Active Transport
Active transportis the movement of a substance across the cytoplasmic mem-
brane against the gradient by using energy provided by the cell. This is similar
to pumping water against gravity through a pipe. Energy must be spent in order
for the pump to work.
A cell makes energy available by removing a phosphate (P) from adenosine
triphosphate(ATP). ATP contains chemical potential energy that is released by
a chemical reaction within the cell. It is this energy that is used to change the
shape of the integral membrane protein-enabling substances inside the cell to be
pumped through the cytoplasmic membrane. For example, active transport is
used to pump sodium (Na+
) from a cell.Group Translocation
Group translocation is a diffusion process that immediately modifies a substance
once the substance passes through the cytoplasmic membrane. The cell must
expend energy during group translocation, which is supplied by high-energy
phosphate compounds such as phosphoenolpyruvic acid (PEP). Group trans-
location occurs in prokaryotic cells.Endocytosis and Exocytosis
Endocytosis and exocytosis are processes used to move large substances or lots
of little ones into and out of a cell. Large substances enter the cell by endocyto-(^76) CHAPTER 4 Prokaryotic and Eukaryotic Cells
How Osmosis Works
- Isotonic solution.“Iso” means the same if a cell is placed in an isotonic solu-
tion. This means that there is the same concentration of solute and solvent
(water) inside and outside of the cell. There is an equalmovement of sub-
stances into and out of the cell. - Hypertonic solution.In a hypertonic solution, the cell is placed in an envi-
ronment where there is a higher concentration of solute. What happens is that
the water inside the cell will move out of the cell by osmosis causing it to
shrink. This shrinking of the cell is called crenation. - Hypotonic solution.When a cell is placed in a hypotonic solution, there is
more water outside of the cell than inside. This means there is more solute con-
centration inside of the cell. The water outside will move into the cell by osmo-
sis causing the cell to swell and ultimately break apart. This is called lysis.