Biomedical Engineering Reference
In-Depth Information
binding sites for hormones, recognition markers for identifying cells as one type or another,
adhesive mechanisms for binding adjacent cells to each other, and channels for transporting
materials across the plasma membrane. The phospholipids are arranged with their “water
loving” (hydrophilic) heads pointing outward and their “water fearing” (hydrophobic)
tails pointing inward. This double-layer arrangement of phospholipids interspersed with
protein channels helps maintain the internal environment of a cell by controlling the sub-
stances that move across the membrane, whereas the cholesterol molecules act as stabilizers
to prevent extensive lateral movement of the lipid molecules.
Some molecules, such as oxygen, carbon dioxide, and water, can easily cross the plasma
membrane, whereas other substances, such as large molecules and ions, must move through
the protein channels.
is the process by which substances move across a selectively
permeable membrane such as a cell's plasma membrane, whereas
Osmosis
refers to the move-
ment of molecules from an area of relatively high concentration to an area of relatively low
concentration. Substances that can easily cross the plasma membrane achieve diffusion equi-
librium when there is no net movement of these substances across the membrane; that is,
the concentration of the substance inside the cell equals the concentration of the substance
outside of the cell. Active transport, which requires an input of energy usually in the form
of ATP, can be used to move ions and molecules across the plasma membrane and is often
used to move them from areas of low concentration to areas of high concentration. This
mechanism helps maintain concentrations of ions and molecules inside a cell that are differ-
ent from the concentrations outside the cell. A typical mammalian cell has internal sodium
ion (Na
þ
) concentrations of 12 mM (12 moles of Na
þ
per 1,000 liters of solution) and extra-
cellular Na
þ
concentrations of 120 mM, whereas intracellular and extracellular potassium
ion (K
þ
) concentrations are on the order of 125 mM and 5 mM, respectively. In addition to
positively charged ions (cations), cells also contain negatively charged ions (anions). A typical
mammalian cell has intracellular and extracellular chloride ion (Cl
) concentrations of 5 mM
and 125 mM and internal anion (e.g., proteins, charged amino acids, sulfate ions, and phos-
phate ions) concentrations of 108 mM. These transmembrane ion gradients are used to make
ATP, to drive various transport processes, and to generate electrical signals.
diffusion
EXAMPLE PROBLEM 3.1
How many molecules of sodium and potassium ions would a cell that has a volume of 2 nl
contain?
Solution
Assuming that the intracellular concentrations of Na
þ
and K
þ
are 12 mM and 125 mM, respec-
tively, the number of molecules for each can be determined by using the volume of the cell and
Avogadro's number.
moles
10
23
molecules
mole
Na
þ
:
10
9
10
13
12
liters
6
:
023
2
liters
¼
1
:
45
molecules
1
;
000
moles
10
23
molecules
mole
K
þ
:
10
9
10
14
125
liters
6
:
023
2
liters
¼
1
:
51
molecules
1
;
000
