ELODEA MEMBRANE TRANSPORT LAB
Background
In order for cells to maintain the chemical reactions occurring inside themselves
substances must move back and forth between a cell’s external and internal
environments. More specifically, reactants need to be acquired by the cell while
harmful products must be eliminated. For example, during respiration your cells
must acquire oxygen and must eliminate carbon dioxide. Other products, like
ATP (an energy molecule) are beneficial and are used by the cell.
As you know, the cell membrane is a fluid structure composed of phospholipids,
cholesterol, and various membrane proteins (adhesion, recognition, receptor,
and transport). The precise location through which a substance travels is
dependent on two important molecular characteristics; the molecule size and
charge (polarity). In general, small uncharged (non-polar) substances move
through the lipid portion of the membrane, while small charged (polar) and larger
molecules must enter the cell through proteins.
In most instances, the movement of substances requires the existence of a
concentration gradient across the cell membrane and substances move from
areas of higher concentration to areas of lower concentration. In such situations,
substances will move by osmosis, diffusion, or passive diffusion (facilitated
diffusion). When substances require movement against their concentration
gradient active transport is used.
In the following laboratory exercise you will investigate the movement of
substances into and/or out of the membrane of an elodea leaf in order to gain a
greater understanding of processes of osmosis and diffusion.
Objective
1. You should understand how and why water moves through a semi-permeable
membrane in various liquid environments.
Materials
1 Elodea Leaf soaked in Distilled Water
1 Elodea Lead soaked in Saltwater
Compound Light Microscope
1 Microscope Slide
1 Cover Slip
Procedure
Elodea Exercise
1. While you are waiting for the eggs to sit, acquire a Compound Light
Microscope, Microscope Slide, Cover Slip, and an Elodea Leaf that has been
soaking in Distilled Water.
2. Place the Elodea Leaf on the Microscope Slide, use a dropper to transfer a
single drop of water on top of the leaf, and then gently place the Cover Slip
over the leaf.
3. Focus on the Elodea Leaf so that you can examine a number of individual
cells, and complete the Elodea Exercise Questions on the worksheet
provided.
4. Then, dispose of the current Elodea Leaf and acquire one that has been
soaking in Saltwater.
5. Place the Elodea Leaf on the Microscope Slide, use a dropper to transfer a
single drop of saltwater on top of the leaf, and then gently place the Cover
Slip over the leaf.
3. Focus on the Elodea Leaf so that you can examine a number of individual
cells, and complete the Elodea Exercise Questions on the worksheet
provided.
MEMBRANE TRANSPORT LAB QUESTIONS
Name: __________________________
Elodea Exercise
1. In the space provide below make a sketch of the Elodea cell that you have
observed, and using your book label the cell wall, chloroplasts, and cell
membrane and central vacuole (hypothesize where it should be).
2. What is the function of the cellular structures and organelles labeled above?
a. Cell Wall
b. Chloroplast
c. Central Vacuole
3. In the space provide below make a sketch of the Elodea cell that you have
observed, and using your book label the cell wall, chloroplasts, and cell
membrane and central vacuole (hypothesize where it should be).
4. How has the position of the Chloroplast changed between the Elodea Leaf in
Distilled Water and Saltwater?
To understand what is going on in these situations you need to read pages 118121 and answer the following questions.
5. Describe the motion of molecules in a fluid.
6. Define diffusion.
7. When does the diffusion of a molecule in solution stop?
8. How are cell membrane considered selectively permeable.
9. Contrast the movement of molecules by passive and facilitated transport.
Based on our previous discussions, what are the contrasting characteristics of
molecules that travel in these two manners?
10. Based on your knowledge of the terms hypertonic and hypotonic, label the
two solutions diagramed below relative to each other.
11. Does water tend to move across a permeable membrane from a hypertonic
to a hypotonic solution OR from a hypotonic to hypertonic solution?
12. Given that there are more solutes (things dissolved in water) outside the
Elodea cell when it is immersed in saltwater, first identify if the surrounding
solution is hypertonic or hypotonic. Then, identify using an arrow which
direction water will tend to move in the cell below. Assuming that the cell
membrane is located between the cell wall and chloroplasts, explain why the
chloroplast become clumped when the Elodea is placed in saltwater.
Cell
13. Given that there are more solutes (things dissolved in water) inside the
Elodea cell when it is immersed in distilled water (more like its real
environment), first identify if the surrounding solution is hypertonic or
hypotonic. Then, identify using an arrow which direction water will tend to
move in the cell below. Then, explain why the chloroplasts become are
spread out in this situation and why it might be important that the cells have a
cell wall.
Cell
14. View the video link on the action of the freshwater paramecium’s contractile
vacuole. Based on the knowledge you have gained regarding osmosis,
hypothesize the function of the contractile vacuole.
15. Discuss issues associated with living saltwater that a saltwater fish would
have to adapt to/overcome?
16. Explain how pouring salt on a slug kills it.