surface area of the membrane, and are part of the cells lining organs that absorb materials. The small intes- tine, for example, requires a large surface area for the absorption of nutrients, and many of its lining cells have microvilli. Some cells of the kidney tubules also have microvilli (see Fig. 1–1) that provide for the effi- cient reabsorption of useful materials back to the blood.
The functions of the cell organelles are summa- rized in Table 3–1.
CELLULAR TRANSPORT
sugar. If a 2% salt solution and a 6% salt solution are separated by a membrane allowing water but not salt to pass through it, water will diffuse from the 2% salt solution to the 6% salt solution. The result is that the 2% solution will become more concentrated, and the 6% solution will become more dilute.
In the body, the cells lining the small intestine absorb water from digested food by osmosis. These
cells have first absorbed salts, have become more
“salty,” and water follows salt into the cells (see Fig.
3–3). The process of osmosis also takes place in the kidneys, which reabsorb large amounts of water (many gallons each day) to prevent its loss in urine. Box 3–1:
Terminology of Solutions lists some terminology we use when discussing solutions and the effects of vari- ous solutions on cells.
BOX 3–1 TERMINOLOGY OF SOLUTIONS
Human cells or other body fluids contain many dis- solved substances (called solutes) such as salts, sugars, acids, and bases. The concentration of solutes in a fluid creates the osmotic pressureof the solution, which in turn determines the move- ment of water through membranes.
As an example here, we will use sodium chloride (NaCl). Human cells have an NaCl concentration of 0.9%. With human cells as a reference point, the relative NaCl concentrations of other solutions may be described with the following terms:
Isotonic—a solution with the same salt concentra- tion as in cells.
The blood plasma is isotonic to red blood cells.
Hypotonic—a solution with a lower salt concentra- tion than in cells.
Distilled water (0% salt) is hypotonic to human cells.
Hypertonic—a solution with a higher salt concen- tration than in cells.
Seawater (3% salt) is hypertonic to human cells.
Refer now to the diagrams shown in Box Figure 3–A of red blood cells(RBCs)in each of these different types of solutions, and note the effect of each on osmosis:
• When RBCs are in plasma, water moves into and out of them at equal rates, and the cells remain normal in size and water content.
• If RBCs are placed in distilled water, more water will enter the cells than leave, and the cells will swell and eventually burst.
• If RBCs are placed in seawater, more water will leave the cells than enter, and the cells will shrivel and die.
This knowledge of osmotic pressure is used when replacement fluids are needed for a patient who has become dehydrated. Isotonic solutions are usually used; normal saline and Ringer’s solu- tion are examples. These will provide rehydration without causing osmotic damage to cells or extensive shifts of fluid between the blood and tissues.
Box Figure 3–A Red blood cells in different solutions and the effect of osmosis in each.
FACILITATED DIFFUSION
The word facilitate means to help or assist. In facili- tated diffusion, molecules move through a membrane from an area of greater concentration to an area of lesser concentration, but they need some help to do this.
In the body, our cells must take in glucose to use for ATP production. Glucose, however, will not diffuse through most cell membranes by itself, even if there is more outside the cell than inside. Diffusion of glucose into most cells requires a glucose transporter, which may also be called a carrier enzyme. These trans- porters are proteins that are part of the cell mem- brane. Glucose bonds to the transporter (see Fig. 3–3), and by doing so changes the shape of the protein. This physical change propels the glucose into the interior of the cell. Other transporters are specific for other organic molecules such as amino acids.
ACTIVE TRANSPORT
Active transportrequires the energy of ATP to move molecules from an area of lesser concentration to an area of greater concentration. Notice that this is the opposite of diffusion, in which the free energy of mol- ecules causes them to move to where there are fewer of them. Active transport is therefore said to be move- ment against a concentration gradient.
In the body, nerve cells and muscle cells have
“sodium pumps” to move sodium ions (Na⫹) out of the cells. Sodium ions are more abundant outside the cells, and they constantly diffuse into the cell (through specific diffusion channels), their area of lesser con- centration (see Fig. 3–3). Without the sodium pumps to return them outside, the incoming sodium ions would bring about an unwanted nerve impulse or muscle contraction. Nerve and muscle cells constantly produce ATP to keep their sodium pumps (and simi- lar potassium pumps) working and prevent sponta- neous impulses.
