Nuclear body
In the eukaryotic cell, there is a distinct nuclear body, called the nucleus. It is surrounded by a double-membrane, called the nuclear envelope, containing pores to allow the passage of RNA out into the cytoplasm. Within the nucleus is contained a nucleolus and chromosomes. The chromosomes of DNA are paired, have a linear structure and are coated by proteins called histones. In a prokaryotic cell, there is no nucleus; instead there is a region in the cytoplasm where the DNA is located, called the nucleoid. It is not membrane-bound. The DNA structure is different in that it is made up of a singular, circular strand. This one closed loop contains about 2000 genes, about 0.2% of the number found in eukaryotes. In addition to this chromosome, prokaryotes have smaller, shorter loops of DNA present in the cytoplasm called plasmids. These replicate independently of the main cellular chromosome.
Mitochondria / Mesosomes
All cells need to respire. In eukaryotic cells, double-membrane bound organelles called mitochondria provide cellular chemical energy by synthesizing ATP (energy compounds). In prokaryotes, invaginations of the plasma membrane serve as surface areas for the reaction. This section of respiratory membrane is called a mesosome, and it contains enzymes associated with respiration (protein generated to drive ATP synthesis).
Chloroplasts / Thylakoids
Chloroplasts, in the plant and algae cells of eukaryotes, trap light energy in a green plastid called chlorophyll, using it to convert carbon dioxide and water into carbohydrates (chemical energy). These organelles are important for organisms that photosynthesize. Some prokaryotes have infoldings of the plasma membrane, like mesosomes. In this case, chlorophylls and enzymes are embedded to form a membrane called thylakoids. These photosynthetic membranes are only present in photosynthetic bacteria of prokaryotes.
Cell wall
Of the eukaryotic cells, only plant, algae and fungi have cell walls. Cell walls are important in preventing mechanical and osmotic damage. It is made of cellulose in green plants and of chitin in fungi. The cell walls of prokaryotes are much more rigid, as they contain a framework of murein (parallel polysaccharides cross-linked by peptide chains to from a 3D network). Cell walls also help determine the shape of the cell. Animal cells are spherical in shape as they lack the cell wall. Bacteria cells come in three common shapes: coccus (round), bacillus (rod) and spirilum (helical); as well as two different types of cell walls: gram-positive and gram-negative.
Cytoskeleton
Eukaryotic cells have a number of protein fibres that help give the cell its shape and support. These include microtubules and filaments for inner cell movement. Prokaryotes are lacking in cytoskeletons.
Flagella / Cilia / Pili
The flagella (and cilia in eukaryotes) aids in cell movement. In eukaryotic cells, they have a distinct arrangement of 9+2 microtubules. In a prokaryote, the flagella (if present) consist of a singe fibril (a cylinder of protein subunits, about 20 nm thick and several µm long). Flagella can be found in prokaryotes all over the cell, or in a group at one or both ends, or as a single tail. Flagella rotate around a ‘bearing’ anchored in the cell wall, producing a corkscrew motion that can propel it through a fluid medium.
Prokaryotic cells also have pili (filamentous structures projecting out of the cell wall). These are involved in transferring plasmids between two prokaryotic cells during mating and attaching the cell to potential hosts.
Ribosomes
The ribosomes of prokaryotes are smaller and float around freely in the cytoplasm. Ribosomes in the eukaryotic cells are larger. They are synthesized in the nucleus and formed in the cytoplasm. Some are bound, forming rough endoplasmic reticulum, whilst others are free in the cell.
Endoplasmic reticulum / Golgi apparatus
The Endoplasmic reticulum (ER) and Golgi apparatus is found only in eukaryotic cells. The ER modifies (synthesizes RNA into protein) and transports pre-made materials to the Golgi apparatus, which modifies and packages these materials. The ER can be split into two types: rough (involved in protein synthesis) and smooth (involved in detoxification and manufactures lipids). Both organelles are membranous: ER is made of tubules, and the Golgi of sacs.
The Endosymbiosis Theory
“Symbiosis” is the term used to describe two species that live together in an intimate relationship where one lives in or on the body of another. “Endosymbiosis” describes a relationship where one organism lives inside of the other (i.e. symbiont within its host).
This theory suggests how prokaryotic cells contribute to eukaryotic cells. The prokaryotes are the symbionts that enter their host: the eukaryote. The prokaryotic cells survive on the nutrients provided by the host and are probably protected by invaginations of the cell membrane surrounding itself. It may become part of a mutualistic relationship with their host (both benefit, neither is harmed), and over a longer period of time, the two organisms become dependent on each other. The prokaryotic cells can no longer survive outside the cell, and thus becomes part of the eukaryote cell’s organelles. The prokaryotes may be consumed by the eukaryote as undigested prey or may invade as an internal parasite. This theory gives an explanation of cellular evolution and suggests how new species first came about.
It was suggested the plasma membrane folded in upon itself to form the nuclear envelope, Endoplasmic reticulum and Golgi apparatus. The chloroplasts (found in some cells) and mitochondria (found in almost all cells) evolved from prokaryotes.
Small aerobic bacteria was ingested by the anaerobic eukaryote and continued to live on the inside of the cell. Each benefited from the relationship: the aerobes became mitochondria, providing the cell with more ATP than could be produced by anaerobic respiration, whilst the cell provided the necessary organic and inorganic compounds. In this way, the cells became more productive and more likely to reproduce. Multiplying led to many more eukaryotes with mitochondria contained within them (natural selection).
Chloroplasts evolved similarly to mitochondria; however they were only ingested by some eukaryotic cells. These cells were able to photosynthesize with their pigments. They produced organic food molecules for their host (energy source) by carbon-fixation, whilst the host gave them inorganic nutrients (i.e. CO2). They both lost their ability to function without the other, those the prokaryotes become permanent cell organelles.
Evidence for the theory
Scientists have found that mitochondria and chloroplasts cannot be formed in a cell that lacks them from the beginning because the eukaryote’s nuclear genes only code for some of the proteins of which the prokaryotes are made of.
It seems likely that mitochondria and chloroplasts were once prokaryotes as they possess many features similar to that of prokaryotic cells:
- Outer membrane is similar to plasma membrane (could have been pinched off to protect prokaryote during early stages of ingestion).
- Inner membrane is similar to bacterial membrane (could have been the original prokaryotic cell membrane, retained even as it becomes the organelle of the eukaryote).
- Own DNA strands are circular like that in prokaryotes, instead of linear in eukaryotes.
- Own ribosomes are smaller than eukaryote’s ribosomes, but similar sized to prokaryotes’.
They are also able to self-replicate independently from the eukaryotic cell because they have their own sets of genes (more similar to prokaryotic genes than eukaryotic genes). It is also highly possible that this theory occurred as eukaryotes are able extremely able with the process of endocytosis.
Evidence against the theory
The mitochondria and chloroplasts could be made from invaginations of the plasma membrane. If the plasma membrane is able to develop into the Golgi apparatus and ER, then it could be possible that mitochondria and chloroplasts are even more highly developed invaginations of the plasma membrane. By relieving the cell of some of its functions, more energy can be directed to evolving highly specialized organelles.
