In this video, we will learn about the structure of plasma membranes and the useful functions they can provide to cells and the organelles within them. We will find out about the fluid mosaic model of the phospholipid bilayer and the roles of the different molecules that might be embedded in it. Finally, we will learn about how different factors can affect the permeability of plasma membranes.
Every living organism possesses cells that are surrounded by a plasma membrane. These membranes act as the gatekeepers of the cell, controlling what can enter and what can leave a cell. Eukaryotic cells, like this plant cell, not only have a plasma membrane or a cell surface membrane around the surface of their cells, controlling what enters and leaves the cell itself, but there are also membranes within the cell surrounding certain organelles. Organelles are subcellular structures that carry out a specific function. The membranes that surround certain organelles allow different conditions and, therefore, different reactions to occur in separate regions of the cell. This separation of cell components is called compartmentalization.
The membranes that surround certain organelles may be a single layer or be made up of two layers, referred to as a double membrane. Organelles that have double membranes include the nucleus, mitochondria, and chloroplasts. Organelles that have a single membrane include lysosomes; the endoplasmic reticulum; the Golgi apparatus, which is sometimes called the Golgi body; and even the large permanent vacuoles that are found in plant cells.
Membrane-bound organelles, like these ones, are a feature of most eukaryotic cells, and they provide a site for certain chemical reactions. For example, the rough endoplasmic reticulum has ribosomes embedded in its membrane to carry out protein synthesis. It’s worthwhile noting that while ribosomes can be attached to the rough endoplasmic reticulum, they can also be freely floating in the cytoplasm. Ribosomes are subcellular structures that do not have a membrane surrounding them.
The inner membrane of mitochondria is the site of aerobic respiration, while the inner membrane of a chloroplast is the site of photosynthesis in plant cells, which allows these cells to make their own food. Depending on their location and composition, membranes can have a whole range of functions. But the key point common to them all is that they act to separate a cell or different sections of a cell from their surroundings through compartmentalization.
Compartmentalization of organelles in the cell is in some way similar to how classrooms in some schools are separated from each other. Just like you might have different lessons in different classrooms at the school, different reactions will be occurring in each organelle of the cell. The plasma membrane separate these organelles, much like the walls in the school can often separate the classrooms. You probably wouldn’t have the paints for art class in the same room as the science lab. And the cell ideally wouldn’t have the chemicals that allow it to carry out respiration in the same organelle as the lysosome that deals with its waste.
Plasma membranes are often described as semipermeable. This means that only certain molecules can pass through them, using proteins embedded in their surface. And sometimes membranes are selectively permeable. Selectively permeable membranes allow only specific substances to pass them based on the current requirements of a cell or organelle. Proteins embedded in these membranes can act effectively as doors, which can be locked or unlocked, depending on the current needs. Let’s look at the structure of the plasma membrane in more detail.
Whether they surround organelles or the cell itself, all plasma membranes in living cells share a common feature. They all have the common basic structure of a phospholipid bilayer. This diagram shows a simplified, typical animal cell and the plasma membrane that surrounds it. We can see that this plasma membrane at the surface of the cell separates the cytoplasm from the extracellular space around it. By magnifying just a section of this membrane, we can see the phospholipid bilayer more clearly.
Plasma membranes consist mostly of molecules called phospholipids, one of which we can see magnified here. Each phospholipid has a hydrophilic phosphate head and a hydrophobic fatty acid tail. The prefix hydro- in both words refers to water molecules. While the suffix -philic means loving, showing how these phosphate heads are attracted to water, the suffix -phobic means fearing, showing how these fatty acid tails are repelled by water molecules. This causes the phospholipids to arrange themselves into two layers, which is otherwise known as a bilayer.
Both the cytoplasm and the extracellular space are filled with water molecules and salt, forming an aqueous environment called plasma, hence why it’s called the plasma membrane. As the hydrophilic phosphate heads are attracted to water molecules, they arrange themselves pointing outwards towards both the extracellular space and the water molecules in the cytoplasm. As the fatty acid tails are repelled by the water molecules, they orient themselves facing inwards towards each other and away from the cytoplasm and extracellular space. These fatty acid tails can have slightly different structures.
Here’s a diagram that shows a phospholipid with one saturated fatty acid and one unsaturated fatty acid in its tail. You might’ve noticed that saturated and unsaturated fatty acids differ in their structure slightly. Saturated fatty acids contain single bonds between all the carbon atoms. This forms a straight chain that is completely saturated with hydrogen atoms. Unsaturated fatty acids contain at least one double bond between the carbon atoms. This forms a kink in the unsaturated fatty acid.