Another example of active transport is the absorp- tion of glucose and amino acids by the cells lining the small intestine. The cells use ATP to absorb these nutrients from digested food, even when their intra- cellular concentration becomes greater than their extracellular concentration.
FILTRATION
The process of filtrationalso requires energy, but the energy needed does not come directly from ATP. It is the energy of mechanical pressure. Filtration means that water and dissolved materials are forced through a membrane from an area of higher pressure to an area of lower pressure.
In the body, blood pressure is created by the 54 Cells
Table 3–2 CELLULAR TRANSPORT MECHANISMS
Mechanism Definition Example in the Body
Diffusion
Osmosis
Facilitated diffusion Active transport
Filtration
Phagocytosis Pinocytosis
Movement of molecules from an area of greater concentration to an area of lesser concentration.
The diffusion of water.
Carrier and transporter enzymes move mole- cules across cell membranes.
Movement of molecules from an area of lesser concentration to an area of greater concentration (requires ATP).
Movement of water and dissolved substances from an area of higher pressure to an area of lower pressure (blood pressure).
A moving cell engulfs something.
A stationary cell engulfs something.
Exchange of gases in the lungs or body tissues.
Absorption of water by the small intestine or kidneys.
Intake of glucose by most cells.
Absorption of amino acids and glucose from food by the cells of the small intestine.
Sodium and potassium pumps in muscle and nerve cells.
Formation of tissue fluid; the first step in the for- mation of urine.
White blood cells engulf bacteria.
Cells of the kidney tubules reabsorb small proteins.
55
•
B
OsmosisC
Facilitated DiffusionD
Active transportE
FiltrationF
PhagocytosisG
PinocytosisSmall protein
Cell of kidney tubule
•
•
•
Lysosome Bacterium White blood cell
• • •
• •
H2O
BP
Glucose Amino acid
RBC Capillary in
tissues
•
•
•
•Cell
membrane
Na+
Tissue fluid
ATP Active transport channel Cytoplasm
Diffusion channel
• •
•
Cell membrane of intestinal cell H2O
Na+
Cytoplasm
• Alveolus of lung
• Capillary
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
CO2 CO2
CO2
CO2
CO2
CO2
• Glucose
• Transporter
Cytoplasm Tissue fluid
• Cell membrane
Figure 3–3. Cellular transport mechanisms. (A) Diffusion in an alveolus in the lung.
(B) Osmosis in the small intestine. (C) Facilitated diffusion in a muscle cell. (D) Active trans- port in a muscle cell. (E) Filtration in a capillary. (F) Phagocytosis by a white blood cell.
(G) Pinocytosis by a cell of the kidney tubules. See text for description.
QUESTION:Which mechanism depends on blood pressure? Which depends on the move- ment of a cell?
pumping of the heart. Filtration occurs when blood flows through capillaries, whose walls are only one cell thick and very permeable. The blood pressure in capillaries is higher than the pressure of the surround- ing tissue fluid. In capillaries throughout the body, blood pressure forces plasma (water) and dissolved materials through the capillary membranes into the surrounding tissue spaces (see Fig. 3–3). This cre- ates more tissue fluid and is how cells receive glu- cose, amino acids, and other nutrients. Blood pressure in the capillaries of the kidneys also brings about filtration, which is the first step in the formation of urine.
PHAGOCYTOSIS AND PINOCYTOSIS
These two processes are similar in that both involve a cell engulfing something, and both are forms of endo- cytosis, endomeaning “to take into” a cell. An exam- ple of phagocytosis is a white blood cell engulfing bacteria. The white blood cell flows around the bac- terium (see Fig. 3–3), taking it in and eventually digesting it. Digestion is accomplished by the enzymes in the cell’s lysosomes.
Other cells that are stationary may take in small molecules that become adsorbed or attached to their membranes. The cells of the kidney tubules reabsorb small proteins by pinocytosis (see Fig. 3–3) so that the protein is not lost in urine.
Table 3–2 summarizes the cellular transport mech- anisms.