topic 1.4- membrane transport Flashcards
state the 4 ways in which particles move across membranes
- simple diffusion
- facilitated diffusion
- osmosis
- active transport
define diffusion
the net, passive movement of molecules from a region of high concentration to a region of low concentration
explain the different abilities of:
- small non-polar molecules
- small polar molecules
- large polar molecules
to diffuse across the phospholipid bilayer
easiest->hardest
1. small non-polar molecules
2. small polar molecules
3. large polar molecules
polar/charged molecules repel hydrophobic tails of phospholipids
give 3 examples of molecules that will move easily by diffusion
oxygen, CO2, glycerol
give a real-life application of simple diffusion
the cornea has no blood supply so its cells obtain oxygen by simple diffusion from the air (it passes through the fluid/tears and into the cornea)
define facilitated diffusion
the passive movement of molecules across the cell membrane via the aid of a membrane protein
what types of molecules use facilitated diffusion?
molecules that are unable to freely cross the phospholipid bilayer (e.g. large, polar molecules and ions)
what two types of protein mediate facilitated diffusion?
channel proteins and carrier proteins
what are carrier proteins?
Integral glycoproteins
how do carrier proteins function?
bind a solute and undergo a conformational change to translocate the solute across the membrane
can carrier proteins bind to any molecules?
no; they will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction
can carrier proteins bind to any molecules?
no; they will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction
can carrier proteins move molecules against the concentration gradient?
may move molecules against concentration gradients in the presence of ATP (i.e. are used in active transport)
are carrier proteins faster or slower than channel proteins?
Carrier proteins have a much slower rate of transport than channel proteins
what are channel proteins?
Integral lipoproteins
how do channel proteins work?
they contain a pore via which ions may cross from one side of the membrane to the other
do channel proteins let any molecules through?
no; they are ion-selective and may be gated to regulate the passage of ions in response to certain stimuli
can channel proteins move molecules against the concentration gradient?
no; they can only move molecules along a concentration gradient (i.e. are not used in active transport)
what is an axon? and what is its function?
- a part of a neuron which consists of a tubular membrane with cytoplasm inside
- used to convey messages rapidly in the form of an electrical impulse
what does a nerve impulse consist of?
rapid movements of Na ions into and K ions out of the axon membrane via facilitated diffusion (through Na and K channels)
potassium channels in axons are —-
voltage gated
what causes voltage across membranes?
- an imbalance of positive and negative charges across the membrane
- more +ve outside axon than inside= K channel closed
- more +ve inside axon than outside = K channel open, so ions diffuse through
describe and draw the 3 steps of facilitated diffusion of potassium in axons
- channel closed; there is a net -ve charge inside the axon and a net +ve charge outside
- channel briefly opens; net +ve charge inside and net -ve charge inside; K ions rush outside by translocating ions to create a voltage difference across the membrane
- channel closed by ‘ball and chain’ within milliseconds
how is the necessary concentration gradient restored for nerve impulses?
active transport, done by a sodium-potassium pump protein
describe and draw the first three steps of the sodium-potassium pump cycle
- the interior of the pump is open to the inside of the axon; three sodium ions enter the pump and attach to their binding sites
- ATP transfers a phosphate group from itself to the pump; this causes the pump to change shape and the interior is then closed
- the interior of the pump opens to the outside of the axon and the 3 Na ions are released
describe and draw the final three steps of the sodium-potassium pump cycle
- 2 K ions from outside can then enter and attach to their binding sites
- binding of K causes release of the phosphate group; this causes the pump to change shape again
- the interior of the pump opens to the inside of the axon and the 2 K ions are released
define active transport
the use of energy to move molecules against a concentration gradient
how is energy for active transport generated?
- The direct hydrolysis of ATP (primary active transport)
- Indirectly coupling transport with another molecule that is moving along its gradient (secondary active transport)
what proteins does active transport involve?
carrier proteins (called protein pumps due to their use of energy)
describe active transport
- A specific solute binds to the protein pump on one side of the membrane
- The hydrolysis of ATP (to ADP + Pi) causes a conformational change in the protein pump
- The solute molecule is consequently translocated across the membrane (against the gradient) and released
define osmosis
the net movement of water molecules across a semi-permeable membrane from a region of low solute concentration/high water potential to a region of high solute concentration/low water potential
why can osmosis happen in all cells?
water molecules, despite being hydrophilic, are small enough to pass through the phospholipid bilayer
some cells have water channels called ——-
aquaporins
describe the function of aquaporins and give examples
they greatly increase membrane permeability to water- eg kidney cells that reabsorb water and root hair cells in plants
what does the fluidity of membranes allow?
materials to be taken into cells by endocytosis or released by exocytosis
define endocytosis
The process by which large substances (or bulk amounts of smaller substances) enter the cell without crossing the membrane
describe endocytosis
- An invagination of the membrane forms a flask-like depression which envelopes the extracellular material
- The invagination is then sealed off to form an intracellular vesicle containing the material
is endocytosis a passive process?
no, the proteins in the membrane that carry out this process requires energy from ATP
give two main examples of endocytosis
- phagocytosis to ingest/kill pathogens
- in the placenta, proteins from the mother’s blood, including antibodies are absorbed into the foetus
describe the movement of vesicles within a cell
- endocytosis; vesicles can then move through the cytoplasm
- proteins are synthesised by ribosomes
- vesicles enter the rough endoplasmic reticulum and accumulate there
- vesicles bud off from the rER
- the Golgi apparatus modifies the proteins
- vesicles bud off from the Golgi and carry the modified proteins to the plasma membrane
- endocytosis
describe exocytosis
- vesicles fuse with the plasma membrane
- the contents of the vesicle are expelled and the membrane flattens out again
give 2 examples of exocytosis
Digestive enzymes released from gland cells by exocytosis (secretion)
expulsion of waste products or unwanted materials (eg water in unicellular organisms through contractile vacuoles)
define a isotonic solution
a solution that has the same osmolarity as a tissue
define a hypertonic solution
a solution that has a higher osmolarity than a tissue
define a hypotonic solution
a solution that has a lower osmolarity than a tissue
describe what will happen to animal cells in hypertonic/hypotonic solutions
hypertonic solutions; water will leave the cell causing it to shrivel (crenation)
hypotonic solutions; water will enter the cell causing it to swell and potentially burst (lysis)
describe what will happen to plant cells in hypertonic/hypotonic solutions
hypertonic solutions; the cytoplasm will shrink (plasmolysis) but the cell wall will maintain a structured shape
hypotonic solutions; the cytoplasm will expand but be unable to rupture within the constraints of the cell wall (turgor)
what is a real life application of isotonic solutions
it is important for any human tissues or organs to be bathed in an isotonic solution during medical procedures. usually, NaCl solution is used (normal saline)- osmolarity= 300mOsm= cytoplasm osmolarity
how could we determine the osmolarity of a tissue?
bathe it in a salt solution for a short time and measure the increase/decrease in mass- when mass change=0, that is the same conc
