Prokaryotes and Eukaryotes Cells

Introduction

In this unit, you will study about organism, which are different from the ones you are familiar with. The aspect you are going to study are Prokaryotes (they are unicellular). The fungi and the algae (part of Eukaryotes) they are multicellular. The fungi undergo a kind of nutrition called heterotrophic, they do mostly not move. Their food is digested outside their bodies and the products of the
digestion are absorbed. Most algae use sunlight to make their food. Prokaryotes are one single cell organisms. You are familiar with bacteria they belong to this group.

 Bacteria as an example of Prokaryotes

Bacteriology is that important branch of microbiology which is concerned with the study of bacteria. Bacteria are the most ancient group of organisms that appeared about 3,500 million years ago. They have a cellular structure but cannot be identified either as animal or plant, but they are often included with fungi. Bacteria range in size from a length of 0.1 to I Oum and an average diameter of
about 0.1 um. They can be found in such environments as soil, dust, water, air, on and in plants and animals. Bacteria fall into the group of smallest organisms called microbes.

Some bacteria can survive at very high temperatures of up to 360-degree centigrade or very low freezing temperatures. Their numbers are enormous and with fungi, their activities are crucial to all other organisms. They are responsible for decay of organic matter and subsequent recycling of nutrients. Even though they cause disease, they are of increasing importance to humans because they can be used in many biotechnological processes.

Cell Wall: Contains a molecule of murein made up of polysaccharide chains that are cross-linked at regular intervals by short chains of amino acid. This makes the cell wall strong and rigid. Thus each cell is bounded by a net-like sac. Even though its tiny pores allow the passage of water, ions and small molecules the rigid wall prevents it from busting. Bacteria fall into two groups according to their wall structure - - Gram positive and Gram negative. Christian Gram (1884) was the biologist that developed the stain, which led to this classification. The murein net of Gram positive bacteria is filled with polysaccharides, and proteins to form a relatively thick wall. The walls of Gram negative bacteria are thinner, but their outer layer is coated with a smooth, thin membrane- like layer of lipids and polysaccharides. This serves as a protector from Iysozme and anti-bacteria enzyme. Gram negative bacteria are resistant to penicillin because of this outer layer.

Cell Surface Membrane: This is the partially permeable membrane that surrounds the living material of the bacterial cell. Respiratory enzymes are located here. In some bacteria it forms mesosomes and/or photosynthetic membranes. Mesosomes: These are infoldings of the cell surface membrane. They perform a role during cell division. They assist in the formation of new cell cross- walls between the daughter cells. Ribosomes: These are the locations where protein synthesis takes place.

Capsules: When the background of a bacteria specimen is stained slimy gummy secretions around)the cell becomes clear. They make up the capsule, or which enable bacteria to stick to surfaces. They provide additional protection to the bacteria.

Flagella: They are fine hair-like protein fibrils that serve as organs of locomotion. There are four ways in which flagella may be arranged. See drawing for types. 

1.  Monotrichate - a flagellum at one end of the cell, e.g. cholera vibrios. 
2. Amphitrichate - a flagellum at each pole of the cell, e.g. alcaligene faecales. 
3.  Lophotrichate - a cluster of flagella at one end of the cell, e.g. pseudomonas. Peritrichate - several flagella spread around the cell surface, e.g. typhoid bacillus. - Pili (singular pilus): also known as fimbrial (see figure 5.1a, ) are shorter and thinner than flagella. The female pilus type is involved in sexual reproduction. - Plasmids: small, self-replicating circle of extra DNA with a few genes but provide extra survival advantage. 

Chemoheterotrophic Bacteria

They obtain their energy requirement from chemicals in their food. There are three further subdivisions of this nutritional category of bacteria- saprotrophs, mutuatists and parasites. Saprotrophs are organisms that obtain their food from dead and decaying matter. They digest their food by secreting enzymes onto the organic matter so the digestion actually takes place outside of the organism. Thereafter the products of digestion which are soluble are absorbed and assimilated within the body of the saprotroph. This class of bacteria and fungi play a major role in the process of decay and recycling of nutrients. Therefore they are known as decomposers. They are useful in the process of producing humus from plant and animal remains. On the other hand, they cause the decay of food that is useful to humans. Mutualists (also known as symbionts). This refers to a process in which two living organisms that have a close relationship derive mutual benefits from each other. For example, the nitrogen-fixing bacterium rhizobium living in the root nodules of legumes. Both organisms benefit from the nutrition process, of each other.

Parasites are living organisms that live in or on other organisms (called host) from which they obtain their food and sometimes shelter. Parasites that cause disease are known as pathogens. There are two types of parasites: obligate parasites and facultative parasites. The former can only survive and grow in living cells while the latter infect their host, bring about its death and continue to live on its remains.

 Photoautrotrophic Bacteria

These are bacteria that carryout photosynthesis and use carbon dioxide as a source of carbon. It is possible that the process photosynthesis first evolved in blue-green bacteria. They are found on the surface of layer of fresh and seawater.

Chemoautrotrophic Bacteria

Also known as chemosynthetic bacteria, they source their carbon from carbon dioxide and obtain their energy from chemical reactions. They do so by oxidising inorganic materials like ammonia and nitrite. The nitrification process is like this:

1. BH+4 oxygen NO-2 + energy
2. NO-2 oxygen NO-3 + energy

 Reproduction and Population Growth in Bacteria

When suitable conditions are available bacteria can grow very rapidly. The following conditions enhance bacteria growth right temperatur e, nutrient availability, pH and ionic concentrations. For obligate aerobes, oxygen must be present but it must be absent for obligate anaerobes. The nucleus to cytoplasm ration determines the optimum size at which a bacteria will begin to reproduce. A bacterium may divide into two identical daughter cell. This is called a sexual binary fission. First the DNA is replicated, then copied, after that the cell division takes place. In fast growing bacteria, reproduction can take place every 20 minutes.

Primitive forms of sexual reproduction where there is an exchange of genetic material also takes place in some bacteria. The process is called Genetic recombination.

Bacteria Opportunistic Infection

This is a situation where a person whose immune system has been severely weakened falls prey to all kinds of bacterial infection. For example when a HIV patient enters the third phase of the development of the disease (AIDS-related complex ARC) several infections will begin to take place. The bacterial infection becomes prolonged and more difficult to treat. This happens because of the
significant drop in the number of T help cells in the patient.

Sum mary

In this unit on Prokaryotes and Eukaryotes, you have learnt that Eukaryotes evolve from Prokaryotes and they contain true nuclear. Bacteria is an example of  Prokaryotes. There are different types of bacteria classified on the basis of their feeding. When condition is suitable, they multiply rapidly.