CHAPTER 4 Flashcards
extracellular fluid contains a
large amount of
SODIUM, and also chlorine ions
but only a small amount of
potasium
the intracellular fluid are considerably greater than those in the extracellular fluid
phosphates and proteins
Most of these penetrating proteins therefore, can function as
transport proteins
Some have watery spaces all
the way through the molecule and allow free movement of water, as well as selected ions or molecules; these are
called
channel proteins
means random molecular movement of substances molecule by molecule, either through intermolecular spaces in the membrane or in combination with a carrier protein.
Diffusion
are usually highly selective for the types of molecules or ions that are allowed to
cross the membrane.
Both the channel proteins and the carrier proteins
bind with molecules or ions that are to be transported;
conformational changes in the protein molecules then move the substances through the interstices of the protein to the other side of the membrane.
called carrier proteins
Transport through the cell membrane, either directly through the lipid bilayer or through the proteins, occurs by one of two basic processes:
Diffusion or Active transport
The energy that causes diffusion is the energy of the
Normal kinetics motion of matter.
Means movement of ions
or other substances across the membrane in combination with a carrier protein in such a way that the carrier protein causes the substance to move against an energy gradient, such as from a low-concentration state to a high concentration state.
Active transport
This continual movement of molecules among one another in liquids or in gases is called
diffusion
Diffusion through the cell membrane is divided into two subtypes called
Simple diffusion and facilitated diffusion
Means 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.
Simple diffusion
The rate of diffusion is determined by the amount of substance available,
The velocity of kinetic motion, and the number and sizes of openings in the membrane through which the molecules or ions can move.
Requires interaction of a carrier
protein.
The carrier protein aids passage of the molecules or ions through the membrane by binding chemically with them and shuttling them through the membrane in
this form.
Facilitated diffusion
Simple diffusion can occur through the cell membrane by two pathways:
- Through the interstices of the lipid bilayer if the diffusing substance is lipid soluble.
- Through watery channels that penetrates all the way of some the large transport proteins.
One of the most important factors that determines how rapidly a substance diffuses through the lipid bilayer is the
Lipid solubility
Are high, so all these can dissolve directly in
the lipid bilayer and diffuse through the cell membrane in the same manner that diffusion of water solutes occurs in a watery solution.
The lipid solubilities of oxygen, nitrogen, carbon dioxide, and alcohols
water is highly insoluble in the membrane lipids, it readily passes through channels in protein molecules that penetrate all the way through the membrane.
water is highly insoluble in the membrane lipids, it readily passes through channels in protein molecules that penetrate all the way through the membrane is
astounding
Are composed of integral cell membrane proteins that form open tubes through the membrane and
are always open.
Pores
permit rapid passage of water through cell membranes but exclude
other molecules.
aquaporins or water channels
Aquaporins have a narrow pore that permits water molecules to diffuse through the membrane in single file
The protein channels are distinguished by two important characteristics:
(1) They are often selectively permeable to certain substances, and
(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)
Permit passage of potassium ions
across the cell membrane about 1000 times more readily than they permit passage of sodium ions.
Potassium channels
were found to have a tetrameric structure consisting of four identical protein subunits surrounding a central pore
Potassium channels
At the top of the channel pore are pore loops that form a narrow
selectivity filter
Lining the selectivity filter are
carbonyl oxygens
One of the most important of the protein channels, the
is only 0.3 by 0.5 nanometer in diameter.
sodium channel
but more important, the inner surfaces of this channel are lined with amino acids that are
strongly negatively charged
The opening and closing of gates are controlled in two principal ways:
- Voltage gating
- Chemical (ligand) gating
This causes a conformational or
chemical bonding change in the protein molecule that opens or closes the gate. This is called
Chemical gating or Ligand gating
One of the most important instances of chemical gating is the effect of acetylcholine on the so called
Acetylcholine channel
This gate is exceedingly important for
the transmission of nerve signals from one nerve cell to another and from nerve cells to muscle cells to cause muscle contraction.
Facilitated diffusion is also called
carrier-mediated diffusion
because a substance transported in this manner diffuses through the membrane using a specific carrier
protein to help. That is, the
Carrier facilitates diffusion
Although the rate of simple
diffusion through an open channel increases proportionately with the concentration of the diffusing substance in facilitated diffusion the rate of diffusion approaches a
maximum, called
Vmax
Among the most important substances that cross cell
membranes by facilitated diffusion are
Glucose and most of the amino acids.
