Prokaryotes are organisms that lack a cell nucleus or any other membrane-bound organelle, while Eukaryotes possess both. The nucleus is one of the most important structures required in living organisms. The nucleus can be thought of as the ‘brain’ of the cell.
Mostly Prokaryotes are single-celled or unicellular, except the Myxobacteria, which is a multicellular organism at some stage in its life cycle. The Eukaryotes are multicellular beings that carry out different functions of life. They contain both membrane-bound organelles (small organs) and a membrane-bound nucleus.
The nucleus controls several important functions. For example, the duplication of cells occurs under the command of the nucleus. A Eukaryotic cell has the capability of cell division. It contains the important structure, Deoxyribose Nucleic Acid (DNA) which is necessary for the function and development of organisms.
Prokaryotes can be exemplified as different members of a group. They carry out separate tasks and work in a group function. Similar cells, group together and work in harmony to carry out one particular function. Hence they do not carry out individual processes of life. The reason Prokaryotes lack important organelles like the mitochondria, ribosome, and nucleus is that they are not required to perform complicated functions. This can explain their single-celled structure and hence their smaller size.
The Eukaryotes are dissimilar to the Prokaryotes in that, they are required to carry out important functions of life. A single organism can be capable of performing multiple tasks. It is a being in itself. It contains important structures like mitochondria, ribosomes, endoplasmic reticulum, etc. Its nucleus contains DNA. Hence complicated tasks like the production of proteins or cell division can be carried out by these cells. Eukaryotic cells must accommodate important sub-structures in their body. This, explains why Eukaryotes are larger in size when compared to Prokaryotes.
A tissue is a structure that is made up of similar types of cells and an organ is described as a structure that is made up of similar tissues. Several organs may group together to form an organ system. The digestive system is made up of several organs like the liver, stomach, intestines, etc. The liver is made up of individual cells called hepatocytes. The stomach has similar muscles that help it contract in harmony. The esophagus has cartilaginous structures that help during peristaltic movements. Therefore, each organ is made up of similar tissues (Hurlbert, 1999). Each tissue is made up of several, but similar prokaryotic cells. Organs include the heart, brain, liver, etc.
The reason there is dissimilarity in size between Prokaryotes and Eukaryotes can be explained by a simple example. If 5 people are delegated the task of building a structure and they are provided all the materials necessary, the plan, and the blueprints of the structure, their work will be made easy. These 5 men have been programmed in a way that building the structure will be practically effortless for them. Consider these 5 men to be individual prokaryotic cells and the tasks that they are performing as a group to be a function. The structure that they are building is tissue and several similar structures could potentially form an organ. Similarly, if another 5 men were to build the same structure but were not provided any materials, plans, or a blueprint of the structure.
The men would have to gather the necessary materials, make a plan and then start their work. Such men would require expertise in the matter as everything that is required to build the structure has not been provided to them. These men can be considered as eukaryotic cells that have to carry out a complicated task. Such men would be of higher intelligence, unlike the previous 5 men who would not have to put too much thought process in carrying out their work.
The functional demands of eukaryotic cells require them to have specific organelles within their bodies. These organelles accommodate space. This space is the reason eukaryotic cells are larger in size when compared to prokaryotic cells. Prokaryotic cells are simple structures that do not need organelles in their bodies.
References
Hurlbert, R.E. (1999) Microbiology 101/102 internet text Chapter ii: eukaryotic vs. PROKARYOTIC CELLS.
Kaiser, G. (2007) Introduction. Cellular Organization: Prokaryotic And Eukaryotic Cells.