1.4 membrane transport Flashcards
1
Q
what are the diff ways particles can move across membranes?
A
- simple diffusion
- facilitated diffusion
- osmosis
- active transport
2
Q
what regulates movement of molecules in and out of cell?
A
plasma membrane
- regulates ability to take in molecules from the environment and expel molecules out
- selectively permeable membrane
- some factors that affect ability of membrane to cross: charge / nature of molecule, size, concentration gradient across membrane
3
Q
look at the permeability of lipid bilayer and comment
A
- hydrophobic (non-polar) molecules are able to dissolve in the lipid layer and pass through the membrane rapidly
- polar molecules are not able to easily pass through the non-polar region within the lipid bilayer
- transport proteins allows for the passage of polar molecules across the membrane via facilitated diffusion or active transport
4
Q
what is diffusion?
A
- tendency for molecules to spread out evenly within an available space
- passive process and does not require any input of additional energy
- molecules individually may move randomly, diffusion of a molecule may show a net movement in one direction
- dynamic equilibrium refers to a state of no net movement of molecules when the rate of molecular movement in one direction is matched with the rate of molecule movement in the other direction
- follows the concentration gradient from high to low
5
Q
what are the factors affecting rate of diffusion?
A
- particle size – smaller particles diffuse faster
- thickness of membrane affects the length of the diffusion pathway, with thin membranes increasing the rate of diffusion
- higher surface area available will increase the rate of diffusion
- higher temperature will increase the rate of diffusion as each molecule has more kinetic energy
- steeper concentration gradient will increase the rate of diffusion
6
Q
what is osmosis?
A
- diffusion of water molecules across a selectively permeable membrane
- water will move from an area of higher water potential (lower solute concentration) to an area of lower water potential (higher solute concentration)
- selectively permeable or partially permeable membrane allows for certain molecules to pass through but not others
- basis for selection for such membranes is usually by size, with smaller molecules generally able to pass through the membrane
7
Q
what is tonicity?
A
- ability of a solution to cause a cell to gain or lose water
- isotonic solution has a solute concentration that is the same as the inside of a cell, and no net osmosis will take place
- hypertonic (hyper = excess) solution: water potential in cell greater than that of solute concentration outside, and the cell will lose water via osmosis
- hypotonic solution: water potential of cell less than that of solute concentration outside, and the cell will gain water via osmosis
8
Q
what is osmoregulation?
A
- process of regulating osmotic potential of cell, and this is known as osmoregulation (e.g. paramecium have contractile vacuoles that can contract and expel excess water from the cell)
- some cells have cell walls (like cellulose cell walls in plant cells) that can prevent the cell from swelling too big and lysing (bursting) as a result of water intake due to osmosis
- cell is considered turgid when the cell swells up due to water intake and presses against the cell wall, and in plant cells this helps the plant to stand upright
- cell is considered flaccid when it loses water and the plasma membrane pulls away from the cell wall
9
Q
why must tissues or organs to be used in medical procedures be bathed in a solution with the same osmolarity as the cytoplasm?
A
- to prevent osmosis
- red blood cells can become crenated or lyse when placed in hypertonic or hypotonic solutions, it is important for red blood cells to be stored in isotonic saline solutions
- similarly, intravenous drips and other solutions that are introduced to a patient’s blood must be isotonic to prevent blood cells from lysing or becoming crenated, thus losing their function
- donor tissues or organs extracted from transplant must likewise be kept in isotonic saline solutions to protect the structural integrity of the cells and preventing them from changing in shape due to differences in osmotic potential of the transport solution
10
Q
what is facilitated diffusion?
A
- in facilitated diffusion, transport proteins speed up the movement of molecules across the plasma membrane
- even though the movement of molecules is facilitated, the movement is passive and must go down a concentration gradient with no input of energy
- channel proteins are a type of transport protein that possess a hydrophilic channel within the protein that certain molecules or ions are able to use as a tunnel to pass through the membrane
- aquaporins are an example of channel proteins specific to the movement of water molecules across the cell membrane
- ion channels are channel proteins that open or close in response to a stimulus (gated channels)
- carrier proteins are another type of transport proteins that bind to specific molecules and undergo a subtle change in shape that translocates the solute binding site (carrying the solute) across to the other side of the membrane
- transport proteins are highly specific to the type of molecules it moves
- carrier proteins allow for the diffusion of large polar molecules across the cell membrane (e.g. sugar and amino acid molecules)
11
Q
compare simple diffusion and facilitated diffusion
A
DIFF
simple: no requirement for carrier molecules
facilitated: via special pores or carrier molecules
simple: similar molecules diffuse at similar rate
facilitated: specific molecules diffuse faster
SIMILARITY
- rate dependent on concentration gradient
- equilibrium reached when concentrations are equal
- energy from atp not required
12
Q
what is active transport?
A
- active transport is active, and uses energy in order to move molecules against a concentration gradient
- as compared to facilitated diffusion, which is passive and can move molecules without expending energy, but can only move molecules down a concentration gradient
- energy is supplied in the form of Adenosine Triphosphate (ATP)
- carried out by specific proteins embedded within the cell membrane
- molecule or ion binds to a specific binding site on a carrier protein
- ATP on inside of the membrane binds to carrier protein and transfers a phosphate group to the protein
- change in shape of carrier protein, moving the molecule or ion across the membrane
- molecule or ion is released on the other side of the membrane and the carrier protein reverts to its original shape
- with expenditure of energy, this can take place against or regardless of the concentration gradient
- cells involved in active transport tend to have: many mitochondria & high rate of respiration
- factors that affect respiration also affects rate of active transport
• temperature
• oxygen concentration
• presence of respiratory poisons like cyanide - examples of active transport processes:
• nerve impulse transmission
• muscle contraction
• absorption of amino acids in the intestines
• absorption of mineral salts by plant roots
• excretion of urea by the kidney
13
Q
what are the sodium-potassium pumps and how do they work?
A
- sodium – potassium pump is an important pump in nerve axons, and follows a repeated cycle of steps that pumps 3 sodium ions out of the cell and 2 potassium ions into the cell per cycle
- each cycle utilises 1 molecule of ATP
- creates an imbalance of charges which is important
to maintain the membrane potential in a nerve axon - average membrane potential in a typical nerve axon is about -70mV
- membrane potential and imbalance of ions across the axon membrane allows for depolarisation and the generation of an electric charge down the axon
- process:
1. interior of pump open to the inside of the axon, and 3 sodium ions enter the pump and binds to the binding site
2. ATP binds and transfers 1 phosphate group to the pump
3. causes pump to change shape and this expels 3 sodium ions out of the cell
4. 2 potassium ions from outside the axon binds to the pump at the binding sites, triggering the release of the phosphate group
5. loss of phosphate triggers pump to return to original shape
6. potassium ions released from the pump due to the shape change and this restores the pump back to its original configuration, ready for the next cycle
14
Q
what are vesicles?
A
- small spherical packages that bud off from the endomembrane system (mainly RER and golgi apparatus)
- carry products (mainly proteins) produced by the ribosomes on the rough ER to the golgi apparatus, or from the golgi apparatus to exit the cell via exocytosis
15
Q
how do large molecules cross plasma membrane?
A
- via bulk transport, which requires the expenditure of energy
- bulk transport occur in cells mainly in the form of endocytosis (taken into the cell) or exocytosis (released by the cell)
