of electrons onto the surface of a specimen • Images look 3-D o Transmission electron microscopes (TEMs): focus beam of electrons through a specimen • Used mainly to study internal structure of cells Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Cell Fractionation/Centrifugation • Cell fractionation: takes cells apart and separates major organelles from one another o enables scientists to determine the functions of organelles • Ultracentrifuges fractionate cells into their component parts • Biochemistry and cytology help correlate cell function with structure Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Cell Fractionation/Centrifugation Cells can be separated from one another on the basis of•size & density and the isolated structures can then be analyzed chemically. The Two Types of Cells 1. Prokaryotes have no membrane-enclosed compartments. 2. Eukaryotes have membrane-enclosed compartments called organelles, such as the nucleus. Prokaryotic Cells Do Not Have a Nucleus • Prokaryotic cells: o Are enclosed by a cell membrane o Have DNA located in the nucleoid region • The rest of the cytoplasm consists of: o Cytosol (water and dissolved material) and suspended particles o Ribosomes—sites of protein synthesis Fig. 6-6 Fimbriae Nucleoid Ribosomes Plasma membrane Bacterial chromosome Cell wall Capsule 0.5 µm (a) A typical rod-shaped bacterium Flagella (b) A thin section through the bacterium Bacillus coagulans (TEM) A Prokaryotic Cell Prokaryotic Cells Do Not Have a Nucleus • Most prokaryotes have a rigid cell wall outside the cell membrane. • Bacterial cell walls contain peptidoglycans. • Some bacteria have an additional outer membrane that is very permeable. • Other bacteria have a slimy layer of polysaccharides, called the capsule. Prokaryotic Cells Do Not Have a Nucleus • Some bacteria, including cyanobacteria, have an internal membrane system that contains molecules needed for photosynthesis. Prokaryotic Cells Do Not Have a Nucleus • Some prokaryotes swim by means of flagella, made of the protein flagellin. • A motor protein anchored to the cell membrane or outer membrane spins each flagellum and drives the cell. Prokaryotic Flagella Prokaryotic Flagella Prokaryotic Cells • Cytoskeleton: Some rod-shaped bacteria have a network of helical actin-like protein structures to help maintain their shape. Concept 6.2: Eukaryotic cells have internal membranes that compartmentalize their functions • ALL organisms composed of one of two o Eukaryotic • Protists, fungi, animals, plants types of cells: • Plasma membrane o Prokaryotic • Semifluid substance called cytosol • Only organisms of Domain Bacteria and Archaea • Chromosomes (carry genes) • No nucleus • Ribosomes (make proteins) • DNA in a nucleus that is bounded by a • DNA in an unbound region called membranous nuclear envelope the nucleoid • Membrane-bound organelles • No membrane-bound • Cytoplasm in the region between the plasma organelles membrane and nucleus • Cytoplasm bound by the plasma • Eukaryotic usually much larger than membrane prokaryotic cells Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Quick reminder of classification! A FEW Plasma Membrane • Plasma membrane: selective barrier that allows sufficient passage of oxygen, nutrients, and waste to service the volume of every cell • General structure of biological membrane = double layer of phospholipids Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-7 Outside of cell (a) TEM of a plasma membrane The cell membrane allows cells to maintain homeostasis! Inside of cell 0.1 µm Carbohydrate side chain Hydrophilic region Hydrophobic region Hydrophilic region Phospholipid Proteins (b) Structure of the plasma membrane Cell Size • The logistics of carrying out cellular metabolism sets limits on the size of cells • Surface Area :Volume o As the surface area increases by a factor of n2, the volume increases by a factor of n3 • Small cells = greater surface area relative to volume = MORE diffusion/cellular processes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments • Eukaryotic cells have a nucleus, cell membrane, cytoplasm, and ribosomes, as well as membraneenclosed compartments called organelles. • Each organelle plays a specific role in the cell. Eukaryotic Cell • Has internal membranes that partition the cell into organelles • Plant and animal cells have most of the same organelles Concept 6.3: The eukaryotic cell’s genetic instructions are housed in the nucleus and carried out by the ribosomes • Nucleus: contains most of the DNA in a eukaryotic cell o contains most of the cell’s genes; usually the most conspicuous organelle • Nuclear