And you might’ve noticed that this portion of the unsaturated fatty acid contains less hydrogen than the same number of carbons on the saturated fatty acid. This is why it’s called unsaturated, because it’s not fully saturated with hydrogen atoms. These kinks in the fatty acid tails mean that the fatty acids cannot fit together as closely as they would with two saturated fatty acid tails. This increases the fluidity of the plasma membrane.
The fluid mosaic model describes how the phospholipids are, to a certain degree, free to move fluidly throughout the membrane as they are not chemically bound to each other, but simply attracted to or repelled by the water molecules. The term “mosaic” refers to the fact that there are molecules, mostly proteins, of different shapes and sizes embedded in various sections of the plasma membrane, much like the tiles of a mosaic. You can see some of these molecules in this diagram showing a top-down view of a part of the plasma membrane. But let’s have a look at a side view so you can see all of these molecules more clearly.
We’ve changed the color of the phospholipids in this diagram to black so we can distinguish them better from the other molecules that are embedded in the phospholipid bilayer. There are various proteins that embed themselves in this phospholipid bilayer. One such type of protein are called intrinsic proteins, or sometimes integral proteins, and they span both layers of the phospholipid bilayer. All intrinsic proteins are globular proteins with hydrophobic regions that fit into the membrane. The intrinsic proteins need to have hydrophobic regions, as you might recall the fatty acid tails of phospholipids are hydrophobic too.
There are two main types of intrinsic proteins: channel proteins and carrier proteins. Channel proteins provide a hydrophilic channel, which allows the diffusion of polar molecules and charged ions through the plasma membrane down or along their concentration gradient. These substances, like this positively charged sodium ion, for example, would otherwise not be able to be transported across the membrane due to the hydrophobic nature of the phospholipid tails. Thankfully, the channel proteins provide them with a hydrophilic channel through this hydrophobic region, allowing them to move passively by diffusion from an area of their high concentration to an area of their low concentration.
Carrier proteins contain specifically shaped receptors that are complementary to a particular molecule’s shape. For example, let’s say this molecule of glucose needs to be transported from the extracellular space outside the cell across the membrane and into the cytoplasm. But glucose is in a higher concentration inside the cell’s cytoplasm than it is in the extracellular space. Thankfully, there’s a carrier protein that’s complementary to the shape of glucose. Due to the specific shape of the receptor in the carrier protein, only glucose can fit into it. It may help to visualize glucose as the only molecule that has a key for the specific protein door.
When glucose binds to the receptor on the carrier protein, the protein changes shape and releases glucose onto the other side of the membrane. In this case, glucose was in a higher concentration inside the cell than it was outside the cell. So, it had to be actively transported into the cell using the carrier protein. Active transport requires an input of energy, which is supplied by a molecule called ATP, in order to transport molecules up or against their concentration gradients. Interestingly, however, most carrier proteins are also capable of passive transport, moving molecules from an area of their high concentration to an area of their low concentration. This process does not require ATP.
Glycoproteins are another example of an intrinsic protein, which remember means that it spans the whole bilayer of the membrane. The prefix glyco- indicates that this protein is attached to a carbohydrate chain. Glycoproteins have two main uses, one of which is to help cells adhere to each other and stick together. Glycoproteins can also act as receptors. The general role of a receptor is to allow a specific chemical, such as a hormone, to bind to it and elicit a response in the cell. This is called cell signaling, and it allows cells to communicate with each other efficiently.
This is the glycolipid. You might’ve noticed that this also contains the prefix glyco-, showing that it’s a carbohydrate chain bound to one of the phospholipids in the bilayer. They are sometimes referred to as antigens or cell markers, as they’re used by the immune system in cell recognition to identify a cell as non-self or self. An antigen that marks a cell as non-self indicates that they’re attached to cells from another organism, which might be a dangerous pathogen.
An example of glycolipids in human cells is in ABO blood grouping. There are glycolipids on the cell surface membrane of red blood cells, like this one, that act as antigens. These antigens are specific for one of four blood groups: A, B, AB, and O. For example, these green glycolipids might indicate that this person has blood type A.
Extrinsic proteins, which are sometimes called peripheral proteins, do not span the whole diameter of the membrane like intrinsic proteins do. Instead, they’re embedded in only one side of the membrane, interacting with the phosphate head of a phospholipid, either on the interior or exterior of the phospholipid bilayer. They can also interact with intrinsic proteins. Extrinsic proteins can sometimes act as receptors or can aid membrane stability.
