AP Biology : Understanding the Cell Membrane

Study concepts, example questions & explanations for AP Biology

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Example Questions

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Example Question #1 : Understanding The Cell Membrane

Living in Arizona, you see a variety of animals that are able to survive in very hot conditions. One of the ways they are able to tolerate such warm conditions is by maintaining the fluidity of their plasma membranes.

How would a desert tortoise's plasma membrane differ from the plasma membrane of a dog that lives in an air conditioned house?

Possible Answers:

The tortoise would have more unstaurated fatty acid tails in its membrane compared to the dog.

The tortoise would have less phospholipids in its bilayer compared to the dog.

The tortoise would have shorter fatty acid tails in its membrane compared to the dog.

The tortoise would have more cholesterol in its membrane compared to the dog.

Correct answer:

The tortoise would have more cholesterol in its membrane compared to the dog.

Explanation:

The plasma membrane is able to maintain the appropriate level of fluidity by manipulating a variety of factors. More cholesterol in the membrane reduces its permeability, which is useful in hot conditions. Desert animals would also be expected to have very few unstaurated fatty acids, and the fatty acid tails would be longer. All of these factors would help the plasma membrane not be too fluid in the hot sun.

Example Question #2 : Understanding The Cell Membrane

Which form of cellular transport is required to move a molecule against its concentration gradient?

Possible Answers:

Facilitated diffusion

Osmosis

Diffusion

Active transport

Correct answer:

Active transport

Explanation:

The natural flow of molecules is from areas of high concentration to areas of low concentration. To act against this gradient, energy must be input. Active transport requires the use of energy to move a molecule up its concentration gradient. Diffusion, osmosis, and facilitated diffusion are forms of passive transport that move a molecule down its concentration gradient.

Example Question #3 : Understanding The Cell Membrane

Which of the following forms of transport uses vesicles to transport large molecules?

Possible Answers:

Active transport

Facilitated diffusion

Osmosis

Endocytosis

Correct answer:

Endocytosis

Explanation:

Endocytosis involves the use of vesicles to transport large molecules into the cell. Facilitated diffusion, osmosis, and active transport use diffusion or protein channels to transport molecules.

Note that exocytosis also uses vesicles, but uses them to export large molecules out of the cell or to incorporate them into the cell membrane.

Example Question #4 : Understanding The Cell Membrane

How are integral proteins introduced into the cellular membrane of an eukaryotic cell?

Possible Answers:

Cytoplasmic ribosomes translate the necessary proteins and send them to the membrane

Cells have all of the necessary membrane proteins upon completing mitosis

Exocytosis supplies the membrane with the necessary proteins, which are present on the secretory vesicle

Endocytosis brings in the necessary proteins and incorporates them into the membrane

Correct answer:

Exocytosis supplies the membrane with the necessary proteins, which are present on the secretory vesicle

Explanation:

Exocytosis allows the membrane of secretory vesicles to be incorporated into the cellular membrane. This expands the membrane surface, while including the desired proteins into the membrane. Due to fluidity and the mosaic model of the membrane, these proteins can then distribute to other areas on the cell surface.

Endocytosis does the opposite process, and involves a pinching off of the cell membrane in order to transport incoming materials. Ribosomes translate the proteins, however, processing of membrane proteins occurs in the endoplasmic reticulum and Golgi body, ultimately packaging membrane proteins in secratory vesicles for exocytosis.

Example Question #5 : Understanding The Cell Membrane

Where would aspartic acid, an amino acid with a negatively charged side chain, most likely be found in a transmembrane protein?

Possible Answers:

Outside or inside the cell

Within the membrane interior, outside the cell, or inside the cell

Outside the cell

Inside the cell

Within the membrane interior

Correct answer:

Outside or inside the cell

Explanation:

Any amino acid with a charged side chain will be polar. Membranes have hydrophobic tails on the interior, and hydrophilic heads facing the outside and inside of the cell. Since polar molecules are charged, they will interact with the hydrophilic parts of the environment, and therefore they will not be found within the membrane interior.

Example Question #6 : Understanding The Cell Membrane

What would happen to a cell that contains 4.0 moles of solute inside it if it were placed in a cup of water with 2.0 moles of sodium chloride?

