The Dawn of Life
We have already looked at the elements that define all biological entities, the four different classes of biomolecules that they create and their primary function in the biological entity, now we can turn our attention to the basic biological unit and its’ organisation. Hopefully at this point the chemistry and the biology will make sense.  Again I must apologise for the number of links on this page but, as usual, we have to get through a lot of technical information rather quickly and it is better to provide links to more detailed information rather than explain it all here. As usual we will be using Wikipedia as our reference material.
THE BASIC BIOLOGICAL STRUCTURAL UNIT
The cell is the basic structural, functional, and biological unit of all known living organisms; it is the smallest unit of life that can replicate independently. Organisms can be classified as unicellular (consisting of a single cell; including bacteria) or multicellular (including plants and animals). There are two types of cell, eukaryotic, which contain a nucleus, and prokaryotic, which do not. Prokaryotes are single-celled organisms, while eukaryotes can be either single-celled or multicellular .
Prokaryotes include two of the domains of life, bacteria and archaea. Prokaryotic cells are simpler and smaller than eukaryotic cells, and lack membrane-bound organelles such as the nucleus. The DNA of a prokaryotic cell consists of a single chromosome that is in direct contact with the cytoplasm. The nuclear region in the cytoplasm is called the nucleoid.
Plants, animals, fungi, slime moulds, protozoa, and algae are all eukaryotic. These cells are about fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume. The main distinguishing feature of eukaryotes as compared to prokaryotes is compartmentalization: the presence of membrane-bound organelles (compartments) in which specific metabolic activities take place. Most important among these is a cell nucleus, an organelle that houses the cell’s DNA. This nucleus gives the eukaryote its name, which means "true kernel (nucleus)".
All cells, whether prokaryotic or eukaryotic, have a membrane that envelops the cell, regulates what moves in and out (selectively permeable), and maintains the electric potential of the cell. Inside the membrane, the cytoplasm takes up most of the cell’s volume. All cells (except red blood cells which lack a cell nucleus and most organelles to accommodate maximum space for haemoglobin) possess DNA, the hereditary material of genes, and RNA, containing the information necessary to build various proteins such as enzymes, the cell’s primary machinery. There are also other kinds of biomolecules in cells. This article lists these primary components of the cell, then briefly describes their function.
The cell membrane, or plasma membrane, is a biological membrane that surrounds the cytoplasm of a cell. In animals, the plasma membrane is the outer boundary of the cell, while in plants and prokaryotes it is usually covered by a cell wall. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of phospholipids, which are amphiphilic(partly hydrophobic and partly hydrophilic). Hence, the layer is called a phospholipid bilayer, or sometimes a fluid mosaic membrane. Embedded within this membrane is a variety of protein molecules that act as channels and pumps that move different molecules into and out of the cell. The membrane is said to be ’semi-permeable’, in that it can either let a substance (molecule or ion) pass through freely, pass through to a limited extent or not pass through at all. Cell surface membranes also contain receptor proteins that allow cells to detect external signalling molecules such as hormones.
The cytoskeleton acts to organize and maintain the cell’s shape; anchors organelles in place; helps during endocytosis, the uptake of external materials by a cell, and cytokinesis, the separation of daughter cells after cell division; and moves parts of the cell in processes of growth and mobility. The eukaryotic cytoskeleton is composed of microfilaments, intermediate filaments and microtubules. There are a great number of proteins associated with them, each controlling a cell’s structure by directing, bundling, and aligning filaments. The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, polarity and cytokinesis. The subunit protein of microfilaments is a small, monomeric protein called actin. The subunit of microtubules is a dimeric molecule called tubulin. Intermediate filaments are heteropolymers whose subunits vary among the cell types in different tissues. But some of the subunit protein of intermediate filaments include vimentin, desmin, lamin(lamins A, B and C), keratin (multiple acidic and basic keratins), neurofilament proteins (NF - L, NF - M).
Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Cells use DNA for their long-term information storage. The biological information contained in an organism is encodedin its DNA sequence. RNA is used for information transport (e.g., mRNA) and enzymatic functions (e.g., ribosomalRNA). Transfer RNA (tRNA) molecules are used to add amino acids during protein translation.
Prokaryotic genetic material is organized in a simple circular DNA molecule (the bacterial chromosome) in the nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different, linear molecules called chromosomes inside a discrete nucleus, usually with additional genetic material in some organelles like mitochondria and chloroplasts (see endosymbiotic theory).
Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called transfection. This can be transient, if the DNA is not inserted into the cell’s genome, or stable, if it is. Certain viruses also insert their genetic material into the genome.
Organelles are parts of the cell which are adapted and/or specialized for carrying out one or more vital functions. Both eukaryotic and prokaryotic cells have organelles, but prokaryotic organelles are generally simpler and are not membrane-bound.
There are several types of organelles in a cell. Some (such as the nucleus and golgi apparatus) are typically solitary, while others (such as mitochondria, chloroplasts, peroxisomes and lysosomes) can be numerous (hundreds to thousands). The cytosolis the gelatinous fluid that fills the cell and surrounds the organelles.
Cells are an arrangement of biomolecules, organised by their function, in such a manner as to provide a controlled environment to enable growth (i.e. metabolism) and ultimately cell division (i.e. replication).
Metabolism is the set of life-sustaining chemical transformations within the cells of living organisms. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments.
Cell division involves a single cell (called a mother cell) dividing into two daughter cells. The mechanics of cell division are beyond the scope of this article but DNA replication, or the process of duplicating a cell’s genome (hereditary information), always happens when a cell divides through mitosis or binary fission.