This post is meant to lay some ground rules by describing some preliminary concepts that most of us might have learned in high school.
Murphy’s law
Let us not forget that the nature had several billion years to play with “Life and its various forms”. As a result, we see that multiple disparate ways of optimisation–in terms of evolution, survival, gene-propagation etc.–have been tried and chosen. Some of the choices were undone and when undoing a harmless choice seemed costly, it was left as was (vestigial organs, for instance). To sum up, whatever seems logical to serve a purpose will probably be true in one or the other life-form. The converse is also true: Whatever can be stated as a general rule will almost always have exceptions.
A Cell and its Organelle
A cell, we all know by now, is the basic building block of life — its structural and functional unit. Cells come in different shapes, sizes, and structures. A cell performs some of the general functions (general in the sense that every cell performs that function in order to survive) while some functions are specific to the kind of cell. Some cells exist independently (e.g. single cell organism) whereas others as an aggregation (e.g. in the form of simple multi-cellular organisms or as a tissue in a complex organism). A cell typically contains sub-units, specialised to perform specific functions, called organelles. For example, mitochondria which performs the task of power/energy generation for the cell. An organelle may or may not be wrapped in a membrane. This distinction broadly speaking serves as one of the criteria for classification of cells (and organisms, in turn).
Before we move on to organelle and classification of cells, let’s remind ourselves of the macromolecules whose aggregation/interaction make various organelles function.
Biological Macromolecules
The large molecules, consisting of different combination of various small organic molecules, are important cell components performing important and wide range of functions required for survival and growth of a cell. These are broadly classified as the following:
- Proteins:
- Proteins are amongst the most interesting molecules as far as Bioinformatics is concerned. They perform diverse functions that are vital for life: they act as enzymes, hormones, transporters; provide structural support; the list is long.
- Structurally, protein molecules are polymers of amino acids. An elementary gist of information on amino acids (structure, classification etc.) can be found here.
- Necessary details on the structure of proteins can be read here.
- Lipids:
- Mostly non-polar (thus hydrophobic) hydro-carbons.
- Include fats, oils, waxes, phospholipids, steroids.
- Most lipids are made up of long chains of fatty acids attached to some alcoholic group (e.g. glycerol or sphingosine in fats and phospholipids; long chain alcohols in waxes). Steroids, on the other hand, have four fused carbon rings.
- Major important biological functions:
- Fats: stored form of energy
- Waxes: provide protective covering to some species of birds and plants
- Phospholipids: amphipathic – “head” containing phosphate group makes up the hydrophilic part while fatty acid chains form the hydrophobic part. Major constituent of cell membranes (outermost layer of an animal cell).
- Steroids: In animal, steroids (e.g. cholestrol) determine the fluidity of cell membranes and act as signalling molecules (e.g. steroid hormones like estrogens, androgens, progestogens etc.).
- Carbohydrates:
- Hydrates of Carbon. Empirical formula (with a few exceptions) – Cm(H2O)n.
- Classified as the following (based on degree of polymerisation i.e. how many units are joined to form the molecule):
- Sugars [1 or 2 monomers]: Monosaccharides (e.g. glucose, fructose, galactose); Disaccharides (e.g. sucrose, lactose)
- Oligosaccharides [3 to 9 monomers]: e.g. raffinose
- Polysaccharides: Starch; non-starch like glycogen, cellulose etc.
- Major important biological functions:
- Vital fuel source: immediate energy source for the cell (glucose)
- Stored energy source: excess simple sugars get converted to space-efficient polysaccharides (to be used later when needed)- glycogen in animals, starch in plants.
- Structural component: cellulose form the cell walls in plants; chitin constitute cell walls in fungi and exoskeleton in anthropods (e.g. insects, lobsters, shrimps); ribose make up the backbone of RNA and deoxyribose that of DNA.
- Nucleic Acids:
- The software of life. The most important molecule for a ‘Genomist’. They deserve a separate post.
Organelles
A simple one-page summary is here.
Classification
A simple way to classify organisms is based on their ability to “self feed”: (i) Autotrophs – they can convert light/chemical energy into their “food” (e.g. plants) (ii) Hetrotrophs – they require other organisms to serve as their food.
Another broad and more important classification of organisms, based on their cellular characteristics, is as follows:
Prokaryotes: Prokaryotic cells are “simple” cells that lack a nucleus (Nucleic Acids suspend in the cytoplasm rather than being enclosed in a membrane) and their organelles are not membrane-bound. They may have organelles like flagella (whip-like structure for locomotion), ribosomes, and plasmids (circular DNA). Examples include bacteria, cynobacteria (bacteria with ability to do photosynthesis) etc.
Eukaryotes: Eukaryotic cells have membrane-bound nucleus and other organelles. For example, microorganisms like fungus, simple algae, protozoa; higher-order organisms like animals and plants.
Note that Viruses are not considered “living” organisms as they can not reproduce on their own (by cell division) and require “host cells” for for making their copies (through assemblies). In effect, viruses are just DNA/RNA enclosed in a protein coat.
Resources
To read about the functions, chemical structures etc. in more details, refer to this Lumen Learning course page.