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I found this among some notes from undergrad that I was cleaning up. It's an answer from a test I wrote back in 2nd-year microbiology in 2001, and I think quite relevant as a primer on some biological concepts.
This essay explains:
- Why there can exist a size difference between eukaryotes and prokaryotes
- The minimal compliment of organelles required in a eukaryote
- How prokaryotes carry out life without those organelles
- How the endosymbiotic theory of eukaryotic evolution relates
- The role of the cell wall in prokaryotes to water balance in the cell
- The difference in the roles of the cell membrane between the two groups
Eukaryotes vs. Prokaryotes
In a eukaryotic organism, the membrane-bound organelles that are its trademark effectively raise the surface area-to-volume ratio, the traditional limit to cell size. In addition, they provide a high level of segregation and organisation for chemical reactions within the cell, arguably increasing efficiency and definitely increasing possible size. If environments are controlled by vacuoles and other organelles, reactions between reagents that would be too rare in the cytoplasm to react can be much faster, simply because the effective volume is smaller. Prokaryotes lack this ability, so in order for reactions involving low-concentration metabolites to occur, the cell must remain fairly small.
Eukaryotes require size for their diverse collection of structures. At the very least, a eukaryotic cell contains a nucleus, a membrane transport system including the endoplasmic reticulum and golgi, mitochondria, and the ever-present (and often transitory) vacuoles and vesicles. This arsenal requires quite a lot of space. Without so much cellular furniture to deal with, a prokaryote can afford to be the "bachelor's suite of life". As a case study, a single mitochondrion is roughly the size of a prokaryotic cell, and eukaryotes can have dozens of these. Mitochondria are, in fact, postulated as symbiotic prokaryotes adapted to life inside larger eukaryotic cells.
Their lack of dining set, chesterfield, and a china cabinet does not mean prokaryotes are less equipped to deal with survival, any less than a bachelor's suite is less condusive to survival than a three-bedroom apartment; in fact, they gain distinct advantages from their simplicity. The tiny prokaryote can do everything a eukaryote can do, and usually much faster. Their cellular membrane performs the ATP synthesis duties of a mitochondrion. DNA is constantly replicating in the bacterial cytoplasm, with neither the elegance nor the encumbrance of formal mitosis. The environment is small enough that reactions can occur in the hydrophilic cytoplasm or hydrophobic plasma membrane with satisfactory frequency. Some eukaryotic organisms, such as Paramecium, require a contractile vacuole to maintain water balance, while a bacterium achieves this passively with a semirigid cell wall that holds in hydrostatic pressure., keeping the cell from bursting. The eukaryotic membrane is little more than a transport region and a shield compared to the versatility of a prokaryote's, which produces ATP and acts as a hydrophobic environment as mentioned as well as anchoring the cell wall and chromosome and performing all the duties of a eukaryotic membrane.
Prokaryotic organisms are smaller and, in many ways, simpler than eukaryotes. This difference, however, could be likened to the difference between the terms "pen" and "ink-utilising cylindrical cone-tipped writing implement": one is certainly bigger, but size isn't everything.
Prokaryotic cells are single celled organisms that were formed at the formation of the earth, so are the most basic life forms. The prokaryotes are organised in the ‘three domain system’ and include bacteria and blue-green algae. Prokaryotes live in many environments including extreme habitats such as hydrothermal vents, hot springs, swamps, wetlands, and the guts of animals. This is a diagram of a prokaryote cell and it shows that they are not very complex cells. They do not contain any membrane bound organelles and they have relatively small ribosomes. They also do not contain and endoplasmic reticulum.
The DNA is not stored in a nucleus but it is stored in the nucleoid. DNA in the cytoplasm and many prokaryotic cells also contain very small loops of DNA called plasmids. The nucleoid region is the area of the cytoplasm, which contains the single bacterial DNA molecule. Some prokaryotic cells have a capsule, which is an outer layer of protection which protects the cell when being engulfed by other organisms. It also helps the cell stick to surfaces and nutrients. The cell wall (which contains murein) protects and gives the cell shape. In the centre of the cell there is the cytoplasm which is composed mainly of water but also contains digestive enzymes, salts, organelles and other organic molecules.
The prokaryotic cell also has a cell or plasma membrane which controls the movement of substances in and out of the cell. The pili are short, hair-like structures which attach the cell to other bacterial cells. Finally, the bacterial flagellum is a long ‘tail’ which helps the cell move.2 Prokaryotic cells can be harmful as they are bacterial cells and therefore can cause disease. Some of these bacteria have developed a resistance to antibiotics, for example, MRSA. The resistance becomes coded on the plasmid DNA and as the bacteria can share the plasmid between each other, the resistance can bass between the cells.
