Ion channels - INCOMPLETE

describe 4 functions of the plasma membrane

physical barrier - establishes a boundary, protects cellular contents and supports cell structure

selective permeability - regulates entry and exit of ions, nutrients and waste molecules through the membrane

electrochemical gradients - establishes and maintains an electrical charge difference across the plasma membrane

communication - contains receptors that recognise and respond to molecular signals

describe the function of some membrane proteins

structural (cytoskeleton) - help give cell support and shape

receptor - helps cell communicate with their external environment via hormones, neurotransmitters etc

channel - allow water, ions and proteins to flow passively through the bilayer

transport - transport molecules across cell membrane

glycoproteins - cell communications and adhesions

how does water move across the cell membrane?

diffuses across quite easily as so small in size

specialised proteins (aquaporins) provide transmembrane ‘pores’ for water which are always open

these are found in regions where water transport is physiologically important (kidney, intestines)

list some substances that diffuse through the plasma membrane

steroids, CO2, O2 - simple

amino acids, glucose - carrier-mediated facilitated

sodium and chloride ions - channel-mediated facilitated

water - osmosis, through aquaporin or bilayer

define osmosis

the diffusion of water from a less concentrated to a more concentrated solution through a selectively permeable membrane

define isotonic, hypotonic and hypertonic solutions

isotonic - same water potential as inside cell

hypotonic - high water potential than inside cell (water moves into cell)

hypertonic - lower water potential than inside cell (water moves out of cell)

how do ions move across the cell membrane?

cannot diffuse across bilayer but can diffuse across membrane via specialised proteins that form ‘ion channels’

these are ‘gated’ and permeability to different ions is determined by the number of ion channels that are open

what does the presence of ion channels generally mean for membrane permeability to different ions, and their concentrations inside and outside the cell?

to K+ is high, to Na+ and Ca2+ is very low

so intracellular fluid is high in K+ and low in Na+ and Ca2+ (opposite for extracellular fluids)

describe the Na-K pump

an enzyme (Na+-K+-dependent ATPase) that hydrolyses ATP

consumes energy from ATP so can do work

transports Na+ out of cell in exchange for K+ against a concentration gradient (active transport)

3Na+ out, 2K+ in

state 2 places where the Na-K pump is very important

nerve/muscle

organs which transport ions (eg, kidneys)

describe how an inwardly-directed electrical driving force on Na+ is created

Na-K pump establishes and maintains more Na+ outside cell (so Na ‘wants’ to move inwards)

resting membrane potential is slightly more negative inside than outside - Na+ is positively charged so is attracted to the inside of the cell

what does the electrochemical driving force on Na+ in the extracellular fluid mean that the cell has?

a source of potential energy

what is a symport?

a type of secondary active transport where a membrane protein moves two different solutes (typically an ion and a nutrient) in the same direction across a cell membrane

describe the role of ion channels

allows ions to flow down concentration gradient and evoke cellular response

important in skeletal/smooth muscle contraction, insulin secretion, nerve impulses

many drug targets are ion channels

what are ionotropic receptors?

ion channels that are normally closed until receive a stimulus (ligand-gated ion channels)

give the concentration of K+, Na+ and Ca2+ in cytoplasm

140mM, 15mM, 0.1microM

give the concentration of K+, Na+ and Ca2+ in extracellular fluid

5mM, 145mM, 1mM

define signal transduction

the process by which an extracellular signal molecule activates a membrane receptor that in turn alters intracellular molecules to be transduced via a certain pathway to activate a cellular response (converts one form of signal to another)

what are first and second messengers?

first - extracellular signalling molecule

second - intracellular molecule

give some examples of first messengers

amines (eg adrenaline), peptides and proteins (eg insulin), steroids (eg hormones)

state the 4 main classes of receptor

ligand-gated ion channels, G-protein coupled receptors, enzyme-linked receptors, nuclear receptors