An Electron Micrograph Of The Eukaryotic Nucleus
Suspended within the cytoplasm, as well as the nucleus, the eukaryotic cell possesses many other membranous organelles that the prokaryotic cell lacks.
The endoplasmic reticulum in the eukaryotic cell comes in two forms, rough and smooth. It is made of flattened cavities lined with membrane sacs. The rough endoplasmic reticulum is responsible for the transport of proteins. It is rough because it is covered in ribosomes, which synthesis the proteins. The function of the smooth endoplasmic reticulum is to transport lipids and steroids around the cell. The prokaryotic cell does not contain this particular organelle. However, there are ribosomes suspended in the cytoplasm that are also used to synthesise proteins. The eukaryotic ribosomes are larger than those in the prokaryotic cell. The eukaryotic ribosomes measure 80S (svedberg units) where as the prokaryotic ribosomes measure 70S. The development of the ER is linked with the development of the nucleus in the eukaryotic cell. Ribosomes are made in the densest part of the nucleus, the nucleolus, and then need to be transported around the cell in order to make proteins.
Also suspended in the eukaryotic cytoplasm are the mitochondrion and also, unique to plant cells, the chloroplasts. The mitochondria in the eukaryotic cell are responsible for producing ATP. They are known as the powerhouses of the cell. A double membrane binds them and the inner membrane is folded into partitions known as cristae. The prokaryotic cell does not contain this specific organelle either. However a fold in the plasma membrane, known as the mesosome, is the site of respiration. Smaller folds in the membrane can also carry out similar functions.
The chloroplasts, typically found in a plant cell, contain chlorophyll and convert light energy into food substances such as glucose. These chloroplasts are also contained in a membrane. Some prokaryotic cells photosynthesise. Those that do possess small chromatophores that contain a chemical similar to chlorophyll called bacteriochlorphyll, a light absorbing pigment. This chemical is chemically simpler than chlorophyll, which indicates that the process of photosynthesis in prokaryotic cells and eukaryotic cells was chemically different.
The cytoplasm of the prokaryotic cell also contains food storage particles, where chemicals are stored that can be converted into energy. In the eukaryotic cell, these are also present in the cytoplasm but are contained within membranous vesicles. The compartmentalisation of these particles means that they can be secreted from the cell in bulk rather than floating around as loose particles.
The cell membrane of the eukaryotic cell is made up of a complex bilayer of phospholipids, spread through this bilayer are chemical marker like glycolipids and glycoproteins, transport proteins and cholesterol. The prokaryotic cell although lacking in membranous organelles, is surrounded by a cell membrane. However, like many other components of the prokaryotic cell, it is simpler and there are fewer types of phospholipids present.
The cell wall of the prokaryotic cell is unique. It is made from an entirely different substance that the eukaryotic cell. This substance, peptidoglycan, consists of a mucopolysaccharide and a polypeptide combined together. The composure of this cell wall is totally different from the cell wall of a plant cell. The cell wall of a plant cell is made up of various different layers with substances including pectin and cellulose. The more complex cell walls of the eukaryotic plant cell, offer support for the cell, in order to support the organism as a whole. The prokaryotic cells, generally exist as single celled organisms, there fore the support of the cell wall is needed. The eukaryotic animal cell on the other hand, which stereotypically lacks a cell wall, exists as an organism of millions of cell. The individual functions of muscle and bone cells contribute to the support needed by the organism.
In addition to the cell wall, some prokaryotic cells possess a defence mechanism, a slimy capsule on the outside of the cell wall that offers extra protection against the ingestion of phagocytes. The eukaryotic cell on the other hand do not possess this defence, however, they do have other mechanism. For example, lymphocytes, phagocytes and chemical markers. The capsule, along with pili are involved in connecting cells
Some prokaryotic cells possess flagella. These organisms, generally single celled, have the ability to move. Unlike the eukaryotic flagellum, this mechanism lacks an internal structure. The prokaryotic flagellum is shaped like a corkscrew and propels the cell; unlike the eukaryotic cell if they possess a flagellum, move in a lashing action.
These two groups of cells have very different means of sexual reproduction. Where the eukaryotic cell has a complex method, involving many complicated organelles such as the centriole and spindle. The prokaryotic cell has no real method of sexual reproduction. However some DNA exchange can occur between the cell walls. Concerning sex pili.
. The final major difference in the two cells is the size. Where the eukaryotic cell measure 10 to 100 μm, the prokaryotic cell only measures 0.5 to 10μm.
So in conclusion, it is evident that the prokaryotic cell is much more primitive than the eukaryotic cell. Its organelles have a much simpler structure and function. Also, the evolution of unicellular organism, such as bacteria, into multicellular organism such as mammals plays a major role in the difference between these two groups. Where prokaryotic cell contain the mechanisms to exist on their own, eukaryotic cells often exist as much more complicated organisms with different cells working together to carry out complicated processes such as mitosis, sexual reproduction, photosynthesis etc. In making these comparisons it is also important to take into accounts the ways in which cells have evolved in order to adapt to their surroundings. As the percentage of gases in the atmosphere has change, so the chemical make up of the earlier cells evolved.
