The Basics: Prokaryotes versus Eukaryotes

Biology is an amazing complex topic, and this complexity is what makes it fun!

Before jumping into advanced topics, however, it is important to have a solid foundation of the basics, which we usually learn before algebra even.  As a quick review, let's talk about prokaryotes and eukaryotes, the two types of cells that should be familiar to most everyone.

The world of living cells is broken down into two major types of cells: prokaryotes and eukaryotes (also called procaryotes and eucaryotes, depending how you would like to spell).  Evolutionarily, we consider prokaryotes to be the ancestors of eukaryotes, which we will discuss further in the future.

Let's start with the prokaryotic cell and its defining characteristics:

• Cell wall:  Prokaryotes have a cell wall, which are tough boundaries that enclose the cellular contents.  The cell wall is pretty tough, too - its composition gives it rigidity that keeps the prokaryotic cell's shape.  The cell wall is covered on the outside by the outer membrane and on the inside by the plasma membrane.  
• Nucleoid:  DNA in prokaryotes isn't organized quite the same as in eukaryotes.  Rather than being organized in a distinct, membrane-bound portion of the cell, prokaryotes organize their DNA in what is considered the nucleoid.  You can think of the nucleoid as a membrane-less compartment where the organism's DNA is found.  Although in the diagram I have drawn a single linear DNA molecule, prokaryotes have a diversity of types of DNA, from single circular DNA to multiple linear and circular DNA molecules.
• Cytoplasm:  The area within the cell wall and membranes is considered the cytoplasm - the fluid portion of the cell.  By no means is the cytoplasm empty space - it is filled with molecules and many events are taking place constantly within the cytoplasm.

Prokaryotes may seem "simple," but they certainly are not.  The above points are a simplification, but are the general characteristics of prokaryotes.

Eukaryotes can be considered more "complex," though certainly not better.  As with prokaryotes, the below characteristics are general and apply to most eukaryotic cells.

• Nucleus:  Unlike prokaryotes, eukaryotes have a define space for their DNA - the nucleus.  The nucleus is surrounded by the nuclear membrane, which is surrounded by the endoplasmid reticulum (discussed next).  The nucleus is a hub of activity in the eukaryotic cell and by its enclosure in the nuclear membrane, distinct events can occur here and not in the cytoplasm.
• Endoplasmic reticulum:  Surrounding the nucleus is the endoplasmic reticulum (ER), which is a compartment involved in moving molecules in and out of the nucleus, as well as regulating gene expression.  The ER can be broken down into the smooth and rough ER.  The rough ER is where proteins are made on the membrane.  In contrast, the smooth ER can be considered where pieces of the membrane are exchanged with the Golgi apparatus (next).
• Golgi apparatus:  The Golig consists of a stack of membrane-bound vesicles, which are mainly involved in protein trafficking.  Here, a number of modifications can be made to proteins to target them to specific parts of the cell, such as the membrane or lysosomes.
• Secretory vesicles:  Some molecules in the cell need to be sent outside the cell, and they can exit through secretory vesicles.  These vesicles often originate in the Golgi, after which they fuse with the plasma membrane and dump their contents in the extracellular space.
• Lysosome:  The trash compactor of the cell, the lysosome is involved in recycling the contents of the cell.  Lysosomes have contents that break down molecules into their components so that the cell can reuse them.  Additionally, lysosomes can be used to destroy invading bacteria and to break down molecules that can be harmful to the cell.
• Mitochondria:  "The powerhouse of the cell" - the mitochondria (singular mitochondrion) are involved in energy production, yes, but they also function is several other aspects, such as cell death.  Mitochondria themselves are like miniature cells within the eukaryotic cell, and they have their own DNA (mtDNA).  Mitochondria have many interesting aspects that will be discussed in a future post.
• Plasma membrane:  Similar to prokaryotes, the plasma membrane in eukaryotes acts as a boundary, separating the cell from the outside.  Unlike prokaryotes, however, eukaryotes (generally) do not have a cell wall.  The plasma membrane is a very dynamic part of the cell - signaling, budding, engulfing.  

As mentioned, the above characteristics are an incredible simplification of prokaryotes and eukaryotes, but let this serve as a starting point into learning about each type of cell.  Both types have their own special attributes and many mysteries that remain to be solved.