The cell is the most basic unit of life. All living things are made out of cells and cells are where the most basic metabolic reactions necessary for life take place. All living things can be divided into two main groups, based on the fundamental structure of their cells. Eukaryotes are made of cells that have a membrane-bound nucleus and specialized organelles. Prokaryotes are made out of cells that lack a membrane-bound nucleus and lack specialized organelles. Plants, animals, and fungi are eukaryotic organisms while bacteria and archaea are prokaryotic organisms.
The main difference between prokaryotes and eukaryotes comes down to the basic structure of the cell. Each kind of cell has a few distinguishing characteristics that set it apart.
Distinguishing features of eukaryotes include:
- a membrane-bound nucleus that contains genetic information
- membrane-bound organelles
- large (10-100 micrometers)
- linearly organized DNA
- unicellular or multicellular
Distinguishing features of prokaryotes include:
- lack of a membrane-bound nucleus
- lack of membrane-bound organelles
- small (1-5 micrometers)
- circularly organized DNA
- unicellular only
Evolutionarily speaking, eukaryotes are the younger of the two kinds of organisms. The first eukaryotes appeared about 2.7 billion years ago, though this number is still uncertain. Prokaryotes, on the other hand, have existed for at least 3.5 billion years and were the first form of life on Earth.
Eukaryotes can be multicellular or unicellular. Multicellular eukaryotes tend to have several cell types each specialized to serve some particular function. An example would be muscle cells (myocytes). Prokaryotes are unicellular. A prokaryotic organism consists of one cell, so every prokaryotic cell is its own organism.
Eukaryotes are organisms made out of eukaryotic cells, cells with a membrane-bound nucleus and membrane-bound organelles. Eukaryotes make up the majority of commonly encountered macroscopic organisms like plants, animals, and fungi. Eukaryotes can be unicellular or multicellular.
Eukaryotes are set apart by their relatively complex internal structure. First, eukaryotes have their DNA separated from the cell by a membrane-bound nucleus. The nucleus contains the genetic information for the organism bound into tight linear chromosomes, along with proteins for gene expression. During cellular reproduction, (mitosis or meiosis) the nucleus breaks down, the chromosomes are copied, and the cell splits to create two distinct cells. Embedded in the membrane of the nucleus are various transport proteins that regulate the flow of materials in and out of the nucleus.
Eukaryotes are also unique in that their cells contain specialized structures called organelles. Organelles are intracellular structures that perform specific functions. Mitochondria, for instance, are organelles that handle the production of ATP, the fundamental unit of chemical energy used by the body. Other organelles, such as ribosomes, assist with the construction of complex proteins and other biological macromolecules. If one conceives of a cell as a small house, the organelles in the cell would be like the separate rooms, each designed to serve a specific purpose. Some common organelles in eukaryotic cells and their functions include:
- mitochondria – produce ATP
- microtubules – serve as the “skeleton” of the cell
- vacuoles – engulf and transport materials and harmful pathogens in and out of the cell
- ribosomes – construct proteins from information in RNA
- Golgi apparatus – packages and tags proteins for intercellular transport
- chloroplasts (plants only) – facilitate photosynthesis
- lysosomes – contain enzymes to digest and remove waste products
Most eukaryotes are at least capable of sexual reproduction. Sexual reproduction in eukaryotes is a complicated process which involves the crossing over and recombination of DNA from 2 parents, which results in offspring with a unique genetic code. Some eukaryotes do produce asexually though, mostly through some form of binary fission, budding, or cloning.
Among eukaryotic cells, there are 3 major kinds, corresponding to plants, animals, and fungi. The three kinds of cells have some unique features that set them apart.
One feature of plant cells that separate them from other eukaryotic cells is the presence of thick cells walls made of cellulose and pectin. Plants cell walls are resistant to mechanical deformation and breakage so they give plants a degree of flexibility. Each plant cell has a large central vacuole that, when filled with water, exerts turgor pressure on the cell wall causing the plant to stand upright. Plants rely on this mechanism for bodily structure because they lack internal skeletons like mammals.
Plants cells also have chloroplasts that allow them to perform photosynthesis. Chloroplasts contain molecules of the pigment chlorophyll. When photons from light strike chlorophyll, the molecules donate an electron, which creates an electron transport train, which generates the energy needed to convert carbon dioxide and water into carbohydrates.
