Transport of Solutes Across Plasma
Membrane (II)
Facilitated Transport Passive
Facilitated Transport Active
Introduction
• In the last lecture, we studied diffusion and osmosis. This
PPT is on facilitated transport of solutes. The term
facilitated means “aided by” or “made possible by”
proteins in the plasma membrane. These proteins will
function as carriers, channels or pumps.
• Study these slides and read relevant pages in Guyton.
• In the lecture we’ll compare and contrast diffusion with
facilitated transport and look at their significance in the
context of physiological processes like absorption of
nutrients in the gastrointestinal tract.
• There are 28 slides
2
How Solutes Cross Membranes
• Solutes move across cell membranes by:
1. Simple diffusion: We studied it last week
2. Facilitated transport: If they are relatively large, polar or
charged (see next slide for examples). Facilitated
transport may be:


•
Passive transport (this is also called facilitated diffusion) OR
Active (this is also called active transport)
Very large solutes (like proteins) move across the plasma
membrane by bulk transport, called:


Endocytosis: For transport into the cell OR
Exocytosis: For transport out of the cell
3
Relative Permeability of Synthetic Bilayers to
Some Solutes (Alberts Fig. 12.2)
4
Facilitated Transport
• Facilitated transport may be characterized as
 Active if it is endergonic OR
 Passive, if it is exergonic. (Passive facilitated transport is also
called facilitated diffusion)
• It is mediated by membrane proteins and it is for solutes
that are:
 large and polar (previous slide for examples)
 for cations and anions (previous slide for examples)
5
Passive and Active Transport Compared
Alberts Fig. 12.4
6
Types of Transport Proteins
• The proteins that mediate facilitated transport are
called:
 Transporters
• Also called carriers, pumps, permeases
• Mediate either active or passive facilitated transport
 Channel proteins
• Ion channels, porins, aquaporins
 Channel proteins mediate passive transport, always
• They are all multipass proteins
7
Characteristics of Transporters
• Proteins that function as transporters
 Are allosteric
 Have binding sites for one or more solutes
 May behave as:
• Uniports or
• Coupled* transporters, and coupled transporters may function as:
 symports (also called symporters) or antiports (also called
antiporters)
 Are solute specific
 Exhibit Michaelis-menten kinetics *
• Specificity, Vmax, Km,
• Inhibition (competitive & noncompetitive)
8
Conformational Change in a Transporter
(Alberts Fig. 12.7)
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Characteristics of Channels
• Some are allosteric some are not allosteric
• They are very selective for specific ions
• Exist as two types, called
 Leak channels (always open) Or
 Gated channels. Gated channels fluctuate between:
•
open <-> close <-> inactivated states
• Types of Gated channels
 Voltage-gated: Open/close in response to changes in Vm
 Ligand-gated: Open/close in response to ligand binding to receptor
 Mechanosensitive: Open/close in response to forces (pressure,
tension…)
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Typical Ion Channel
(Alberts Fig. 12.20)
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Selectivity of Ion Channels
(Alberts Fig. 12-19)
12
Gated Ion Channels
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Facilitated Transport of
Non-Charged Solute
• Passive transport of non-charged solutes is




