Cell Membrane Review
NOTE: Some of the answers below are written in short hand. They are not to be considered suitable as test
answers, for example.
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Describe the fluid mosaic model of the cell membrane.
- The membrane is flexible and not rigid (due to the nature of the phospholipids). It is composed of a
variety of molecules, primarily phospholipids with proteins embedded.
Describe the structure of a phospholipid.
- It has a polar (hydrophilic) head, and two nonpolar (hydrophobic) tails. The molecule is said to be
amphipathic.
How is the structure of the phospholipid important for its function?
- Being amphipathic, phospholipids spontaneously arrange as a lipid bilayer and as such, create a means
to be selective of molecules that can penetrate (based on their size and polarity).
Define the terms hydrophilic and hydrophobic.
- Hydrophilic: when a molecule has a structure that interacts well with water (e.g. polar, charged).
Hydrophobic: when a molecule has a structure that does not interact well with water (e.g. nonpolar).
What are the two important jobs of the membrane?
- Include/exclude materials from the interior of a cell, and cell recognition.
Why is it important for the cell membrane to be selectively permeable?
- To prevent undesired materials from entering, and to retain the materials that the cell requires.
What factors contribute to the selective permeability of the cell membrane?
- Nonpolar intermembrane space, the tightly packed phospholipids, and transport proteins (for both
active and passive transport).
In general, what kinds of things pass easily through the membrane and which do not?
- In general, small nonpolar molecules will pass easily and large polar molecules will not.
If a tiny hole is made in a cell membrane, it usually seals itself immediately. What properties of the
membrane allow this to occur?
- This will occur spontaneously because of the amphipathic nature of the phospholipids and their
automatic response to exposure to water.
Hormones are chemical messengers that travel throughout the body. Protein hormones (polar) attach
themselves to receptors on the cell surface while lipid hormones (nonpolar) actually enter the cell.
Explain this difference in hormonal behavior.
- Protein hormones, being polar, will not be able to pass through the nonpolar intermembrane space,
therefore there are membrane proteins designed to receive them on the outside. The opposite is true
for lipid hormones, which are nonpolar.
What are glycoproteins and what is their function?
- Proteins with a sugar attached and facing the extracellular fluid. They function in cell recognition.
Discuss three factors that affect diffusion rates.
- As P, D. As T, D. As C, D. Also, as S, D. This question requires explanations so please view
the smart notes on edline.
Describe the fundamental difference between active and passive transport.
- Active requires energy (ATP) and passive does not.
Explain the process of osmosis.
- Water moves from an area of high [water] to and area of low [water], OR in other words, from low
[solute] to and area of high [solute].
Imagine that a cell has been in a slightly hypotonic solution for some time and is now isotonic with the
solution. Has the movement of water molecules stopped? Explain.
- No, the water is always moving. Once equilibrium is reached, the water is simply in and out of the cell
at an equal rate.
Which of the following conditions describe a hypotonic solution that a cell is placed in?
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(a) a higher solute concentration outside the cell
(b) a lower solute concentration outside the cell
(c) a equal solute concentration both inside and outside the cell
(d) none of the above
What happens to a cell placed in a hypotonic solution?
- There will be a net flow of water into the cell because of it’s higher [solute], therefore the added
water will cause the cell to swell.
Water moves mainly in which direction through the pores of a cell placed in a hypertonic solution?
- Net flow of water is OUT of the cell.
You are fertilizing the lawn one day and spill some fertilizer in one spot. What do expect to observe
over the next few days? Explain.
- Spilling fertilizer will cause the grass under it to be bathed in a hypertonic solution. The net flow of
water would be out of the cell and cause the cells to plasmolyze and eventually die.
Why is turgor pressure not used in reference to animal cells?
- Turgor pressure refers to the pressure that water within the cytoplasm applies on the cell wall, but
animal cells do not have a cell wall.
How does facilitated diffusion differ from simple diffusion and osmosis? Why is it a benefit to cells?
- Facilitated diffusion requires transport proteins and simple diffusion and osmosis do not.
Identify whether each of the following is passive or active transport.
(a) osmosis P
(b) exocytosis A
(c) simple diffusion P
(d) phagocytosis A
(e) molecular
transport A
(f) endocytosis A
(g) facilitated diffusion P
Suppose that the concentration of carbon dioxide in the fluid outside a cell became higher than that on
the inside. Predict what would happen. What prevents this from happening normally?
- The CO2 would begin to diffuse back into the cell, but this is prevented because we are constantly
breathing and releasing it into the atmosphere.
A marathon runner collapses after running on a hot day. Although the runner consumed adequate water
along the route, blood testing showed that many of his red blood cells had burst. Why was this the
case? Hint - on hot days, runners usually drink fluids that contain sugar and salt.
- The runner was sweating and losing a large amount of salt, yet by drinking water instead of beverages
with high solute concentrations, his blood essentially became a hypotonic solution. The red blood cells
therefore experienced an influx of water that caused them to burst. A reduced red blood cell count
means that the transport of oxygen within the body becomes greatly reduced and that would have led
him to collapse.
When gardeners bring in fresh vegetables from the garden, they sometimes soak them in saltwater
before rinsing them and soaking them in freshwater. Why would they do this? Hint – this is a helpful
alternative to engineered pesticides.
- The salt water (hypertonic) would cause any microbes to shrivel and die, and then the fresh water
(hypotonic) would not only rinse away the salt, but the cells would experience and increase in turgor
pressure and would seem exceedingly fresh.
Explain why it is not a good idea to drink distilled water or saltwater.
- Distilled water lacks solutes and could lead to hypotonicity of the blood and body fluids, where as salt
water could lead to hypertonicity.
Grocery stores spray their vegetables with water to preserve their freshness. Explain how this makes
the vegetables appear fresh.
- Hypotonic solution, therefore the net flow of water moves into the cells and increases the turgor
pressure within them. This makes them look healthier then perhaps they are.