, is activated by insulin, which can
increase the rate of facilitated diffusion of glucose as much
as 10-fold to 20-fold in insulin-sensitive tissues.
glucose transporter 4 (GLUT4),
Co is concentration outside
Ci is concentration inside
At normal body temperature (37°C), the electrical difference that will balance a given concentration difference of univalent ions—such as sodium (Na+) ions—can be determined from the following formula, called the
Nernst equation
actually means the sum of all the forces of the different molecules striking a unit surface area at a given
instant.
Pressure
By far the most abundant substance that diffuses through the cell membrane is
water
This process of net movement
of water caused by a concentration difference of water is
osmosis
The exact amount
of pressure required to stop osmosis is called the
Osmotic pressure
The osmotic pressure exerted by particles in a solution,
whether they are molecules or ions, is determined by
the number of particles per unit volume of fluid, not by
the mass of the particles
To express the concentration of a solution in terms of numbers of particles, the unit called the
osmole
is the osmolar concentration expressed as osmoles per liter of solution rather than osmoles per kilogram of water
Osmolarity
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
Active transport is divided into two types according to the source of the energy used to cause the transport:
- primary active transport
- secondary active transport
the energy is derived directly from breakdown of adenosine triphosphate (ATP) or of some other high-energy phosphate
compound
Primary active transport
The energy is derived secondarily from energy that has been stored in
the form of ionic concentration differences of secondary molecular or ionic substances between the two sides of a cell membrane, created originally by primary active
transport.
secondary active transport
responsible for maintaining the
sodium and potassium concentration differences across
the cell membrane, as well as for establishing a negative
electrical voltage inside the cells.
pump
is a complex of
two separate globular proteins:
carrier protein
a larger one called the
“ α subunit”
with a molecular weight of about 100,000, and a
smaller one called the “β subunit”
- It has three receptor sites for binding sodium ions on
the portion of the protein that protrudes to the inside
of the cell. - It has two receptor sites for potassium ions on the
outside. - The inside portion of this protein near the sodium
binding sites has ATPase activity
One of the most important functions of the Na+-K+ pump is to control the v
volume of each cell
the larger protein has
three specific features that are important for the functioning of the pump:
The normal mechanism for preventing this is the
Na+-K+ pump
Another important primary active transport mechanism is the
calcium pump
Are normally maintained at extremely low concentration in the intracellular cytosol of virtually all cells in the body, at a concentration about 10,000 times less than that in the extracellular fluid.
Calcium ions
At two places in the body, primary active transport of
hydrogen ions is important:
(1) in the gastric glands of the
stomach and
(2) in the late distal tubules and cortical collecting ducts of the kidneys.
have the most potent primary active mechanism for transporting hydrogen ions of any part of the body
Deep-lying parietal cells
r. Under appropriate conditions, this diffusion energy of sodium can pull
other substances along with the sodium through the cell
membrane. This phenomenon is called c
co-transport
sodium ions again attempt to
diffuse to the interior of the cell because of their large
concentration gradient
counter-transport
are especially important mechanisms in transporting
glucose across renal and intestinal epithelial cells
Sodium-glucose co-transporters
occurs in the same manner as for glucose, except that it uses a different set of transport proteins
Sodium co-transport of the amino acids
Five amino acid transport
proteins have been identified, each of which is responsible
for transporting one subset of amino acids with specific
molecular characteristics
Sodium co-transport of glucose and amino acids
occurs especially through the
epithelial cells of the intestinal tract and the renal tubules of the kidneys
Two especially important counter-transport mechanisms
(transport in a direction opposite to the primary ion) are
- sodium-calcium counter-transport
- sodium-hydrogen counter-transport
occurs in several
tissues
Sodium-hydrogen counter-transport
Sodium-calcium counter-transport and sodium-hydrogen
counter-transport
Sodium-calcium counter-transport
An especially important example is in the — 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.
proximal tubules
At many places in the body, substances must be transported all the way through a cellular sheet instead of simply
through the cell membrane. 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, and (5) membrane of the choroid plexus of the brain and other membranes.
This figure shows
that the epithelial cells are connected together tightly at
the luminal pole by means of junctions called
“kisses”
The basic mechanism for transport of a substance through a cellular sheet is :
(1) active transport through the
cell membrane on one side of the transporting cells in the
sheet, and then
(2) either simple diffusion or facilitated diffusion through the membrane on the opposite side of the cell.
