General physiology 4

1. Transport of Substances
Through Cell Membrane

4. • The lipid bilayer is not miscible with either the
extracellular fluid or the intracellular fluid.
• It constitutes a barrier against movement of
water molecules and water-soluble substances
between the extracellular and intracellular
fluid compartments.
• Lipid soluble substances can penetrate this
lipid bilayer, diffusing directly through the lipid
substance.

5. • The protein molecules in the membrane have
entirely different properties for transporting
substances.
1. Transport proteins.
2. Channel proteins.
3. Carrier proteins.
• Transport through the cell membrane, either
*directly through the lipid bilayer or through the
*proteins, occurs via one of two basic processes:
A. Diffusion.
B. Active transport.

7. “Diffusion”
• Means random molecular movement of
substances molecule by molecule, either
through intermolecular spaces in the membrane
or in combination with a carrier protein.
DIFFUSION THROUGH THE CELL MEMBRANE:
Diffusion through the cell membrane is divided into
two:
1. Simple diffusion
2. Facilitated diffusion.

8. 1- Simple diffusion:
• That kinetic movement of molecules or ions
occurs through a membrane opening or through
intermolecular spaces without any interaction
with carrier proteins in the membrane.
• The rate of diffusion is determined by:
A. The amount of substance available.
B. The velocity of kinetic motion.
C. The number and sizes of openings in the
membrane through which the molecules or ions
can move.

9. Diffusion of Lipid-Soluble Substances
Through the Lipid Bilayer:
• Lipid solubility of the substance:
• The lipid solubilities of oxygen, nitrogen,
carbon dioxide, and alcohols are high.
• The rate of diffusion of each of these
substances through the membrane is directly
proportional to its lipid solubility.

11. Diffusion of Water and Other Lipid-Insoluble
Molecules Through Protein Channels:
• Even though water is highly insoluble in the
membrane lipids, it readily passes through
channels in protein molecules that penetrate all
the way through the membrane.
• Many of the body’s cell membranes contain
protein “pores” called aquaporins that
selectively permit rapid passage of water through
the membrane.
• The aquaporins are highly specialized, and there
are at least 13 different types in various cells of
mammals.

13. DIFFUSION THROUGH PROTEIN PORES AND CHANNELS—
SELECTIVE PERMEABILITY AND “GATING” OF CHANNELS.
• Pores are composed of integral cell membrane proteins that
form open tubes through the membrane and are always open.
• The diameter of a pore and its electrical charges provide
selectivity that permits only certain molecules to pass through.
For exam water channels, permit rapid passage of water
through cell membranes but exclude other molecules.
• The protein channels are distinguished by two important
Characteristics:
(1) they are often selectively permeable to certain substances.
(2) many of the channels can be opened or closed by gates
that are regulated by electrical signals (voltage-gated
channels) or chemicals that bind to the channel proteins
(ligand-gated channels).

14. Selective Permeability of
Protein Channels:
• Many of the protein channels are highly
selective for transport of one or more specific
ions or molecules.
• Potassium channels permit passage of
potassium ions across the cell membrane
about 1000 times more readily than they
permit passage of sodium ions.

15. Gating of Protein Channels:
• Gating of protein channels provides a means of
controlling ion permeability of the channels.
• Some of the gates are actual gate-like extensions
(close the opening or opening the channel).
• The opening and closing of gates are controlled in
two principal ways:
1. Voltage gating.
2. Chemical (ligand) gating.
3. And in rare cases Mechanical gating.

17. 1. Voltage gating:
• The molecular conformation of the gate or of its chemical
bonds responds to the electrical potential across the cell
membrane.
• Voltage-gated ion channels are a class of transmembrane
proteins that form ion channels that are activated by
changes in the electrical membrane potential near
the channel.
• The membrane potential alters the conformation of
the channel proteins, regulating their opening and closing.
• Voltage-gated ion-channels are usually ion-specific, and
channels specific to sodium (Na+),
potassium (K+), calcium (Ca2+), and chloride (Cl–) ions
have been identified.

19. 2. Chemical (ligand) gating.
• Some protein channel gates are opened by the
binding of a chemical substance (a ligand)
with the protein, which causes a
conformational or chemical bonding change in
the protein molecule that opens or closes the
gate.
• One of the most important instances of
chemical gating is the effect of acetylcholine
on the so-called acetylcholine channel.

21. 2- Facilitated Diffusion:
• Facilitated diffusion is also called carrier-mediated
diffusion because a substance transported in this
manner diffuses through the membrane with the
help of a specific carrier protein.

22. “ OSMOSIS”:
• Is the movement of water molecule from one part
to another.
• By far the most abundant substance that diffuses
through the cell membrane is water.
• Yet normally the amount that diffuses in the two
directions is balanced so precisely that zero net
movement of water occurs.
• Therefore, the volume of the cell remains constant.
• Osmotic Pressure: the amount of pressure required
to stop osmosis.

