Eukaryotes are organisms whose cells each have a nucleus and organelles with their own membranes. Prokaryotes are simpler, single-celled organisms without a nucleus and only one interior space. This difference represents a structural advantage that allows eukaryotic cells to organize themselves into multicellular organisms. The interior organelles, including the nucleus, isolate the various cell processes and make them easier to control.
Without a nucleus, prokaryotic cells multiply through a hard-to-control binary fission process. This means they can reproduce rapidly when resources and space are available, but such fast, uncontrolled growth is not wanted when a cell forms part of a larger organism. Instead, each cell has to coordinate its growth and division with all the other cells of the organism. Eukaryotic cells have the structural complexity to do this while prokaryotic cells don't have that capability.
Features and Characteristics of Prokaryotic Cells Under Microscope
The prokaryotic domains are Bacteria and Archaea; each of these domains is divided into kingdoms and smaller taxonomic categories. As single-cell organisms with no nucleus or organelles, they are characterized by the following prominent features:
- Single cells have a cell wall.
- The single cells have a cell membrane.
- Cells contain a DNA strand.
- Cells contain ribosomes.
- The cells have a flagellum.
The single cells of bacteria and archaea are exposed to the environment and thus need a cell wall to protect them. Under a microscope, the cell wall is a thick, clearly visible structure surrounding the cell. On the inside of the cell wall is a cell membrane that controls which substances can cross into and out of the cell.
Inside the cell membrane is a tightly coiled single strand of DNA. The strand is circular, and when the cell starts dividing, the strand uncoils and assumes its circular shape prior to being copied. Once the strand has been duplicated, the two copies move to opposite ends of the cell and the cell splits into two.
Floating freely in the cell cytoplasm are ribosomes that produce the proteins required by the cell. At one end of the cell, a whiplike structure called a flagellum is attached to give the cell mobility. The prokaryotic cells use their simple structure as an evolutionary advantage. Their DNA is unprotected and mutates freely while their fast rate of reproduction allows quick adaptation to new situations and changes in the surroundings.
The Structure of Eukaryotic Cells
If you compare the structures of prokaryotic and eukaryotic cells under a microscope, the cells look quite different. Like prokaryotic cells, eukaryotic cells have a membrane and ribosomes, but the following differences are visible:
- Cells have no cell wall.
- Cells have a nucleus.
- The DNA is in several strands inside the nucleus.
- There are mitochondria and lysosomes, each with their own outer membrane.
- Additional membrane-bound organelles are Golgi bodies and the endoplasmic reticulum.
- Cells have two centrioles.
It's clear that the cells making up eukaryotes have a different structure than prokaryotic cells. While they are complex and reproduce in a more complicated fashion, it is not obvious why exactly that gives eukaryotes a structural advantage.
How Eukaryotic Cells Function
The eukaryotic cells have their own independent functions, but they often function as part of a larger organism. In plants and animals, they import substances from other cells and export waste products and useful proteins, hormones and enzymes. When they engage in an activity, what they export signals to other cells what they are doing. They don't have a cell wall because they don't need one for protection, and it would get in the way of the intercellular exchanges.
Instead of performing their synthesis of cell substances and their conversion of energy in the general space inside the cell membrane, they have specialized regions inside specific organelles where these activities take place. Conversion of glucose to the energy storage molecule ATP is carried out in the mitochondria. Breaking down of cell debris and waste takes place in lysosomes. Golgi bodies and the endoplasmic reticulum synthesize proteins, carbohydrates and lipids. The membrane-bound organelles of eukaryotic cells are specialized in the production of specific cell substances.
Eukaryotic Cell Reproduction
The cells of eukaryotes have two ways of multiplying: sexual and asexual reproduction. Asexual reproduction takes place when more of the same kind of cell is needed, such as in the skin cells of animals. Sexual reproduction is used when a new complex organism such as a plant or animal is created. In asexual reproduction, the number of cells increases while in sexual reproduction, the number of organisms multiplies.
Both kinds of reproduction are complicated multistage operations. For asexual reproduction, the cell nucleus splits into two identical parts in a process called mitosis. Each nucleus has complete copies of the cell DNA, and when the cell splits, each part receives a share of the organelles.
For sexual reproduction, cells are produced with different sexual characteristics in a process called meiosis. For example, in animals, the two types of cells are the sperm cells and the egg cells. Two cells with different sexual characteristics and usually from different organisms of the same species re-unite to form a new organism. In animals the sperm cell fertilizes an egg cell, and the combination grows into a new animal.
The Eukaryote Structural Advantage
The differences between the cells of eukaryotes and prokaryotes give eukaryotes advantages in several areas. When we list the features that are found in eukaryotes but not prokaryotes, what are advantages provided by these differences? The main structural differences lie in the nucleus, the organelles and the cell outer wall. These differences give rise to specific advantages and capabilities for eukaryotes that prokaryotes don't have. As a result, prokaryotes remain simple single-cell organisms. While single-cell eukaryotes also exist, some eukaryotes have made use of these advantages to evolve into higher plants and animals.
The presence of a nucleus in eukaryotic cells gives eukaryotes two advantages. The nucleus represents an additional protective enclosure of the DNA. As a result, eukaryotic DNA is less susceptible to mutations. The nucleus also makes reproduction easier to control. The complicated nucleus-based reproductive processes have many points that can act as a stop to coordinate growth and cell multiplication with the other cells of the organism.
The integration of organelles into the eukaryotic cells concentrates functions into their own interior spaces. This means that processes such as energy production and waste elimination are much more efficient in eukaryotic cells than in prokaryotes. When mitochondria produce the cell's energy, cells can have more or fewer mitochondria, depending on the role they play in the organism. Without organelles, the whole prokaryotic cell has to do everything, and the level of efficiency is lower.
The absence of a cell wall in complex eukaryotes is the advantage that allows the eukaryotic cells to organize themselves into structures such as organs, bones, plant stems and fruit. These cells work together and differentiate themselves depending on their surrounding cells. A cell wall would prevent such close interactions. While prokaryotic cells sometimes clump together in simple structures, they don't differentiate the way eukaryotic cells in complex organisms do.
The major structural advantage of eukaryotes over prokaryotes is the ability to form advanced, multicellular organisms. While eukaryotes can survive as both single-cell and multicellular organisms, prokaryotes don't have the ability to form complex structures or organisms.