1.4 Membrane transport Flashcards
The phospholipid bilayer is
selectively permeable.
Some molecules pass through easily (________________), or go through a “tunnel” (_____________-). Others need energy to get through (____________-).
simple diffusion, facilitated diffusion, active transport
Large molecules use their own membrane to get through (_______________-).
endocytosis/exocytosis
Polar heads:
attracted to other polar (charged) molecules
Non-polar tails:
will repel any charged molecule, therefore preventing passage of ions through the membrane.
Vesicles
small spheroidal packages that bud off of the rER and the Golgi apparatus
They carry proteins produced by ribosomes on the RER to the Golgi apparatus, where they are prepared for export from the cell via another vesicle
Vesicles also allow the plasma membrane to
increase in a growing cell.
Phospholipids and membrane proteins are
synthesized and become inserted into the rER membrane.
When vesicles bud off the rER and join the plasma membrane,
the plasma membrane increases in area a little bit – this is also used to increase size of organelles.
endocytosis
part of plasma pulled inwards
droplet of fluid is enclosed when vesicle pinches off
vesicle can then move through cytoplasm carrying their contents
exocytosis
proteins are synthesized by ribsomes and then enter the rough ER
vesicles bud off the rough ER and carry the proteins to the golgi apparatus
golgi apparatus modifies proteins
vesicles bud off the golgi apparatus and carry the modified proteins to the plasma membrane
vesicles fuse with the plasma membrane.
contents of vesicles are expelled
membrane then flattens out again
Endocytosis:
The taking in of external substances by an inward pouching of the plasma membrane, forming a vesicle
Exocytosis:
The release of substances from a cell (secretion) when a vesicle joins with the cell plasma membrane.
vesicle fuses with membrane during endocytosis and exocytosis by:
approaching the plasma membrane. all membranes are made of the phospholipid bilayer, so share the same properties.
The membranes begin to fuse. Remember the fluidity of the plasma membrane - the phospholipids can flow around each other.
For a moment, there is a single phospholipid bilayer a the point of contact.
The membrane pore opens, allowing the contents to pass through. Notice that throughout the whole process, there is never an unbroken section of the bilayer.
Constitutive secretion
occurs continuously in cells, depending on their function
Regulated secretion
is in response to a trigger e.g. the release of neurotransmitters
types of endocytosis:
phagocytosis
- cell eating
- solids
pinocytosis
- cell drinking
- liquids/fluids
receptor-mediated endocytosis
- receptors involved
- coated vesicle
- coated pit
Outline the key ideas of endocytosis
Mechanism whereby cells take in solids and/or solutions
Involve the formation of vesicles
Infolding of the cell membrane to surround the new contents
Called pinocytosis when fluids are taken into the cell
Called phagocytosis when organisms/food particles (solids) are engulfed
May be receptor mediated
diffusion
passive net movement of particles from areas of high concentration to low concentration. often through a permeable membrane
Solutions consist of a
solute (e.g. salt) dissolved in a solvent (e.g. water)
Concentration is the measure of the amount of
solute in the solution.
passive
does not require energy
net movement
overall movement of particles
concentration gradient
the difference of concentrations of a substance in two locations
factors affecting rate of diffusion
- SA
- length of diffusion pathway
- concentration gradient
maximising surface area for absorption
- alveoli in lungs
- membrane folds in mitochondria and in cristae in chloroplasts
- root hairs for mineral ion uptake
- villi for absorption of food molecules
reducing length of diffusion path
- membranes are very thin
- folded membranes increase SA:V ratio
- more membrane in smaller volume = shorter distances that molecules need to diffuse
Osmosis
the passive net movement of water molecules from an area of low [SOLUTE] to high [SOLUTE] through a selectively permeable membrane.
Water molecules pass through the cell membrane from an area of
low [solute] to one of high [solute].
Water can pass through the cell membrane even though it is hydrophilic because.,…
it is small.
Aquaporin
an integral protein that, as its name suggests, acts as a pore in the membrane that speeds the movement of water molecules
Osmolarity
the measure of the concentration of a solute inside of a fluid or a cell.
Cells can be in three types of osmotic environments (solutions):
hypotonic
- low osmolarity
- net water gain
- solute concentration higher in cell
- water moves into cell
hypertonic
- high osmolarity
- net water loss
- solute concentration higher outside cell
- water moves out of the cell
isotonic
- same osmolarity
- no net gain or loss
The importance of osmotic control preventing damage to cells and tissues
Common medical procedures in which an isotonic saline solution is useful:
fluids introduction to a patient’s blood system via an intravenous drip, e.g for rehydration
used to rinse wounds, skin abrasions etc.
keep areas of damaged skin moist before applying skin grafts
eye drops/wash
frozen ice used pack donor organs for transportation
Facilitated diffusion
the passive net movement of molecules from an area of high concentration to an area of low concentration through a selectively permeable membrane with the help of protein channels.
Large and polar molecules can’t get across the membrane via simple diffusion.
Transmembrane (polytopic) proteins recognise…
a particular molecule and help it to move across the membrane.
Cells can control which types of channel proteins are synthesized and placed in the plasma membrane –
this is how they can control which substances diffuse in and out.
Active transport
the active net movement of molecules from an area of low concentration to an area of high concentration (against the concentration gradient). This process requires ATP (energy).
Integral protein pumps
use the energy from the hydrolysis (splitting) of ATP to move ions or large molecules across the cell membrane.
uniport
transport of only one molecule, without coupling to the transport of another molecule or ion.
symport
the type of transport in which two different molecules can move through a membrane in same direction by using a common carrier mechanism.
antiport
the coupled transportation of two different molecules or ions passing through a membrane in opposite directions
Potassium channels in axons are…
voltage gated. They enable the facilitated diffusion of potassium out of the axon
- At one stage during a nerve impulse there are relatively more positive charges inside.
- This voltage change causes potassium channels to open, allowing potassium ions to diffuse out of the axon.
- Once the voltage conditions change the channel rapidly closes again.
The sodium–potassium pump
follows a repeating cycle of steps that result in three sodium ions being pumped out of the axon (of neurons) and two potassium ions being pumped in. Each time the pump goes round this cycle it uses one ATP. The cycle consists of these steps:
- 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 three sodium ions are released.
- Two potassium ions from outside can then enter and attach to their binding sites.
- Binding of potassium causes release of the phosphate group; this causes the pump to change shape again so that it is again only open to the inside of the axon.
- The interior of the pump opens to the inside of the axon and the two potassium ions are released. Sodium ions can now enter and bind to the pump again (#1).
Estimation of osmolarity
a simple lab with many possible variations.
This is an ideal opportunity to practise and improve your understanding of the following IA criteria:
Analysis
Evaluation
Communication
Taking accurate measurements
