Bio HL1 Test 4 - Kim

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Water soluble globular proteins

hydrophobic amino acids clustered in core, minimizes contact with H2O and stabilizes structure through hydrophobic interactions.
hydrophilic amino acids positioned on outer surface, forms H-bonds and ionic interactions with H2O

integral proteins (embedded in membrane)

hydrophobic amino acids found in areas that contact fatty acid tails of membrane, enables stable embedding

hydrophilic amino acids found in areas exposed to H2O, line channel, face cytoplasm and extracellular fluid

membrane proteins have…

diverse structures, locations, and functions

integral proteins structure

embedded in one or both lipid layers of membrane, typically spans membrane completely (transmembrane), has a pore that connects cytoplasm with extracellular fluid

integral proteins function

transport as channel or carrier, cell signaling as receptors, can function as enzymes

peripheral proteins structure

attached to one surface of membrane, hydrophilic and bound to phospholipid heads or integral proteins

peripheral proteins function

structural support (attachment to cytoskeleton or ECM), cell signaling

facilitated diffusion

passive movement of large or polar molecules through chained proteins, down concentration or electrochemical gradient

allows only specific solutes (e.g. Na+, H2O, C6H12O6) to cross, without ATP

central pore has specific diameter or is lined with certain polar residue to ensure passage of one type of solute, makes membrane selectively permeable by filtering solutes according to size, charge, and polarity (e.g. negatively-charged pores only allow positively-charged ions to cross)

channel proteins

open channel allows solutes to diffuse through pore, can move in both directions (in/out), from high—>low, without ATP

closed channel blocks diffusion despite existing gradient, cells regulate diffusion (solute and direction) by producing and placing different channel proteins

aquaporins

specialized protein channels that increase membrane permeability to H2O, H2O molecules are in constant random motion due to kinetic energy, allows rapid diffusion of H2O down its [] gradient, impermeable to other solutes

Gated Ion Channels

Membrane proteins that open/close in response to specific stimulus which allow ions to cross

Voltage-gated channels on nerve cells

neurotransmitter (ligand)-gated channels on synapses

voltage-gated ion channels

channels that open/close in response to changes in membrane potential (charge); essential for action potential in neurons

voltage exists across membrane due to imbalance of ions

when above -55 mV (threshold), sodium channels open

Na+ diffuses into neurons (influx)

around 30 mV, potassium channels open

K+ diffuses out of neuron (efflux)

ligand-gated ion channels

channels that open when a chemical messenger (neurotransmitter) binds to a receptor

nicotinic acetylcholine receptors (nAchR)—receptor for acetylcholine (movement) located in neuromuscular junctions

binding of ACh opens pore

Na+ diffuses into neuron, changes membrane voltage, causes other Na+ channels to open and neuron to fire

ACh detaches and pore closes

active transport’s three characteristics

moves solutes against their [] gradient (low —> high), utilizes pump proteins, requires energy via ATP hydrolysis

pump proteins (carrier proteins)

transmembrane proteins that bind to and move specific solutes across the membrane

hydrolysis of ATP causes conformational change, ensures controlled transport of solutes

Sodium-Potassium Pump (Na+ / K+) & process

reestablishes membrane potential after neuron fires by transporting Na+ and K+ against their concentration gradients using ATP
process:

3 Na+ bind to intracellular sites of pump (more Na+ inside), 1 phosphate attaches after hydrolysis of ATP, conformation changes, moving Na+ across, 2 K+ bind to extracellular sites of pump (more K+ outside), phosphate released, pump changes to original shape, moving K+ across

Sodium-Potassium Pump results

maintains unequal ion distribution across membrane (more negative inside cell), for each ATP, 3 Na+ are pumped out and 2 K+ are pumped in, electrochemical gradient is required for neuron signaling, muscle contractions, and secondary active transport

