Cellular Processes

1
Cellular Processes
Diffusion, channels and transporters
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Cellular Membranes

• Two main roles
• Allow cells to isolate themselves from the
  environment, giving them control of intracellular
  conditions
• Help cells organize intracellular pathways into
  discrete subcellular compartment, including
  organelles

3
Membrane Structure

• Lipid bi-layer phospholipids, primarily
  phosphoglycerides
• Other lipids
• Sphingolipids alter electrical properties
• Glycolipids communication between cells
• Cholesterol increase fluidity while decreasing
  permeability

Figure 3.20
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Membrane Proteins

• Can be more than half of the membrane mass
• Two main types
• Integral membrane proteins tightly bound to the
  membrane, either embedded in the bilayer or
  spanning the entire membrane
• Peripheral proteins weaker association with the
  lipid bilayer

We will discuss membrane proteins that allow for
flow of ions or for transport of molecules
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Membrane permeability
Lipid bilayer
6
Membrane proteins we will discuss
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Membrane Transport

• Three main types
• Passive diffusion
• Facilitated diffusion
• Active transport

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Passive Diffusion

• Lipid-soluble molecules (alcohol, CO2)
• No specific transporters are needed
• No energy is needed
• Depends on concentration gradient
• High ? low
• Steeper gradient results in higher rates
• Gradients can be chemical, electrical or both
  depending on the nature of the molecule
• e.g., Membrane potential electrical gradient
  across a cell membrane

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Facilitated Diffusion

• Hydrophilic molecules
• Protein transporter is needed - Uniporter
• No energy is needed
• Depends on concentration gradient
• Examples amino acids, nucleosides, sugars
  (glucose)

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Facilitated Diffusion, Cont.

• Three main types of proteins
• 1. Ion channels form pores, channel has to be
  open

• Open/close in response to a membrane potential

b) Open via specific regulatory molecules
c) Regulated through interactions with
subcellular proteins
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Facilitated Diffusion, Cont.
2. Porins like ion channels, but for larger
molecules Cool stuff aquaporin allows water to
cross the plasma membrane 13 billion H2O
molecules per second! But, as pointed out by T.
Todd Jones that is only 0.000000000000018 ml of
water. 3. Permeases function more like an
enzyme. Binds the substrate and then undergoes a
conformation change which causes the carrier to
release the substrate to the other side. Ex.
Glucose permeases
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Facilitated Diffusion - Uniporter

• GLUT1 mammalian glucose transporter
• Uses concentration gradient of glucose to drive
  transport
• Can work in reverse
• Used by most mammals

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Electrical Gradients

• All transport processes affect chemical gradients
• Some transport processes affect the electrical
  gradient
• Electroneutral carriers transport uncharged
  molecules or exchange an equal number of charged
  particles
• Electrogenic carriers transfer a charge, e.g.,
  Na/K ATPase ? exchanges 3Na for 2K

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Membrane Potential

• Difference in charge inside and outside the cell
  ? electrochemical gradient
• Active transporters establish this gradient
• Two main functions
• Provide cell with energy for membrane transport
• Allow for changes in membrane potential used by
  cells in cell-to-cell signaling
• Can be determined by Nernst equation and Goldman
  equation

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Nernst equation

• Used to calculate the electrical potential at
  equilibrium
• Recall ?G RTln(Xi/Xo) zFEm
• Chemical component electrical
  component
• At equilibrium zFEm RTln(Xo/Xi)
• Equilibrium potential is
• Ex (RT/zF) ln Xo/Xi
• where R gas constant, T absolute temperature
  (Kelvin),
• z valence of ion, F Faradays constant
• Example K out 0.01 M K in 0.1 M T 22oC
• So, EK (1.9872295)/(123062) ln (0.01/0.1)
• -58 mV at 22oC

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Nernst equation
Each ion has a different potential given the
difference in concentration gradients.
Must have pores or channels to create potential!
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Nernst equation and ion concentrations
Differences in Nernst potential reflect
differences in chemical gradients!
We will discuss the protein pumps that are
necessary to maintain these gradients.
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Active Transport

• Protein transporter is needed
• Energy is required
• Molecules can move from low to high concentration

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Active Transport, Cont.

• Two main types distinguished by the source of
  energy
• Primary active transport uses an exergonic
  reaction ie ATP
• Secondary active transport couples the movement
  of one molecule to the movement of a second
  molecule

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Primary Active Transport

• Hydrolysis of ATP provides energy
• Three types
• P-type pump specific ions, e.g., Na, K, Ca2
• F- and V-type pump H
• ABC type carry large organic molecules, e.g.,
  toxins

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P-class pumps Na/K ATPase pump

• pumps 2 K in and 3 Na out? important for many
  cellular functions (osmotic balance of cells)?
  uses ATP as energy source? can be blocked with
  poisons like ouabain or digitalis? the potential
  built up in the Na ions will be used by many
  different
• processes i.e. cotransporters, neuronal
  signaling etc.

