Name ______________________________
Ms. De Pinto
Period_______
Date_______________
Transport Across a Cell Membrane
The cell membrane is one of the great multi-taskers of biology. It provides structure for
the cell, protects cytosolic contents from the environment, and allows cells to act as
specialized units. A membrane is the cell’s interface with the rest of the world - it’s gatekeeper,
if you will. This phospholipid bilayer determines what molecules can move into or out of the
cell, and so is in large part responsible for maintaining the delicate homeostasis of each cell.
Semi-Permeability
Some cells function best at a pH of 5, while others are
better at pH 7. The steroid hormone aldosterone is made
in the adrenal gland, but affects mostly the kidney.
Sodium is more than ten times more concentrated outside
of cells rather than inside. If our cells couldn’t control
what crossed their membranes, either no molecules
would make it across, or they’d be traveling willy-nilly
and the internal environment would always be in flux. It’d
be like taking every item on a menu and blending it
together before serving (not the tastiest idea).
So how do cells maintain different concentrations of
Cartoon representing the cell membrane as
a shield preventing things from entering the
proteins and molecules despite the pressures on them
cell which is represented as a castle.
to be homogenous? Cell membranes are
semipermeable, meaning they have control over what
molecules can or cannot pass through. Some molecules can just drift in and out, others
require special structures to get in and out of a cell, while some molecules even need an
energy boost to get across a cell membrane. Each cell’s membrane contains the right mix of
these structures to help that cell keep its internal environment just right.
Movement Across a Membrane and Energy
There are two major ways that molecules can be
moved across a membrane, and the distinction
has to do with whether or not cell energy is
used. Passive mechanisms like diffusion use no
energy, while active transport requires energy
to get done.
Diffusion: the Simple and the
Facilitated
Cartoon representing passive transport as
rolling a boulder down a hill and active
transport as rolling a boulder up a hill.
Diffusion is the movement of particles down their
gradient. A gradient is any imbalance in
concentration, and moving down a gradient just means that the particle is trying to be evenly
distributed everywhere, like dropping food coloring in water. We call this evening-out moving
“downhill”, and it doesn’t require energy. The molecule most likely to be involved in simple
diffusion is water - it can easily pass through cell membranes. When water undergoes simple
diffusion, it is known as osmosis.
Image showing purple ink diffuse from a tiny drop into a beaker of water.
Simple diffusion is pretty much exactly what it sounds like – molecules move down their
gradients through the membrane. Molecules that practice simple diffusion must be small and
nonpolar*, in order to pass through the membrane. Simple diffusion can be disrupted if the
diffusion distance is increased.
Facilitated diffusion is diffusion that is helped
along (facilitated by) a membrane transport
channel. These channels are proteins that allow
molecules to pass through the membrane. These
channels are almost always specific for either a
certain molecule or a certain type of molecule (Ex:
an ion channel), and so they are tightly linked to
certain cell functions.
Active Transport
Sometimes the body needs to move molecules against their gradient. This is known as moving
“uphill”, and requires energy from the cell - imagine how much easier it is to shake the trail
mix together than it would be to then separate all the pieces again.
Ex: The sodium-potassium pump (Na+/K+) in the cell uses the energy released from the
splitting of ATP (adenosine triphosphate) to pump three sodium ions out of and two
potassium ions into the cell. The energy is moving sodium from a concentration of 10mM to
one of 145 mM (low to high).
Ex: One other location for such an ATP pump is the proton/potassium exchanger
(H+/K+ ATPase) found in the stomach. These
proton pumps are responsible for creating the acidic
environment of the stomach, and can cause acid
reflux. Proton pump inhibitors like omeprazole are
prescribed to patients with ulcers or acid reflux to
help reduce the acidity of their gut.
Illustration showing active transport of sodium and
potassium across the cell membrane via the sodiumpotassium ATPase pump.
Let's Review
Transport across a cell membrane is a tightly regulated process, because cell function is highly
dependent on maintain strict concentrations of various molecules. When a molecule moves
down its concentration gradient is it participating in passive transport; moving up the
concentration gradient requires energy making it active transport.
Transport
Molecules Moved
Uses energy?
Simple Diffusion
Small, nonpolar
No
Facilitated Diffusion
Polar molecules, larger ions
No
Active Transport
Molecules moving against their gradient
coupled to the hydrolysis of ATP
Yes
Name ______________________
Ms. De Pinto
Period_______
Date_______________
Diffusion vs. Active Transport Worksheet
Directions: Use the reading, your notes, and your knowledge of Living Environment to answer the
following questions.
1. Define:
a. Passive Transport:
b. Simple Diffusion:
c. Facilitated Diffusion:
d. Active Transport:
e. Osmosis:
2. Is there a difference between Passive Transport and Diffusion? Explain your reasoning.
3. What is the difference between passive transport and active transport?
4. Why is the plasma membrane described as being “semipermeable”?
5. Identify which type of transport fits best with the following statement:
a. Molecules move from a high concentration to a low concentration. ______________
b. Molecules move from a low concentration to a high concentration. ______________
c. Molecules move UP a concentration gradient. ______________
d. Molecules move DOWN a concentration gradient. ______________
e. Molecules move without the use of energy. ______________
f. Molecules require energy to move. ______________
g. Molecules use transport proteins that require energy usage to cross the plasma
membrane. ______________
h. Molecules use transport proteins to cross the plasma membrane, but they do not require
ATP to do so. ______________