25. “ACTIVE TRANSPORT”
• When a cell membrane moves molecules or ions “uphill”
against a concentration gradient (or “uphill” against an
electrical or pressure gradient), the process is called active
transport.
• A large concentration of K is needed in the ICF perhaps it’s low
concentration in the ECF.
• A large concentration of Na is needed in the ECF perhaps it’s
low concentration in the ICF.
• Some energy source must cause excess movement of
potassium ions to the inside of cells and excess movement of
sodium ions to the outside of cells.
• Different substances that are actively transported through at
least some cell membranes include sodium, potassium,
calcium, iron, hydrogen, chloride, iodide, and urate ions,
several different sugars, and most of the amino acids.

26. • Active transport is divided into two types according
to the source of the energy used to facilitate the
transport:
1- Primary active transport.
2- Secondary active transport.
• In both types transport depends on carrier proteins
that penetrate through the cell membrane, as is true
for facilitated diffusion.
• In active transport, the carrier protein functions
differently from the carrier in facilitated diffusion
because it is capable of imparting energy to the
transported substance to move it against the
electrochemical gradient.

27. PRIMARY ACTIVE
TRANSPORT:
• The energy is derived directly from breakdown of
adenosine triphosphate (ATP) or some other high-
energy phosphate compound.
• Sodium-Potassium Pump Transports Sodium Ions
Out of Cells and Potassium Ions Into Cells.
• The Na+-K+ Pump Is Important for Controlling Cell
Volume. (protect against swelling of the cell).
• Electrogenic Nature of the Na+-K+ Pump. ( gives the
positively charge outside and negatively charge
inside the cell; this electrical potential is a basic
requirement in nerve and muscle fibers for
transmitting nerve and muscle signals. )

28. 1. It has three binding sites for sodium ions on the portion of the protein that
protrudes to the inside of the cell.
2. It has two binding sites for potassium ions on the outside.
3. The inside portion of this protein near the sodium binding sites has adenosine
triphosphatase (ATPase) activity.
Sodium-Potassium Pump

29. Primary Active Transport of
Calcium Ions:
• calcium pump - Calcium ions are normally
maintained at an extremely low concentration in the
intracellular cytosol of virtually all cells in the body.
• This level of maintenance is achieved mainly by two
primary active transport calcium pumps.
1. Which is in the cell membrane, pumps calcium to
the outside of the cell.
2. The other pumps calcium ions into one or more of
the intracellular vesicular organelles of the cell,
such as the sarcoplasmic reticulum of muscle cells
and the mitochondria in all cells.

30. Primary Active Transport
of Hydrogen Ions:
• Primary active transport of hydrogen ions is
important at two places in the body:
(1) in the gastric glands of the stomach.
(2) in the late distal tubules and cortical
collecting ducts of the kidneys.

31. SECONDARY ACTIVE
TRANSPORT:
• is transport of molecules across the cell
membrane utilizing energy in other forms than
ATP.
• This energy comes from the electrochemical
gradient created by pumping ions out of the cell.
• Co-transport (Symport): is the name of a process
in which two substances are simultaneously
transported across a membrane by one protein,
or protein complex which does not have ATPase
activity.

32. Co-Transport of Glucose and Amino
Acids Along with Sodium Ions:
• Glucose and many amino acids are transported into most
cells against large concentration gradients; the mechanism
of this action is entirely by co-transport.
• the concentration of sodium ions is high on the outside
and low inside, which provides energy for the transport.
• Na will not transported inside the cell until glucose bind to
the transport protein then a conformational change will
occur.
• Sodium co-transport of the amino acids occurs in the same
manner as for glucose, except that it uses a different set of
transport proteins. At least five amino acid transport
proteins have been identified.

34. Sodium Counter-Transport of Calcium
and Hydrogen Ions:
• a cell membrane transport mechanism
that transports two molecules at once
through the membrane in opposite directions.
• Two especially important counter-transport
mechanisms:
1. sodium-calcium counter-transport .
2. Sodium-hydrogen counter-transport.

35. • Sodium-calcium counter-transport occurs through
all or almost all cell membranes, with sodium ions
moving to the interior and calcium ions to the
exterior; both are bound to the same transport
protein in a counter-transport mode.
• Sodium-hydrogen counter-transport occurs in
several tissues. An especially important example is
in the proximal tubules of the kidneys, where sodium
ions move from the lumen of the tubule to the
interior of the tubular cell while hydrogen ions are
counter-transported into the tubule lumen.

36. Sodium Counter-Transport of Calcium
and Hydrogen Ions:

38. ACTIVE TRANSPORT THROUGH
CELLULAR SHEETS:
• Transport of this type occurs through the:
(1) Intestinal epithelium,
(2) Epithelium of the renal tubules,
(3) Epithelium of all exocrine glands,
(4) Epithelium of the gallbladder,
(5) Membrane of the choroid plexus of the brain.
• The basic mechanism for transport of a substance
through a cellular sheet is
(A) active transport through the cell membrane on one side
of the transporting cells in the sheet.
(B) either simple diffusion or facilitated diffusion through
the membrane on the opposite side of the cell.

39. Active transport through
cellular sheets: