Question 22 - Chemical and Physical Foundations of Biological Systems Practice Test for the MCAT

When red blood cells are placed in a solution with a lower concentration of sodium ions, water molecules pass through the cell membrane from the outside to the inside of the cell in order to balance out the concentration of sodium ions. This is an example of what type of transport? (You may consult the attachment.)

A cell membrane defines the limits of the cell and serves as the boundary between the cell and its surroundings. It is made of a double layer of lipids that blocks the passage of polar substances. The image below shows a typical cell membrane:


Retrieved from:

One important concept related to cell membranes is membrane fluidity. Cells need to keep a certain level of flexibility in order to maintain their stability. Increasing temperature, cholesterol content, and unsaturation of the fatty acid tail of the phospholipids in the bilayer increases the membrane fluidity.

Substances pass through the cell membrane by a process called “transport”. Transport can be passive if no energy is consumed or active if energy is required to transport substances across the membrane. The movement of an uncharged solute across a membrane depends on its concentration gradient, while the movement of an ion depends both on its concentration gradient and the electric potential of the membrane. Membranes are said to be polarized if the resting membrane potential is different than zero, depolarized if the membrane potential is higher than the resting membrane potential, and hyperpolarized if the membrane potential is lower than the resting membrane potential.

The following table shows the characteristics of the different types of solute transport across a membrane:

Type of Solute
Requires Energy Requires a
Membrane Protein
Specific for a
Certain Solute
Simple Diffusion No No No
Facilitated Diffusion No Yes Yes
Osmosis No No Yes
Active Transport Yes Yes Yes

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