membrane transport 1,2&3 Flashcards
plasma membrane is a semi fluid
phospholipid bilayer
in plasma membrane describe the middle and then the inner/ outer surfaces
- hydrophilic heads at the inner and outer surfaces
- hydrophobic tails in the middle
integral proteins
peripheral proteins
embedded in the bilayer
bound to inner or outer membrane surface
functions of proteins of the plasma membrane
- transport across the membrane
- receptors involved in cell signalling
- enzymes that catalyze reactions
- anchoring of cells to each other and substrates
is the lipid bilayer miscible with the extracellular fluid or intracellular fluid (cytoplasm)
no; acts as a barrier against movement of water and water-soluble substances between the extracellular environment and the cytoplasm of the cell
what substances are able to diffuse directly across the lipid bilayer
lipid-soluble substances
examples:
- gases: O2, N2, CO2
- small, uncharged polar molecules: alcohol
plasma membrane is ____ permeable
selectively
channel proteins
water filled tunnel through the protein, allow free movement of water, some ions and molecules
carrier proteins
bind with ions or molecules, conformational change in protein moves ion/molecule to other side of membrane
diffusion and facilitated diffusion are examples of _____ transport
passive
what causes molecules and ions to diffuse
random motion; termed heat aka brownian motion
random motion will cause molecule and ions to diffuse across membrane until
equilibrium is reached
in drug delivery, what is important in diffusion in a particular direction
the net rate
what factors affect the net rate of diffusion
Fick’s law of diffusion:
- concentration gradient
- permeability of membrane to substance
- surface area of membrane
- molecular weight of substance
- distance across which diffusion must occur
concentration gradient
chemical driving force
ex rate at which Na+ diffuses into the cell is proportional to concentration of Na+ outside of cell
permeability of membrane
- phospholipid bilayer
- the more lipid soluble the molecule (fewer polar or ionized groups) the more easily it will traverse the membrane
surface area of membrane: the greater the SA of membrane the
greater the number of molecules that are able to hit the membrane and pass through
why is SA a limiting factor of cell size
as animals/ cells get larger, their SA:V ratio gets smaller
do larger or smaller molecules move slower
larger move slower
do larger or smaller molecules find it harder to pass through membrane protein channels
larger
an _____ exists across a membrane when one side is relatively more pos and one more neg
electrical potential
membrane electrical potential
electrical driving force
charged ions will move across membrane to achieve state of equilibrium
equilibrium potential
state of balance between movement of ions due to chemical driving force and electrical driving force: so when the electrical force is equal to but opposite the direction of chemical force
the electrical potential difference that will balance a given concentration difference of univalent ions can be determined by the
nernst equation
pressure different across a membrane
can also cause the movement of molecules from one side of a membrane to another
pressure
is the sum of all the forces of the different molecules striking a membrane
generally, the pressure is greatest on the side with the greater number of molecules
pressure gradient force
greater energy available to cause net movement of
molecules from side of high pressure to low pressure
osmosis: if two aqueous solutions are separated by a membrane that allows only water molecules to pass
water will move into the
most concentrated solution
tonicity
refers to the strength of a solution in relationship
to osmosis
Isotonic solution
solute concentrations are the same on both sides of the cell membrane
cells stay same size
hypotonic solution
solution has a lower concentration of solute (more water) than the cell
water enters the cell
the cell swells (turgor) & may burst (lysis)
Hypertonic solution:
solution has a higher percentage of solute (less water)
than the cell
water leaves the cell
the cell shrinks
red blood cells in less than 0.9% NaCl will
swell: hypotonic
red blood cells in a solution with a concentration higher
than 0.9% NaCl
will shrink; hypertonic
The movement of water from a hypotonic to a hypertonic solution causes an increase in ______ in the hypertonic compartment
eventually, this hydrostatic pressure will equal the
osmotic pressure, and the movement of water will ___
hydrostatic pressure
stop
most abundant substance to diffuse across cellular membranes
water
Typically, the cytoplasm of cells has a higher
concentration of ____ than extracellular fluid
which means it has higher _____
solutes
osmolarity
cells control their intracellular osmolarity by
pumping out ions
diffusion through cell membranes occurs via 2 processes:
1) simple diffusion: directly through lipid bilayer, CHANNEL proteins
2) facilitated diffusion; through specific CARRIER protein
what molecules can use simple diffusion to cross membrane
lipid soluble such as CO2, O2, steroids and alcohols
or water soluble / water through hydrophilic channel proteins
channel proteins
- integral protein
- have central canal/ pore that is hydrophilic
- selectively permeable by diameter of pore (size), electrical charge or protein structure
- opened or closed by gates
aquaporins
type of channel protein that allows passage of water
potassium channel (simple diffusion, channel protein)
what passes and in which direction
how many protein subunits does it consist of
what exists at the surface of the pore/ function?
