Cell Biology - 1.4 Membrane Transport Flashcards
(48 cards)
Passive transport
- a way in which materials move across the cell membrane (by diffusion, facilitated diffusion, osmosis (and active transport)) - does NOT use energy as molecules move from a high to low concentration (ie. down the concentration gradient)
Passive transport involves the movement of material along a concentration gradient (high concentration ⇒ low concentration)
Because materials are moving down a concentration gradient, it does not require the expenditure of energy (ATP hydrolysis)
- simple diffusion
- osmosis
- facilitated diffusion
Active transport
(ion pumps and phagocytosis) - USES ENERGY as molecules move from a LOW TO HIGH concentration AGAINST the concentration gradient
Active transport involves the movement of materials against a concentration gradient (low concentration ⇒ high concentration)
Because materials are moving against the gradient, it requires the expenditure of energy (e.g. ATP hydrolysis)
Can either be:
- direct
- indirect
Semi-permeable
- the membrane is Semi-permeable ie. it allows some molecules through but not all molecules (controls what goes in and out) - some small molecules can easily diffuse across, but most need help
(only certain materials may freely cross – large and charged substances are typically blocked)
Concentration gradient
- the changing in concentration of a substance as it travels over a distance
- where concentration is different across an area = there is a high and low concentration area
- the gradient of concentrations of different particles within an area (eg. the spray in a classroom)
- a concentration gradient exists until the diffused substance is evenly distributed
- if there is a large difference in concentration between the two areas, there is a steep concentration gradient so the diffusion is faster
Simple Diffusion (Passive Transport)
it is the passive movement of particles (atoms, ions molecules) from an area of higher concentration to an ara of lower concentration down a concentration gradient.
- requires no energy
- diffusion occurs as a consequence of the random motion of particles
(eg. glucose diffuses into a cell and is used to make energy by respiration - movement of small or lipophilic molecules (e.g. O2, CO2, etc.))
Factors of diffusion rates
- molecule size - small = move faster
- temperature - higher temperature = move faster
- concentration - diffusion is faster down the steeper gradients
Facilitated diffusion (Passive Transport)
(Simple diffusion but requires proteins to support it)
movement of large or charged molecules via membrane proteins (e.g. ions, sucrose, etc.)
It is the passive transport of molecules or ions across a cell membrane via specific transmembrane integral proteins
THE EXCEPTION = POLAR molecules (excluding water) CANNOT diffuse freely across the plasma membrane - due to the hydrophobic nature of the fatty acid tails of the phospholipids that make up the lipid bilayer - all polar molecules that enter the cells are transported by proteins in the form of transmembrane channels
Lipid bilayer
The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells.
it is a part of the membrane that is virtually impermeable - materials (eg. proteins, hormones, water, etc) are transported across it by being “facilitated” by integral proteins that span the membrane.
Channel proteins (ion channels)
Integral lipoproteins which contain a pore via which ions may cross from one side of the membrane to the other
Channel proteins are ion-selective and may be gated to regulate the passage of ions in response to certain stimuli
Channel proteins only move molecules along a concentration gradient (i.e. are not used in active transport)
Channel proteins have a much faster rate of transport than carrier proteins
Osmosis (passive transport)
Water always wants to go: 1. high water -> low water, 2. low salt -> high salt
“the movement of water molecules from an area of high concentration to an area low water concentration through a semi-permeable membrane”
is the net movement of water molecules across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration (until equilibrium is reached)
- does not use energy to make it happen
- (dependent on solute concentrations)
Osmolarity
“measure of the concentration of solutes in a solution” as defined by the number of osmoles of a solute per litre of solution (osmol/L)
can be categorised as hypertonic, hypotonic or isotonic
Hypotonic
- high water levels
- hypotonic area
- low solute concentration
When a cell is placed in a hypotonic solution it will gain water and swell (become TURGID/lysis) - in a plant cell the cell membrane will be constrained by the cell wall - it could BURST
(Vacuole swells and pushes against the cell membrane === turgent)
water flows into cell
Isotonic
- equal concentration (of solute in both areas - in and outside the shell)
no change = water outside and inside stays equal
Hypertonic
- lower water level
- high solute
- hypertonic area
When a cell is placed in a hypertonic solution it will lose water and shrink (become FLACCID/crenation) - in a plant cell the cell membrane will pull away from the cell wall - in extreme cases the cell cannot recover and is said to be PLASMOLYSED = PLASMALISED
Water flows out of cell
Tonicity
determining if a solution is hypotonic, isotonic or hypertonic
- > high water solution = water wants to go into the cell (opposite applies for cell in salty solution) —> If cells absorb too much water they will pop!!
