Cell membrane

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.