GRE Subject Test: Biology : Cell Membrane and Organelles

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

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Example Question #1 : Understanding Membrane Structure

Which of the following conditions would result in a more fluid membrane?

Possible Answers:

A membrane in cold temperatures

Fully saturated fatty acid tails

Long fatty acid tails

Fatty acid tails with double bonds

Correct answer:

Fatty acid tails with double bonds

Explanation:

There are many factors that determine the fluidity of cell membranes. Membranes that are composed of fully saturated, long fatty acid tails are generally less fluid then the opposite conditions. In addition, lower temperatures result in a less fluid membrane.

Membranes that have fatty acid tails with double bonds are more fluid because the double bonds make it difficult for multiple phospholipids to float next to one another. The shape of the double bond adds a another dimension to the lipid, preventing the tails from packing together. Unsaturated fatty acids are thus more fluid than saturated fatty acids.

Example Question #1 : Understanding Membrane Structure

Which of the following factors would you expect to see in a cell membrane for an animal living in a very hot environment?

Possible Answers:

Longer fatty acid tails

Reduced integral proteins

More unsaturated fatty acid tails

Reduced presence of cholesterol

Correct answer:

Longer fatty acid tails

Explanation:

In higher temperatures, the cell membrane is going to become increasingly fluid due to the increased movement of the phospholipids. The cell membrane can control its fluidity in high temperatures by both increasing the saturated fatty acid tail content of the phospholipids, as well as making the fatty acid tails longer. Cholesterol can also help by acting as a buffer at high temperatures.

Example Question #1 : Cell Membrane And Organelles

What polymer is commonly found in the cell walls of fungi?

Possible Answers:

Glycogen

Cellulose

Peptidoglycan

Chitin

Correct answer:

Chitin

Explanation:

Although cell walls often serve very similar functions for different species, the composition of the cell walls can vary significantly. Plant cell walls employ cellulose, while bacteria use peptidoglycan. Fungal cell walls use the polymer chitin.

Example Question #1 : Understanding The Cell Wall

In bacteria, what is the polymer that makes up the cell wall?

Possible Answers:

Glycogen 

Peptidoglycan 

Cellulose

Chitin 

Starch

Correct answer:

Peptidoglycan 

Explanation:

The correct answer is peptidoglycan. Cellulose composes the cell walls of plants, whereas chitin composes the cell walls of fungi. Starch and glycogen are stored polymers of glucose in plants and animals, respectively.

Example Question #1 : Cell Membrane And Cell Wall

Which statement correctly describes a Gram-positive bacterial cell?

Possible Answers:

Contains no outer membrane outside of the peptidoglycan layer

Contains a periplasmic space

Stains red in the acid-fast stain

Has a very thin peptidoglycan layer

Stains red in the Gram stain

Correct answer:

Contains no outer membrane outside of the peptidoglycan layer

Explanation:

A Gram-positive cell has the following basic structural characteristics: stains dark purple in the Gram stain, has a thick peptidoglycan layer, and possesses no outer membrane beyond this layer. Thus, there is also no periplasmic space. Acid-fast stains are only used for specific bacteria that have waxy cell walls.

Example Question #4 : Cell Membrane And Organelles

Leucine is a hydrophobic amino acid. In which of the following portions of a transmembrane protein would a leucine residue most likely be located?

Possible Answers:

Cytosolic face

Membrane-spanning region

Exoplasmic face

Leucine would not be found in a transmembrane protein

Correct answer:

Membrane-spanning region

Explanation:

Because leucine is a hydrophobic amino acid, it would make sense that it would be most stable in a hydrophobic environment. The interior of the phospholipid bilayer is a hydrophobic environment; therefore, leucine and other hydrophobic amino acids are more commonly found in the membrane-spanning portions of transmembrane proteins.

Polar and hydrophilic amino acids are most commonly found in the cytosolic and exoplasmic regions of the membrane, as these regions interact with the aqueous environment outside of the membrane.

Example Question #2 : Cell Membrane And Organelles

A researcher finds a large amount of an unusual sugar in the cells of a recently discovered species. Which of the following are potential mechanisms the cell is using to import the sugar?

I. Passive diffusion

II. Receptor-mediated endocytosis

III. Symport

Possible Answers:

I, II, and III

II and III

III only

I and II

Correct answer:

II and III

Explanation:

Because the molecule is a sugar, it is too large to passively diffuse across the plasma membrane and contains polar regions that would make this impossible.

There are several other mechanisms by which the sugar could enter the cell. One of these is receptor-mediated endocytosis. In this process the sugar would bind to receptors on the plasma membrane, which stimulates a budding event and eventually leads to the formation of a vesicle inside the cell.

Symport is another potential mechanism. In symport, the import of a molecule is coupled with the import of another molecule through the same transmembrane protein. For example, glucose has a symport mechanism with sodium ions.

Example Question #1 : Cell Membrane And Organelles

What type membrane protein changes its shape when allowing substances to cross the membrane?

Possible Answers:

Glycoproteins

Channel proteins

Carrier proteins

Kinases

Correct answer:

Carrier proteins

Explanation:

Of the given choices, only channel and carrier proteins allow substances to cross the membrane. While channel proteins create an open pore through which substances can cross, carrier proteins will change their shape in order to allow substances to cross the membrane.

Example Question #1 : Understanding Membrane Functions

Which of the following is a function of membranes?

Possible Answers:

Selectively controlling import and export of nutrients

Separating the internal and external environments

Housing proteins used in important metabolic pathways

All of these are membrane functions

Correct answer:

All of these are membrane functions

Explanation:

The question asks about membranes in general, not just the cell plasma membrane; therefore, all of the answers are true. The plasma membrane's most important functions are protecting the internal environment of the cell and selectively allowing nutrients into the cytoplasm (semi-permeability). The final answer describes a function of the inner-membrane of the mitochondria, which houses the electron transport chain.

Example Question #1 : Understanding Membrane Functions

Name the type of transport that requires an expenditure of energy in order to move a compound across the membrane.

Possible Answers:

Diffusion

Facilitated diffusion

Active transport

Osmosis

Correct answer:

Active transport

Explanation:

Energy is not necessary when a compound is being moved down its electrochemical gradient. Diffusion, facilitated diffusion, and osmosis all involve a compound moving from a higher to a lower concentration. Since this is the spontaneous direction of flow, no energy input is required.

In order to move a compound against its electrochemical gradient, energy is needed. This type of transport is called active transport.

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