- must compare it (the solution) to something eg A to B
solution “equation”
solution = solvent (eg. H2O) + solute (dissolved)
Isotonic saline solutions - application
they are used in medical procedures (eg. kidney transplants and intravenous therapy to rehydrate patients) - they must be bathed in the same osmolarity as human cytoplasm
Application - tissues or organs to be used in medical procedures must be bathed in a solution the same osmolarity as the cytoplasm to prevent osmosis
osmoregulation
The process of controlling the amount of water in cells
Why osmosis is important to plants
- plants absorb water through the roots and reabsorption of water by the proximal and distal convoluted tubules of the nephron
- needed for photosynthesis
- allows them to gain water and stay alive -> “support”
(Freshwater - cells gain water as the cytoplasm contains dissolved substances = incise cell has lower water concentration than outside = USEFULL AS: keeps water flowing into cells ensuring they are turgid
saltwater - cells tend to lose water = concentration of water is much higher in cells than sea)
Practical: Osmosis in living potato cells
COMPULSORY PRACTICAL YOU MUST KNOW
Skill = Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions
Aim = determine the osmolarity of potato cells IV = concentration of sucrose/sugar in H2O DV = Osmosis in potato cells CV = potato types, volume of sugar solutions, etc.
Method:
- use cork borer to cut 10 cylinders from potato (equal length)
- weigh each potato
- place into boiling tube x 2 (one with 0.7 mol/l sugar solution, “” 0.5, “” 0.3, “” 0.1, ““0)
- leave overnight, next day re-weigh potatos
- calculate averages and put onto graph
Reasons for results for hypertonic = when the sucrose concentration was greater the percentage decrease was great -> due to the H2O in the potato diffused out of the potato —- opposite applies for hypotonic solutions.
Salt in the human body
Our bodies naturally require salt & because of this when put into a SLIGHTLY hypertonic solution the cells will withhold a “normal” mass - ie. cells were not killed.
Potassium Ion Channels - example of facilitated diffusion
- the most common type of ion channels - they form potassium-selective pores that span cell membranes
Integral proteins with a hydrophilic inner pore via which potassium ions may be transported
The channel is comprised of four
transmembrane subunits, while the inner pore contains a selectivity filter at its narrowest
region that restricts passage of alternative ions
Potassium channels are typically voltage-gated and cycle between an opened and closed conformation depending on the transmembrane voltage
Structure and Function of Potassium Channels in Axons:
- Axons of neuron contain potassium channels that as used during an action potential (They are closed when the axon is polarised but open in response to depolarisation of the axon membrane allowing K+ ions to exit by facilitated diffusion which repolarizes the axon - they will only remain open for a very short time before a blobular sub-unit blocks the pore, the channel then returns to its original closed conformation)
Potassium Ion Channels - Process
- they are used to move K+ ions across the membrane (In axons of neurons, the channels work with the sodium-potassium pump)
=> the axons of nerve cells transmit electrical impulses by translocating ions to create a voltage difference across the membrane: - at rest the the sodium-potassium pump expels sodium ions from the nerve cell, while potassium ions are accumulated within the cell
- When the neuron fires, these ions swap locations via facilitated diffusion via sodium and potassium channels.
Active Transport
- is the movement of molecules from an area of low concentration to an area of high concentration against the concentration gradient and REQUIRES energy (ATP)
- Integral protein pumps use the energy from the hydrolysis of ATP to move ions or large molecules across the cell membrane
eg.
1. uptake of glucose in the intestines in humans
2. the exchange of Na and K ions in nerve axons
3. uptake of mineral ions into root hair cells of plants
