Active Transport:
What Can Cells Do When
Diffusion Doesn’t Work?
M. Schultz
The Cell Membrane is Selectively
Permeable
• Only small molecules with a neutral
charge get across easily.
• They only flow from high to low
concentrations.
• A cell can’t use diffusion if
– it needs to get in large or charged molecules.
– it needs to get molecules from low to high
concentrations.
+
+
Polymer
0
+
+
+
+
-
Monomer
+
+
+
Small and neutral molecules get into and out of the cell easily.
The Cell Membrane
Animation
Facilitated Diffusion: A Type of
Passive Transport
• This does not require energy, but it does
require a helper because the molecule the
cell wants to get in is too big to fit through
the membrane just anywhere.
• This is how you get glucose into your cells.
Glucose Transporter
Animation
Active Transport
I. This requires energy (ATP) on the part of the
cell.
II. The cell needs to use active transport when
A. The molecule is too large and the
membrane has to change shape.
B. The cell needs to pump materials against
the concentration gradient (from low to
high).
Endocytosis: A Type of Active
Transport
Some molecules are just too big to get past
the membrane even with special helpers.
Animation
There are two types of Endocytosis
I. Phagocytosis is taking in large solid
particles.
II. Pinocytosis is taking in large amounts of
liquid.
Exocytosis: Another Type of Active
Transport
This is when a cell spits out large particles of
waste.
Sodium Potassium Pumps: A Type
of Active Transport
The cell must use pumps when it is trying to
get materials from where there is a low
concentration to a side where there is a
higher concentration.
This is movement against the concentration
gradient. Animation