This week we are studying the cell.

We have already discussed the importance of the cell in defining life (The Cell Theory) and talked about why this is a meaningful definition of life. I also spent a little time discussing viruses and how they defy this definition, but are often included or excluded depending upon the view or purpose of the investigator / student. (i.e. Viruses fail to be alive if the Cell Theory is used as the definition, but they are often considered alive by microbiologists for the purpose of classification, discussion of evolution, etc).

We began by recalling when in history people first realized that there was a microscopic world existing at all. This led into a talk about classification and how life falls into two major groups of cells, Prokaryotic and Eukaryotic. There are a number of key differences between these types of  cells, but I focus on just a few: 

1. Prokaryotic cells tend to be smaller

2. Prokaryotic cells have closed circles of DNA, Eukaryotic cells have linear chromosomes

3. Prokaryotic and Eukaryotic ribosomes are different from one another

4. Prokaryotic cells lack membrane-bound organelles (most notably, the nucleus)

We discussed other features, but I think these are the hallmark differences. Prokaryotic cells span two domains of life, the bacteria (which I tend to focus on) and the archae (which are more ancient and often extremophiles). Eukaryotic cells fall into four kingdoms: animalia, plantae, fungi and protista. With a quick discussion about some differences between these groups, I shelved all but animals and said that this was the group we would focus on for the remainder of the semester (with some exceptions such as photosynthesis).

What makes these four kingdoms similar is their Eukaryotic cell type. As I stated above, one feature of Eukaryotic cells is their membrane-bound organelles. These organelles are how the cell divies up its many tasks into separate functions and gets each of them done by some specific structure. In addition to discussing true organelles, we also discussed other structures and their functions (Ribosomes, plasma membranes, cytoplasm, cytoskeleton)

We finished up Tuesday’s class after just introducing all of the players. Today we will be putting some of them together to show how they function as parts of a larger organization. The three things I have in mind to walk through are: 

1. Energy Pathway – how solar energy gets converted into chemical energy, how that energy is stored (not getting into this part much) and then how that energy is brought back out and converted into a more usable form (ATP) that is put to work to make cells do things.

2. The Central Dogma – fleshed out this time with names of some of the processes. Initially focusing on how information is transformed into something that can actually do work (proteins). Then discussing how these proteins are made in a little more detail (cytoplasmic vs secretory proteins). This lets us talk about the ER, Golgi, Ribosomes and even ends with exocytosis.

3. Phagocytosis – I’m an immunologist, so I think about how macrophages attack cells and other foreign particles all the time. This is a good way to reverse the process of exocytosis and talk about endocytosis. Following endocytosis, we can then bring lysosomes and peroxisomes into play and discuss how they function to break down these ‘non-self’ items so that they become harmless (I’ll end by quickly tying this into the immune system’s antigen display mechanism – but without any detail).

That may be enough for them today. Depending upon questions, things can either go much quicker or drag out for the balance of the class. I expect that we will finish this material with enough time to at least get started with the next chapter – membranes. I like this chapter anyway and I think it’s the chapter that puts the students into the ‘mind’ of the cell the best. If you focus on a membrane and how it handles transport and diffusion, you are zoomed in so close, that suddenly, the cell feels large and familiar.

Lastly, I am really hoping to find an great animation of cellular processes that made the email loop of Penn a couple years ago. Cross your fingers – I have no idea where I might get a copy.