Chapter 1. Membranes

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You have been asked to participate in one of the most ambitious scientific experiments to date: the creation of artificial cells! A team of scientists is creating an entire working cell—all its components—and you are one of the collaborators. We have scientists working on the nucleus of this cell and its DNA, on the internal organelles like mitochondria and ER, on its metabolic processes, etc. Your specific project will be working on the cell membrane—the outer covering that holds everything together and regulates what can come in and go out of the cell. You’ll have to determine how best to construct this membrane (i.e. what components are necessary and how they should be arranged). You’ll also need to control how chemicals are transported across this membrane. And because all of the cell organelles, like the nucleus and the mitochondria, are also surrounded by a membrane, everybody else’s work depends on you. It’s a complicated task that is crucial for this exciting project.

Are you ready to begin?

Background Info

Why aren’t humans water soluble? We know that the human body is at least two-thirds water, and we know that it contains many molecules such as sugars and salts that dissolve in water. So why don’t we melt in the rain?

[Insert figure: Wicked Witch melting from Wizard of Oz]

The reason why it’s safe to take a shower has a lot to do with how a cell is constructed. When you think about it, a cell is a complicated, busy place. Cells have to be able to change shape, otherwise we literally couldn’t move. They also have to take in the materials they need to survive, such as oxygen and glucose, while at the same time getting rid of the things they don’t need, like carbon dioxide and toxins. And they have to contain and be surrounded by water without dissolving in water. All of these things are made possible by the plasma membrane.

The plasma membrane surrounds the contents of the cell, forming the boundary between the cell and the outside world. Every cell has a plasma membrane. The internal structures of cells, like the nucleus and mitochondria, are also surrounded by the same type of membrane. So knowing how the membrane works tells us a lot about how the cell works.

The plasma membrane is usually described as a fluid mosaic. There are different types of molecules that make up a membrane—which is where the mosaic part comes from. And the way in which these molecules interact makes the membrane a fluid, which can move and change shape easily.

There are four major classes of molecules found in the membrane

Phospholipids, cholesterol, proteins, and carbohydrates are the four major classes of molecules found in the membrane. We need to understand these molecules to really understand how a membrane functions, so let’s look at them.

The majority of the membrane is made up of phospholipids. These are molecules that have a phosphate group at one end and two fatty acid (lipid) tails at the other.

[Insert figure: phospholipid structure]

The phosphate group is negatively charged, so it’s attracted to positively charged substances. Water molecules have a positive charge on their hydrogen ions, so the phosphate groups on the phospholipids are attracted to water. They are called hydrophilic (water-loving) because of this. On the other hand, the fatty acid tails are uncharged. They are not attracted to the water molecules—in fact, they are repelled by them. They are called hydrophobic (water-fearing).

Another reason why the membrane is a fluid is the effect of another important membrane molecule, cholesterol. Most of us have bad associations with that word, but cholesterol is actually a very necessary part of the membrane. Cholesterol molecules are lipids, so they fit into the membrane between the fatty acid tails of the phospholipids. Cholesterol acts to stabilize the membrane. At cold temperatures, it helps keep the membrane from freezing solid; at high temperatures, it helps keep the membrane from melting. We definitely need cholesterol—just not too much of it.

Carbohydrates cover the surface of most membranes. They are very important for cell signaling and identification, and are so common that cells are often described as being “sugar-coated.” Carbohydrates allow two cells to recognize one another and stick together, forming tissues and organs. They also help a sperm to recognize and fertilize an egg. And unfortunately, they help bacteria recognize cells to infect. Because they are charged, they cannot embed themselves into the membrane. Instead, they are either attached directly to the surface of the membrane or to proteins or lipids on the surface.

Proteins do a lot of the work that happens at the surface of a cell. Some membrane proteins are enzymes, helping chemical reactions to take place. Others are receptors, receiving signals from the environment and helping the cell respond to them. And many proteins are involved in transport of materials into or out of the cell. These types of proteins will be an important focus of this lab.