Animal cells are distinct in that most animal cells lack cell walls entirely. The lack of a rigid cell wall allows animal cells to take on a variety of shapes. Animal cells tend to have smaller vacuoles than plant or fungal cells, and they tend to have more mitochondria.
Fungal cells are different from both plant and animal cells in that their cell walls contain chitin, the same substance that makes up the exoskeleton of arthropods, crustaceans, and insects. Fungal cell walls consist of a dense inter-connected weave of chitin fibers. The basket-like structure gives the fungal cell wall a lot of strength and durability.
Fungal cells lack chlorophyll so they cannot photosynthesis, but they do contain different kinds of pigments that give fungi their wide range of colors. These pigments often serve to protect the cell from ultraviolet radiation. Fungal cells also regulate transport across their cell membranes using ergosterol, as opposed to cholesterol in animal cell membranes.
Prokaryotes differ from eukaryotes in that their cells do not contain a membrane-bound nucleus that contains their DNA. Instead, the genetic material of prokaryotes floats free in the intracellular cytoplasm. The metabolic reactions that sustain the cell also take place in the cytoplasm. All prokaryotes are unicellular organisms, meaning that every prokaryotic cell is an individual organism. All prokaryotic organisms are bacteria or archaea.
Most prokaryotes have thick cells walls made out of peptidoglycan and external structures like flagellum that they used for mobility. Prokaryotes can come is a wide variety of shapes, including spheres, spirals, rods, etc. Prokaryotes also tend to be much smaller than eukaryotes, on the order of 1-5 micrometers. Prokaryote DNA is normally stored in a single circular chromosomal loop, instead of the linear tightly packed chromosomes of eukaryotes.
Prokaryotes also lack internal organelles. While they do seem to have some quasi-specialized regions, prokaryotic cells do not have well-defined and separate organelles like eukaryotic cells do. This lack of internal organelles does not necessarily mean that prokaryotes are internally simple: on the contrary, prokaryotes have some complex internal mechanisms that are very different than eukaryotes. Many prokaryotes are not obligate oxygen-breathers, so they have developed an entirely distinct set of metabolic reactions than those found in eukaryotes.
All prokaryotes reproduce asexually, mostly through binary fission. During binary fission, a single cell will split into two copies with identical DNA. Though they cannot sexually reproduce, many prokaryotes can engage in a form of DNA transfer and recombination. Prokaryotes, in particular, bacteria, are capable of directly inserting segments of their DNA into the DNA of other prokaryotes. This process is often called “horizontal” gene transfer as it happens between 2 already extant organisms, as opposed to the “vertical” gene transfer from parent to offspring in sexually reproducing species.
Although prokaryotes are generally defined as strictly unicellular, some species of prokaryotes conglomerate into colonies called “biofilms.” Biofilms have been shown to have some degree of phenotypic variation over time and space, and engage in a rudimentary form or intercellular signaling. These considerations have led some biologists to argue that there are, in fact, multicellular prokaryotic organisms.
Relationship Between Prokaryotes and Eukaryotes
Of the two kinds of organisms, prokaryotes are the older, having first emerged some 3.5 billion years ago. It is currently believed that the first eukaryotic cells emerged as a result of a larger cell engulfing smaller prokaryotes. These prokaryotes adapted and developed to become the intracellular organelles found in eukaryotic cells. Evidence for this theory includes that fact that some mitochondria contain DNA similar in shape and structure to prokaryotic DNA, chloroplasts in plants share many similarities with single-celled photosynthetic bacteria, and ribosomes in both prokaryotes and eukaryotes share similarities. The theory that eukaryotic life emerged from the incorporation of smaller prokaryotic organisms into larger complexes is called endosymbiosis.
To summarize, eukaryotes and prokaryotes are the two main kinds of living organisms, each distinguished by the fundamental structure of their cells. Eukaryotes have cells with a membrane-bound nucleus and specialized organelles. Prokaryotes lack a membrane-bound nucleus and lack organelles. Eukaryotes can be unicellular or multicellular while prokaryotes are strictly unicellular.
Eukaryotes include plants animal and fungi. Prokaryotes include bacteria and archaea. Prokaryotes are the older of the two and first evolved 3.5 billion years ago. Eukaryotes emerged around 2.7 billion years ago, though this number is not certain. It is believed that the first eukaryotic cells emerged as a result of the incorporation of smaller prokaryotes into a larger cell body.