envelope: encloses the nucleus, separating it from the cytoplasm • Nuclear membrane: double membrane; each membrane consists of a lipid bilayer • Ribosomes: use the information from the DNA to make proteins Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-9a Nuclear envelope ENDOPLASMIC RETICULUM (ER) Flagellum Rough ER NUCLEUS Nucleolus Smooth ER Chromatin Centrosome Plasma membrane CYTOSKELETON: Microfilaments Intermediate filaments Microtubules Ribosomes Microvilli Golgi apparatus Peroxisome Mitochondrion Lysosome Eukaryotic Cells Not found in plant cells Fig. 6-9b NUCLEUS Nuclear envelope Nucleolus Chromatin Rough endoplasmic reticulum Smooth endoplasmic reticulum Ribosomes Large central vacuoles are unique to plant cells! Central vacuole Golgi apparatus Microfilaments Intermediate filaments Microtubules Mitochondrion Peroxisome Chloroplast Plasma membrane Cell wall Plasmodesmata Wall of adjacent cell CYTOSKELETON Eukaryotic Cells Nucleus • The nucleus is usually the largest organelle: • Location of DNA and DNA replication • Site where DNA is transcribed to RNA • Contains the nucleolus, where assembly of ribosomes from RNA and proteins begins Nucleus • Pores regulate the entry/exit of molecules from nucleus • Nuclear lamina: maintains shape of nucleus; composed of protein • Chromatin: genetic materials formed by DNA and proteins o Can condense into chromosomes • Nucleolus: located within the nucleus; site of ribosomal RNA (rRNA) synthesis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Ribosomes: Protein Factories • Ribosomes: translate the nucleotide sequence of a mRNA molecule into a polypeptide. • They occur in both prokaryotic and eukaryotic cells • Consists of one large and one small subunit. o carry out protein synthesis in two locations: • In the cytosol (free ribosomes) • On the outside of the endoplasmic reticulum or the nuclear envelope (bound ribosomes) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-11 Cytosol Endoplasmic reticulum (ER) Free ribosomes Bound ribosomes Large subunit 0.5 µm TEM showing ER and ribosomes Small subunit Diagram of a ribosome Concept 6.4: The endomembrane system regulates protein traffic and performs metabolic functions in the cell • Components of the endomembrane system: o Nuclear envelope o Endoplasmic reticulum o Golgi apparatus o Lysosomes o Vesicles o Plasma membrane • Components are either continuous or connected via transfer by vesicles • Endoplasmic reticulum (ER): accounts for more than half of o continuous with the nuclear envelope o Two distinct regions of ER: • Smooth ER, which lacks ribosomes • Rough ER, with ribosomes studding its Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings surface The Endomembrane System vesicles shuttle substances between the various components, as well as to the cell membrane. Functions of the ER • Smooth ER o o o o Synthesizes lipids Metabolizes carbohydrates Detoxifies poison Stores calcium • Rough ER o Has bound ribosomes, which secrete glycoproteins (proteins covalently bonded to carbohydrates) o Distributes transport vesicles, proteins surrounded by membranes o Is a membrane factory for the cell Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Golgi Apparatus: Shipping and Receiving Center • Golgi apparatus: consists of flattened membranous sacs called cisternae • Functions of the Golgi apparatus: o Modifies products of the ER o Manufactures certain macromolecules o Sorts and packages materials into transport vesicles Fig. 6-16-1 Nucleus Rough ER Smooth ER Plasma membrane Fig. 6-16-2 Nucleus Rough ER Smooth ER cis Golgi trans Golgi Plasma membrane Fig. 6-16-3 Nucleus Rough ER Smooth ER cis Golgi trans Golgi Plasma membrane Lysosomes: Digestive Compartments • Lysosome: membranous sac of hydrolytic enzymes that can digest macromolecules • Lysosomal enzymes can hydrolyze: o proteins o fat o polysaccharides o nucleic acids • Phagocytosis: some cells engulf another cell by; forms food vacuole • Lysosome fuses with food vacuole digests molecules Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-14a Nucleus 1 µm contain hydrolases (digestive enzymes), and are the site where macromolecules are hydrolyzed into monomers! Lysosome Lysosome Digestive enzymes Plasma membrane Digestion Food vacuole (a) Phagocytosis Lysosomes: Digestive Compartments • Autophagy: is the programmed destruction of cell components. • Cells break down their own materials, and even entire organelles, within lysosomes. Fig. 6-14b Vesicle containing two damaged organelles 1 µm Mitochondrion fragment Peroxisome fragment Lysosome Peroxisome Vesicle (b) Autophagy Mitochondrion Digestion