Our bodies are capable of a huge range of tasks, from the ordinary – breathing, walking,
eating – to the extraordinary - inventing new technology, sculpting pottery, running marathons.
To achieve these goals, our bodies depend on our organs (heart, lungs, brain etc) working together.
Looking closer, we see that each organ is made up of millions of individual cells working together.
Looking closer still, we see that cells themselves are quite complex!
Every cell is essentially a tiny factory. Overall, they break down chemical fuel for energy and
produce proteins and other molecules. If we take one final step closer, to look inside of a cell,
we can see these activities occur in compartments called organelles, or ‘little organs’.
When you think of a factory, you probably think of a large, multi-story building that makes stuff.
The building has protective exterior walls, which is just like the cell membrane or plasma membrane.
But the factory also has doors with swipe card entry to let in the right people
and supplies. Similarly, the plasma membrane, which is made up of lipids and proteins,
is a selectively permeable barrier that allows only certain molecules to come in or leave. This
usually means good things like nutrients and hormones, although sometimes viruses can trick
the cells and enter by similar mechanisms. Once the materials are through the plasma membrane,
some end up in another membrane-bound organelle called an endosome – an internal delivery service.
The endosome can deliver the material to many other organelles that we will learn about soon.
The first compartment inside of our cells that we’ll visit today is the most
well-known organelle - the nucleus. This is the combined IT department and CEO headquarters.
It is where critical information is stored and decisions are made. Crammed inside of the nucleus
of every single cell is two meters of DNA; fibers that contain all the information necessary for
making new proteins, and for cell replication. The nucleus, like the plasma membrane and many other
organelles, is surrounded by a lipid membrane. However, it has relatively big pores called
“nuclear pores” that allow RNAs and some proteins to pass between the inside (the nucleoplasm)
and the outside (the cytoplasm). The cytoplasm is the name for the entire space between the
nucleus and the plasma membrane. Inside of the nucleus sits the nucleolus, an organelle
within an organelle. The nucleolus, which is an example of “non-membrane bound” organelle,
is involved in making ribosomes - particles that synthesize proteins.
Now let’s look more at manufacturing. Sitting just outside the nucleus is the endoplasmic
reticulum (or ER), a network of sausage-like tubes extending through the cytoplasm
that is involved in making proteins and lipids for cell membranes. Once these materials are
made in the ER, they are sorted, packaged, and delivered to the cell’s shipping department - the
Golgi apparatus. The Golgi apparatus repackages these proteins in small shipping containers
called vesicles that get delivered to various other organelles and to the plasma membrane.
How does this delivery occur? Analogous to the way in which packages can be taken around a factory
along hallways, vesicles are transported by motor proteins along long protein fibers
called microtubules and actin filaments, which make up the cell’s cytoskeleton.
In addition to making new products, cells also have to get rid of expired products. The main
waste disposal department is an organelle called the lysosome, which can degrade or recycle waste.
Factories need an energy source to keep all the machinery working,
and so do cells. The food that we eat - fat, carbohydrates, and sugars - is broken down by
the cell, and ultimately needs to be converted to adenosine triphosphate, or ATP, for energy.
Lets take fat as an example. The breakdown of fat is facilitated by peroxisomes which pass
the products to mitochondria. A mitochondrion is the power plant that takes in fuel derived from
fat and converts it into ATP, the main energy currency that cells use for their many activities.
As our tour concludes, I hope that you can appreciate that the cell is a complex environment,
with many parts working together to keep our bodies going. Thus, not surprisingly,
individuals with genetic disorders in one of the cells’ organelles often have severe disease.
As scientists learn more about the intricate work of organelles in health,
stress and disease, new therapies to combat these disorders may become possible. And we
can look forward to understanding more about how these many tiny organelles make our cells tick.