[Insert figure: figure illustrating structure of plasma membrane]

The membrane controls transport of materials into or out of the cell

The membrane is the boundary between the cell and the outside environment. It must act as a barrier, keeping out things that could harm the cell. But at the same time, it always has to be able to take in things that it needs, like glucose, oxygen, and ions (charged atoms, often called electrolytes). In order to meet these two opposing goals the membrane is selectivelypermeable, meaning that some substances can pass through the membrane easily, while some cannot. Because we know how the membrane is constructed, we can make some predictions about its permeability.

• Molecules that are small can often push their way between the phospholipids and cross the membrane. This includes gases like oxygen and carbon dioxide,
• Nonpolar (uncharged) molecules can easily pass through the oily, hydrophobic interior of the membrane to get across. For example, lipids can interact with the fatty acids in the center of the membrane, so they can travel through that part of the membrane easily. Steroid hormones fall into this group.

Some kinds of molecules cannot travel through the membrane easily.

• Large molecules also have difficulty getting through the membrane. Again, glucose and amino acids have this characteristic.
• Charged molecules have a very hard time traveling through the membrane. When they reach interior of the membrane, the hydrophobic fatty acids will repel the charged substance, kicking it out of the membrane. This group includes several important substances like ions, many amino acids, and glucose.

[Insert figure: figure illustrating which molecules can/cannot pass through the membrane]

If important molecules like glucose and amino acids can’t travel through the membrane, how do they get into the cell? This is where membrane proteins become essential. There are a variety of proteins that help to transport molecules from one side of the membrane to the other.

How are materials transported across the membrane?

Because the cell is constantly using up oxygen during cellular respiration, there is always more oxygen outside it than inside it. This is called a concentration gradient, where the concentration of a molecule is higher on one side of the membrane than the other. There is a concentration gradient for almost all molecules and ions in the human body.

The tendency of any molecule is to move from where it is at high concentration to where it is at low concentration: it’s much easier to roll downhill than to run uphill. This is called passive transport, because it doesn’t require energy. If passive transport of a molecule goes on long enough, it will reach equilibrium, where the concentration is the same on both sides of the membrane. Passive transport is also called diffusion.

[Insert figure: figure illustrating passive vs. active transport]

In living cells, the diffusion of water is so important that it gets its own name, osmosis. Water balance is essential for survival, so it is critical to understand how water will move into or out of the cell.

The body is an aqueous solution. This means that the body is mostly water, with other substances dissolved in it. Water is the solvent, while substances like sugar and ions dissolved in the water are called solutes. The more solutes dissolved in the water, the fewer free water molecules are available. While it is not totally accurate, we can think of a solution that is high in solutes as being low in water.

For most cells, the internal concentration of salt (sodium chloride, NaCl) is about 0.9%. If the concentration of salt outside the cell membrane is different from this, water molecules will move across the membrane by osmosis. To determine which way the water will move, we need to compare the solute concentration, or tonicity, of the two different solutions, inside and outside the cell.

• Two solutions that have the same solute and water concentrations are called isotonic.
• A solution with more solutes is hypertonic to one with fewer solutes. The one with fewer solutes is hypotonic to the one with more solutes.

What happens to cells under these different conditions?

• A cell placed in an isotonic solution is in equilibrium. Because the concentration of water is the same inside the cell as it is outside, there will be no net movement of water.
• A solution is hypotonic to the inside of the cell if it contains less than 0.9% salt. This means that there is more water outside the cell than inside it, and therefore water will move into the cell. For an animal cell, this can be a disaster. The cell membrane can only stretch so far, and the cell may burst like an overfilled water balloon. For a plant cell, the cell wall will prevent this from happening. The cell will fill up with water, but it will not burst.
• A solution is hypertonic to most cells if it contains more than 0.9% salt. This means that there is more water inside the cell than there is outside it, so water will move out of the cell. The cell will shrivel up and shrink as it loses water.

