Test: Cell membrane - 1 - MCAT MCQ

When phospholipids are added to water, they can form a bilayer structure due to the arrangement of water molecules around the hydrophobic tails of the phospholipids. Water molecules tend to form an ordered structure around hydrophobic molecules, creating a cage-like arrangement. This ordered structure is energetically unfavorable. By forming a bilayer, the hydrophobic tails of the phospholipids are shielded from the water, reducing the formation of the cage-like structure and leading to a more favorable arrangement. Therefore, lipids cause water to arrange in an ordered, unfavorable cage-like structure, and forcing lipids into a bilayer reduces this effect.

Cholesterol plays a crucial role in modulating the fluidity of the cell membrane. At high temperatures, cholesterol acts as a stabilizing agent by restraining the movement of phospholipids and reducing membrane fluidity. It prevents the phospholipid molecules from packing too closely together, thereby maintaining the integrity and stability of the membrane. This is important because high temperatures can cause the phospholipids to become more fluid and disorganized.

On the other hand, at low temperatures, cholesterol acts as a fluidity buffer by preventing the phospholipid molecules from packing too tightly. It inserts itself between the phospholipids and increases the space between them, allowing for more movement and flexibility in the membrane. This helps to maintain membrane fluidity and prevents the membrane from becoming overly rigid and potentially losing its functionality.

The cell membranes of polar bear paws would most likely have an increased concentration of unsaturated phospholipids compared to a typical animal cell. Unsaturated phospholipids contain fatty acid chains with double bonds, which introduce kinks in the hydrocarbon chain. These kinks prevent the phospholipids from packing closely together, increasing membrane fluidity. This increased fluidity is advantageous in colder environments because it helps to prevent the cell membrane from becoming too rigid and losing its functionality.

Polar bears live in extremely cold environments, and their paw pads are exposed to low temperatures. By having a higher concentration of unsaturated phospholipids in their cell membranes, the polar bears can adapt to the cold temperatures and maintain proper membrane fluidity. The unsaturated phospholipids allow the cell membranes to remain flexible and functional even at low temperatures.

Aquaporin proteins and potassium channel proteins are not specifically related to membrane fluidity and are involved in other processes such as water transport and ion channel regulation, respectively. Therefore, they are not the most likely macromolecules to have an increased concentration in the cell membranes of polar bear paws compared to a typical animal cell.

Ethylene is a small, nonpolar molecule composed of carbon and hydrogen atoms. Nonpolar molecules, like ethylene, can diffuse across a membrane more quickly compared to polar molecules. The lipid bilayer of the cell membrane is primarily composed of hydrophobic tails of phospholipids, creating a barrier to polar molecules. Since ethylene is nonpolar, it can easily dissolve in the lipid bilayer and diffuse through it rapidly.

Glucose, on the other hand, is a polar molecule that cannot passively diffuse across the lipid bilayer due to its hydrophilic nature. It requires specific transport proteins or channels to facilitate its movement across the membrane.

Therefore, ethylene would diffuse through a membrane most quickly among the given options due to its nonpolar nature, allowing it to readily pass through the hydrophobic interior of the lipid bilayer.

Glucose, being a polar molecule, cannot easily pass through the cell membrane via simple diffusion because of the hydrophobic interior of the lipid bilayer. Instead, glucose typically enters the cell through facilitated diffusion, which involves the use of carrier proteins. Carrier proteins specific to glucose bind to the molecule on one side of the membrane and undergo a conformational change to transport glucose across the membrane and release it on the other side. This process does not require energy expenditure by the cell and allows glucose to move down its concentration gradient. It is important to note that facilitated diffusion is a passive process and does not require the input of cellular energy.

Simple diffusion and facilitated diffusion are both passive processes that involve the movement of molecules across a membrane from an area of higher concentration to an area of lower concentration. However, there are notable differences between the two processes.

In simple diffusion, molecules passively move across the membrane directly through the lipid bilayer without the involvement of any specific proteins. The rate of simple diffusion is influenced by factors such as the concentration gradient, temperature, and molecular size. Simple diffusion does not require transport proteins and is not limited by their availability.

On the other hand, facilitated diffusion involves the movement of molecules across the membrane with the assistance of specific transport proteins, such as channel proteins or carrier proteins. These proteins facilitate the movement of molecules by creating selective channels or by binding to the molecules and undergoing conformational changes. The rate of facilitated diffusion is dependent on the number of available transport proteins in the membrane. If there are limited numbers of transport proteins, the rate of facilitated diffusion can be limited.

Option B correctly states that the rate of facilitated diffusion is limited by the number of transport proteins in the membrane, which is a notable difference compared to simple diffusion.

Osmosis is the movement of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. The rate of osmosis is influenced by several factors:

I. Intracellular solute concentration: The concentration of solutes inside the cell affects the osmotic pressure and determines the direction and rate of water movement.

II. Extracellular solute concentration: The concentration of solutes outside the cell creates an osmotic gradient and influences the movement of water into or out of the cell.

III. Polarity of solutes: The polarity of solutes does not directly affect the rate of osmosis. Osmosis is primarily driven by the concentration gradient of solutes, not their polarity.

IV. Molecular weight of solutes: The molecular weight of solutes does not directly affect the rate of osmosis. Osmosis is driven by the concentration gradient rather than the size or weight of solute molecules.

V. The presence of aquaporins: Aquaporins are specialized channel proteins that facilitate the movement of water across the cell membrane. The presence of aquaporins can significantly increase the rate of osmosis by providing a pathway for water molecules to move more efficiently.

Option A correctly includes factors I, II, and V, which are the key determinants of the rate of osmosis across a cell membrane.

The primary purpose of the sodium-potassium pump is to maintain the electrochemical gradient across the cell membrane, which is crucial for the proper functioning of nerve cells and other cells in the body. The pump actively transports three sodium ions (Na⁺) out of the cell while simultaneously importing two potassium ions (K⁺) into the cell. This exchange helps create a concentration gradient and electrical potential difference across the membrane.

By pumping Na⁺ out and K⁺ in, the sodium-potassium pump contributes to the establishment and maintenance of the resting membrane potential, which is necessary for nerve cell excitability. The resting membrane potential allows nerve cells to respond to stimuli and propagate action potentials, which are electrical signals that enable communication within the nervous system.

Additionally, the exchange of Na⁺ and K⁺ by the pump helps store potential energy in the form of an electrochemical gradient. This stored energy can be utilized when needed for various cellular processes, such as the generation of action potentials, active transport of other molecules, and maintenance of osmotic balance.

Obligate anaerobes cannot survive in the presence of oxygen and would likely be killed by such a therapy, treating the infection. The other types of bacteria listed can all survive in the presence of oxygen and would likely not be treated using this therapy.

Lysosomes are vesicular organelles that digest material using hydrolytic enzymes. They are surrounded by a single membrane. Both mitochondria and nuclei are surrounded by double membranes, eliminating choices (B) and (C). Ribosomes must not be surrounded by membranes because they are found not only in eukaryotes, but also in prokaryotes, which lack any membrane-bound organelles, eliminating choice (D).