Cellular structure and function

Definitions & Key takeaways

Cells are the smallest structural and functional unit of life. All living things are made up of cells, from single-celled organisms to the largest animals. Cells have small parts called organelles that carry out vital functions necessary for a cell's life. Major organelles include: The cell membrane: surrounds and protects the cell. The nucleus: contains the DNA. The endoplasmic reticulum: helps in protein synthesis, lipids, and steroids. The mitochondria: are responsible for providing the cell with energy.

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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. In addition, cells have a number of organelles, which are like little rooms within our apartment.
And together, the cytosol and the organelles make up the cytoplasm of the cell. Now, the nucleus is a highly specialized organelle, which is home to our genetic material.
In terms of rooms, you can picture the nucleus as the office, with bookshelves containing various books and manuals. How to do home repair, how to do interior remodeling, how to talk to your neighbors, how to take out the trash, and so on.
Extending the analogy, each book would be like a gene that contains the exact information - like a blueprint - needed to build a specific protein.
Drilling down deeper - the exact letters and words of the books are written in the language of DNA. Now, as it turns out DNA is actually one long molecule with some parts that code for genes connected together by other parts that don’t code for genes.
If the DNA molecule in a cell were stretched out, it would be about 2 meters long! Fortunately, the DNA is tightly coiled and wound in multiple steps so that it packs down really tight.
First off, it’s coiled around proteins called histones. Zoomed in to that level, the DNA looks like pearls on a string.
But then that “necklace” gets folded further and further, creating an extremely neatly folded and tightly packed form of DNA, called a chromosome.
Many chromosomes create chromatin, a shapeless mass of DNA floating in the nucleus.. Now, to make a protein, the DNA of a particular gene has to be transcribed or copied over into RNA - specifically messenger RNA or mRNA.
Special structures inside the cell, called ribosomes, can then use the mRNA copies to produce proteins! The ribosomes are made by a structure within the nucleus called the nucleolus out of ribosomal RNA or rRNA and proteins.
They’re like the workshop of our cell apartment, with mini 3D printers, printing up various spare parts or tools for the apartment - which is the analogy for protein synthesis.
The ribosome converts mRNA into a string of amino acids which forms the protein in a process called translation. Ribosomes rely on a triplet code, where every three nucleic acids along a sequence of mRNA correspond to one of the twenty common amino acids or to a stop codon which tells the ribosome to stop building the protein.
So to help visualize all of this, imagine the ribosome floating around in the cytosol. Once that ribosome binds to mRNA, it uses the cytoskeleton to help make it’s way over to an organelle called the endoplasmic reticulum - or ER for short.
The ribosome attaches to the plasma membrane surface of the endoplasmic reticulum and will start translating the mRNA into a protein that gets directly injected it into the lumen - or the inside - of the ER.
The ER supports lots of ribosomes on its surface, each of which is busy translating proteins. To create enough surface area to do this, the ER is really a continuous stack of flattened phospholipid membrane.
Sort of like a huge deflated balloon, that’s all crumpled up - low in volume, but with a lot of surface area available! The part of ER that produces protein is called “rough ER”, since the ribosomes resemble tiny little bumps that make the ER look “rough” under electron microscopy.
There’s also a “smooth” type of endoplasmic reticulum, which is involved in making lipids, like cholesterol and phospholipids, that the cell can use for producing more cell membrane.
In some cells, like the liver, the smooth ER helps with detoxifying harmful chemicals, so in this case, kind of like the spa or bathroom in the apartment.
In other cells, like glands, the smooth ER uses cholesterol, to go on to create various steroid hormones. For this reason, some specialized hormone-producing cells, like the ones found in the testes or the ovaries, have lots of smooth ER.
Another organelle called the Golgi apparatus takes the proteins, lipids, and hormones that are generated in the rough ER and smooth ER, and packages them into a vesicle made of a double layer of phospholipids that are then distributed around the cell.
Think of the Golgi apparatus like the garage of the apartment, where you have all your packed-up boxes, that you sometimes then move and store in other rooms.
There are transport vesicles, which move within the cell, and secretory vesicles which move molecules out of the cell, a process known as the exocytosis - like taking the garbage out of the garage.
An example of a vesicle that stays in the cell would be the lysosome, which contain enzymes that can help break down other molecules, as well as large organelles that may be damaged.
Finally, there are the mitochondria, which are organelles that serve as the power generators, like in the utility closet of our apartment.
In the cytoplasm, glucose goes through glycolysis - which is a metabolic pathway that cleaves the 6-carbon glucose into two halves, which are 3-carbon molecules called pyruvate.
From there, the pyruvate enters the mitochondria and goes through the citric acid cycle or Krebs cycle, as well as the electron transport chain, in order to produce adenosine triphosphate - or ATP.
These processes require O2, so this is termed aerobic respiration. The byproducts of this set of reactions are CO2 and H2O.
Ultimately, about 32 molecules of energy-rich ATP are made for every single molecule of glucose taken in by the cell. Alternatively, if there’s not enough glucose to go around, our cell can try burning fatty acids in the mitochondria as a source of fuel - in a process called beta oxidation.
Now, as it turns out - mitochondria can only work with “medium sized” fatty acids. So to make it work, a special organelle called the peroxisome chops long fatty acids down into medium sized ones.
The process generates dangerous hydrogen peroxide, but the peroxisome has an enzyme called peroxidase, which safely converts the hydrogen peroxide into water and oxygen.
OK, quick recap! The phospholipid bilayer forms the cell membrane which are the walls of the cell, and the internal structure of the cell is made up of a cytoskeleton and cytoplasm.
The key organelles include the nucleus, which houses the DNA, the endoplasmic reticulum which generates proteins, lipids, and steroids, and the mitochondria, which provides the cell with energy using aerobic respiration, which burns glucose, or beta oxidation, which burns fatty acids.