NROB60 Chapter 3.doc

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Human Biology
Janelle Le Boutillier

NROB60 – Chapter 3 Introduction: - Stimulus must be translated into neural signals that travel rapidly up the long sensory nerves of the body. o In the spinal cord, the signals are transferred to interneurons - Even simple reflexes requires the nervous system to collect, distribute and integrate information - Electrical charge in the cytosol of the axon is carried by electrically charged atoms (ions) instead of free electrons o The axon is not well insulated and is bathed in salty extracellular fluid which conducts electricity o The axonal membrane however has properties that enable it to conduct a special type of signal  The nerve impulse or action potential – overcomes the biological constraints • They do not diminish over distance; they are signals of fixed size and duration - Cells capable of generating and conducting action potentials, include both nerve and muscle cells o They are said to have excitable membrane o The action in action potentials occurs at the cell membrane - In the resting neuron, the cytosol along the inside surface of the membrane has a negative electrical charge compared to the outside o This difference in electrical charge across the membrane is called the resting membrane potential The Cast of Chemicals: - 3 main elements: o Salty fluids on either side of the membrane o The membrane o The proteins that span the membrane Cytosol and Extracellular Fluid: - Water is the main ingredient of the fluid inside the neuron o Electrically charged atoms (ions) are dissolved in this water and are responsible for the resting and action potentials Water: - Uneven distribution of electrical charge - The oxygen atom has a greater affinity for electrons than does the hydrogen atom o Oxygen atom acquires a net negative charge (due to extra electrons) o Hydrogen atoms acquire a net positive charge - H2O is a polar molecule, held together by polar covalent bonds o Makes water an effective solvent of other charged or polar molecules Ions: - Atoms or molecules that have a net electrical charge are known as ions. - Ionic bond – when ions are held together by the electrical attraction of oppositely charged atoms - Each positively charged ion will be covered by water molecules oriented so that the oxygen atom will be facing the ion o Negatively charged ion will face the hydrogen atoms  These clouds of water that surround each ion are called spheres of hydration - The electrical charge of an atom depends on the difference between the number of protons and electrons. o Monovalent – when the difference is 1 o Divalent – when the difference is 2 - Cations – ions with a net positive charge - Anions – ions with a negative charge The Phospholipid Membrane: - Hydrophilic – substances including ions and polar molecules, water-loving o Atoms bonded by nonpolar covalent bonds have no basis for chemical interactions with water  Occurs when the shared electrons are distributed evenly in the molecule so that no portion acquires a net electrical charge.  Hydrophobic – compounds that do not dissolve in water, water-hating The Phospholipid Bilayer: - The main chemical building blocks of cell membranes are phospholipids o Contain long nonpolar chains of carbon atoms bonded o hydrogen atoms o Has a phosphate group attached to one end of the molecule  The polar head (phosphate group) is hydrophilic, tail is nonpolar - It is arranged in a phospholipid bilayer, effectively isolating the cytosol from the extracellular fluid Protein: - Examples that are made up of protein molecules o Enzymes – catalyze chemical reactions in the neuron o Cytoskeleton – gives a neuron its special shape o Receptors – are sensitive to neurotransmitters Protein Structure: - Proteins are molecules assembled from combinations of 20 amino acids o All amino acids have a central carbon atom which is covalently bonded to 4 molecular groups:  Hydrogen atom  Amino group  Carboxyl group  R-group – variable group, determine the chemical relationships in which each amino acid can participate - Amino acids assemble into a chain connected by peptide bonds, which join the amino group of one amino acid to the carboxyl group of the next o Polypeptides – proteins made of a single chain of amino acids - The 4 levels of protein structure: o Primary structure – like a chain, in which the amino acids are linked together by peptide bonds o Secondary structure – when the protein molecule is synthesized it can coil into a spiral-like configuration called alpha helix. o Tertiary structure – interactions among R groups can cause the molecule to change its 3 dimensional conformation by bending, folding and assuming a globular shape. o Quaternary structure – polypeptide chains can bond together to form a larger molecule  Each of the polypeptides contributing to a protein in this structure is called a subunit Channel Proteins: - Regions where nonpolar R groups are exposed will be hydrophobic and tend to associate with lipid - Regions with exposed polar R groups will be hydrophilic and tend to avoid a lipid environment - Ion channels across a membrane requires 4-6 similar protein molecules to form a pore between them o Important properties:  Ion selectivity - is specified by the diameter of the pore and the nature of the R groups lining it • Ex. potassium channels are selectively per
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