Title: Cellular Processes
1Cellular Processes
Diffusion, channels and transporters
2Cellular 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
3Membrane 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
4Membrane 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
5Membrane permeability
Lipid bilayer
6Membrane proteins we will discuss
7Membrane Transport
- Three main types
- Passive diffusion
- Facilitated diffusion
- Active transport
8Passive 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
9Facilitated Diffusion
- Hydrophilic molecules
- Protein transporter is needed - Uniporter
- No energy is needed
- Depends on concentration gradient
- Examples amino acids, nucleosides, sugars
(glucose)
10Facilitated 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
11Facilitated 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
12Facilitated Diffusion - Uniporter
- GLUT1 mammalian glucose transporter
- Uses concentration gradient of glucose to drive
transport - Can work in reverse
- Used by most mammals
13Electrical 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
14Membrane 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
15Nernst 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
16Nernst equation
Each ion has a different potential given the
difference in concentration gradients.
Must have pores or channels to create potential!
17Nernst 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.
18Active Transport
- Protein transporter is needed
- Energy is required
- Molecules can move from low to high concentration
19Active 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
20Primary 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
21P-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.
22P-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
23P-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.
24P-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
-
25V-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.
26V-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
27Secondary 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
28Secondary 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
29Membrane 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
30Membrane 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
31Membrane 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
32Co-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
33Membrane Potential and Co-transporters
HCO3-/Cl- antiporter
34Co-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-
35Co-transporters
Exchangers are regulated by internal pH and
increase their activity as the pH in the cytosol
falls