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William Huggon

Biology lecture 2 (Pg 79-110) - CELLULAR PROCESSES REGULATE ANIMAL PHYSIOLOGY o Physical and chemical processes underlie biological processes o Transport of materials across cell membranes is important for maintaining homeostasis and physiological processes such as conduction of nerve impulses, regulation of body fluids and uptake of nutrients, etc. - THE CELL o Stability of chemical reactions responsible for animal physiology is maintained within cells through the action of cell membranes:  Maintain cell integrity  Control cell activities  Electrical activity of cell membranes leads to cell-cell signaling - ALLANIMAL CELLARE SURROUNDED BY A FLUID PLASMA MEMBRANE o Integral proteins: serve as –  passive transport pores and channels  active-transport pumps and carriers  membrane –linked enzymes  chemical signal receptors o Peripheral protein: membrane-linked proteins that do not extend through membrane, can act as enzymes, receptors, etc. o Glycoproteins: have oligosaccharide side chains vital for cell recognition and communication o ….BUT …membrane composition varies, i.e. myelin sheath surrounding axons has little membrane proteins - Membrane-associated proteins have polar and non-polar regions o Non polar: domains contain amino acid with hydrophobic side chains and are buried in the lipid bilayer o Polar: domains contain hydrophilic amino acid side chains and projects into aqueous media - MOVEMENT ACROSS THE CELLULAR MEMBRANE o Membranes can be:  Permeable: substances can move across the cell membrane (permeable, semi-permeable or selectively permeable)  Impermeable: substances cannot move across the cell membrane o Passage across the membrane can be:  Passive: from areas of high to low concentration (gradient)  Active: molecules move against the concentration gradient (fueled by energy from the hydrolysis of ATP) Mechanisms for transmembrane movements o Materials can cross the cell membrane by:  PASSIVE DIFFUSION (no energy required) • Movement of molecules from one location to another solely as a result of their random thermal motion • Net fluxes always proceed from regions of higher concentration to lower concentration. • Net fluxes depend on several factors: temperature, mass of the molecule, surface area, and medium through which the molecules move • Permeability to: o Lipid-soluble molecules (i.e. steroids) o Gases o Uncharged polar molecules (ethanol, urea) o Water (but very slow) - Channels. Carriers, and pumps allow transport across biological membranes o Passive transport  Channels • Voltage-gated • Ligand-gated • Water aquaporins  Carriers (aka secondary active transport) • Uniport • Co-transport (symport) • Counter-transport (antiport) o Active transport  ATP powered pumps - PASSIVE TRANSPORT (FACILIATED DIFFUSION)- no energy required o Unlike passive diffusion, requires protein molecules that assist in the transmembrane movement of solutes o Movement along a concentration gradient o Permeability to:  Small water-soluble (polar) molecules  Ions (i.e. Na, K, Ca, Cl)  Water (through aquaporin channels) o Passive transport – WATER  Osmosis is the net diffusion of water across a membrane  ISOTONIC SOLUTION: cell volume remains unchanged  HYPOTONIC SOLUTION: water enters the cell due to higher osmotic concentration cytoplasm relative to the solution  HYPERTONIC SOLUTION: water leaves the cell and the cell shrinks in colume  NOTE: AQUAPORIN PROTEINS ARE WATER CHANNELS. THEY CAN BE INSERTED AND REMOVED FROM THE CELL MEMBRANE TO ALTER PERMEABILITY TO WATER. THIS IS UNDER HORMONAL REGULATION - MECHANISM OF PASSIVE TRANSPORT o Passive transport mechanism can be through a pore or open channel o Can also include the solute combining with a CARRIER (transporter) protein in the membrane (CARRIER-MEDIATED PASSIVE TRANSPORT - CARRIER PROTEIN CONFIGURATIONS o Uniporters: transport a single type of solute in one direction o Symporters: simultaneously transport 2 different solutes in the same direction o Antiporters: simultaneously transport 2 different solutes in the opposite direction  These terms are also applied to active transport systems - ACTIVE TRANSPORT- energy is required o Can be classified as primary active or secondary active transport o Primary: direct expenditure of ATP-dependent membrane pumps to transport substances AGAINST concentration gradient o A model of primary active transport  Sodium-potassium pump: is an ATPase with binding sites for ATP and Na+ on its cytoplasmic side and binding site for K+ on its external surface (result: cotransport of Na+ and K+ in opposite directions) 1. Transporter binds 3 Na+ from cytosol 2. Phosphorylation by ATP favours conformational change 3. Na+ is released