The final structure we’ll discuss is cholesterol, shown here in blue. Cholesterol is an abundant lipid in the membrane of animal cells. It has a hydrophilic head that can interact with the hydrophilic phosphate groups on the phospholipids and with the cytoplasm or the extracellular space, which you all remember are both aqueous environments. It has a bulky hydrophobic tail, which can interact with the also hydrophobic fatty acid tails of the phospholipids. Through these interactions that modulate the packing of phospholipids in the membrane, cholesterol can either increase or decrease the fluidity of the membranes at very different temperatures. Let’s take a look at this now.
Temperature has a significant impact on the fluidity of plasma membranes. At low temperatures, phospholipids tend to move less and become more tightly organized in a crystal-like structure. Therefore, low temperatures make membranes more rigid, which can interfere with key functions like the passage of gases. More rigidity also makes membranes more prone to breaking. Cholesterol limits this phenomenon by disrupting the phospholipids’ tight and regular organization with this bulky tail, thereby increasing membrane fluidity. This makes the membranes less rigid at low temperatures and less prone to breaking.
At higher temperatures, more thermal or heat energy is supplied to the phospholipids in the bilayer. This increases the kinetic energy of the phospholipids, which means that they move around faster. This greatly increases the fluidity of the membrane. If the membrane is more fluid, its permeability increases as there are many more spaces through which substances can move.
Another issue is that the embedded proteins may become denatured, which is an irreversible change of shape, if the temperature rises above their optimum level. This might mean that they can no longer carry out their transport roles. At higher temperatures, however, when cholesterol is present in the membrane, the cholesterol tail keeps strong interactions with the phospholipids around it and pulls them together, which prevents the membrane from becoming too fluid. So, cholesterol can effectively act as a buffer that maintains a stable membrane fluidity over a wide range of temperatures.
Organic solvents can also have a significant effect on cell membranes. You may recall that the fatty acid tails of a phospholipid are hydrophobic. This also means that they are nonpolar. When nonpolar solvents or organic solvents like ethanol that are less polar than water interact with the phospholipid bilayer, they interact with the hydrophobic fatty acid tails, which can disrupt and disorder the membrane and make it more permeable to other substances. An example of this occurring is with antiseptic or alcohol wipes that disrupt the membranes of bacterial cells, killing the bacteria.
There are some techniques that can be used to evaluate the permeability of membranes. One example is colorimetry, which is a technique that measures the concentration of colored compounds or pigments in a solution. Let’s see how this might work by looking at two different solutions: one containing distilled water only and another that’s had a beetroot sample sitting in it for 30 minutes. Colorimetry is especially useful when the experimental cells contain pigment that is highly concentrated.
Beetroot cells, like the one we can see here, have large vacuoles that contain a lot of the red pigment betalain that gives them their distinctive dark-red color. And they’re often used in colorimetry. When a segment of beetroot is placed into a solution of water, some of the red pigment will leak out of the beetroot cells through their cell surface membranes and into the solution. The more permeable the cell surface membrane is, the more pigment will leak out. After 30 minutes, the beetroot sample can be removed and the solution can be placed into a vial called a cuvette.
Each of these cuvettes can be individually placed into a device called a colorimeter. The colorimeter passes light through the solution that’s placed within it and measures the percentage of light that’s transmitted through the pigment and is picked up by a detector. The colorimeter can then work out how much light was absorbed by the pigment itself, and this is displayed on the colorimeter’s screen. If all the light passes through the sample, as would likely be the case for distilled water, this shows that no light would’ve been absorbed by any pigments or compounds in the solution. So, in this case, the absorbance would be zero percent and the transmittance would be 100 percent.
However, with our beetroot sample that contains betalain pigment, the pigments would absorb some of the light passing through the solution. So the transmittance that’s picked up by the detector would be a lot lower, and the absorbance that’s calculated would be a lot higher. Using filters on the path of light improves the accuracy of measurements by selecting only the light wavelength that is absorbed by the pigments in the sample. If we change the conditions in our sample, for instance, changing the temperature or adding a solvent, any increase in the absorbance that we measure will represent an increase in the membrane permeability caused by these factors, as more pigment will have leaked out of the cells and into the solution.
Let’s review the key points we’ve covered in this video. We’ve learned some of the functions of plasma membranes. We learned about the fluid mosaic model and how it describes the composition of plasma membranes and how temperature and organic solvents can affect membrane fluidity and permeability. Finally, we learned how colorimetry can measure membrane permeability.