Lights, Camera, Acting Transport!
Pre-Activity Worksheet
ANSWER KEY
LIGHTS, CAMERA, ACTING TRANSPORT!
This worksheet will prepare you for your upcoming activity on membrane transport. You can use Sections
7.1-7.3 in your Campbell textbook or sections 5.1-5.4 in Raven to help you.
1.
Fill out the following chart comparing different mechanisms of transport across a plasma membrane:
Types of molecules transported?
(List size, polarity, charge)
Requires a carrier or channel protein?
(Yes/No)
Requires an additional energy source?
(Yes/No)
Do any molecules travel against their concentration
or electrochemical gradients? (Yes/No)
Simple
Diffusion
Facilitated
Diffusion
No
Larger
Polar
Charged
Yes
Primary
Active
Transport
Larger
Polar
Charged
Yes
Secondary
Active
Transport
Larger
Polar
Charged
Yes
Small
Nonpolar
Neutral
(Either)
(Carrier only)
(Carrier only)
No
No
Yes, ATP
No
No
Yes
Not directly
One does, one
doesn’t
2.
What is meant by the term “membrane potential”? What is an electrochemical gradient?
“Membrane potential” refers to the electrical potential energy (i.e., voltage) stored across a biological
membrane when positive and negative charges are separated. It results from an unequal distribution of
ions on either side of the membrane. An electrochemical gradient combines the electrical forces of the
membrane potential with the concentration gradients of molecules on either side of the membrane. The
diffusion of molecules across the membrane depends on the combined forces of the electrical and
concentration gradients.
3.
What type of membrane protein does the cell use to generate a membrane potential?
Na+/K+ pump, proton pump, or electrogenic pump are all acceptable answers.
4.
Briefly describe the 6 steps involved in ion transport using the Na +/K+ pump.
1) _3 Na+ ions from the cytoplasm bind to the Na+/K+ pump. ________________________________
2) _ATP in the cytoplasm phosphorylates the Na+/K+ pump. ________________________________
3) _The Na+/K+ pump changes shape, releasing the 3 Na+ ions outside of the cell. _______________
4) _2 K+ ions from the outside bind to the pump, and the phosphate is released. _________________
5) When the phosphate is lost, the protein pump returns to its original shape.__________________
6) _2 K+ ions are released to the interior of the cell, and the cycle begins again._________________
5.
What specifically would happen to a cell if its Na+/K+ pump were defective? (Hint: “It would die” is
not an acceptable answer.)
The cell would be unable to maintain its Na+ and K+ concentration gradients across the plasma
membrane. Therefore, it would no longer have the potential energy stored in this electrochemical
gradient available to do work.
6.
What is a cotransporter?
A cotransporter is a membrane protein that couples the “uphill” (i.e., active) transport of one
molecule or ion with the “downhill” diffusion of an ion along its electrochemical gradient.
7.
Why are electrochemical gradients important to the process of co-transport?
The potential energy stored in electrochemical gradients provides the energy source for co-transport.
8.
Is the Na+/K+ pump considered a cotransporter (Yes/No)? _No.___ Why or why not?
The Na+/K+ pump is not a cotransporter because: 1) both Na+ and K+ move against their
concentration gradients; and 2) ATP (not an electrochemical gradient) is the energy source for this
type of active transport.