Cellular structure and function

The cell is the basic unit that makes up every living organism.

It’s the smallest form of life that can replicate on its own, but cells in our body differ quite a lot from one another.

The human body alone has over 200 distinct cell types - from long skinny neurons that can get over 1 meter long to macrophages that gobble up pathogens to myocytes that contract to let you to flex your muscles.

But despite their differences, they share lots of similar features.

So let’s imagine the average cell as a small apartment. First, we want some walls to distinction between what’s “outside” and what’s “inside.” These walls are the cell membrane or plasma membrane, and they’re made out of a double layer of phospholipid molecules.

Phospholipids have a “head” made out of negatively charged phosphate, which is hydrophilic - meaning it likes water.

Phospholipids also have a “tail” made out of two fatty acids, which are hydrophobic - meaning, they avoid water.

In water, phospholipids form a bilayer - where the hydrophobic tails are oriented inwards, where there is no water, and the hydrophilic heads oriented outwards, in contact with water molecules.

So the plasma membrane forms a wall with water on both sides.

This wall is semipermeable - meaning it allows some things through, like oxygen or carbon dioxide - but doesn’t allow other things through, like glucose and sugars.

Fortunately, we have the “doors” and “windows” on this apartment, and they’re made of special protein channels that are, essentially, tiny tunnels through the phospholipid bilayer.

These channels allow water and specific ions like sodium and potassium to come in and out of the cell.

Now, like any well built apartment, our cell has a sturdy framework called the cytoskeleton.

The cytoskeleton is made out of proteins like microfilaments, microtubules, and intermediate filaments, which all provide structural stability.

The cytoskeleton is also very dynamic, allowing the cell to change shape by selectively contracting and extending filaments - which is important in some cell functions, like muscle contraction, cell division, and even cell movement!

The cytoskeleton also helps structures within the cell move from one area to another.

Now that the apartment is built - let’s fill it in. The cell is filled with intracellular fluid called cytosol, which contains various ions like sodium and potassium.