Membrane Transport Diffusion Solution solvent water and solute dissolved molecules Due to random movement thermal energy solute molecules will show net movement from region of high concentration to a region of low concentration solute moves down the concentration gradient Rate of diffusion o Increases with temperature o Increases with concentration gradient o Increases with surface area of membrane o Decreases with distance If solute molecules can penetrate the membrane then diffusion can occur across the membrane o Lipids small gas molecules ions through protein channels in membrane nonpolar molecules Osmosis Diffusion of water across a semipermeable membrane through aquaporin channels in plasma membrane Water moves from low solute concentration solution to high solute concentration solution water diffuses from high water concentration to low water concentration Water will diffuse by osmosis across the membrane until solute concentration is the same on both sides of the membrane Osmotic pressure how strongly a concentrated solution pulls water by osmosis across the membrane o Pure water 0 o Equal osmotic concentration isotonic o Less concentrated hypotonic o More concentrated hypertonic If pure water moves into the cell it will cause the cell to swell and burst lyse Osmolality vs molality Molality is the number of compound molecules per liter of water Osmolality is the number of solute particles per liter Carrier Mediated Transport Mechanisms Mediated by carrier proteins that span the plasma membrane Properties o Specificity for a specific molecule o Limited number of transporters can be saturated with a max o Closely related molecules can compete to transporters on the transport rate cell surface o Facilitated diffusion gradient o Transported molecule is moved down its concentration o Doesn t require extra energy from the cell o Because facilitated diffusion requires using a limited number of transporters transport can be regulated by increasing or decreasing the number of transporters Primary active transport o Membrane carrier protein is an ATPase that breaks down ATP to release energy o Energy is used to transport molecule against its concentration gradient up the gradient will require energy o Ca2 ATPase pump hydrolysis of ATP and release of ADP causes conformational change in transporter releasing Ca2 outside the cell moves calcium from low to high o If you run out of ATP the transport will no longer take place Coupled active transport o Cell uses energy to establish steep concentration gradient for o Co transporter allows molecule 1 to move down concentration o Couples energy of first molecule to co transport molecule 2 up o Most cells maintain a steep Na and K gradient across their molecule 1 gradient its gradient membranes o Na K ATPase pump 3 Na ions bind to inside of carrier protein ATP is hydrolyzed Release of ADP moves 3 Na to outside of cell Release of Pi moves 2 K to inside of cell o Cotransport symport both molecules move in the same o Countertransport antiport molecules move in opposite direction directions Membrane Potential 02 01 2012 Resting Membrane potential Vm Cells have an unequal distribution of charge across their membrane more positive charges on the outside and more negative charges on the inside cell Charge separation is caused by movement of ions in and out of the Ions are moved by chemical diffusion down concentration gradients and by electrical attraction and by active transport Concentration of ions inside and outside reaches equilibrium due to equilibrium potential Each ion species feels two forces pulling on it through open ion channels o 1 Chemical driving force depends on concentration o 2 Electrical driving force depends on electrical potential gradient across membrane difference across membrane These forces can act in same direction or opposite directions across the membrane o Example K has chemical driving force out of cell but electrical driving force into cell o Example Na ions has both chemical driving force and electrical driving force into cell The electrical potential that balances the concentration gradient is called the equilibrium potential If there are open channels for an ion the electrical and chemical driving forces will try to force the ions to move across the The ion will move across the membrane until the change in electrical charge causes the cell s Vm to reach the ion s equilibrium membrane potential 2 forces on ions what Nernst Equation diffusion down concentration gradient electrical attraction toward opposite charge Positive ions will be pulled by electrical force from the outside to the inside no matter for K chemical and electrical forces are in opposite direction for Na chemical and electrical forces are in the same direction Eion 61 charge log ion out ion in Membrane potential Vm or RMP The cell s overall membrane potential Vm is a combination of the Eion of all the ions that can permeate the membrane with a greater contribution for the ions with the greatest permeability i e with the most open ion channels Vm approaches the Equilibrium Potential of the most permeable ion o For neurons and most cells the most permeable ion is K o So the membrane potential Vm at 70 mV is close to EK at 90 mV Goldman Equation finds the compromise membrane potential accounting for each permeant ion Neurons 02 01 2012 Structure of Vertebrates Peripheral compartment o Everything outside of the brain and spinal cord heart lungs gastrointestinal tract liver kidneys skeletal muscle skin etc o Neurons and nerve fibers outside the brain and spinal cord Central nervous system o Brain at front of body o Spinal cord running down the back o Protected by skull and vertebra o Sensory receptors clustered in head vision hearing taste smell Functions of Nervous System Sensory motor integration detect changes in the environment or in the body via sensory receptors coordinate responses across the body Initiate response via skeletal muscle somatic nerves for voluntary movement or via smooth muscle and glands autonomic nervous system Neurons nerve cells point to point communication across the body to coordinate responses integrate electrical and chemical signals at dendrites and cell body depending on inputs neuron sends electrical and chemical signals down axon to synapse on target cell Sensory neurons afferents carry sensory information into the CNS Motor Neurons efferents carry impulses out of CNS to make muscles move or effect target organs
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