what are the types of transport?
- active
- passive
What is passive transport?
the movement of material along a concentration gradient (high concentration ⇒ low concentration)
Why does passive transport not need energy?
Because materials are moving down a concentration gradient, it does not require the expenditure of energy (ATP hydrolysis)
What are the types of passive transport?
- simple diffusion
- osmosis
- facilitated diffusion
What is active transport
the movement of materials against a concentration gradient (low concentration ⇒ high concentration)
What is diffusion?
the net movement of molecules from a region of high concentration to a region of low concentration
- This directional movement along a gradient is passive and will continue until molecules become evenly dispersed (equilibrium)
- Small and non-polar (lipophilic) molecules will be able to freely diffuse across cell membranes (e.g. O2, CO2, glycerol)
what impacts the rate of diffusion?
- temp
- molecular size
- steepness of the gradient
What is osmosis?
Osmosis 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)
What causes water to move across the membrane?
- Water is considered the universal solvent – it will associate with, and dissolve, polar or charged molecules (solutes)
- Because solutes cannot cross a cell membrane unaided, water will move to equalise the two solutions
- At a higher solute concentration there are less free water molecules in solution as water is associated with the solute
- Osmosis is essentially the diffusion of free water molecules and hence occurs from regions of low solute concentration
What is osmolarity?
Osmolarity is a measure of solute concentration, as defined by the number of osmoles of a solute per litre of solution (osmol/L)
Tissue organs in medical procedures
Tissues or organs to be used in medical procedures must be kept in solution to prevent cellular dessication
This solution must share the same osmolarity as the tissue / organ (i.e. isotonic) in order to prevent osmosis from occurring
What is facilitated diffusion?
Facilitated diffusion is the passive movement of molecules across the cell membrane via the aid of a membrane proteins
When is facilitated transport used?
by molecules that are unable to freely cross the phospholipid bilayer (e.g. large, polar molecules and ions)
What mediated facilitated transport?
by two distinct types of transport proteins – channel proteins and carrier proteins
Carrier proteins
- Integral glycoproteins which bind a solute and undergo a conformational change to translocate the solute across the membrane
- Carrier proteins will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction
- Carrier proteins may move molecules against concentration gradients in the presence of ATP (i.e. are used in active transport)
- Carrier proteins have a much slower rate of transport than channel proteins (by an order of ~1,000 molecules per second)
Channel proteins
- 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
when are sodium-potassium pumps used?
in active transport
- At rest, the sodium-potassium pump expels sodium ions from the nerve cell, while potassium ions are accumulated within
when are potassium channels used?
in facilitated diffusion
- When the neuron fires, these ions swap locations via facilitated diffusion via sodium and potassium channels
Potassium Channels
- 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
Active transport involves the use of carrier proteins (called protein pumps due to their use of energy)
- A specific solute will bind 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
Sodium-Potassium Pump
An integral protein that exchanges 3 sodium ions (moves out of cell) with two potassium ions (moves into cell)
The process of ion exchange against the gradient is energy-dependent and involves a number of key steps:
- 3 sodium ions bind to intracellular sites on the sodium-potassium pump
- A phosphate group is transferred to the pump via the hydrolysis of ATP
- The pump undergoes a conformational change, translocating sodium across the membrane
- The conformational change exposes two potassium binding sites on the extracellular surface of the pump
- The phosphate group is released which causes the pump to return to its original conformation
- This translocates the potassium across the membrane, completing the ion exchange
Sodium potassium pump
a carrier protein that uses ATP to actively transport three sodium ions out of a cell and two potassium ions into the cell
Axon potassium channels
Uses facilitated diffusion. Is voltage gated, when more negative on the outside (during nerve impulse) gate is open, potassium ions pass through and gate is rapidly closed due to a ball attached by amnio acids.
Uncontrolled osmosis in plant tissues
- the effects of uncontrolled osmosis are moderated by the presence of an inflexible cell wall
In hypertonic solutions, the cytoplasm will shrink (plasmolysis) but the cell wall will maintain a structured shape
In hypotonic solutions, the cytoplasm will expand but be unable to rupture within the constraints of the cell wall (turgor)
