Cell membrane
Definitions & Key takeaways
The cell membrane is an essential cellular structure consisting of a lipid bilayer with embedded proteins. The lipid bilayer is a double layer of phospholipids containing hydrophilic (water-loving) head groups and hydrophobic (water-hating) tails. The head groups interact with water, while the tails interact with each other to form the bilayer. The proteins in the membrane are embedded in the lipid bilayer and play various roles in cell function, including signal transduction, cell adhesion, and transport. The cell membrane regulates what comes in or out of the cell, known as selective permeability.
The cell membrane is an important structural element of the building block of life - the cell. Its main role is to define what’s inside - the intracellular space - and what’s outside - the extracellular space.
It also regulates what comes in or out of the cell - that’s called selective permeability. The cell membrane is basically made up of a bilayer of phospholipid molecules.
Phospholipids are amphiphilic molecules, meaning “both-loving”. Now, the phospholipid is made out of three things - their head, which is made out of negatively charged phosphate, a tail - made out of two fatty acids, and a skeleton made out of glycerol, that brings everything together.
Their “head” is hydrophilic - meaning it likes water. Meanwhile, their “tail” is lipophilic - meaning, it loves fats.
These lipophilic parts also exclude water - so they’re not just lipophilic, they’re also hydrophobic. In water, phospholipids form a bilayer - where the hydrophobic tails are oriented inwards, where there are no water molecules, and the hydrophilic heads oriented outwards, in contact with water molecules.
So the plasma membrane forms a wall with water on both sides. The cell membrane is also semipermeable.
That means that the membrane allows some molecules to pass through, but not others - and it’s mostly based on the molecule’s size, polarity, and charge.
There are roughly five categories. Small and nonpolar molecules, like oxygen or carbon dioxide will diffuse through the membrane quickly.
Small, polar molecules, like water, will be able to pass through, but it happens relatively slowly. That’s because even though the middle of the phospholipid bilayer is hydrophobic, the occasional molecule of water can sort of slip through because it’s such a small molecule.
Now, large and nonpolar molecules, such as retinol - also known as Vitamin A1 - can also cross the cell membrane thanks to them being non-polar - but once again, the crossing is really slow, because the molecule is so large.
Now, as you might guess, large, polar molecules, like glucose, are unlikely to pass the cell membrane on their own. Highly polar, charged ions like Na+, K+, Cl-, or molecules that possess a charge, like amino acids stand no chance at passing the cell membrane In addition to phospholipid bilayers, membranes also contain cholesterol.
Without cholesterol, at low temperatures, the phospholipids pack tightly together and become less fluid, and that makes the membrane brittle.
Without cholesterol, at high temperatures, the phospholipids separate from one another and that makes the membrane leaky and weak.
So the role of cholesterol is twofold. At low temperatures, it squeezes in between phospholipid molecules and keeps them from packing too tightly together to keep the membrane more fluid.
And at high temperatures, cholesterol pulls phospholipid molecules together, decreasing the space between them. So cholesterol makes the cell membrane fluid and durable, no matter the weather.
This is particularly important for cells like red blood cells that have cell membranes that see a lot of wear and tear over time, and get exposed to different temperatures.
In fact, that’s why red blood cells have membranes with even higher levels of cholesterol than normal. Cell membranes have various membrane proteins embedded within them.
There are integral proteins, that span the cell membrane bilayer, peripheral proteins, which are found on the inner or the outer edge of the membrane, and lipid bound proteins, which can be found hanging out in between the phospholipid layers.
Some of these membrane proteins are transport proteins, and they’re examples of integral proteins because they span across the membrane.
Transport proteins help move molecules that can’t freely diffuse across the membrane, to get in and out of the cell. There are two types of transport proteins: channels and carriers.
Channel proteins open to form a sort of a tunnel through the membrane, through which water and ions can flow right through.