Indirect Active Transport (Cotransport)

movement of solute against its concentration gradient by using energy from ion gradient rather than ATP hydrolysis

sodium-dependent glucose cotransporters

energy released as Na+ moves down its electrochemical gradient is used to move glucose against its concentration gradient; electrochemical gradient is maintained by Na+ / K+ pump, and is still considered active because energy is still required

cotransporters in epithelial cells of small intestines

enables glucose absorption even when its concentration is lower in lumen; moves into bloodstream via facilitated diffusion, seen in epithelial cells of proximal convoluted tubule in nephron, prevents glucose loss in urine

membrane is held together by

weak hydrophobic interactions, thus the bilayer can bend, pinch, and fuse to create vesicle, which allows for large amounts of solute to enter/exit cell, requires ATP and is considered active transport

endocytosis

solutes and H2O enter the cell via vesicle formation from plasma membrane: e.g. fetus with mother’s antibodies, white blood cells with pathogen

two types of endocytosis

phagocytosis and pinocytosis

phagocytosis

uptake of solid substances (sent to lysosome)

pinocytosis

uptake of liquids/dissolved substances (faster than protein channels)

exocytosis and examples

materials leave the cell when vesicles fuse with plasma membrane and expel contents, replaces phospholipids lost initially during endocytosis: e.g. release of insulin from pancreas and secretiuon of neurotransmitters at synapses

central nervous system

brain, spinal cord (dorsal)

nervous system consists of

central nervous system, peripheral nervous system

peripheral nervous system

connects CNS with rest of body

neurons

specialized cells that relay messages via electrical impulses, converts sensory information (sound or scent) into electrical signals

cell body (soma) structure

contains nucleus, organelles, and cytoplasm, carries out metabolic processes for survival

nerve fibres structure

dendrite—multiple, shorter fibers, recieves chemical information from other neurons and conducts electrical impulses towards soma

axon—single, elongated fiber, conducts electrical impulses away from soma to other neuron/effectors

diameter of axon affects on speed of nerve impluses

squids have giant axons with diameter of ~500um (human axon is 1um), internal resistance to ion flow is low, so impulses travel more quickly, even without myelin, often used when speed is vital

myelination

myelin sheath: insulating layer of lipid-rich membrane that wraps axons of some neurons, composed of oligodendrocytes (CNS) and schwann cells (PNS), gaps in between are called nodes of ranvier, electrical impulses jump from one to next, increases speed of transmission, electrical impulses occur along entire axon in non-myelinated neurons

synapses

specialized junction between neurons and neurons or neurons and effector cells (muscles or glands)—presynaptic neuron contains vesicles filled with neurotransmitters, synaptic cleft is the narrow gap (~20nm) in between, postsynaptic cell contains specific receptors for released NTs

synaptic signal travels in

one direction only: presynaptic neuron → synaptic cleft → receptors on postsynaptic cell

oscilliscope traces

measure and display changes in membrane potential over time, use two electrodes, one inside and one outside of membrane

x-axis—time (ms)

y-axis—membrane potential (mV)

measures impulses per second

membrane potential

voltage difference across membrane due to unequal ion distribution

resting potential

stable membrane potential when neuron is not firing

stable membrane potential #

~-70 mV, inside is more negative than outside

three reasons why resting potential is negative

Na+/K+ pump: 3 Na+ out for every 2 K+ in

two ions diffuse back across the membrane—more permeable to K+; K+ diffuses out faster than Na+ diffuses in

presence of negatively charged proteins inside

action potential

neural firing—change in membrane potential that produces electrical impulses

“electrical” because it involves the movement of positively charged ions

depolarization (+30 mV)

membrane potential becomes “less” negative—stimulus opens Na+ (voltage gated) channels, Na+ rushes in (influx) due to greater [] outside, neuron becomes more positive

repolarization

restores membrane to resting potential, after Na+ enters, K+ (voltage gated) channels open, K+ rushes out (efflux) due to greater [] inside, neuron becomes more negative

hyperpolarization

membrane becomes more negative than resting potential, exiting K+ surpasses

refractory period

state of recovery required for neuron to fire again, prevents AP from moving backwards, ionic distribution is reversed (more Na+ in and more K+ out), Na+/K+ pump works to restore resting potential

threshold potential

critical level of depolarization required to trigger an action potential, stimulus must be strong enough to break threshold (-55 mV), only then will voltage-gated Na+ channels open, any signal < -55 mV will not trigger an AP

