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Molecular Biology Flashcards > Cell Membrane Transport II - Carriers and Pumps > Flashcards

Cell Membrane Transport II - Carriers and Pumps Flashcards

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1
Q

Learning outcomes

A
  • 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
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2
Q

Membrane transport processes

A

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

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3
Q

Pores and Channels

A
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4
Q

Channels can be highly selective:
selectivity filter of bacterial K+ channel

A
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5
Q

Rate of transport

A
  • 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
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6
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8
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8
Q

Carriers (transporters)

A
  • 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)
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9
Q

Carriers mediated transport

A
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9
Q

The presence of carriers affects the
rate of transport

A
  • 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
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10
Q

Types of carrier mediated transport

A
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10
Q

The presence of carriers affects the
rate of transport

A
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11
Q

Types of carrier mediated transport

A

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

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11
Q

Co-transport: secondary
active transport

A
  • 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
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11
Q

Co-transport: secondary
active transport

A
12
Q

Primary active transport

A
  • Active transport uses ATP to power movement
  • Substance is ‘pumped’ against its concentration
    gradient
  • Phosphorylation triggers a conformational
    change
  • Conformational change alters binding affinity
12
Q

Co-transport: secondary
active transport

A
13
Q

Primary active transport

A
14
Q

Na+
/K+ pump - primary active transport

A
  • 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
14
Q
A
15
Q
A
16
Q
A
16
Q

Epithelial solute transport

A

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

17
Q

Three classes of ATP driven pumps

A
  1. P-type pumps –phosphorylate themselves during the pumping cycle. Ion pumps setup & maintaining
    concentration gradients across membranes (e.g. Na+
    , K+
    , H+
    , Ca2+)
  2. ABC transporters (ATP-Binding Cassette transporters) -primarily pump small molecules across cell membranes
    (e.g. cholesterol).
  3. 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
18
Q
A
18
Q

Intracellular Ca2+ is kept low by pumps

A

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)

19
Q

Intracellular Ca2+ is kept low by pumps

A
20
Q

Ion gradients, channels and
transporters in a typical cell

A
21
Q

Ion gradients, channels and
transporters in a typical cell (more!)

A
22
Q

Endocytosis and exocytosis

A
  • 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
22
Q

Endocytosis and exocytosis

A
23
Q
A

Molecular Biology Flashcards (20 decks)

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