The Living World

The world of living things, is our world, a world of interdependence and relatedness, it includes an incredible number of diverse and fascinating organisms। All of these organisms are, or should be important to us, our world is their world, their health is our our health and our life is their life। We are not separate or apart. We are joined, united, interactive and dependant on each other.

Nobody can ever know all the name of all creatures and plants and fungi that inhabit this world, there are far to many for the human mind to really even think about। Many, if not most, of the individual species that make up this wonderful rainbow of life are not even known yet to science. However the number of organisms that is known to science is already immense, well over 1 million.

What is Life?
We are alive, we talk about living things all the time। We also use the word life regularly, but what do we really mean by these words life, alive and living?
These questions are not as easy to answer as you may like, as soon as you start to look closely at the way we use these words our simple definitions of what they mean seem full of holes। Scientists and philosophers are still arguing about where the boundaries between life and non-life lie। It is in inherent in the way we as humans think that we like to assign everything to categories with hard well defined borders। It is unfortunate for us that a serious study of the world around us seems to indicate that hard well defined borders do not actually exist in many cases।

Cellular Magic

The same problem applies to what is alive and what isn't. Nobody really thinks steel rods are alive yet the stresses and strains which effect living muscles or plant stems are basically the same as the stresses and strains which effect steel rods. OK steel rods do not grow reproduce themselves but in the right environment some crystals do, are these crystals alive?

Today we might think of life as a process. A process that involves cellular systems in a series of activities. By cellular systems we mean ones which have a definable inside, separated somehow, from an equally definable, outside. Generally we define the life giving activities of these systems as 1)a tendency to trap energy (either directly as radient energy or indirectly as matter, or both) within the system. This results in a build up of greater complexity inside the cells. 2) A further tendency to convert the materials brought into the system into new forms which are more useful to the system and to excrete unwanted products, both those brought into the system and those resulting from internal activity. 3) Finally, and most importantly in some ways, to reproduce themselves.

All the things that science currently accepts as living exist as either single cells, or as a collection of cells working together (unicellular life or multicellular life). These cells exist as an area of cytoplasm enclosed by a membrane. Sometimes this membrane, called the 'Cell Membrane' is further enclosed in a more rigid 'Cell Wall' (i.e. Plants, Fungi, and most Protista, Bacteria and Archaea). All use DNA as there means of storing information concerning the mechanics and methods of reproduction. All contain proteins of some sort.

Unicellular organisms are the Prokaryotes (Bacteria and Archaea), and some Eukaryotes:- the Protista, some Fungi and some Plants. Multicellular organisms are the rest of the Eukaryotes:- most Fungi, most Plants and Animals. In single celled organisms the cells are all the same most of the time in any given species. In multicellular organisms individual groups of cells have become specialised to perform particular roles in the life of the organism. The life of the organism is dependant on the correct working of all the different groups, each of which is dependant on all the others for its continued existence. In simple multicellular organisms such as sponges all the cells are very similar, in more complicated multicellular organisms the degree of specialisation of cells is much greater resulting in cells that are very different from one another. In humans, there are 1014 different types cells making up over 200 different kinds of tissues.

As far as we know life began as single celled (unicellular) organisms about 3,800 million years ago (3,800,000,000 years ago). For most of the history of life on this planet these single celled organisms have dominated the scene completely, and they are still essential to all life. Over millions of years they slowly changed the world by changing the composition of the atmosphere and creating soil. Multicellular organisms arose much, much later in time when some of these unicellular organisms learned that they could be more successful if they worked together. Multicellular organisms first appeared in the fossil between 650 and 550 million years ago. However, because at this stage fairly complicated groups such as Trilobites already exist, scientists believe that multicellular life probably first appeared between 1600 MYA and 1000 MYA. For reasons that aren't known the fossil record is pretty scarce before the Cambrian (600 - 500 MYA) However once multicellular life forms got started they blossomed amazingly, the first huge explosion of diverse life forms occurred in the early Cambrian, and several other such sudden increase in the number of species have occurred since. Now multicellular life is amazingly successful and occurs almost everywhere. For multicellular organisms to be successful these individual cells have had to learn how to communicate with each other, how to control the distribution of resources, how to make sure the correct number of each different sort of cell exists and that they are all in the correct places as well as devising new methods of reproduction which ensure that all the different sorts of cell are present in the new organisms. This has involved not nly changes in DNA, but changes in the very way DNA works. It has taken a long time for the complexity of multicellular activity to develop to the extent where human beings with human brains and minds are possible.

Classification

As human beings we have divided the living world up into ६ Kingdoms living organism. Viruses and other disease causing non-cellular entities are not considered to be living because they can not reproduce themselves. They use, and are dependant on the reproductive machinery of other living things in order to reproduce themselves. They do not have cells, all the known organisms in the ६ Kingdoms of life have a structure based on the cell.

The six kingdoms of living things are divided into two major groups, Prokaryotes and Eukaryotes। There are two prokaryote kingdoms and four eukaryote kingdoms.There are huge fundamental differences between the ways these two groups go about living.

Eukaryotes have a separate membrane bound nucleus, numerous mitochondria and other organelles such as the Golgi Body within each of their cells. These areas are separated off from the main mass of the cell's cytoplasm by their own membrane in order to allow them to be more specialised. The nucleus contains all the Eukaryote cell DNA for instance and the Mitochondria are where energy is generated. The exception to this rule are red blood cells which have no nucleus and do not live very long.

Prokaryotes do not have a nucleus, mitochondria or any other membrane bound organelles। In other words neither their DNA nor any other of their metabolic functions are collected together in a discrete membrane enclosed area. Instead everything is openly accessible within the cell, though some bacteria have internal membranes as sites of metabolic activity these membranes do not enclose a separate area of the cytoplasm.

The Six Kingdoms

Kingdom	When Evolved	Structure	Photosynthesis
Prokaryotes:-
Bacteria	3 to 4 billion years ago	Unicellular	Sometimes
Archaea	3 to 4 billion years ago	Unicellular	No
Eukaryotes:-
Protista	1.5 billion years ago	Unicellular	Sometimes
Fungi	1 billion years ago	Unicellular or Multicellular	No
Animalia	700 million years ago	Multicellular	No
Plantae	500 million years ago	Multicellular	Yes