6 Membrane Dynamics and Osmosis Lab
AP Biology
Name: ____________ Name: ____________ Name: ______________
Name: _____________ Date: ____________ Period: ______________
Cell membranes surround each and every living cell (as far as I
know). Cell membranes are often described as selectively permeable
membranes because they allow certain molecules in and out, and they
exclude certain molecules from entering or leaving. Simply stated,
membranes allow many different kinds of small particle/compounds to enter
and leave such as oxygen, carbon dioxide, and water. These compounds will
move from an area of higher concentration to lower concentration (that is to
say with the concentration gradient). This process requires no energy on the
part of the cell. These compounds and many others, are constantly moving
across cell membranes even after an equilibrium (when there is the same
number of a particular molecule on each side of a membrane-see diagram
below:) has been reached.
Beginning
Middle
End- Equilibrium.
Molecules STILL movebut the same number of
water molecules move in
one direction as in the
opposite direction
Page 2 (Cont. Handout #6 Cell membrane dynamics/Osmosis AP Bio.)
Cell surface receptors found on ALL cells can hold onto and “grab”
those signal molecules based on compatible positive and negative charge
match-ups (i.e. “plus” charges will be attracted to “negative” charges). The
NUMBER of cell surface receptors on a cell surface for any particular signal
molecule has a lot to do with the cell responding to that signal. The more
receptors and the more corresponding signal molecules, the more likely the
cell will respond to those signals.
The experiment we will be doing simulates a selectively permeable
membrane and the response to signal molecules. The green boxes have
hundreds of holes in them that are all approximately the same size. You will
be given a certain amount of small clear glass beads that will represent
water molecules. Along with these clear glass beads you will get a similar
amount of larger metal marbles that represent “signal molecules”.
Cells try to maintain a balance of water into and out of the cell.
Aquaporins are transmembrane protein “canals” that allow water to pass into
and out of the cell quickly.
Water molecules
As you can see, there are many
“tunnels” in the membranes shown
above. For our purposes, I “made”
these cells with many holes to try
and demonstrate the membrane
dynamics- that is- how the cell
controls the movement of water into
and out of the cell.
Moreover, I attached powerful
magnets to these “green” cells that
represent cell surface receptors.
Each group will have a cell with a
slightly different number of receptors
to show the effect of varying receptors
attracting different amounts of signal
molecules that the cell can respond to.
Aquaporin “tunnel”
Page 3 (Cont. Handout #6 Cell membrane dynamics/Osmosis AP Bio.)
This lab is in three parts. In each part you will “pour” the contents of
a beaker into the reaction chamber - a small wooden box containing your
green “cell”. This is the same as pouring water and signal molecules over an
actual cell. Some of the glass marbles will fall into the cell and some will
not enter the cell and fall to the bottom of the box. Some of the metal
“signal” molecules will attach to the cell receptors and some will fall to the
bottom of the box. For each section, you will count and record the type and
number of glass marbles that “STAY” in the box, or metal marbles that
attach to the outside of the box in Table 1 (next page).
Follow the directions carefully and record your results in the table- next
page.
Section 1:
1.) Fill plastic beaker with 100 small clear glass marbles and 20 larger
metal marbles:
2.) Pour marbles over the green cube (slowly). Do not worry if a few
marbles fall out in the green box when you remove the green cell from
the wood box. Try not to lose any marbles on the floor during transfer!
3.) Carefully remove the green cube from the box and place it over small
cardboard box and remove all marbles from within the cell by gently
rotating the box until all marbles are out of box and picking off the signal
molecules. Count and record your results in Table one.
Section 2:
4.) Clear all marbles out of box. Count out 200 small clear glass marbles
and 30 larger metal marbles. Place in plastic beaker.
5.) Repeat steps 2 and 3.
Section 3:
6.) Clear all marbles out of box. Count 300 small clear glass marbles and
40 larger metal marbles. Place in plastic beaker.
