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6.2.3. Eukaryotic cells

Eukaryotic cells

The eukaryotic cells are much more complex than prokaryotes, both structurally and functionally. Like prokaryotes, they also have a plasma membrane  and ribosomes, they differ from them in that they have a nucleus that separates the DNA from the cytoplasmcytoplasmic organelles,  and the cytoskeleton.

The plasma membrane is very similar in all eukaryotic cells, distinguished by membrane receptors, the proteins located on the outside.

Three types of structures are observed inside the cell: the endomembrane system, the energy-transducing organelles, and the non-membrane structures.

The endomembrane system is made up of membranous organelles that occupy almost the entire cytoplasm, each with its own function. It is constituted by the endoplasmic reticulum, continuing the nuclear membrane, the Golgi apparatus, related to the membranes of the endoplasmic reticulum, the vacuoles and the lysosomes.

The energy transducing organelles are mitochondria and chloroplastsThey have a double membrane. The mitochondria are responsible for obtaining energy from the oxidation of organic matter, and chloroplasts , from light energy.

The lacking membrane structures found in the cytoplasm are the ribosomes, the centrioles, and microtubules  and microfilaments  which form the so - called endoskeleton cell or cytoskeleton.

The nucleus of eukaryotic cells is surrounded by the nuclear envelope, a membranous double layer with abundant pores, which separates the nucleoplasm  from the cytoplasm. Inside, in the nucleoplasm, is the DNA associated with histones, and some condensation of material called the nucleolus.

Another characteristic of eukaryotic cells is the cytoskeleton, protein filaments that extend from the nuclear to the plasma membrane , which maintain the shape of the cell, facilitate cell mobility (using structures such as cilia and flagella), and play a role in important role both in intracellular trafficking (for example, vesicle and organelle movements) and in cell division.

Células procariotas

Células eucariotas

Its size is small, between 1 and 5 microns. They are only visible under a microscope.

They are larger than prokaryotes. Many measure between 20 and 50 μ, the yolk of a chicken egg 2 cm, some neurons more than 1 meter, etc.

Its cellular organization is very simple, exclusive of bacteria.

Its organization is complex, present in the rest of living beings.

They have few shapes: spherical (cocci), rod-shaped (bacilli), orthographic coma (vibrios), or spiral (spirilla).

They have very varied shapes.

They are always unicellular, although they can form colonies.

They can constitute unicellular or multicellular organisms. In these there are very specialized cells and, therefore, with very different shapes.

It has a very thick cell wall, which is usually surrounded by a mucous capsule that favors the daughter cells to remain united, forming colonies.

Plants, most algae and fungi have a cell wall, chemically simpler than that of prokaryotes.

In animal cells it is absent, although they may present an extracellular matrix, a secretion membrane.

Its plasma membrane is devoid of cholesterol.

Its plasma membrane has cholesterol.

It lacks the cytoskeleton and most of the cellular organelles: it only has ribosomes and storage structures.

The membranous organelles are the mesosomes. Cyanobacteria also have thylakoids.

They have a cytoskeleton and a great variety of membranous organelles (mitochondria, chloroplasts, endoplasmic reticulum, lysosomes, vesicles, Golgi apparatus, ...)

Animal cells also have centrioles.

It has small ribosomes (70S).

It has large ribosomes (80S).

They have no defined nucleus. They do not have a nuclear envelope.

DNA is condensed in a region of the cytoplasm called the nucleoid. Nucleoli are not distinguished.

They have a defined nucleus surrounded by a nuclear envelope.

Inside the nucleus they have one or more nucleoli.

Its genetic material is composed of multiple linear DNA molecules, which are condensed in chromosomes or decondensed in chromatin depending on the moment of the cell cycle.

DNA is organized into a single circular double-helix molecule that, although it may be associated with proteins, does not form nucleosomes.

It only has a single chromosome.

In addition, they present plasmids, small circular double-stranded DNAs.

In its chromosomes, DNA is linear and double helix and is associated with histones forming nucleosomes.

Also, there is circular DNA in chloroplasts and mitochondria.

 

The mRNA does not require maturation.

The preRNA needs maturation to be mRNA.

Transcription and translation take place in the cytoplasm.

Transcription takes place in the nucleus and translation takes place in the cytoplasm.

There is no meiosis. Its cell division is direct, by bipartition.

Reproduction is asexual, although there may be phenomena of parasexuality (exchange of genetic material).

Its cell division is by mitosis or by meiosis.

 

Catabolism can be by fermentation, by aerobic respiration, or by anaerobic respiration. It is carried out in the mesosomes.

Catabolism is always by aerobic respiration, and it is carried out in the mitochondria.

Occasionally, there may be fermentation.

Photosynthesis occurs in some bacteria, and takes place in mesosomes and thylakoids.

Photosynthesis only occurs in some plant cells, and takes place in chloroplasts.

They present submicroscopic flagella.

They can present cilia and flagella of complex structure surrounded by the plasma membrane.