K+ binds 4. Dephosphorylation favours original conformation 5. K+ is released to cytosol. Cycle can repeat. o NA+, K+ ATPase  Maintains ion concentration gradients  Electrogenic: one extra+ charge leaves the cell o Secondary active transport (energy is indirectly required)  When the movement of one molecule across the membrane depends on the ATP-dependent concentration gradient of another molecule  Can be symport or Antiport - COUPLED TRANSPORTERS o Cotransport (symport)  Sodium gradient is used to carry sugars and amino acids into the cell  Energy in the gradient is derived from energy that drives Na+/K+ pump o Antiport:  E.g. sodium gradient is used to carry Ca+ out of the cell  Energy in the gradient is derived from energy that drives Na+/K+ pumps  Another examples is the Na+/H+ antiporter in the proximal tubule of kidney - ENDOCYTOSIS o Cell ingests macromolecules via formation and fusion of membrane bounded vesicles o If fluids ingested: pinocytosis o If solid ingested: phagocytosis 1. Ligand molecules bind to surface receptors molecules, which accumulate in coated pits formed by clathrin molecules bound to membrane 2. Coated pits is invaginated 3. Coated vesicle is formed 4. Coated vesicle fuses with the existing vacuole shedding clathrin 5. Fused complex undergoes further processing 6. Clathrin and receptor molecules are recycled to plasma membrane - EXOCYTOSIS o Constitutive secretion: proteins continually secreted from the cell regardless of environmental cues o Regulated secretion: proteins are packaged but only secreted in response to a signal and increase in cytoplasmic Ca2+ - GAP JUNCTION: direct communication b/w cells o Direct passage of molecules between neighboring. i.e. through two membranes (electric synapse, cardiac muscle) o Adjoining cells have an array of connected hexagonal channels o Central channel allows passage of molecules smaller than 2 nm LECTURE 5: NERVOUS SYSTEM AND LOCOMOTION - Two major types of muscles (based on the arrangement of actin andm myosin) o Striated  Skeletal and cardiac  Actin and myosin arranged in parallel (sacromeres) o Smooth  Actin and myosin are not organized and do not have sarcomeres - - - SKELETAL MUSCLE MORPHOLOGY o Muscles consist of long, cylindrical, multinucleated NUSCLE FIBERS arranged in parallel o Each muscle fiber is made up of many parallel myofibrils made up of sarcomeres o Sarcomere is the functional unit of the muscle cell o Sliding filament theory: muscles contract when cross-bridge interactions occurs between myosin heads and actin filaments (ATP ad Ca2+ dependent) o Watch video on sliding filament theory o Sliding filament theory  Myosin head attaches to actin  Power stroke: myosin head pivots pulling the actin filament toward the center  The cross bridge detaches when a new ATP binds with the myosin  Cocking of the myosin head occurs when ATP  ADP+ P. Another cross bridge can form - SPECIALIZATION AT THE NEUROMUSCULAR JUNCTION o The axon of a presynaptic motor neuron forms many branches along the surface of a muscle fiber o Muscle membrane has various TRANSVERSE JUNCTIONAL FOLDS o ACTIVE ZONES in the presynaptic membrane are rich in synaptic vesicles and are located above the junctional folds o An Action potential in the presynaptic neuron can depolarize postsynaptic muscle membrane and cause action potential along the muscle fiber. - VERTEBRATE SKELETAL MUSCLE o Most vertebrate skeletal muscles are neurogenic muscles and receive signals from a motor neuron o Neurogenic muscles: muscles in which contraction is initiated in the CNS o The sarcolemma is rich in acetylcholine receptors (nicotinic) - ACH: the primary neurotransmitter at the vertebrate neuromuscular junction o Acetyl CoA is synthesized in the mitochondria o Choline acetyl transferase catalyzes the conversion of choline and acetyl coA to Ach o The Ach is packaged into synaptic vesicles o Ch is released into the synapse o Ach binds to its receptors on the post-synaptic cell o Acetylcholinesterase (AChE) breaks don Ach into choline and acetate, terminating the signal in the post synaptic cell o The presynaptic cell takes up and recycles the choline, and the acetate diffuses out of the synapse  NOTE: MOTOR NEURONS OF INVERTEBRATES RELEASE OTHER NEUROTRANSMITTERS…I.E. GLUTAMTE - Excitation-contraction coupling o Action potential arriving at the neuromuscular junction causes an action potential in the fiber, followed by a muscle twitch o LATENT PERIOD: of a few msec before the muscle generates tension o EXCITATION-CONTRACTION COUPLING, where electrical excitation of the plasma membrane leads to activation of a muscle contraction o WHAT IS THE CAUSE OF LATENT PERIOD? - Aps penetrate into muscles
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