7.) Repeat steps 2 and 3.
Page 4 (cont. Handout #6 Cell membrane dynamics/Osmosis AP Bio.)
Table 1: Data for number of molecules of water “absorbed” by the green cell
and the number of “signal” molecules “absorbed” by the cell. (30 points)
Section
Small Marble Count
“Water”
Large Metal Marble Count
“signal”
1
2
3
Using the data above, graph the number of molecules found “inside”
the green cell compared to the section of the experiment and simultaneously
graph the number of metal marbles that “stuck” to the receptor molecules.
There is graph paper on page 6 of this handout. (Your teacher will
demonstrate how to set this graph up and plot the data points.)
Answer the following questions on this handout only. (5 points each)
_____ 1.) Cell membranes:
a.) are found around all living cells
b.) are found only around cells that are not bacteria
c.) are found only around animal cells not plant cells
d.) are found only around bacterial cells
_____ 2.) Cell membranes function to
a.) to hold together all other parts of the cell and give cell shape
b.) To separate what is inside the cell from what is outside the cell
c.) to control what goes into and out of the cell
d.) all of the above are correct
_____ 3.) Osmosis is the movement of…
a.) any molecule across a cell membrane
b.) the movement of water across a cell membrane
c.) the movement of some salt or solute across a plasma membrane
d.) none of the above is correct
Page 5 (Cont. Handout #6 Cell membrane dynamics/Osmosis AP Bio.)
_____ 4.) (T/F) Water will move across a cell membrane from an area of
high concentration to low concentration.
_____ 5.) Aquaporins are…
a.) cell surface receptors that tell the cell when it needs water
b.) transmembrane proteins that move sodium and potassium across the cell
membrane
c.) transmembrane proteins that move water into and out of the cell
d.) another name for the bi-lipid layer of the cell membrane
_____ 6.) During osmosis, equilibrium means…
a.) that there is an equal movement of water into and out of the cell
b.) that there is a net movement of water into the cell
c.) that there is a net movement of water out of the cell
d.) that there is NO movement of water across the cell membrane
_____ 7.) As the number of small clear marbles “poured” over the green
cell increased…
a.) the smaller the number of small clear marbles entered the “green” cell
b.) the larger the number of small clear marbles entered the “green” cell
c.) the larger the number of large marbles entered the “green” cell
d.) the smaller the number of large marbles entered the “ green” cell
_____ 8.) Speculate why the number of signal molecules (metal marbles)
changed in each section.
a.) the increase in large marbles represented molecules that were toxic to the
cell
b.) the increase of large marbles may represent an increased “need” for
target cell product
c.) the increase of large molecules were there as “controls” for the
experiment
d.) none of the above are correct
_____ 9.) What compound is the basis of all life?
a.) potassium b.) chlorine c.) sodium d.) water
Page 6 (Cont. Handout #6 Cell membrane dynamics/Osmosis AP Bio.)
_____ 10.) Why was the number of clear marbles changed for each section?
a.) to show that the more water was poured over the cell the more water the
cell would absorb
b.) to show that water will only move in the cell regardless of the amount of
water is poured over the cell
c.) to show that water will only move out of the cell regardless of how much
water is poured over the cell
Page 7 (Cont. Handout #6 Osmosis and Cell Signaling AP Bio.)
Date: __________________________
Lesson Plan for handout #6 AP Biology
Objective: TLWD ability to define and explain how water molecules are
transported through cell membranes, using aquaporins and no energy.
Moreover, students will be able to quantitate the response of cells to varying
amounts of signal molecules. Students will also obtain data, place the data
in a data table and construct a graph using the data from that table when
given handout #6.
Content: passive transport/osmosis
NJCCCS: 5.3.12.A.3
Method: Power point/Internet animation (see below).
Comment: Google search- “osmosis” + “cell” Page 1 look for large red
blood cell
Homework: Complete #6