physio baby guyton chap 4

Extracellular fluid has higher concentrations of sodium, calcium, bicarbonate, and chloride, compared with intracellular fluid.

Intracellular fluid has higher concentrations of potassium, phosphates, magnesium, and proteins compared with extracellular fluid.

Channel proteins provide a watery pathway for movement of (mainly) ions across the membrane.

Carrier proteins bind with specific molecules and then undergo conformational changes that move molecules across the membrane

Diffusion means random movement of molecules either through intermolecular spaces in the cell membrane or in combination with a carrier protein. The energy that causes diffusion is the energy of the normal kinetic motion of matter.

Active transport means movement of substances across the membrane in combination with a carrier protein and also against an electrochemical gradient. This process requires a source of energy in addition to kinetic energy

Simple diffusion means that molecules move through a membrane without binding to carrier proteins.

Simple diffusion can occur by way of two pathways:(1) through the interstices of the lipid bilayer, and (2) through water-filled protein channels that span the cell membrane

Facilitated diffusion requires a carrier protein. The carrier protein aids in passage of molecules through the membrane, probably by binding chemically with them and shuttling them through the membrane in this form.

The Rate of Diffusion of a Substance Through the Cell Membrane Is Directly Proportional to Its Lipid Solubility. The lipid solubilities of oxygen, nitrogen, carbon dioxide, anesthetic gases, and most alcohols are so high that they can diffuse directly through the lipid bilayer of the cell membrane

Water readily penetrates the cell membrane and can also pass through transmembrane protein channels. Other lipid-insoluble molecules (mainly ions) of a sufficiently small size can pass through the water-filled protein channels.

Gating of Protein Channels Provides a Means for Controlling Their Permeability. The gates are thought to be molecular extensions of the transport protein, which can close over the channel opening or be lifted from the opening by a conformational change in the protein molecule itself. The opening and closing of gates are controlled in two principal ways:

Voltage gating. In this instance, the molecular conformation of the gate is controlled by the electrical potential across the cell membrane. For example, the normal negative charge on the inside of the cell membrane causes sodium gates to remain tightly closed. When the inside of the membrane loses its negative charge (i.e., becomes less negative), these gates open, allowing sodium ions to pass inward through the sodium channels. The opening of sodium channel gates initiates action potentials in nerve fibers

Chemical gating. Some protein channel gates are opened by the binding of another molecule with the protein, which causes a conformational change in the membrane protein that opens or closes the gate. This process is called chemical (or ligand) gating. One of the most important instances of chemical gating is the effect of acetylcholine on the “acetylcholine cation channel” of the neuromuscular junction.

Facilitated Diffusion Is Also Called Carrier-Mediated Diffusion

. Molecules transported by facilitated diffusion usually cannot pass through the cell membrane without the assistance of a specific carrier protein

Facilitated diffusion involves the following two steps: (1) the molecule to be transported enters a blind-ended channel and binds to a specific receptor, and (2) a conformational change occurs in the carrier protein, so the channel now opens to the opposite side of the membrane where the molecule is deposited.

Facilitated diffusion differs from simple diffusion in the following important way. The rate of simple diffusion increases proportionately with the concentration of the diffusing substance. With facilitated diffusion, the rate of diffusion approaches a maximum value as the concentration of the substance increases. This maximum rate is dictated by the rate at which the carrier protein molecule can undergo the conformational change.

Among the most important substances that cross cell membranes by facilitated diffusion are glucose and most of the amino acids.

Substances Can Diffuse in Both Directions Through the Cell Membrane. Therefore, what is usually important is the net rate of diffusion of a substance in one direction. This net rate is determined by the following factors:

Osmosis Is the Process of Net Movement of Water Caused by a Concentration Difference of Water.

, the volume of a cell remains constant. However, a concentration difference for water can develop across a cell membrane. When this happens, net movement of water occurs across the cell membrane, causing the cell to either swell or shrink, depending on the direction of the net movement. The pressure difference required to stop osmosis is the osmotic pressure.

The Osmotic Pressure Exerted by Particles in a Solution Is Determined by the Number of Particles per Unit Volume of Fluid and Not by the Mass of the Particles.

The Osmole Expresses Concentration in Terms of Number of Particles

One osmole is 1 gram molecular weight of undissociated solute

180 grams of glucose, which is 1 gram molecular weight of glucose, is equal to 1 osmole of glucose because glucose does not dissociate.

normal osmolality of the extracellular and intracellular fluids is about 300 milliosmoles per kilogram, and the osmotic pressure of these fluids is about 5500 mm Hg.

An electrochemical gradient is the sum of all the diffusion forces acting at the membrane. These forces include the forces caused by a concentration difference, an electrical difference, and a pressure difference.

When a cell membrane moves a substance uphill against an electrochemical gradient, the process is called active transport.

Active Transport Is Divided Into Two Types According to the Source of the Energy Used to Effect the Transport.

Primary active transport. The energy is derived directly from the breakdown of adenosine triphosphate (ATP) or some other high-energy phosphate compound.

Secondary active transport. The energy is derived secondarily from energy that has been stored in the form of ionic concentration differences between the two sides of a membrane, originally created by primary active transport. The sodium electrochemical gradient drives most secondary active transport processes

Primary Active Transport

The Sodium-Potassium Pump Transports Sodium Ions out of Cells and Potassium Ions Into Cells.

The sodium-potassium (Na+-K+) pump, which is present in all cells of the body, is responsible for maintaining the sodium and potassium concentration differences across the cell membrane, as well as for establishing a negative electrical potential inside the cells.

The pump operates in the following manner:

1)Three sodium ions bind to a carrier protein on the inside of the cell, and two potassium ions bind to the carrier protein on the outside of the cell.

2)The carrier protein has adenosine triphosphatase (ATPase) activity, and the simultaneous binding of sodium and potassium ions causes the ATPase function of the protein to become activated.

3)The ATPase function then cleaves one molecule of ATP, splitting it to form one molecule of adenosine diphosphate and liberating a high-energy phosphate bond of energy.

4)This energy is then believed to cause a conformational change in the protein carrier molecule, extruding the sodium ions to the outside and the potassium ions to the inside

The Na+-K+ Pump Controls Cell Volume.

when the cell begins to swell, the Na+-K+ pump is automatically activated, moving to the exterior still more ions that are carrying water with them. Therefore, the Na+-K+ pump performs a continual surveillance role in maintaining normal cell volume

Co-Transport and Counter-Transport Are Two Forms of Secondary Active Transport

Co-transport. The diffusion energy of sodium can pull other substances along with the sodium (in the same direction) through the cell membrane using a special carrier protein

Counter-transport. The sodium ion and substance to be counter-transported move to opposite sides of the membrane, with sodium always moving to the cell interior. Here again, a protein carrier is required.

Glucose and Amino Acids Can Be Transported Into Most Cells by Sodium Co-Transport

Calcium and Hydrogen Ions Can Be Transported Out of Cells Through the Sodium Counter-Transport Mechanism.

Calcium counter-transport occurs in most cell membranes, with sodium ions moving to the cell interior and calcium ions moving to the exterior; both are bound to the same transport protein in a countertransport mode.

Hydrogen counter-transport occurs especially in the proximal tubules of the kidneys, where sodium ions move from the lumen of the tubule to the interior of the tubular cells, and hydrogen ions are countertransported into the lumen.