Na+/K+ pump will re-establish resting potential

all-or-none principle

once initiated, AP is always the same strength (height), independent of stimulus

AP Propagation

spread of an AP along the axon without losing strength, occurs due to local currents created by ion diffusion, short-distance movements of charged ions along the axon, Na+ diffuses into axon, positive charge spreads inside toward adjacent areas that are at resting potential (occurs outside as well), causes these areas to depolarize and reach threshold, opens Na+ channels, initiates AP

saltatory conduction

rapid transmission of an action potential along a myelinated axon

myelinated axon

multiple phospholipid bilayers in myelin sheath prevent ion movement, channels and pumps are clustered in nodes of ranvier, or exposed areas in between, they force AP to ‘jump’ from one node to the next, increases speed up to 100x

unmyelinated axons

every Na+ and K+ channels on axon must open for AP to spread

synaptic transmission will or will not be an essay

IT WILL BE AN ESSAY (LOCK IN)

neurotransmitters transmit signals between…

neurons or neurons and effector cells—muscles/glands

action potential arrives at

the presynaptic membrane

AP opens voltage-gated _____ channels, causing _____ _______.

calcium; Ca2+ influx

after Ca2+ influx in presynaptic membrane… (rest of process)

Ca2+ binds to proteins on synaptic vesicles, initiates movement towards synapse, exocytosis releases NTs at synapse, excess are recycled (reuptake) or hydrolyzed, binds to specific receptors on postsynaptic neuron, opens ligand-gated channels, causing Na+ influx, initiates AP

Excitatory Postsynaptic Potentials (EPSP)

temporary depolarization of membrane that increases likelihood of action potential:

NT diffuses across synaptic cleft and binds to transmembrane receptor, opening neurotransmitter-gated channels (Na+, Ca2+)

acetylcholine

released in neuromuscular junctions, initiates muscle contractions, binds to postsynaptic receptor for a short time—not long enough for one AP—must continually be removed to prevent fatal convulsion or paralysis

Acetylcholinesterase (synaptic enzyme)

breaks down ACh into two parts: acetate + choline

choline is transported into presynaptic neuron and reused

acetate diffuses away from synapse and enters bloodstream

removal of ACh ends transmission

exogenous chemicals

substances that originate from outside the body that alter normal physiological processes—can interfere with receptors or NT removal

neonicotinoids

synthetic pesticides that bind to nicotinic acetylcholine receptors—insects have a greater proportion of such receptors and tend to bind much more strongly

causes prolonged receptor activation which leads to paralysis and death—cannot be degraded by acetylcholinesterase—has been blamed for collapse of honeybee populations

cocaine

stimulant—blocks dopamine and serotonin reuptake transporters—NTs remain in synaptic cleft and increase postsynaptic neuron firings—sustains euphoria, unrelated to any reward activity

inhibitory postsynaptic potentials (IPSP)

inhibitory neurotransmitters reduce the likelihood of action potentials

cause hyperpolarization in postsynaptic neurons by opening Cl- (enters) or K+ (exits) channels that make the cell more negative

GABA

calms the nervous system to process sensory input in an organized way

summation effect

more than one neuron forms synapses with postsynaptic neurons—one EPSP is not enough to break threshold

summation is when multiple EPSP and IPSPs combine to fire APs—either one must fire repeatedly (temporal) or several must fire at the same time (spatial)

if depolarization > hyperpolarization, threshold is reached

perception of pain

free nerve endings of sensory neurons in skin detect pain—they are unencapsulated, or exposed directly to tissues, used to sense high temperature, acids, and chemicals like capsaicin in chili

contain ion channels for positively charged ions (Na+), once threshold is met, APs fire and travel to spinal cord and brain

brain percieves pains, not site of stimulation

emergent properties: consciousness

state of complex awareness of self and environment, including perception, thought, and subjective experience

consciousness is an emergent property resulting from collective neural activity rather than individual neurons—interaction produces outcomes not predictable from single components alone—idea that the system is greater than the sum of its parts