The rate of transport of a substance into a cell is drastically reduced
when the formation of ATP is blocked. The transport system must be a
||both active transport and simple diffusion
||both simple and facilitated diffusion
Active transport systems use an energy source (usually ATP) to drive
the movement of molecules against a concentration gradient. The best
example of active transport is the Na+/K+ATPase. This membrane protein
transporter moves Na+ out of the cell and K+ into the cell, building
up high Na+ outside and high K+ inside the cells.
Nearly a third of the energy we use each day drives this transport
system. The transmission of signals through our brain and the control
of water flow into and out of cells requires the activity
of the Na+/K+ATPase. This system is a good example of the coupling
of ATP hydrolysis with an unfavorable reaction (ion movement against
a concentration gradient).