what is in lined with/ function
- permits passage of K+ out of the cell
- consists of 4 protein subunits surrounding a central pore
- at the surface of the pore are pore loops: selectivity filter
- lined with carbonyl oxygens: interact with hydrated K+, strip water & allow K+ to pass through, Na+ too small to interact with carbonyl O2
sodium channel (simple diffusion, channel proteins)
what are inner surfaces lined with, what is their function
what is too large to fit through
- inner surfaces lined with amino acids that are strongly negatively charged, pull small Na+ away from water molecules
- hydrated K+
ions too large to fit through channel
gating of protein channels controls
permeability of channels
_____ of channel proteins opens or seals pore (to open or close gates)
conformational change
***the protein doesn’t go through conformational change like carrier proteins tho, I feel like this is confusing but this is just talking about the opening/closing of gates
opening and closing of gates in channel proteins is controlled by what 2 mechanisms
voltage gating
ligand (chemical) gating
describe voltage gating of protein channels
- Molecular conformation of the gate, or its chemical bonds, responds to the electrical potential across cell membrane
- Cells in general have a small net excess of negative ions clustered beneath the plasma membrane
- If the cell is depolarised, differential between inside & outside of cell reduced
- gate opens & ions enter
examples of voltage gating
- sodium & potassium channels
- neurons & muscle cells
- calcium channels
describe ligand gating of protein channels
- Gates are opened by the binding of another molecule
- causes a change in conformation or chemical bonds
- gate opens
example of ligand gating protein channels
neurotransmitter channels in nerve & muscle cells
what facilitates diffusion in facilitated diffusion
carrier proteins; specific for particular molecules,
still down a concentration gradient!!
how many glucose carrier proteins are identified
5
glucose transporter 4 is a type of glucose carrier protein, what is it activated by
insulin
increases rate of diffusion by 10-20x
in simple diffusion the rate is proportional to the concentration gradient
in facilitated diffusion, it is different in 2 key ways; the rate is proportional to:
1) the number of carrier proteins available (saturation of carrier proteins= vmax)
2) similar molecules can compete for binding w the carrier protein
facilitated diffusion uses _____ proteins
CARRIER!!!!
not channel (thats simple diffusion)
active transport
when molecules or ions moving against concentration gradient
requires energy
active transport: where do K+ and Na+ have high concentrations
- K+: high concentrations intracellularly
- Na+: high concentrations extracellularly
active transport requires ____ proteins
carrier
two types of active transport
1) primary: energy derived from breakdown of ATP,
2) energy that stored as ionic concentration gradient derived originally from primary active transport
describe ex of primary and secondary active transport in regards to NA+-K+ pump
- Na+-K+ pump uses ATP to drive Na+ out of the cell (primary)
- Creates a strong chemical driving force
to move Na+ back into the cell - As Na+ moves down its concentration
gradient, it may take another molecule
with it (the amino acid glutamine) (secondary)
primary active transport; sodium potassium pump
what direction does it pump
pumps Na+ out of the cell & simultaneously pumps K+
into the cell, both against their concentration gradients
what is sodium-potassium pump responsible for
- responsible for maintaining concentration gradients & so also negative electrical voltage inside the cell
- also responsible for transmitting nerve impulses
sodium potassium pump: what part of the carrier protein has receptor sites for binding Na+, and how many does it have
inside of the cell
has 3 receptor sites for binding Na+
sodium potassium pump: what part of the carrier protein has receptor sites for binding K+, and how many does it have
outside of the cell
has 2 receptor sites for binding K+
sodium potassium pump: when Na+ binds intracellularly, what happens
- ATPase enzyme cleaves
adenosine triphosphate (ATP) - generates adenosine diphosphate (ADP) & a single
phosphate group, releasing energy - released energy causes conformational change
- Na+ released extracellularly
sodium- potassium pump: what happens after Na+ is released extracellularly
- extracellular K+ then binds
- triggers the release of the phosphate group
- causes conformational change
- K+ released intracellularly
all the steps of sodium potassium pump
- Na+ binds intracellularly
- ATPase enzyme cleaves
adenosine triphosphate (ATP) - generates adenosine diphosphate (ADP) & a single
phosphate group, releasing energy - released energy causes conformational change
- Na+ released extracellularly
- extracellular K+ then binds
- triggers the release of the phosphate group
- causes conformational change
- K+ released intracellularly
in nerve cells ___% of cells energy is used to drive sodium potassium pump
70
sodium potassium pump has important role in controlling cell _____
how
volume
- inside the cell are proteins & molecules that cannot
leave - most are negatively charged
- attract large numbers of positive ions
- causes osmosis of water into cell
- 3 Na+ expelled for every 2 K+ gained
- equals a net loss of ions
- osmosis of water out of the cell
sodium potassium pump:
___ Na+ expelled for every ___ K+ gained
3
2
sodium potassium pump also plays important role in transmission of ______
nerve impulses
3 Na+ expelled for every 2 K+ gained; deficit of positive ions inside the cell –> electrical potential
primary active transport: calcium pump
- calcium uniport
- Ca2+ extremely low intracellularly
- pumps Ca2+
- out of the cell
- into sarcoplasmic reticulum of muscle cells
- into mitochondria
how does Na+ facilitate secondary active transport
- When Na+ transported out of the cell by primary active transport a large concentration gradient develops
- equates to a large energy store!
