I was at an awesome brewery in Flagstaff over the weekend (shout out to Historic Brewing Company) with my husband.  Over a beer and homemade chips, we start discussing science.  Now, don't imagine that just because I'm a scientist that I insist on talking about it all the time.  Most of the time we're talking about our dogs or work or school or this blog.  This weekend, the science discussion actually started by talking about grading curves and how they are sometimes fair and sometimes not, and they sometimes measure student's performance and sometime the teacher's (sophomore year physics - I'm referring to you!).  From here, the discussion gets a bit fuzzy, but we eventually starting discussing cells and start arguing about the difference between eukaryotic and prokaryotic cells.

Before I get into the details of the argument, let's talk about what a cell is. Cells are the lego blocks of all living organisms.  Cells contain the genetic material and can divide and replicate into new cells (in a process called the cell cycle).  If you take any part of an organism and zoom in, you will see cells. Cells don't all look or function the same - as you may imagine because cells in your body do different thing than cells in a plant or cells in a fungus or a bacterial cell. And even different cells in your body look and function differently. Blood cells are round and flow through blood vessels.  Blood vessels are made out of multiple layers of cells surround by muscle cells to help contract and expand the vessels to move the blood. Muscle cells are different depending on where in your body they are located. For example, heart muscle cells look and function different than muscle cells found in your bicep. Nerve cells (called neurons) in the brain often have lots of branching so that they can connect to other nerve cells to transmit signals.  And just to clarify, I know I said that you could zoom into ALL parts of your body and see cells, however there are exceptions to everything.  For example, hair and nails are make out of a hard, tough protein called keratin and not cells.

Even though cells may look different and function differently, all cells have a few things in common:

  1. All cells are surrounded by a membrane, often called the plasma membrane, that is typically made up of fats (called lipids) and proteins.  You can think of this as a plastic bag that can hold stuff inside of it.
  2. The stuff inside this cell membrane includes cytoplasm - you can think of this as a jello inside the plastic bag made up of lots of proteins.
  3. Inside all cells is the genetic material (DNA) that is required to make and replicate the cell and for the cell to function.

Even though different types of cells have different shapes and different functions, there are two main types of cells:

  1. Prokaryotic cells - these are cells that do not have membrane bound structures inside the cell membrane (floating around in the jello-like cytoplasm).
  2. Eukaryotic cells - these cells have membrane bound structures called organelles. We'll discuss specific organelles in future posts, but some organelles that you may already be familiar with are the nucleus, which is another membrane-bound structure that contains the DNA, or mitochondria, which are the energy-producers of the cell, or chloroplasts, which are the organelle that converts sunlight into energy in plants.

So, if a cell has a nucleus, it's eukaryotic and if not, it's prokaryotic. And this is where the argument started.  My husband insisted that eukaryotic cells are defined by the fact that they are only found in multi-cellular organisms.  Multi-cellular refers to any organism that's made up of more than one cell - like humans who have about 100 trillion cells, redwood trees, spiders, birds, seaweed, whales, algae, mushrooms, etc.  Although it is true that eukaryotic cells are mostly found in multi-cellular organisms, protists are single-celled and contain a nucleus - making them eukaryotic. Conversely, most prokaryotes are single cells - like bacteria and plankton.  However, some of these single-celled prokaryotes can stick together and work together as a community in a slimy biofilm that is very similar to being multi-cellular.

Why in the world does this matter? Why did I spend any of my energy on a Friday night (and a bit on Saturday when we rekindled the discussion) even discussing this point?  It's because definitions matter and understanding the details matter - especially in science. But that ultimately isn't the point of this post - it's about cells.  Without cells, life as we know it wouldn't exist.  Without an understanding of cells and how they work, we can't understand what it means when they dysfunction and cause disease in humans.

Dr. Cathy Seiler is the Program Manager for the tissue biorepository at St. Joseph's Hospital and Barrow Neurological Institute. She has her BA in Biochemistry and Molecular Biology from Boston University and PhD in the Biological Sciences from Cold Spring Harbor Laboratory. Her research and teaching focuses on genetics, cancer, and personalized medicine. Find her on Facebook at www.facebook.com/thingsitellmymom

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