An example would be an aquaporin channel, which opens and closes to let water in. In contrast, carriers are very specific - they have special binding sites that only allow certain molecules to bind to them, and they also have gates at both ends, which open sequentially.
One example of a carrier protein is the glucose transporter called GLUT4. By default, the transporter is open to the outside, waiting for a glucose molecule to float by.
When a glucose molecule binds to a special site within the transporter, the outer gate closes, and the inner gate opens.
The glucose then unbinds, and float freely into the cell, while the transporter will reset, awaiting for the next glucose molecule.
There’s also a special family of transport proteins, which are essentially enzymes. These transport proteins use the energy from adenosine triphosphate, or ATP, to actively pump various ions in or out of the cell, against their concentration gradient.
That gives them their name - ATPases. In other words, these ATPases push ions from the side with a low concentration, over to the side with a high concentration.
An example is the Na+/K+ ATPase, which pumps 3 ions of sodium out of the cell in exchange for 2 ions of potassium into the cell - both going against their concentration gradient.
Cell membranes also contain cell adhesion molecules, or CAMs, which are integral transmembrane proteins that anchor the cell in place by attaching it to other cells.
These cell adhesion molecules come together and create junctions, called adherens junctions or tight junctions. An example of a cell adhesion molecules would be the cadherin, a membrane protein whose job is to hold tight to neighboring cell cadherins, at an adherens junction.
Think of them as velcro, attaching the two cells together. The tight junction is, well, even tighter, and creates a fully waterproof seal through the creation of protein complexes, and that sort of resembles rivets joining metal plates together, ensuring waterproofing.
Cell adhesion molecules and junctions are essential for the structural integrity of multicellular tissues, they’re like the mortar and the cell are like bricks - together forming a strong brick wall.
Next, there are cell surface receptors, which receive an input from the outside and transmit it to the inside of cell. The input is usually something that other cells produce, like hormones, cytokines, or growth factors - so cell surface receptors are sort of like the cells’ mailbox - receiving news from the world.
If a hormone binds to a receptor, the receptor changes its shape, and that triggers a chain of event within the cell. For example, when there’s a lot of glucose in the blood, the pancreas secretes insulin, which is a peptide hormone that binds to insulin receptors on various cells.
Now, this receptor also functions as a tyrosine kinase. Kinases are enzymes that add phosphate groups so when insulin binds to the insulin receptor, the receptor autophosphorylates - meaning, it transfers a phosphate group from an ATP molecule onto the receptor.This triggers a cascade of intracellular events.
The end result is that the cell mobilizes spare GLUT4 transporters, which are mostly chilling inside of the cell on small bubbles of phospholipid bilayer, called a vesicle.
The vesicle with the transporters then moves to the membrane, and fuses with it, therefore allowing more GLUT4 transporters to do their job - which speeds up glucose intake.
It’s like the cell is getting a love letter from the pancreas, that says “open the door to get a sweet treat”. How cool is that?
Aside from cholesterol and proteins, cell membranes also contain various carbohydrates, which are usually bound to proteins, creating glycoproteins, or bound to lipids - creating glycolipids.
These glycoproteins and glycolipids are unique to each cell and help cells recognize one another - a bit like identification cards.
In fact, immune cells often distinguish normal cells from invading pathogens based on the glycoproteins and glycolipids on the cell surface.
Now, the proteins and cholesterol within the bilayer aren’t static - but float horizontally through the phospholipids. In fact, if you look at a cell membrane from above it looks a bit like a mosaic with parts that can move around, so that’s why the cell membrane is sometimes referred to as a fluid mosaic model.
Alright, as a quick recap - the phospholipid bilayer contains cholesterol, which makes it more durable at all temperatures, as well as various proteins.
Transport proteins help move molecules across the membrane, cell surface receptors receive signals from other cells, and adhesion proteins, keep the cell anchored to neighboring cells.
Finally, the cell membrane contains carbohydrates in the form of glycoproteins and glycolipids, which help cells recognize one another.
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