Membrane Dynamics cont’d
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How many substrates can a carrier move?
Active Transport
Secondary Active Transport
Transepithelial transport
How many substrates can a carrier
move?
How many substrates can a carrier
move?
Direction of substrate movement.
Carrier mediated transport into cells:
- net movement as long as there is a
concentration gradient across the
mb  Facilitated Diffusion
Diffusion and carrier proteins
Disequilibrium drives
facilitated diffusion of glucose.
Function during disequilibrium
Ca++
Ca++
Ca++
Calcium entry
Some molecules need to be in
disequilibrium.
• Levels of extracellular calcium and
extracellular sodium (Na+) are high.
Na+
Na+
Ca++
Ca++
Active Transport
• Na+ is removed from cells against its
concentration gradient
• Need ATP energy for this work
Active Transport: Na+/K+ ATPase
Na+
K+
Low levels of intracellular Na+
Active Transport: Na+/K+ ATPase
1.
3 Intracellular Na+
Ions bind
Onto Na+/K+ ATPase
Active Transport: Na+/K+ ATPase
2.
ATP hydrolysis
Active Transport: Na+/K+ ATPase
• 3.
The 3 Na+ ions are
released
Into the ECF
Active Transport: Na+/K+ ATPase
• 4.
Binding of 2 K+ ions from ECF
Active Transport: Na+/K+ ATPase
• 5.
Intracellular release of 2K+ ions
Active Transport: Na+/K+ ATPase
Secondary (indirect) Active
Transport
Symport driven by Na+ concentration gradient for trans-epithelial transport,
Sodium-glucose symporter
Sodium-glucose symporter
Sodium-glucose symporter
Transepithelial transport:
-Primary active transport
-Secondary active transport
-Facilitated diffusion
Must have
low levels of intracellular Na+
To drive transepithelial transport
Intracellular glucose provide energy for
primary and secondary active transport.
Where does transepithelial
transport occur?
• Glucose absorption in the intestine
• Glucose absorption in the nephron
• Glucose is moved from the mucosal
surface of the epithelium to the serosal
surface.
• Glucose is moved from the apical surface
of the cells to the basal surface of the
cells.
How does water move in the body?
• The cell membrane is semi-permeable
• Water can move freely
• Water is in equilibrium between cells and
extracellular fluids (osmotic equilibrium)
• Ions and solutes are disequilibrium
• Osmosis water moves along its
concentration gradient across a semipermeable membrane
Distribution of solutes in the body
fluid compartments
plasma
Interstitial fluid
Intracellular fluid
Ions and solutes are in disequilibrium
Ions and solutes are in disequilibrium
• Water can cross the cell membrane
Na+
Na+
K+
K+
proteins
Osmosis
• water moves along its concentration
gradient across a semi-permeable
membrane
• Water moves to dilute a solute
Osmosis
Osmotic pressure is pressure exerted
to counter the movement of water
to dilute something
Osmolarity
• Describes the number of particles in solution
• Know this and the direction of water movement
can be predicted
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# of particles in 1 liter of solution
Is expressed as osmoles/L, or OsM
If very dilute: milliosmoles/L, or mOsM
Human body, approx 300 mOsM
Osmolarity: number of particles in 1L
• 1 M glucose = 1 OsM glucose
• 1M NaCl = 2 OsM NaCl, because NaCl
disassociates to 2 ions in solution.
Na+ Cl-
Compare the osmolarity of 2 solutions:
• Solution A
• Solution B
• 1 OsM glucose
• 2 OsM glucose
• A is hyposmotic to B
• B is hyperosmotic to A
• (A has fewer particles
than B)
• (B has more particles
than A)
Compare the osmolarity of 2 solutions:
• Solution B
• Solution C
• 2 OsM glucose
• 1 OsM NaCl
• B is hyperosmotic to C
• C is hypotonic to B
• (B has more particles/L
than A)
• (C has fewer particles/L
than B)
Compare the osmolarity of 2 solutions:
• Solution A
• Solution C
• 1 OsM glucose
• 1 OsM NaCl
• A is isosmotic to C
• C is isosmotic to A
Osmosis, the diffusion of water across the
cell membrane, has consequences on cells
• After water leaves a cell, the volume
changes (it can shrink)
Tonicity
• Describes how the cell volume will change
in a solution
P is penetrating solute
N is nonpenetrating solute
Water moved into the cell to dilute the solutes.
• Cell gains volume in a hypotonic solution
• Cell looses volume in a hypertonic solution
• Cell keeps the same volume in an isotonic
solution.
Tonicity indicates how the cell
volume will change in a solution
• In a hypotonic solution, the cell has a
higher concentration of a nonpenetrating
solute than the solution, water moves in.
• In a hypertonic solution, the cell has a
lower concentration of nonpenetrating
solute than the solution, water leaves the
cell
During intavenous injection:
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0.9% (normal) saline  isotonic
D5--.9% saline (5% dextrose)  isotonic
D5W  hypotonic
0.45% saline  hypotonic
• Vs dehydration  hypotonic
• Vs blood loss  isotonic