4 1 Cells provide compartments for biochemical reactions 1 Cell theory Chapter 4 Cells the working units of life a Cells are fundamental units of life b All living organisms are composed of cells c All cells come from preexisting cells d Conceptual implications i Studying cell biology is in some sense the same as studying life 1 Principles that underlie the functions of a single bacterial cell are similar to those governing the approx 60 trillion cells in an adult human ii Life is continuous 1 All human cells came from single cell zygote or fertilized egg 2 Small cell size is a practical necessity arising from the decrease in the surface area to volume ratio of any object as it increases in size a Cells small b c surface area must be large to accommodate changes between cell and environment b Volume of a cell determines amount of metabolic activity it carries out per unit of time c Surface area of a cell determines the amount of substances that can enter it from the outside environment and the amount of waste products that can exit to the environment 3 Microscopes a Light microscopes i Use glass lenses and visible light to form images ii Smallest detail that can be seen is 0 2 micrometers in diameter which is about 1 000 times smaller than an object the human eye can see iii Used to visualize living cells and general cell structure b Electron microscopes i Use electron beam focused by magnets ii Size limit is 2 nanometer which is 100 000 times smaller than something human eye can see iii Can be used to visualize most structures within preserved cells 4 Chemical analysis of cells usually begins with breaking them open to make a cell free extract a If conditions are right the properties of cell free extract are the same as inside the cell 5 Plasma membrane consists of phospholid bilayers with proteins a There is both compositional and functional diversity within this general framework b Roles i Acts as selectively permeable barrier preventing some substances from crossing it while permitting other substances to enter and leave the cell 1 Allows cell to maintain homeostasis stable internal environment that is distinct from surrounding environment ii Important in communicating with adjacent cells and receiving signals from the environment iii Often has proteins protruding from it that are responsible for binding and adhering to adjacent cells or to a surface 1 Plays important structural role and contributes to cell shape 6 Prokaryotes a Archaea and Bacteria nucleus b Eukaryotes i Have in common a prokaryoitc cellular organization ii Typically does not have membrane enclosed internal compartments does not have a i Eukaryotic cell organization is found in members of domain Eukarya ii Includes protists plants fungi and animals iii Contain membrane enclosed compartments called organelles where specific metabollic functions occur 1 Most notable is nucleus where most of cell s DNA located and where gene expression begins 4 2 Prokaryotic Cells do not have a Nucleus 1 Prokaryotic structure a Plasma membrane encloses cell regulates traffic in and out of cell b Nucleoid is region in cell where DNA is located c Rest of material enclosed in plasma membrane is cytoplasm two components i Cystosol consists mostly of water containing dissolved ions small molecules and soluble macromolecules ii Ribosomes complexes of RNA and proteins are sites of protein synthesis d Substances in cytoplasm are in constant motion 2 Specialized structures a Cell walls Many prokaryotes have one i Located outside plasma membrane ii Cell walls of bacteria contain peptidoglycan 1 Polymer of amino sugars cross linked by covalent bonds to peptides iii Enclosing cell wall of some bacteria is capsule 1 Not essential to bacterial life b Internal membranes i System that helps with function such as photosynthesis 1 Photosynthesis requires membranes ii Other prokaryotes have internal membrane folds that are attached to the plasma membrane 1 Bacterium with enclosed compartments would have several evolutionary advantages a Chemical Rxns more efficient b Certain biochemical activities could be segregated within compartments i Made of protein called flagellin ii Complex motor protein spins each flagellum on its axis like a propeller iii Motor protein anchored to plasma membrane and some in outer membrane of cell wall i Helical network of filamentous structures that extend down length of cell inside plasma c Flagella d Cytoskeleton membrane 4 3 Eukaryotic Cells have a Nucelus and other membrane bound compartments 1 Eukaryotes have plasma membrane ribosomes and cytoplasm just like prokaryotes 2 Contain many membrane enclosed organelles that compartmentalize biochemical rxns 3 Ribosomes a Consist of one larger and one smaller unit i Each subunit consists of 1 3 large RNA molecules called ribosomal RNA rRNA ii Also consists of multiple smaller protein molecules that are bound noncovalently to one another and to the rRNA b Role is to translate nucleotide sequence of a messenger RNA molecule into a polypeptide chain c Not membrane enclosed compartment Float freely in cytoplasm 4 Nucleus a Contains most of cell s DNA b Location of DNA and DNA replication c Where DNA is transcribed to RNA d Contains nucleolus region where ribosomes begin to assemble from RNA and proteins e Enclosed by two lipid bilayers that form nuclear envelope f Seperates DNA transcription occurs in nucleus and translation in cytoplasm g Membrane perforated by nuclear pores which connect interior of nucleus to cytoplasm h i Allows molecules to enter leave nucleus Inside nucleus DNA proteins form chromatin i Chromatin occurs in form of long thin threads called chromosomes 5 Endomembrane system Includes nuclear envelope ER Golgi apparatus and lysosomes a b Vesicles shuttle substances between various components of endomembrane system as well as plasma membrane c Endoplasmic Reticulum i ii Rough ER Interior compartment lumen is separate and distinct from surrounding cytoplasm 1 Rough because of the many ribosomes attached to outer surface of membrane 2 Ribosomes become attached when they begin synthesizing protein destined for modification within RER a Protein enter RER if it contains specific short sequence of amino acids b One inside proteins chemically modified to alter functions and chemical tag for delivery c RER participates in transporting proteins to other locations in cell transported in vesicles that pinch off from ER d Most membrane bound proteins are made on RER