Because the cell membrane is leaky to sodium and potassium, sodium ions diffuse into
the cell and potassium ions diffuse out. The cell maintains the gradients of these ions by
pumping sodium out of the cell and bringing potassium back in.
3 Na+ expelled
3 sodium ions (Na+) from the
cytosol bind to the inside surface
of the sodium-potassium pump.
Na+ binding triggers ATP to
bind to the pump and be split
into ADP and P (phosphate).
The energy from ATP splitting
causes the protein to change
shape, which moves the Na+
to the outside.
2 potassium ions (K+)
bind to the outside
surface of the pump
and cause the P to
The release of the P
causes the pump to
return to its original
shape, which moves
the K+ into the cell.
If Na+ is moved out of the cell and K+ is moved into the cell by
active transport, then they are moving________their concentration gradients.
A ctive tra n s p o rt Active transport is the process in
which energy is used to move substances across a mem-
brane against a concentration gradient (that is, from lower
concentration to higher concentration). The source of ener-
gy depends on which of three transport mechanisms is used: •
Pumps use energy from splitting ATP to power the
movement of substances. The most common example
is the sodium-potassium pump (Na+/K+ pump, or
Na+/K+-ATPase), which is found in all cells (Figure
sodium out of the cell and transports potassium into
concentration gradient. For example, many cells,
such as heart and muscle cells, have a sodium-calcium
calcium out of the cell against its concentration
gradient by coupling this movement to the movement
of sodium from outside the cell to inside the cell
(down the sodium concentration gradient).
coupling it to the movement of electrons across
some membranes. As you will see in Chapter 14, the
movement of electrons from one protein to another
in the inner mitochondrial membrane provides the
energy to transport proteins out of the mitochondrial
The active and passive transport mechanisms just
described exist throughout the membranes of organelles
and the cell itself. They work very well for moving small
substances—such as ions, water molecules, glucose mol-
ecules, and amino acids—across membranes. However,
large proteins and even invading bacteria require other
bulk transport processes.
E n d o cy to sis a n d e x o c y to sis To transport large
materials, a membrane engulfs the material into the
inside of a small round sac, or vesicle (VES-i-kul).
These sacs move about the cell interior with energy
supplied by ATP and contractions of microfilaments,
which pull them along. There are two major types of
vesicle transport, endocytosis and exocytosis. Endocy-
62 CHAPTER 3
Cells and Tissues