[Insert figure: figure illustrating osmosis and cells in solutions of varying tonicity]

Larger molecules like glucose will also diffuse through the membrane, but they need help from a transport protein. Because our cells are always using up glucose, the glucose transporter is a very important membrane protein.

The opposite of passive transport is active transport. In this case, a molecule is moving uphill, from where it is at low concentration to where it is at high concentration. Active transport ensures that that the molecule never reaches equilibrium. Active transport is work, so it takes energy. As in other cellular activities that require energy, the energy source for active transport is adenosine triphosphate (ATP). One very important example of active transport is the Na⁺, K⁺ transporter.

Sodium (Na⁺) and potassium (K⁺) are two of the most important electrolytes in the body. In order for our nervous system and muscles to function, there must be a concentration gradient for these ions. This is why electrolyte imbalances can cause muscle cramps and in extreme cases even seizures.

Every nerve impulse and muscle contraction causes large amounts of sodium to enter the cell. To restore levels back to normal, sodium must be pumped out of the cell by the Na⁺, K⁺ transporter. At the same time, potassium is pouring out of the cell, so it must be pumped back in by the transporter.

If this transporter stops working, our muscles stop contracting, our heart stops beating, and our brain shuts down, so it is a very important membrane protein.

[Insert figure: figure illustrating the transport of Na+ and K+, the concentration differences on either side of the membrane, and the use of ATP.]

Now that you know how membranes work for cells, you are ready to build one. Your finished product will have all of the properties of a membrane, including its permeability and transport capabilities. Your work will make it possible to build organelles and ultimately an artificial cell!

Pre-lab Quiz

Before we can begin to design this experiment, you’ll have to understand the background material covering evolution and natural selection—how it works, what some of the possibilities are, what the supporting evidence is, etc. Make sure you've read the background material thoroughly, and then answer the following questions. You must get a score of 90% or better on these questions to proceed to the first experiment.

Experiment Notebook

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Lab Report

Vocabulary List

Active transport: the movement of a substance from where it is at low concentration to where it is at high concentration. This process requires energy.

Aquaporin: a channel protein that allows water to move across the membrane.

Bilayer: a double layer of molecules. The membrane contains a bilayer of phospholipids.

Cholesterol: a lipid that controls the fluidity of the membrane.

Concentration gradient: condition in which there is more of a substance on one side of the membrane than on the other side.

Diffusion: the movement of a substance from where it is at high concentration to where it is at low concentration. This process does not require energy.

Equilibrium: condition in which the concentration of a substance is the same on both sides of the membrane.

Fluid mosaic: phrase used to describe the membrane because it is very flexible and contains several different types of molecules.

Hydrophilic: water loving molecule that will interact with water.

Hydrophobic: water fearing molecule that does not interact with water.

Hypotonic: a solution containing fewer solutes than the one it is being compared to.

Hypertonic: a solution containing more solutes than the one it is being compared to.

Ion: an electrically charged atom.

Integral membrane protein: a protein that is embedded in the membrane and reaches from one side of the membrane to the other.

Isotonic: two solutions with the same concentrations of solutes.

Na⁺, K⁺ transporter: membrane carrier protein for the active transport of Na⁺ and K⁺ ions.

Osmosis: the diffusion of water across a membrane.

Passive transport: the movement of a substance by diffusion which does not require energy.

Peripheral membrane protein: a protein that is attached to only one side of the membrane.

Phospholipid: molecule with a phosphate group at one end and two fatty acids at the other.

Plasma membrane: forms the outer boundary of the cell. It is very similar in structure to the membranes of organelles.

Selective permeability: some molecules, but not all, can cross the membrane.

Solute: a molecule or molecules dissolved in a solution.

Solution: a mixture of molecules dissolved in a solvent.

Solvent: the substance in which the solvent is dissolved.

Tonicity: the solute concentration.

Transport protein: a protein located in the membrane that moves a molecule from one side of the membrane to the other.