Possible Answers:

Half of the NaCl would move into the cell to make even concentrations

Nothing

It would shrivel

It would burst

It would swell

Correct answer:

Nothing

Explanation:

This question is tricky because we need to remember that if there are 2.0 moles of NaCl in the water, then there are 4.0 moles of solute because it will dissociate to one  and one ; therefore, this is an isotonic environment and there will be no net movement of water. When a compound dissociates in solution, it is the ion concentration that will affect the movement of water, rather than the amount of initial solid.

Example Question #7 : Understanding The Cell Membrane

The negative charge inside cells is primarily maintained by __________.

Possible Answers:

the membrane being less permeable to potassium ions than to sodium ions

the plasma membrane being impermeable to water

the sodium-potassium pump

the membrane being less permeable to sodium ions than to potassium ions

Correct answer:

the sodium-potassium pump

Explanation:

If there was no expenditure of energy when determining the voltage across the plasma membrane, there would be equal electrical charge on both sides of the bilayer as the ions travel to reach equilibrium. This means that ATP must be used in order to establish a resting potential, keeping the ions away from electrical equilibrium.

The sodium-potassium pump is an example of how ions can be pumped against their electrochemical gradients in order to establish a negative voltage inside the cell. The cell membrane is not permeable to sodium or potassium.

Example Question #8 : Understanding The Cell Membrane

Of the following, which is most likely to pass through the cell membrane via passive transport?

Possible Answers:

A charged ion

Water, when traveling from a highly hypertonic environment into a cell

A peptide hormone

Glucose

A steroid molecule

Correct answer:

A steroid molecule

Explanation:

Passive transport is simple diffusion, without the aid of any protein channels. Since the plasma membrane is composed of a lipid bilayer, the molecules that most readily cross it are small and hydrophobic.

Remember that water is polar; hydrophobic molecules are nonpolar, like lipids and oils. Steroids are derived from cholesterol and are extremely lipid-soluble, so they can cross the membrane unassisted. Large, polar molecules (such as glucose or peptides) and charged ions need channel proteins to facilitate their crossing. Water can diffuse across the membrane, but only when it is moving down its concentration gradient. The answer choice given has water moving from a hypertonic (high-solute, or low-water) environment into a cell, which cannot occur passively.

Example Question #9 : Understanding The Cell Membrane

What is the major factor limiting the size of a cell?

Possible Answers:

Concentration of water in the cytoplasm

Ratio of surface area to volume

Number of membrane-bound organelles 

Availability of nutrients

Correct answer:

Ratio of surface area to volume

Explanation:

While the availability of nutrients is a tempting answer, it is important to remember that even in incredibly nutrient-rich environments cells reach a maximum size. The concentration of water in the cytoplasm and the amount of membrane bound organelles will not really have any effect on the size of the cells.

The correct answer is the surface area to volume ratio because it dictates the amount of chemical activity carried out per unit of time. The volume of the cell determines its metabolic needs: larger cells hold more biological material and require more energy and maintenance. The surface area of the cell determines its ability to transport nutrients and absorb nutrients, providing the tools to maintain the cell's volume. A large surface area to volume ratio is essential for the cell.

Example Question #10 : Understanding The Cell Membrane

What are the main components of a phospholipid?

Possible Answers:

Two fatty acids, a glycerol backbone, and a cholesterol molecule

A polar head group, two fatty acids, and a glycogen backbone

A polar head group, two fatty acids, and a glycerol backbone

Three fatty acids and a glycerol backbone

Correct answer:

A polar head group, two fatty acids, and a glycerol backbone

Explanation:

Phospholipids found in the plasma membrane are comprised of a polar head group, two fatty acids, and a glycerol backbone. The phosphate group carries a negative charge, allowing it to interact with polar aqueous environments. The two fatty acid tails form the hydrophobic region of the bilayer interior. The glycerol backbone forms the structural component for linking the polar and non-polar regions.

Glycogen is a polymer of glucose sugars, and is not found in phospholipids. Triglycerides are composed of three fatty acids and a glycerol backbone.

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