Transport of Solutes Across Plasma Membrane (II)

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Transport of Solutes Across Plasma Membrane (II)

• Facilitated Transport Passive
• Facilitated Transport Active

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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 well 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

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How Solutes Cross Membranes

• Solutes move across cell membranes by
• Simple diffusion We studied it last week
• 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

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Relative Permeability of Synthetic Bilayers to
Some Solutes (Alberts Fig. 12.2)
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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)

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Passive and Active Transport ComparedAlberts
Fig. 12.4
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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

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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)

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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 lt-gt close lt-gt 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)
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Gated Ion Channels
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Facilitated Transport ofNon-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

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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

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Effect of Electrochemical Gradient on Ion Flux
(Study legend Fig. 12.8)
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Active TransportStudy Guyton Ch. 4

• Carried out by
• Coupled Transporters
• ATP-Driven Pumps
• Light-Driven Pump (not in human cells)

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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 ATPases described in the next six
  slides

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Types of Proteins involved in Primary Active
Transport

• The proteins involved in active transport are
  classified as
• Tranport ATPases or ATP-driven pumps (Na/K
  pump)
• Light-driven pumps (like bacteriorhodopsin)
• Coupled transporters
• There are four types of transport ATP-ases
• 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

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P-type ATPases

• Example The Na/Kpump
• 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)3-

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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.

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ABC-type ATPasesExample 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 ABC-type ATPase involved in Cl-
  transport

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The End

• Transport Processes (2)
• Facilitated transport passive
• Active transport