All organisms consist of small cells, typically too small to be seen by a naked eye, but big enough for an optical microscope . Each cell is a complex system consisting of many different building blocks enclosed in membrane bag. There are unicellular (consisting only of one cell) and multicellular organisms. Bacteria and baker’s yeast are examples of unicellular organisms - any one cell is able to survive and multiply independently in appropriate environment.
There are estimated about 6x1013 cells in a human body, of about 320 different types. For instance there are several types of skin cells, muscle cells, brain cells (neurons), among many others. The number of cell types is not well-defined, it depends on the similarity threshold (what level of detail we would like to use to distinguish between the cell types, e.g., it is unlikely that we would be able to find two identical cells in an organism if we count the number of their molecules). The cell sizes may vary depending on the cell type and circumstances. For instance, a human red blood cell is about 5 microns (0.005 mm) in diameter, while some neurons are about 1 m long (from spinal cord to leg). Typically the diameter of animal and plant cells are between 10 and 100 microns.
There are two types of organisms - eukaryotes and prokaryotes, and two types of cells respectively. Bacteria belong to the prokaryotes. However, most organisms which we can see, such as trees, grass, flowers, weeds, worms, flies, mice, cats, dogs, humans, mushrooms and yeast are eukaryotes. The distinction between eukaryotes and prokaryotes is rather important, because many of the cellular building blocks and life processes are quite different in these two organism types. This is believed to be the result of different evolutionary paths. Evolution is an important concept in biology, there is a proverb saying that things only make sense in biology in the context of evolution. Most scientists believe that life first emerged on Earth around 3.8 billion years ago. The oldest fossilised bones that have been found resembling bones from anatomically modern humans are about 100,000 – 200,000 years old. Nobody really knows how life emerged on Earth, but there is lots of scientific evidence regarding how it may have evolved.
Viruses are not quite living organisms, but when inside a living host cell they show some features of a living organism. Viruses are too small to be seen in an optical microscope, but are big enough to reveal their structure in an electron microscope (the characteristic size of the virus is about 0.05-0.1 micron, while the wavelength of green light is about 0.5 micron).
Prokaryotic cells are smaller than eukaryotic cells (a typical size of a prokaryotic cell is about 1 micron in diameter) and have simpler structure (e.g., they do not have any inner cellular membranes that are always present in Eukaryotes, see below). Prokaryotes are single cellular organisms, but note that being a single cell does not mean that an organism is a prokaryote. Being smaller than eukaryotes does not mean that prokaryotes are any less important – for instance it is quite likely that the number of bacteria living in the mouth and digestive tract of a human are larger than the number of eukaryotic cells in the same individual and many of these bacteria are necessary for a human being to live a normal life (these numbers are rather difficult to estimate, rather a hypothesis). Prokaryotes are sometimes also known as microbes.
A model of a eukaryotic cell (picture taken from On-Line Biology Book )
A eukaryotic cell has a nucleus, which is separated from the rest of the cell by a membrane. The nucleus contains chromosomes, which are the carrier of the genetic material (Section 3). There are internal membrane enclosed compartments within eukaryotic cells, called organelles, e.g., centrioles, lysosomes, golgi complexes, mitochondria among others (see picture above), which are specialised for particular biological processes. The mitochondria are found in all eukaryotes and are specialised for energy production (respiration). Chloroplasts are organelles found in plant cells which produce sugar using light. Light is the ultimate source of energy for almost all life on Earth. The area of the cell outside the nucleus and the organelles is called the cytoplasm. Membranes are complex structures and they are an effective barrier to the environment, and regulate the flow of food, energy and information in and out of the cell. There is a theory that mitochondria are prokaryotes living within eukaryotic cells.
An essential feature of most (prokaryote and eukaryote) living cells is their ability to grow in an appropriate environment and to undergo cell division. The growth of a single cell and its subsequent division is called the cell cycle. However, not all cells continually grow and divide, for example neurons only undergo an initial growth phase. Prokaryotes, particularly bacteria, are extremely successful at multiplying - it is likely that natural selection has favoured single celled organisms able to grow and divide quickly. Multicellular organisms typically begin life as a single cell, usually as a result of fusion of a male and a female sex cell (gametes). The single cell has to grow, divide and differentiate into different cell types to produce tissues and in higher eukarotyes, organs. Cell division and differentiation need to be controlled. Cancerous cells grow without control and can go on to form tumours. Development of single cells into complex organisms is in itself an area of study called developmental biology. This year’s Nobel prize for Physiology or Medicine has been awarded to scientists for the discoveries of key regulators of the cell cycle.
Cells consist of molecules.
Cells consist of molecules.