movement across membranes Flashcards
what are channel proteins
integral - span across membrane
make hydrophillic tunnels for target molecules to pass through
form a narrow aqueous pore
very specific; depends on
- size
- charge
passive - no energy needed for movement
may be gated (voltage or ligand)
usually ions move through them (eg Na+, K+) or water (aquaporins)
what are carrier proteins
have specific binding sites
carrier undergoes a conformational change
involved in facilitated diffusion and active transport so can be passive OR active
3 types
what are the 3 types of carrier proteins
- uniport
- symport
- antiport
what are uniport carrier proteins
transport a single substance
what are symport carrier proteins
transport 2 substances in same direction
what are antiport carrier proteins
transport 2 substances in the opposite direction
what are the 3 main forces that drive substances into or out of cells
chemical
electrical
electrochemical
what determines the direction in which substances move into/out of cells
based on the presence of a gradient
substances either move with the gradient or against it with help
what is Nernst equation/diffusion potential details
E ion = In
how do chemical drives work
based on conc differences across the membrane
all substances have a conc gradient
force is directly proportional to the conc gradient
what is RT In
how do you describes ion diffusion work done as a ratio
how do electrical driving forces work
also known as membrane potential
based on the distribution of charge across the membrane
only charged substances eg Na+ , K+
force depends on size of membrane potential and charge of ion
what is zFW
descrives electrical work done (V = EIon)
how do electrochemical driving forces work
combines chemical and electrical forces
net direction is equal to the sum of chemical and electrical forces
only charged substances eg Na+ , K+
describe membrane equilibrium
when diffusional and electrical forces balance
RTIn = zFEion
Eion = In
what are the 2 types of membrane transport
passive and active
what are 2 types of passive transport
simple diffusion
active diffusion
what are two types of active transport
primary
secondary
what is passive transport
does not require an input of energy
substance moves down its gradient from high to low
simple diffusion - gases
facilitated diffusion - mediated by proteins (channel or carrier)
describe glucose movement
GLUT 4 carrier protein
expressed in skeletal muscle and adipose tissue
glucose is taken up by facilitated diffusion
expression is unregulated by insulin
in the absence of insulin, glucose cannot enter cell
insulin signals the cell to insert GLUT 4 transporters into the membrane, allowing glucose to enter cell
what is GLUT 1
the glucose carrier protein
it is present in many cells, including the brain
it transports glucose across the blood-brain barrier via facilitative diffusion
what is GLUT1 deficiency syndrome
very rare disorder
mutations in gene that encodes GLUT 1
less functional GLUT 1- reduces the amount of glucose available to brain cells
symptoms include seizures, microcephaly, developmental delay
what is active transport
the movement of solutes from a region of low concentration to a region of high concentration against the concentration gradient.
Need transmembrane carrier protein and ATP is required.
2 types - primary and secondary
describe primary active transport
directly uses a source of energy, commonly ATP
example of primary active transport
Na+/K+-ATPase
pumps 3 Na+ out of the cell and 2K+ into the cell
utilises the hydrolysis of ATP to ADP and Pi
what is wilsons disease
rare disorder
caused by mutation ins ATP7B gene - this is a protein that is a Cu2+-ATP present in the liver that transports copper into bile
results in deposition of copper in liver and other tissues eg brain, eyes
symptoms include liver disease, tremor,
describe secondary active transport
transport of a substance against its gradient coupled to the transport of an ion usually Na+ or H+, which moves down its gradient
uses energy from the generation of the ions electrochemical gradient (usually by primary active transport)
example of secondary active transport
eg Na+/glucose cottransporter proteins (SGLT)
- present in intestinal lumen and renal tubules
- transports glucose from low to high concentration
- Na+/K+ATPase generates a sodium gradient to enable co transport of sodium and glucose
what is glucose - galactose malabsorption
rare disorder caused by mutation in SGLT 1
SGLT1 transports glucose and galactose from the intestinal lumen
less functional SGLT 1 - inability to transport glucose and galactose - resulting in their malabsorption
symptoms include severe, chronic diarrhoea, dehydration
what is cellular signalling
communication between cells takes place via signalling molecules eg hormones, neurotransmitters and growth factors
signalling molecules bind to to receptors
what are two types of receptors
intracellular - eg steroid hormones
cell surface - eg peptide hormones
what are second messegers
affect gene expression in the nucleus either directly or through signalling cascades
eg cAMP, IP3, DAG, Ca2+ amplification
what are Gateway to intracellular signals:
Examples; open a channel, activate a intracellular
enzyme, induce second messenger (peptide hormone binds to receptor) & migrate
nucleus to receptor-ligand complex
what is enzyme linked receptor
e.g. tyrosine kinase - transfers a phosphate group from ATP to a protein in a cell thus acts like an on/off switch
lead to protein and receptor phosphorylation
what is ion channel linked receptor
participate in rapid signalling events found in electrically active cells like neurons, also referred to as ligand gated ion channels,
lead to changes in membrane potential or ionic concentration within cell
eg cholinergic nicotinic receptors
what are G protein-coupled receptors
- sense molecules outside the cell and activate inside signal transduction pathways to ultimately illicit a cellular response
lead to protein phosphorylation
mutations in G subunit can lead to cholera
what do intracellular receptors lead to
protein phosphorylation and altered gene expression
what is endocytosis
energetic process to absorb/engulf molecules into a cell. Some extracellular fluid is usually engulfed too along with the molecule etc. - a portion of the membrane is invaginated to form a membrane-bound vesicle called an endosome
where can endocytosis occur
in neutrophils and macrophages - they implement phagocytosis (eating)
whereby they engulf entire cells/macromolecules to form a phagosome
what is pinocytosis
fluid endocytosis
involves taking in dissolved solutes via vesicles
how does pinocytosis work
is receptor mediated - specific
found in depressed areas (coated pits) - allows the cell to get the molecules it needs.
1. Ligands bind to receptor
2. this complex is engulfed -releasing the ligand into the cytosol (fluid portion of the cytoplasm outside the cell
organelles)
what is exocytosis
Vesicle from the golgi apparatus, fuse with the plasma cell membrane, resulting in the expulsion of waste or the secretion of enzyme/hormones
what is cystic fibrosis
caused by mutation in CFTR protein
chloride channel
found in many tissues eg gut, pancreas, lungs and skin
“secretory epithelium”
abnormal function results in sticky viscous mucus
no osmotic drag
drugs that target membrane transporters
cardiac glycosides
proton pump inhibitors
loop diuretics
thiazide diuretics
what do cardiac glycosides do
eg digoxin
act on Na+/ K+Atpase in cardiac muscle cells
indirect action to increase intra cellular (Ca2+)
what do proton pump inhibitors do
eg omeprazole
act on H+/K+-Atpase in gastric parietal cells
what do loop diuretics do
eg furosemide
act on NKCC2 cotransporter in thick ascending limb of loop of henle
what do thiazide diuretics do
eg bendroflumethiazide
act on NCC cotransporter in distal tube
what is facilitated diffusion
the movement of solutes from a region of their high concentration to a region of their low concentration through protein channels
(WITHOUT CARRIER PROTEINS).
This continues until dynamic equilibrium is reached.
example of where facilitated diffusion is used
Glucose - protein assisted which is regulated by insulin. Voltage gate
channels activated by action potentials
what is wilsons disease
rare disorder
caused by mutation ins ATP7B gene - this is a protein that is a Cu2+-ATP present in the liver that transports copper into bile
results in deposition of copper in liver and other tissues eg brain, eyes
symptoms include liver disease, tremor,