Mediated by transporters
Driven by magnitude of gradient (S)
Down gradient and toward equilibrium
Net flow is in either direction (into or out)
• Exhibits Michalis-Menten kinetics
14
Facilitated Transport of Ions
• Facilitated transport of ions is mediated by channels- leak
or gated
 Is down gradient and towards equilibrium
 Is driven by electrochemical gradient
 The electrochemical gradient takes into account both the
membrane potential plus the concentration gradient
• Depending on the charge of the ion, the membrane potential
may favor or oppose influx or efflux of the ion
15
Effect of Electrochemical Gradient on Ion Flux
(Study legend Fig. 12.8)
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Active Transport
Study Guyton Ch. 4
Carried out by
1. Coupled Transporters
2. ATP-Driven Pumps
3. Light-Driven Pump (not in human cells)
Active Transport(1): Significance
• Active transport is important for:
 Intake of nutrients and solutes from the extra-cellular fluid even when the
concentration of these solutes is higher inside the cell.
 For moving wastes or excess ions out of the cell even their concentrations
is higher in the extracellular fluid.
 For maintaining non-equilibrium concentrations of certain ions across the
plasma membrane (or across membrane of certain organelles
• Last item is essential for sustaining life. Many cellular functions (like nerve
impulse conduction) depend on these concentration gradients.
• Active transport may be classified as:
 1) primary or direct if coupled to ATP hydrolysis
 2) secondary or indirect if not directly coupled to ATP hydrolysis. This
type is coupled to the potential energy in a Na+ or H+ gradient
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Active Transport (2): Some facts
•
•
•
•
•
•
Mediated by uniports, symports or antiports
Kinetics more complex than Michalis’
Against gradient and away from equilibrium
Is inherently unidirectional or vectorial
Is energy - dependent
May be classified as:
 primary or direct
 Secondary or indirect
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Uniport, Symport, Antiport
(Study legend fig. 12.13)
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Indirect Active Transport
• Uses potential energy in Na+ or H+ concentration
gradient
 Na+ in animals
 H+ in plants, bacteria and mitochondria
• One solute is transported down its gradient
concomitantly (together) with another solute
transported against its gradient
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Direct Active Transport
• Direct active transport is coupled to:
 ATP hydrolysis or to
 Light energy (in some prokaryotes)
• Direct active transport depends on four types of transport
ATP’ases described in the next six slides
22
Types of Proteins involved in Primary Active
Transport
•
The proteins involved in active transport are classified as:



•
Tranport ATP’ases or ATP-driven pumps (Na+/K+ pump)
Light-driven pumps (like bacteriorhodopsin)
Coupled transporters
There are four types of transport ATP-ases
1.
2.
3.
4.
§
P-Type (P stands for phosphate group)
V-Type (V stands for vacuoles, vessicles)
F-Type (F stands for factor)
ABC-Type (ATP Binding Cassette-Type)
Relevant details are given in the next 4 slides
23
P-type ATPases
• Example: The Na+/K+pump
• They are found in the plasma membrane of most animal
cells, plants and fungi and in the sarcoplasmic reticulum of
muscle cells.
• They are reversibly phosphorylated by ATP.
Phosphorylation-dephosphorylation is an intrinsic event in
the transport process.
• All of them transport cations (Ca2+, H+, Na+ and K+)
• Sensitive to inhibition by the vanadate ion (VO4)3-
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The Na+/K+ ATPase
(Alberts Fig. 12-11)
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V-type ATPases
• Found mostly in the membrane of plant vacuoles and in
that of lysosomes.
• They pump H+ ions and help maintain a proton gradient
that ranges between 10x to 10,000x
• They consist of two multimeric subunits, an integral and a
peripheral one. Only the peripheral component (it faces
the extracellular fluid) gets phosphorylated. It has binding
sites for ATP and ATPase activity
• Phosphorylation is not an integral part of the transport
process.
• They are not inhibited by (VO4)326
F-type ATPases
• They are commonly found in the inner mitochondrial
membrane (cristae). Examples: F0F1 particles.
• They can use the energy derived from ATP hydrolysis to
generate proton gradient OR can use a proton gradient to
synthesize ATP.
 Remember that, in mitochondria, Fo functions as a pore for H+ ions
to flow through and F1 as ATP synthase. The synthase is activated
as H+ flows down its electrochemical gradient.
27
ABC-type ATPases
Example: The MDR Transport Protein
• MDR stands for multidrug resistance
• They were originally identified in bacteria but are quite
common in humans (48 different genes have been
identified).
• They transport a wide variety of solutes (ions, sugars, AA,
peptides…) BUT are specific for a particular solute.
• They are clinically significant because
 They confer resistance to certain antibiotics and antineoplastic
drugs because they pump these drugs out treated cells
 The abnormal protein responsible for cystic fibrosis is an ABCtype ATPase involved in Cl- transport
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The End:
Transport Processes (2)
Facilitated transport passive
Active transport