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P-class pumps Na/K ATPase pump
Binding of phosphate from ATP drives conformation
change that allows ions to be transported to
appropriate sides ? an asparate residue becomes
phosphorylated and the energy transfer changes
the proteins conformational shape
Na binding sites switch from high affinity on
inside to low affinity on outside to allow for
binding of Na on inside and release of Na ions
on outside. K binding sites with from high
affinity on outside to low affinity on inside for
the same reason
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P-class pumps Ca 2 ATPase pump

• pumps 2 Ca2 ions out for every 1 ATP molecule
  used
• Uses ATP to drive Ca 2 out against a very large
  concentration gradient
• Internal Ca 2 binding sites have a very high
  affinity
• (in order to overcome extremely low Ca2
  concentrations inside cell)
• Energy transfer from ATP to the aspartate of the
  Ca2 ATPase causes
• a protein conformational change and Ca2
  transported across membrane
• Ca2 binding sites on outside are low affinity
  and Ca2 is released
• The transfer of energy from the ATP to the pump
  triggers a conformational
• change that moves the protein and allows the
  translocation of
• Ca 2 across the membrane
• At the same time the Ca2 binding sites change
  from high to low affinity.

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P-class pumps Ca 2 ATPase pump cont.

• In muscle cells the Ca2 ATPase is the major
  protein found in the membrane
• of the sacrcoplasmic reticulum (SR)
• 80 of the protein in the SR is the Ca2 ATPase
• SR is a storage site for Ca2 that is release to
  drive muscle contraction
• Ca2 ATPase will remove excess Ca2 from the
  cytoplasm and pump it into
• the lumen of the SR

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V-class pumps proton pumps

• These pumps transport H only
• Found in lysosomes, endosomes and plant vacuoles
• Transport H ions to make the lumen or inside of
  the lysosome acidic
• (pH 4.5 - 5.0)
• Many of these pumps are paired with Cl- channels
  to offset the electrical
• gradient that is produced by pumping H across
  the membrane.

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V-class pumps proton pumps

• H is transported into the lysosome
• Cl- flows in to keep a balance
• If Cl- doesn't flow in then there is rapid build
  up of potential (charge) across
• the membrane which would block the further
  transport of H.
• This would occur long before the lumen becomes
  acidic because
• not that many ions need to be transported to
  produce the voltage potential

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Secondary Active Transport

• Use energy held in the electrochemical gradient
  of one molecule to drive another molecules
  against its gradient
• Antiport or exchanger carrier molecules move in
  opposite directions
• Symport or cotransporter carrier molecules move
  in the same direction

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Secondary Active Transport
Uniporter One molecule. Amino acids,
nucleosides,sugars Symporter/cotransporter
movement in the same directions. Na/glucose
cotransporter in the intestine Antiporter/Exchange
r Cl-/HCO3- exchanger in the red blood cell
Example of a Cotransporter
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Membrane Potential and Na

• Animal cells are more negative on the inside than
  on the outside
• ( -80 to -70 mV)
• Mostly due to K ions (inside gt outside) created
  via Na /K pump, K leak channels and anions
  inside the cell (proteins etc)
• Remember K will move down its concentration
  gradient
• Nernst potential for K is - 80 to -70 mV.
• Why is this important???
• Transport of Na down the chemical gradient and
  the electrical gradient. Makes Na a powerful
  co-transporter!

Favors movement of Na into the cell
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Membrane Potential and Co-transporters
Na/glucose co-transporter

• Used by cells in the intestine to transport
  glucose against
• a large concentration gradient
• This is a symporter both in the same direction
• ?G for 2 Na is -6 kcal/mol

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Membrane Potential and Co-transporters
3 Na/Ca2 antiporter

• Important in muscle cells
• Maintains the low intracellular concentration of
  Ca2
• Plays a role in cardiac muscle
• Ca2i 0.0002 mM and Ca2o 2 mM
• So ?G RTln (2/0.0002) 5.5 kcal.mol
• ?G zFEm 2(23062)(0.070Volts) 3.3 kcal/mol
• Total 8.8 kcal/mol
• ?So must transport 3 Na in for 1 Ca2 out

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Co-transporters
HCO3-/Cl- antiporter

• Regulate pH
• Carbon dioxide from respiration
• CO2 H2O ? H2CO3 ? H HCO3- in the presence of
• carbonic anhydrase (enzyme)
• Note 80 of the CO2 in blood is transported as
  HCO3-.
• This is generated by red blood cells (RBC)
• RBC have a protein (AE1) and this is the
• HCO3-/Cl- antiporter
• Pumps 1 X 109 HCO3- every 10 msec.
• Clears the CO2 and Cl- transport ensures that
  there isn't a
• build up of electrical potential

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Membrane Potential and Co-transporters
HCO3-/Cl- antiporter
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Co-transporters
Other transporters that regulate pH
Na/H antiporter Remove excess H when cells
become acidic
NaHCO3-/Cl- co-transporter HCO3- is brought
into the cell to neutralize H in the cytosol
HCO3- H ? H2O CO2 in the presence of
carbonic anhydrase. Driven by Na Couples the
influx of HCO3- and Na to an efflux of Cl-
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Co-transporters
Exchangers are regulated by internal pH and
increase their activity as the pH in the cytosol
falls