Also, as prokaryotic cells reproduce asexually with binary fission, they pass on the resistance to the daughter cells. On the other hand, prokaryotic cells can be very beneficial as they are a species that are important to humans. The bacterial species are used in the production of cheese and yoghurts. In the intestine of all mammals, the bacterial cells help to produce vitamin K, which is vital in the digestion of certain foods. Bacterial cells are also necessary for digesting the dead and waste material. The skin of humans is covered by a ‘normal flora’ of bacteria. This helps to prevent any harmful microorganisms getting into the body. 3
Eukaryotic cells organisms with cells that contain a nucleus and other organelles which are bounded by membranes. The main difference between eukaryotic and prokaryotic cells is the fact that, prokaryotic cells contain no membrane bound organelles. The most significant membrane bound organelle which is not found in a prokaryotic cell is the nucleus, which contains all the genetic material. Most eukaryotic cells also contain other membrane-bound organelles such as mitochondria and the Golgi apparatus. In addition, plants and algae contain chloroplasts. Many unicellular organisms are eukaryotes, such as protozoa. All multicellular organisms are eukaryotes, including animals, plants and fungi.4 5
This is a diagram of an animal eukaryotic cell and it shows that it is a lot more complex than the prokaryotic cell (above). It is also about ten times larger. The nucleus stores the majority of the cell’s genetic material. It is surrounded by a nuclear envelope which is made of two membranes with fluid in between. The chromatin is inside and it is comprised of DNA and proteins. It has the instructions for making protein, which may be used for regulating the activity of the cell. When the cell divided the chromatin condenses into visible chromosomes. The nucleolus is at the centre of the nucleus and creates RNA and ribosomes which pass into the cytoplasm to make proteins. The rough endoplasmic reticulum (ER) contains ribosomes and carries the proteins made there. On the other hand, the smooth ER makes lipids, and it does not contain ribosomes.
The Golgi apparatus or Golgi body receives proteins from the ER and modifies them, which may involve adding sugar molecules to them. The proteins then are packaged in vesicles so they can be transported, possibly outside of the cell. The mitochondria have two membranes with fluid in between. The inner membrane is highly folded to form cristae. The mitochondria produce most of the adenosine triphosphate (ATP) during respiration. This provides the energy for the cell. Lysosomes contain digestive enzymes which break down materials, such as invading organisms.6 7
Plant eukaryotic cells differ in several distinctive ways from animal eukaryotic cells. In the plant cell there is a large central vacuole, which is enclosed by a membrane known as the tonoplast. This maintains the cell’s shape, controls movement of molecules between the cytosol and sap, stores useful material and digests waste, proteins and organelles. Plant cells also are surrounded by a cell wall made from cellulose, outside of the cell membrane. In plant cells there are chloroplasts which contain the green pigment chlorophyll, which allows the plant to photosynthesis.
Eukaryotic cells have membranous organelles which help to group the cell so that each reaction in the organelle is separate from those of other organelles. Eukaryotic cells also have a variety of organelles that carry out different functions. Prokaryotic cells came before eukaryotic cells so they do not have the organelles that eukaryotic cells have. Eukaryotic cells have a membrane bound nucleus, while prokaryotic cells store the DNA in a region called the nucleoid. Prokaryotic cells also have the outermost layer, the capsid, while eukaryotic cells do not have this layer. The nuclear membrane around the DNA allows for the regulation of different materials into and out of the nucleus. This is the same for each organelle too. The most important plasma membrane (cell membrane) is the one that surrounds the eukaryotic cell itself. This plasma membrane separates the extracellular matrix from the cytoplasm, and helps regulate what enters and exits.
Number of chromosomes:
More than one
One–but not true chromosome: Plasmids
Usually unicellular (some cyanobacteria may be multicellular)
True Membrane bound Nucleus:
Animals and Plants
Bacteria and Archaea
Meiosis and fusion of gametes
Partial, undirectional transfers DNA
Lysosomes and peroxisomes:
Absent or rare
May be absent
DNA wrapping on proteins:
Eukaryotes wrap their DNA around proteins called histones.
Multiple proteins act together to fold and condense prokaryotic DNA. Folded DNA is then organized into a variety of conformations that are supercoiled and wound around tetramers of the HU protein.
Present (in plants)
Absent; chlorophyll scattered in the cytoplasm
Microscopic in size; membrane bound; usually arranged as nine doublets surrounding two singlets Submicroscopic in size, composed of only one fiber
Permeability of Nuclear Membrane:
Plasma membrane with steroid:
Only in plant cells and fungi (chemically simpler)
Usually chemically complexed
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