13 - passive & active transport mechanisms Flashcards
what are channel-mediated transport processes?
Channel-mediated: transmembrane channel proteins form open pores through the membrane, allowing the free diffusion of small molecules of the appropriate size and charge.
(e.g. Na+ ions moving through a Na+ channel)
what are careier-mediated transport processes?
Carrier-mediated: transmembrane carrier proteins bind a specific molecule on one side of the membrane then undergo a
conformational change that allows the molecule to pass through the membrane and be released on the other side.
(e.g. glucose is transported by a GLUT1 carrier)
what are the best-characterised channel proteins?
ion channels, which mediate the passage of ions across plasma membranes.
where are ion channels present?
Present in the membranes of all cells, but especially well studied in nerve and
muscle.
explain ion channels
transport is extremely rapid (many times faster than carrier proteins)
highly selective (narrow pores restrict passage to ions appropriate size and charge)
- specific channels enable the passage of Na+, K+, Ca2+, and C-
most ion channels are not permanently open
- the opening of channels is regulated by “gates” that transiently open in response to specific stimuli.
how to carrier mediated transport proteins work?
Polar molecules can cross the cell membrane through carrier proteins (e.g. sugars, amino acids, and nucleosides)
– unlike channels, carriers are not open to both sides of the
membrane simultaneously
each transport protein specific to one molecule.
what are the three classes of transport protein?
- uniport
- symport
- antiport
how do uniporters work?
- polar molecule bonds to specific site on transport protein
- conformational change occurs. polar molecule is released into the cell.
what is facilitated diffusion?
carrier- and channel mediated transport can be an example of facilitated diffusion: facilitating movement of polar molecules/ions down their concentration gradient.
why can polar molecules and ions not cross the plasma membrane?
due to the hydrophobic tails of the phospholipids.
what do membrane transport proteins allow?
Membrane transport proteins allow certain polar molecules
and ions across the membrane
…
Proteins involved in facilitated diffusion are very specific to a
particular substrate.
Facilitated diffusion is a PASSIVE process. what does this mean?
Facilitated diffusion is a PASSIVE process. This means that the polar molecule or ion can only move DOWN the diffusion gradient.
why is there a limit to how fast facilitated diffusion can occur?
There are only so many transport proteins for a particular substrate on the surface of a cell. Therefore, there is a limit to how fast facilitated diffusion can occur.
what are the limitations of facilitated diffusion?
The fact that this is a PASSIVE process means that eventually, the concentration of the substrate inside and outside the cell will be equal.
Therefore, a driving force is required to maintain the diffusion gradient.
This is achieved through ACTIVE TRANSPORT
what is active transport?
cell pumps polar molecule or ion against the diffusion gradient using energy.
active transport is central to how cells maintain different intracellular concentrations.
what are some examples of active transport?
Active transport proteins can work both ways: they can pump ions into the cell or out of the cell.
in animal cells:
Intracellular K+ is greater than extracellular K+, therefore
K+ must be actively pumped into the cell.
conversely:
Intracellular Na+ is less than extracellular Na+, therefore
Na+ is actively pumped out of the cell.
what are exchange pumps?
Some transport proteins
operate by pumping out one
ion whilst taking in another
type of ion.
these are exchange pumps.
what is an important example of exchange pumps?
An important example is the
Sodium-Potassium Exchange
Pump, or the Sodium- Potassium ATPase.
substrates required:
- 1 ATP
- 3 Na+ ions
- 2 K+ ions
explain active transport
Active transport is a major factor in establishing differences in intracellular and extracellular ion concentrations.
it requires energy (e.g. ATP) to operate.
It moves polar molecules and ions AGAINST their concentration gradient
An important example of an active transport protein is the
Sodium-Potassium Exchange Pump. This is an example of an antiporter protein.
what charge does the inside face of the plasma membrane have?
net negative charge
what net charge does the outer face of the plasma membrane have?
net positive charge.
what does the difference in charge result from?
from selective permeability of the membrane and the transport processes discussed.
what is resting membrane potential?
Opposites attract… but difference in charge is held apart by the membrane (e.g. a membrane potential)
In a resting cell, this called the resting membrane potential and is measured in mV (typically -80 to -40 mV)
what do cells use resting membrane potential for?
Cell use this energy for key physiological processes e.g.
– secretion from glands
– transmission of information in nerves
– muscle contraction
what is secondary active transport?
As an ion diffuses into the cell down its concentration gradient, it carries another ion or polar molecule with it, they are co-transported.
Active transport is then used to pump the ions out of the cell.
what is bulk transport
Lipophilic molecules enter cells through diffusion across the membrane
Hydrophilic molecules (polar molecules and ions) can enter cells through transport proteins.
What about larger substrates, such as:
– Proteins
– Bacteria and viruses for removal and destruction
Different types of bulk transport:
– exocytosis
– endocytosis:
* phagocytosis
* pinocytosis
* receptor mediated endocytosis
why are vesicles the key to bulk transport
Small membrane-enclosed sacs containing a different molecular composition to their surroundings
Composed of lipid bilayer with distinct inner and outer faces
Produced by the endomembrane system for
exocytosis
what is exocytosis?
Macromolecules such as proteins and
polysaccharides must sometimes be
secreted outside the cell
what are the three types of vesicle produced by the Golgi apparatus?
sensory vesicles: exocytosis of macromolecules
membrane-renewal vesicles: deliver new lipids and proteins to the cell membrane.
lysosomes: contain digestive enzymes and remain inside the cell.
give an example of endocytosis
Cell engulfs a large
extracellular object via cellular
extensions called pseudopodia.
Object is surrounded creating a large membrane enclosed sac (phagosome).
Lysosomes fuse with phagosome to digest extracellular object
Phagocytosis performed by
specialised cells (e.g.
macrophages)
explain receptor-mediated endocytosis
- target molecules (ligands) bind to receptors in plasma membrane.
- Areas coated with ligands form deep pockets in plasma membrane surface.
- Pockets pinch off, forming endosomes known as coated vesicles.
- Coated vesicles fuse with primary lysosomes to form secondary lysosomes
- Ligands are removed and absorbed into the cytoplasm.
- the lysosomal and endosomal membranes separate.
- The endosome fuses with the plasma membrane, and the receptors are again available for ligand binding.
what are the key points of receptor mediated endocytosis?
selective process - receptors in plasma membrane cluster together in coated pits; the intracellular side contains the protein clathrin.
Enables cell to acquire bulk quantities of a specific substrate
once released from the receptor, the ligand can enter the cell for use in intracellular processes.
give two exampled of receptor mediated endocytosis
uptake of cholesterol
uptake of iron, attached to special transport proteins.
what is pinocytosis?
Formation of endosomes (pinosomes) filled with extracellular fluid
Not receptor mediated, target is the local extracellular fluid
Deep groove or pocket forms in plasma membrane and then pinches off, forming vesicle inside the cell
