Biology Day 3: What is the fundamental unit of life?


The fundamental unit of life - the smallest singular thing that could be called an organism - is known as the cell. It is the smallest unit that has all the properties that we typically attribute to life, and the smallest organisms are singular cells. Diagrams of the cell typically present it as being simple, but it’s anything but - it’s a complex network of molecules that all work together to complete the functions of life, to create new parts, replicate, and process energy. However, by making the cells simple in diagrams, we can better understand all the individual components. 

Cells are made up of many different types of molecules that we will be looking at in the future, however cells typically include such things as water, nucleic acids, proteins, carbohydrates, lipids, and salts. These parts all come together to create structures in the cell, provide organization, carry out important functions necessary to life (such as the processing of energy), and replicate the cell. There are many different types of very unique cells - from bacteria cells, to plant cells, to skin cells to blood cells. However, they all typically have these types of molecules and use them in similar ways (evidence that contributes to the general idea that all of life is related). 

The first basic type of cell is the prokaryotic cell. There are three fundamental groups of organisms on our planet: Archaea, Bacteria, and Eukaryota. Archaea and Bacteria, though as fundamentally different from one another as they are from Eukaryota, are collectively known as prokaryotes. This means that their genetic material - their nucleic acids - are floating within the cell and are not separated from the rest of the cell by a membrane. However, this does not make their cells any simpler than the cells of eukaryotes, who have their genetic material contained within a separate membrane. Prokaryotic cells typically have water - called cytoplasm - enclosed by a membrane. This membrane is further surrounded by a structure called the cell wall. Many prokaryotes also have this cell wall further surrounded by a capsule. 

The cytoplasm is very different in composition than the liquid surrounding the cell - it has different concentrations of many molecules. Typically, the bacterial cell is littered with structures called ribosomes which float about the cell - ribosomes are the molecules that help to create proteins. Besides ribosomes, the cytoplasm also has genetic material - both in the bacterial chromosome (the bulk of the genetic material) and in small, circular plasmids. There is also the cytoskeleton within the cell - structural filaments that serve to organize and transport components of the cell throughout the cytoplasm. The cytoskeleton is important in cell division, protection, shape determination, and polarity (meaning, the orientation of cell components). Many types of bacteria also have structures in which to store nutrients such as glycogen and vacuoles. There are also other types of small structures within the cell. Inclusions are small molecules, including granules, that are found throughout the cell, such as lipid droplets. Gas vacuoles are membrane found vesicles that provide buoyancy to cells that need it. Microcompartments are membrane-bound structures in the cell (organelles) that surround and enclose enzymes. Carboxysomes are found in bacteria that harness their own energy (rather than obtain it from other organisms) that house the components needed to do so. Finally, magnetosomes are found in bacteria that can align themselves along a magnetic field, to allow the bacteria to find optimal oxygen concentrations. 

Bacteria also have many many small proteins on the surface of the cell. Pili are small protein tubes that extend from the outer membrane and are present in high numbers on the cell. They help the cell stay attached to a surface or to other cells. Sometimes they are also used to help the cell move. Sometimes an S layer, otherwise known as a surface layer, is also on the external surface of the cell - it is a array of crystalline proteins that is used as a permeability barrier for molecules. Many other large molecules are secreted by the bacterial cell - glycocalyx made of carbohydrates and protein that protect the bacteria from being eaten. Flagella are also found on many bacteria cells - they are very long, whip like structures that protrude from the cell wall and help the bacteria move. Sometimes bacteria have a single flagella (monotrichous), sometimes multiple tufts of flagella at one end of the cell (lophotrichous), sometimes a flagella on either end of the cell (amphitrichous), and sometimes multiple flagella all around the cell (peritrichous). 

Archaea and Bacteria, though similar in many ways, have a significant number of differences. While all of the structures listed above are found in Bacteria, not all are found in Archaea. Archaean bacteria are also very different in many of the biological molecules they contain and how they organize their genes and their enzymes. Furthermore, their cell membranes are linked together differently than in bacterial cells. In fact, many aspects of archaean biology are extremely similar to eukaryotes. This has implications for the ancestry of life and how all three groups are related to one another evolutionarily; however, that’s another topic for another time. 

Speaking of topics going on too long - next week we will look at the eukaryotic cell, which is also extremely complex - but still similar in a surprising number of ways to the prokaryotic cell. 


Sources for the text:

i work in a lab that studies small tropical fish and these were the notes on someone’s spreadsheet that they left open. i found them very endearing and relatable

Thunder Cell of May 13, 2015 passing by! by Mitch Mirror
Via Flickr:
Even if the main part of this thunderstorm was raging over Germany, some thunder cells traveled also over Switzerland and could be observed, bringing impressive lightning and cloud formations. Some even see an evil face in this one :-)