The Basics: Cells
Just as atoms can be described as the building blocks of chemistry, cells could be referred to as the building blocks of life, they are certainly the building blocks of complex life. You may think that anything so small could not have much variety, after all the majority of biological molecules are based off just four elements carbon, hydrogen, oxygen and nitrogen, with the occasional use of sulphur and phosphorus thrown in for good measure. However, using only these elements the average human cell can produce up to 100,000 different proteins. It really is miraculous how so few building blocks can create something so complex and beautiful. So, what exactly is a cell? Why is there so much variety? And how do we classify them?
What is a cell?
In a very loose sense a cell is like a house, each cell contains a simple barrier, a wall between it and the outside. This barrier, or cell membrane, is selective, it will only let certain molecules through, like having a key for the door. Once on the inside, some cells will resemble open plan houses whereas others will have different rooms or compartments for different functions.
Despite these similarities cells come in a wide variety of shapes and sizes, as shown below you. Here is an example of a neuron and a plant cell, they have dramatically different shapes and purposes.
Photographs of a spinal cord motor neuron right and a cross-section tip of an Allium root, left. Taken by Berkshire Community College Bioscience Image Library.
From these pictures you can easily see that outer membrane and also the variety in both shape and internal structures. Cells can have a variety of different internal structures depending on type and species. One commonality is that all cells contain DNA, in some it is just a densely packed area, but in others it has its own membrane bound compartment. In fact, it is this difference underpins the two major categories of cell types; prokaryotes and eukaryotes. Prokaryotes have only one external membrane whereas eukaryotes have internal membranes that surround different structures called organelles.
Another difference is whether each cell creates an organism on its own, a single celled organism, the ultimate lone survivor, or, whether they team up together to create multicellular organisms. Complex organisms are multicellular, each cell takes on a specific role within the organism, but each individual cell can no longer sustain life on their own, the ultimate co-dependant relationship.
Prokaryotes
Prokaryotes are simpler than eukaryotes, they are all about open plan living. The fluid inside the cell is called the cytoplasm and in prokaryotes this contains all the cellular machinery needed to sustain life. Their DNA is contained in a circular form, ring shaped, with all prokaryotic cells having at least one that contains their genomic DNA but they can have more. Most prokaryotes reproduce asexually, copying its DNA and dividing into two daughter cells.
Eukaryotes
Eukaryotes are far more complex than prokaryotes, rather than open plan living eukaryotes have internal membrane bound compartments for distinct functions. Like a house that has loads of different rooms for different purposes, eukaryotes have a nucleus which is a membrane bound compartment for DNA, lysosomes which are the waste recyclers and ribosomes which create new proteins. The presence or absence of some organelles depend species. Animal cells contain a membrane bound mitochondria to generate the energy whereas in plant cells this is achieved by a chloroplast.
The number and type of organelles in the cell also depends on how specialised the cells is. Eukaryotes can live as single or multi-cellular organisms. As a single celled organism the cell will retain all the functions needed for life but in multicellular organisms the cells can specialise into preforming specific roles as the cross section of a leaf shows below.
A cross section of the leaves of the beach grass Ammophila. In response to long periods of drought the leaf curls. The right hand picture shows the overall structure and the right-hand picture is a close up showing the individual cells. Taken by Berkshire Community College Bioscience Image Library.
Single Cellular Organisms
Both prokaryotes and eukaryotes are capable of being single celled organisms. They are capable of synthesising everything they need for life, ingesting molecules, generating new ones and dividing to create new daughter cells. Through the secretion of proteins, some can form biofilms which creates a better environment for all but important distinction is that they retain the ability to survive as free-living organisms.
Multicellular organism
On the other hand, only eukaryotes are capable of becoming multicellular organisms, sometimes referred to as complex life. In this environment cells become specialised. To begin with each cell is capable of becoming any cell but it is like a one way street, each turn takes you further down the road and there are no legal U-turns. Thus cells increasingly specialise until they lose the ability to be anything else. Thereby an organism is created with specialised tissues for specific functions, transporting water, contracting, storing fat deposits or carrying electrical signals. In this way a single cell and form together to create more complex organisms.
Summary
Cells are fundamental to life. All the living organisms you can see around you, and those too small to see, are made up from cells. The are two major subtypes of cells, prokaryotes with only and external membrane and eukaryotes that also have internal membranes. Prokaryotes are single celled organisms whereas eukaryotes can be single or multi-cellular organisms. In multi-cellular organisms the cells take on different appearances and therefore function within the organism. These are the major distinctions between cells but they harbour the countless variety found within the entirety of living organisms.
Further reading
A much more detailed look at how cells function
MedlinePlus.gov – What is a cell?
For a list of the different organelles that can exist within eukaryotic cells and a brief description of their function.
Learn More from the Basics Series
A brief overview of how DNA works as the mechanism through heritable traits are passed down.
Most DNA is translated into proteins, but what exactly is a protein?
How to change DNA, this molecule that underpins how traits are inherited, and why would we want to.
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