- Na+ trying to diffuse down its chemical gradient into
the cell - allows other molecules to be pulled through with Na+
- co-transport
Involves a carrier protein that can bind Na+ & the molecule to be co-transported - once they are both attached, Na+ gradient causes both Na+ & other molecule to be transported into cell
symport (secondary active transport)
both moving in same direction, co-transport
antiport (secondary active transport)
moving in different directions, counter-transport
uniport (primary active transport or facilitated diffusion)
specific to one molecule
uniport, symport or antiport:
calcium
uniport
uniport, symport or antiport:
Na-glucose
symport
uniport, symport or antiport:
Na-K
antiport
uniport, symport or antiport:
Na-Ca
antiport
describe sodium-glucose symport
- 2 binding sites:
- one for Na+
- one for glucose
- The conformational change necessary to allow Na+ to enter
the cell can only occur if glucose also binds - both Na+ & glucose move in same direction
- co-transport
- Important in renal & intestinal cells
describe how Na+ facilitates counter-transport
(different directions)
- Na+ again tries to move down its concentration gradient
into the cell - molecule to be transported needs to move out of the
cell - Na+ binds to a binding site on the extracellular portion
of the carrier protein - counter-transported molecule binds to its binding site
intracellularly - once both have bound, a conformational change occurs &
energy released from Na+ moving down its concentration
gradient transports the other molecule out of the cell
sodium-calcium antiport (secondary active transport)
which direction do they move
which cells does it occur in
- Na+ moves intracellularly
- Ca2+ moves extracellularly
occurs in all cells
what are the two vesicular transport mechanisms
1) endocytosis; into cell
* phagocytosis
* pinocytosis
* receptor-mediated
2) exocytosis; out of cell
* constitutive
* regulated
endocytosis is for
molecules that are too large to enter via carrier proteins
endocytosis has key roles in
- cell signaling: endocytosis of hormone-receptor
complexes terminates signaling - cellular defense: ingestion of pathogens to be degraded by lysosomes, ingested products follow different pathways
- degraded (lysosomes)
- recycled
- stored
phagocytosis (endocytosis)
- ‘cell eating’
- large particles
- only a few, specialised cells perform phagocytosis
- phagocytes
- includes macrophages & neutrophils
- phagocytose pathogens, damaged & apoptotic cells or debris
pinocytosis (endocytosis)
- ‘cell drinking’
- extracellular fluid & solutes; large proteins
- occurs continually in most cells
- rate varies between cells
receptor mediated endocytosis
- highly specific as to which molecules endocytosed
- allows for ingestion of minor components of
extracellular fluid, without large volumes of ECF - specific receptors on membrane surface
direction of movement exocytosis
trans face of the Golgi apparatus to the cell membrane
in exocytosis vesicles contain
synthesised proteins & waste products
in exocytosis vesicles formed by
membrane proteins & lipids from Golgi
replenish those lost from the cell membrane during
endocytosis
2 forms of exocytosis
1) Constitutive Exocytosis
* occurs in all cells
* components of the extracellular matrix
* waste products
* initial packaging in endoplasmic reticulum
* modified & repackaged in Golgi
* replenishes lipids & proteins of cell membrane
2) Regulated Exocytosis
* products stored for release at specified time
* hormones, digestive enzymes, neurotransmitters
* vesicle membrane does not replenish cell membrane
Solutes are pumped into the _____
extracellular/interstitial fluid
water drawn into interstitial fluid by ____
returned to vascular system via _____
osmosis
lymphatic system
