In AP® Biology, there is a lot of information about cells. Cells have many organelles to memorize and understand. One of the organelles that you must know for the AP® Bio exam is the cell membrane. In this AP® Biology Crash Course Review, we will go over the important information that you need to know for the AP® exam about cell membranes. We will first go over the structure of the membrane. After we understand its structure, we will be able to use that knowledge to understand its function in the cell. Finally, we will go over a question that you might see on your AP® Biology exam.

Structure

The cell membrane is primarily made up of phospholipids. Phospholipids are composed of a phosphate head and a fatty acid tail. The phosphate head is hydrophilic meaning “water-loving,” while the fatty acid tail is hydrophobic “water-fearing.” Because of this, the phospholipids arrange themselves in a bilayer. The phosphate heads are on the outside of the bilayer, leaving the fatty acids on the inside, away from all of the water. The cell membrane acts as a thin barrier, which separates the living cell from the aqueous environment.

The hydrophilic heads are polar as well. Because of this, polar molecules are not able to freely pass through the membrane; this limits the traffic into and out of the cell and allows certain substances to cross more easily than others.

Additionally, cholesterol is found in the cell membrane. Cholesterol molecules are amphipathic, meaning that they have both hydrophilic and hydrophobic regions. Cholesterol functions to moderate the fluidity of the membrane and stabilize it. There are also proteins in the membrane, let’s review that next.

Membrane Proteins

Polar molecules must be able to pass through the membrane. In order for the molecules to pass through the membrane, there must be membrane proteins. Membrane proteins can aid in the crossing over of specific material across the cell membrane.

There are three classes of membrane proteins:

1. Channels: Channel proteins act as cylinders carved into the membrane. Specific ions are able to enter into the channel, allowing them passage into or out of the cell.

2. Carriers: Carrier proteins carry a molecule across the cell membrane. The molecule is bound to the protein causing a conformation change. The conformation change places the molecule on the opposite side of the membrane.

3. Pumps: Pump proteins use active transport to bring in molecules. Active transport is done against the direction of the electrochemical gradient by breaking the phosphate bond of an ATP molecule.

These membrane proteins allow certain molecules to exit and leave the cell. Though they cause the cell membrane to be permeable, they are very specific in which molecules are able to move. These membrane proteins are also called integral proteins because they penetrate the lipid bilayer across the entire membrane. There are another set of proteins called peripheral proteins, which are loosely bound to the surface of the membrane and serve as antigens. They do not assist in the transport of molecules but rather as identity markers.

These proteins assist smaller molecules in moving across the membrane but there are size restrictions to these proteins.

Large Molecules Across the Cell Membrane

Large molecules will not be able to be moved using membrane proteins; they are simply too small. The large molecules must use moving vesicles and vacuoles. There are two modes of transportation: endocytosis and exocytosis. Endocytosis is when the cell is accepting a vesicle or vacuole, while exocytosis is when the cell is excreting a vesicle or vacuole.

We will first review the method of exocytosis. In exocytosis, the cell assembles the molecules from the cytoplasm into a secretory vesicle. The secretory vesicle will then fuse with the cell membrane, opening the vesicle and allowing its contents to spill into the extracellular space. One example of exocytosis is found in the neuronal cells. When a neuronal cell has been excited, it will secrete a neurotransmitter into the extracellular space near its neighboring neuron. The neighboring neuron will receive the neurotransmitter and understand it as a message.

There are three major ways that endocytosis occurs in the cell:

1. Phagocytosis: Phagocytosis is when the cell is ingesting something as food. The “food” will be taken in when the cell membrane fuses around the molecule. The contents will then be sent to the lysosome for digestion.

2. Pinocytosis: Pinocytosis is a much less specific process. The cell takes in the molecules and is able to process them based on the signal that it receives.

3. Receptor-mediated endocytosis: Receptor-mediated endocytosis is triggered by a molecular signal. Usually, a cell will receive the signal by the binding of another molecule to the antigen. This will cause the cell to take up the surrounding molecules to process the signal. An example of this is found in the neuronal cell. As we discussed, one cell will release neurotransmitter signals that the other cell will receive. The neighboring cell will receive the neurotransmitter by receptor-mediated endocytosis.

Water

Water has a different form of transportation across the membrane called osmosis. The direction of osmosis is determined by comparing the solute to the solvent. The universal solvent in biology is water; therefore, water will always move across a concentration gradient. Water will move into an environment that has a higher concentration of solute.

The process of osmosis is important, because water must be able to move freely in order for the cell to survive. Hypotonic cells, cells with more water than solute, will lyse due to too much water pressure. Think about hypotonic cells like balloons that have filled with too much air and burst. Too little water poses a problem as well; hypertonic cells, cells which have a higher concentration of solute, will shrink and die. If you would like to visualize this, think about a dried piece of fruit.

AP® Biology Exam Question

In order to test our knowledge, let’s review a question about the cell membrane found on the AP® Bio exam in 2007.

Membranes are essential components of all cells. Identify three macromolecules that are components of the plasma membrane in a eukaryotic cell and discuss the structure and function of each.

Here is an example of a full point answer:

The cell membrane is made up of phospholipids, cholesterol, and proteins. Phospholipids form a bilayer with hydrophilic heads pointing out and hydrophobic tails pointing in. The phospholipid bilayer provides selective permeability and fluidity to the membrane, allowing certain nonpolar molecules to pass through the membrane. Cholesterol is an amphipathic steroid that is embedded in the bilayer. Cholesterol functions to stabilize the membrane. Finally, there are many proteins found on the bilayer. Proteins are made up of amino acids. The proteins on the membrane function to transport molecules across the membrane and also to identify the cell.

Summary

In this AP® Biology Crash Course Review, we have covered the information you must know about cell membranes for the AP® Biology exam. We first went over the structure of the membrane, then we went over other components found in the membrane that assist in its function. Finally, we went over a question that you might see on your AP® Biology exam.

Thank you for reading this article, Cell Membranes: AP® Biology Crash Course Review! We really appreciate your feedback, let us know how we did! If you want to study more with us, check out our article, Cell Organelles: AP® Biology Crash Course Review!

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