Cell Membrane Transport II - Carriers and Pumps Flashcards
Learning outcomes
- To understand the mechanisms, energy source and requirement of integral
membrane proteins associated with: - passive diffusion
- carrier mediated transport
- primary and secondary active transport
- Explain how transport rates are affected by carriers and channels
- Understand how coupled transport enables the movement of solutes against an
electrochemical gradient - Explain the two-step process involved in the net transport of solutes across
epithelia - Understand exo-and endo-cytosis
Membrane transport processes
Simple passive diffusion
* Through the membrane
Facilitated diffusion
* Through channel or transporter
* Through symport (same direction)
* Through antiport (opposite direction)
Active transport
* Transport through a pump – coupled to chemical
process
* Primary active transport (using a fuel, often ATP)
* Does not require concentration gradient, but can
cause one
* Secondary active transport (using existing
concentration gradients)
Endo/ exocytosis
* Transport of substances into (endocytosis) or out
(exocytosis) of the cell via vesicles
Pores and Channels
LMV
Channels can be highly selective:
selectivity filter of bacterial K+ channel
Rate of transport
- Higher concentration gradient means a higher rate of
diffusion - No rate limit for simple diffusion
- Pores and channels increase the permeability, but it is
still simple diffusion - Carrier mediated transport shows saturation
Carriers (transporters)
- Carriers bind the solute and change conformation
- Rate limited by speed of each carrier and total number of carriers
- Called facilitated diffusion
- Energy is from the concentration gradient
- Works equally well in both directions (net movement depends on concentration gradient)
Carriers mediated transport
The presence of carriers affects the
rate of transport
- Carrier mediated transport shows saturation -maximum transport rate Jmax.
- Km is the affinity constant -solute concentration ([x]) at which JX is half of the maximal flux (Jmax).
- Lower Km, the higher the apparent affinity of the transporter for the solute.
- Fixed number of membrane bound carriers for each solute
- Limited speed at which the carrier can operate
- Transport rate can be reduced by competition for binding sites
- Transport rate is temperature dependent, depends on activation energy
- They can be “blocked” pharmacologically by specific drugs & toxins
Types of carrier mediated transport
The presence of carriers affects the
rate of transport
Types of carrier mediated transport
Uniporters: transport a single solute from one side
of the membrane to another
Co-transporters: transfer of one solute depends on
the simultaneous transport of a second solute
(coupled transport).
Two subtypes of coupled transport:
Symport: transports second solute in the same
direction as the first
Antiport: transports second solute in opposite
direction as the first
Co-transport: secondary
active transport
- SGLT1 (sodium-dependent glucose transporter)
- Na+
-glucose transporter moves both Na+ and glucose
into cells - Utilizes the Na+ electrochemical gradient to transport
glucose against its concentration gradient
Co-transport: secondary
active transport
Primary active transport
- Active transport uses ATP to power movement
- Substance is ‘pumped’ against its concentration
gradient - Phosphorylation triggers a conformational
change - Conformational change alters binding affinity
The secondary active transport does not require ATP directly, but the electrochemical gradient produced by the primary active transport is utilized by the ATP.
Co-transport: secondary
active transport
Primary active transport
Na+
/K+ pump - primary active transport
- Pumps Na+ and K+
in sequence not together - Transports 3 Na+ out for 2 K+
in using one ATP
molecule - Phosphorylation changes binding affinity
- Electrogenic (net movement of charge)
- Moves Na+ and K+ against their concentration
gradients - Maintains intra and extracellular Na+ and K+
concentrations
Antiport and symport processes are associated with secondary active transport
Epithelial solute transport
Glucose uptake by intestine:
* Primary active transport via Na+
/K+ pump keeps
intracellular Na+
low
* Na+
/Glucose cotransporter facilitates glucose
entry on apical side (Na+ gradient powers
glucose uptake into cell)
* High Glucose in cell powers facilitated diffusion
by carrier on basolateral side
* Requires differential expression of transport
proteins on apical and basolateral membranes
Three classes of ATP driven pumps
- P-type pumps –phosphorylate themselves during the pumping cycle. Ion pumps setup & maintaining
concentration gradients across membranes (e.g. Na+
, K+
, H+
, Ca2+) - ABC transporters (ATP-Binding Cassette transporters) -primarily pump small molecules across cell membranes
(e.g. cholesterol). - V-type pumps -turbine-like protein machines, constructed from multiple different subunits. Transfers H+
into
organelles such as lysosomes to acidify their interior.
F-type ATPases, structurally related to V-type but work in reverse: use H+ gradient across membrane to drive the
synthesis of ATP from ADP and phosphate
VAP
Intracellular Ca2+ is kept low by pumps
Intracellular Ca2+ concentration = ~ 10-4 mM
Extracellular Ca2+ concentration = ~ 1 – 2 mM
Ca2+ pumps are expressed on different membranes
to keep intracellular Ca2+ low
Cell membrane: PMCA (plasma membrane Ca2+-
ATPase)
Ca2+ pumps on sarcoplasmic reticulum: SERCA
(sarcoplasmic and endoplasmic reticulum ATPase)
Intracellular Ca2+ is kept low by pumps
Ion gradients, channels and
transporters in a typical cell
Ion gradients, channels and
transporters in a typical cell (more!)
Endocytosis and exocytosis
- Vesicle mediated transport of
substances either into
(endocytosis) or out
(exocytosis) of the cell - Good for bigger things such
as proteins - Can be very specific
Endocytosis and exocytosis
