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BIOLOGY 2B03 (244)
Kim Dej (39)


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McMaster University
Kim Dej

Introduction to Cell Biology What is a cell?  Smallest living unit  Can grow, reproduce, process information, respond to stimuli, communicate, carry out chemical reactions  Contain hereditary material  Surrounded by plasma membranes Prokaryote Both Eukaryote  Single cell or  Selectively-  Cell containing organism permeable membrane-  Lack membrane- plasma bound nucleus bound nucleus and membrane and organelles subcellular  Cytoplasm that have specific compartments  Contain DNA and  Smaller, simpler, ribosomes functions more abundant  Identical essential  Larger, more  Nucleoid is single metabolic complex, less circular DNA reactions abundant molecule  Cell activities are  Cell reproduces by cell fission compartmentaliz  Organisms may be ed unicellular or exist in colonies e.g. red blood cell, neuron e.g. bacteria, cyanobacteria Cells to tissues Simple—single cells embedded in extracellular matrix e.g. alga Complex—arrangements of cells in sheets, chains, balls; need direct connections e.g. intestinal epithelial sheets Molecule Building Block Function DNA Nucleotides Hereditary material RNA Nucleotides Protein synthesis Protein Amino acids Cell structure and function Lipids Fatty acids Membrane structure Carbohydrates Sugars Energy production Proteins Functions - metabolic activities - act as enzymes - transportation - structural proteins - cell signaling - transmembrane proteins (receptor, channel) - associate with DNA (histones), enhancers - organelle identity and function Structure - determines function - specific and highly diverse Building blocks = amino acids - monomers; attached in linear array to form primary structure = polypeptide - four components attached to central alpha carbon - 20 different kinds with same general structure - side chains (R) affect protein structure and function - differ in size, shape, charge, hydrophobicity, reactivity - solubility—bond with water through H bonding Hydrophilic: charge-polarized e.g. molecules with –OH/O or – + NH 2NH 3 @ pH 7 Hydrophobic: not electrically polarized; repelled by water; tend to be on inside; often aggregate e.g. oils, fats, saturated hydrocarbons How to make a protein - form peptide bond by covalently bonding amino acids together into peptides - occurs in ribosomes - two subunits; large and small - mRNA in, polypeptide out Protein structure 1) Primary - linear sequence of AAs is determine by nucleotide sequence of encoding genes - number of different polypeptide sequences = 20 where n = # of AAs 2) Secondary - conformation of a part of polypeptide, which spontaneously assumes random coil structure - note that protein folding is a RANDOM process; starts as soon as it exits ribosome; end result is an STABLE average - interactions between side chains are noncovalent; weak attractive forces; stabilize periodically stable structures i) ionic—attraction between positive cation and negative anion ii) hydrogen—interaction between positive H and unpaired electrons iii) Van der Waals/London Dispersion forces—result of transient dipole between two noncovalently bonded atoms iv) hydrophobic effects—aggregation of nonpolar molecules in aqueous medium Three types of arrangements: 1) alpha-helix  Rigid cylinder similar to DNA helix  carbonyl oxygen hydrogen bonded to amide hydrogen four residues away (C-term. direction)  forms independently of specific R groups; they only work to determine hydrophobicity of outer surface  pattern is 3.6 residues/turn 2) beta-sheet  laterally packed strands 5-8 AA long  formed by H bonds between carbonyl and amino groups of backbone of adjacent strands  forms independently of R groups; they work to determine hydrophobicity quality and stabilize interactions  may be arranged parallel or anti-parallel 3) connectors = turns/loops/hinges  involve 3-4 AA that are responsible for bent part of polypeptide Motifs - combinations of secondary structures - display 3D shape - associated with particular function 1) Coiled-coil = leucine zippers - 2 or more aliphatic alpha-helices wrapped together; held together by hydrophobic interactions - repeating segment of 7 AAs (heptads) -
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