What Are Prokaryotic Cells?

Prokaryotic cells are cells that make up prokaryotes. The main feature of this type of cell is that it has no nucleus bounded by the nuclear membrane, no nucleoli, and only a nucleus. Evolutionary status is low. Organelles have only ribosomes, have cell walls, and differ in composition from eukaryotic cells. Cells are small, have no shaped nuclei, no chromosomes, and DNA does not bind to proteins. ^[1]

Prokaryotic cells

Prokaryotic cells are composed of

Prokaryotic cells have no nuclear membrane,

Cell wall

Most prokaryotic gene structures exist in the form of operons, that is, multiple genes that perform similar functions are gathered together, under the control of the same promoter, and have a terminator downstream. Spacers of varying length exist between the two genes, such as genes for enzymes involved in lactose metabolism. At distance

Bacteria multiply in two ways. Some bacteria are in an unfavorable environment, or when they are depleted of nutrients, they form endospores, also known as spores. They are dormants with strong resistance to adverse environments. Formed internally, so often called endospores.

The vitality of spores is very tenacious. Bacillus in some lake sediments is still active after 500-1000 years. Clostridium botulinum spores can withstand boiling at 100 ° C for 5-9.5 hours at pH 7.0. The spore is composed of the following parts inside and outside:

1. Spore protoplast: the core contains concentrated protoplasts.

2.

The traditional taxonomy classifies organisms into animal kingdom and plant kingdom according to their nutritional mode, exercise ability and cell structure.

Prokaryotes are composed of prokaryotic cells and are all single-celled organisms, often called bacteria.

According to appearance characteristics, prokaryotes can be roughly divided into six types of "three bacteria and three bodies", namely bacteria (narrow sense), actinomycetes, cyanobacteria, mycoplasma, rickettsial bodies, and chlamydia.

bacterial

Prokaryotic cell biology (6 photos)

It is the organism with the widest distribution and the largest number of individuals in the natural world, and is the main participant in the material cycle of nature. Bacteria are mainly composed of cell wall, cell membrane, cytoplasm, nucleoplasm, etc. Some bacteria also have special structures such as capsule, flagella, and pili. Most bacteria are between 0.5-5 microns in diameter. They can be divided into three categories according to their shape: cocci, bacilli, and spirilli.

Actinomyces micron

Actinomycetes are a class of prokaryotic microorganisms that have hyphae, reproduce with spores, and are positive for Gram staining. Because of its branched hyphae and colony morphology similar to molds, actinomycetes were previously thought to be "microbes between bacteria and fungi". However, the results of research conducted using modern biological technology show that actinomycetes are actually prokaryotic microorganisms belonging to the category of bacteria, but their cell morphology is branched hyphae. From a phylogenetic perspective, actinomycetes (except high-temperature actinomycetes) belong to the high G + C / mol% group in this large branch together with all G + bacteria.

Cyanobacteria

Cyanobacteria, also known as cyanobacterium, can perform photosynthesis similar to higher plants

Cyanobacteria

(Using water as an electron donor and releasing oxygen), it is different from the photosynthesis mechanism of photosynthetic bacteria, so it is considered the simplest plant. Cyanobacteria do not have chloroplasts, but they do contain phycocyanin (blue, but low in content) and chlorophyll (green and high in content). They are autotrophic organisms capable of photosynthesis. Most species of bacteria are living organisms that saprophytic or parasitic. Neither cyanobacteria nor bacteria have formed nuclei. Cyanobacteria cell genetic information carrier is a circular DNA molecule like other prokaryotic cells, but the amount of genetic information is large and can be compared with higher plants. Because the volume of cyanobacteria cells is much larger than other prokaryotic cells, the diameter is generally 10 microns, or even 70 microns (Vibrio), so the cyanobacteria cells cannot be seen with the naked eye. Cyanobacteria are single-celled organisms, and some cyanobacteria often exist as filamentous cell populations. For example, the nodoc commune var.flagtlliforme belongs to the cyanobacteria filamentous body; cyanobacteria grow in grasslands and deserts in the northwest. Because of its homophonic sound with "fortune", it has been scrambled to eat, and has been listed as a protected organism in China; green mandarin is actually a symbiotic body of nitrogen-fixing cyanobacteria and aquatic fern full river red. Cyanococcus, Candida, Oscillator, and Cabbage (cell population is black-blue) are all cyanobacteria, so not all cyanobacteria are green.

Mycoplasma

Mycoplasma is usually 0.2-0.3 microns in size and is currently considered the smallest cell that can pass through a bacteria filter. No cell wall,

Mycoplasma

Can not maintain a fixed shape and appear polymorphic. There is more cholesterol in the cell membrane, accounting for about 36%. This is necessary to maintain the integrity of the cell membrane. Any substance that can act on cholesterol (such as amphotericin B, saponin, etc.) can cause the destruction of the mycoplasma membrane. Make mycoplasma dead.

Mycoplasma genome is a circular double-stranded DNA with a small molecular weight (only one-fifth that of E. coli) and limited synthesis and metabolism. One end of Mycoplasma pneumoniae has a special terminal structure, which can make Mycoplasma adhere to the surface of respiratory mucosal epithelial cells and is related to pathogenicity.

Chlamydia and Rickettsia

Chlamydia is small, 200-500 nanometers in diameter, and can pass through bacterial filters. Rickettsia is slightly larger and most of it cannot pass through the filter membrane. They all have DNA and RNA, peptidoglycan-containing cell walls that are characteristic of Gram-negative bacteria, but the enzyme system is incomplete and must live in the host cell, known as the energe parasite.

Trachoma is caused by chlamydia. Because it can form inclusion bodies, it was initially considered to be a large virus. In 1956, the famous Chinese microbiologist Tang Feifan and his assistant Zhang Xiaolou and others isolated the pathogen of trachoma for the first time.

Chlamydia has a special life history. Individuals with infectivity are called elementary bodies. They are small and have tough cell walls. In the host cell, the protozoa gradually elongates to form a non-infectious individual, called the initial body. It is a thin-walled spheroidal cell with a large volume. A microcolony is formed, and subsequently a large number of daughter cells are differentiated into protozoa with the ability to infect.

Rickettsia

Rickettsia is also obligately parasitic, mainly in arthropods, and some can be introduced into the human body through fleas, lice, ticks, and mites, such as typhus and trench fever.

American doctor HTRichetts first discovered it as the pathogen of Rocky Mountain typhus in 1909 and sacrificed the disease in 1910, henceforth it was called Rickettsia. It differs from chlamydia in that its cells are large, non-filterable, have strong synthetic ability, and do not form inclusion bodies.