The cells

The cell is the fundamental structure of all living things. It is made up of the cytoplasm, delimited by a membrane called the plasma membrane and containing at least one DNA molecule (where the genetic information is found) and the enzymes necessary for its growth and multiplication. There are two types of cells, prokaryotic and eukaryotic .

The discovery of the cell was made by  Robert Hooke in 1665 by looking at a cork wall under a microscope. Looking at the wall, he observed cells or cells that corresponded to dead cell walls.

After the discovery of the cell, research in biology was radically changed and enhanced, formulating the cell theory as a fundamental part of biology that explains the constitution of living beings.

Cell theory

The modern concept of cell theory can be summarized in the following principles:

  1. The cell is the structural unit of living matter and all living things, so all of these are made up of cells. The cell is the physiological unit of life.
  2. The development of the vital functions of living beings occur within the cell.
  3. The cells come from pre-existing cells.

The prokaryotic cell

The plasma membrane of prokaryotic cells is made up of … lipids, complex proteins, enzymes …

The most important function of the plasma membrane, as well as in eukaryotic cells, is to control the composition of intracellular fluids through the transport of ions and molecules from outside the cell and vice versa.

The genetic material is formed by a single DNA molecule that is not delimited by any structure as it happens in the eukaryotic cell with the nucleus. Inside prokaryotic cells, DNA is packed into a structure called a nucleoid. The nucleoid DNA is connected to the plasma membrane through the mesosomes.

Prokaryotic organisms, having developed in anaerobic environments, have very complex metabolic processes, such as glycolysis (or glycolysis) . This consists of the degradation of glucose under anaerobic conditions, and it is present in all cells where ATP synthesis occurs, an important source of chemical energy.

The eukaryotic cell

The eucaroite cell is much more complex than the prokaryotic cell since, in addition to structurally differentiating, there are also many more cellular organelles than in the prokaryotic cell.

Cell membrane

The eukaryotic cell is made up of a plasma membrane that surrounds the cell and is made up of phospholipids and is organized in two layers. Each phospholipid molecule consists of two hydrophobic tails and a hydrophilic head, thus being an amphipathic molecule . This membrane, also called the cell membrane, defines the limits of the cell and ensures the retention of its contents. The phospholipids are oriented in the double layer so that the hydrophobic tails of each molecule face inward. The hydrophilic head, looks outwards. The lipid double layer is the basic structural unit of all membranes and serves as a permeability barrier for most water-soluble substances.

The membrane proteins have hydrophobic and hydrophilic portions . The hydrophobic part is located towards the inside, while the hydrophilic regions are located towards the outside of the membrane, which is why they are called glycoproteins. We can distinguish integral and peripheral proteins. The former are membrane-spanning proteins. As I have explained before, there are two polar R groups, an area of ​​the protein that is outside the bilayer (hydrophilic bond), while the non-polar R groups, an area of ​​the protein that is in contact with the bilayer (hydrophobic bond).

The peripheral proteins are those that do not pass through the membrane. These proteins are attached to the polar heads of phospholipids (ionic / hydrophilic bond). The main characteristics are for example fluency. The main functions are various such as:

  • promote selective permeability (transport), such as channel, carrier, pump proteins
  • give specificity to each cell,
  • recognition and adherence,
  • reception and transmission of signals,
  • enzyme function
  • and finally a structural bridge between the cytoskeleton and the extracellular matrix.

Cellular transport

The lipid double layer is impermeable to polar ions and molecules, such as O2, CO2, N2, and fatty acids. In order to transport these molecules to the inside or outside of the cell membrane, there are two types of transport:

  1. Passive transport
  2. Active transport.

The first ( passive transport ) is based on the difference in concentration of substances inside and outside the cell. If the concentration is higher inside the cell, the molecules will move outwards; vice versa.

In the second ( active transport ), the molecules move according to a concentration gradient, that is, from a zone of less concentration to a greater concentration. The plasma membrane is a semi-permeable membrane, not all molecules can cross it, and this can condition the direction of the flow of water molecules. This phenomenon is better known as osmosis.

The energy required for active transport is higher and the best known pump is that of Na + and K +. The K + ions are more concentrated inside the cell and that is why they try to get out, while the Na + ions are more concentrated in the extracellular liquids, they try to enter the cells, that is why all animals pump against concentration gradients, the Na + towards the outside and K + towards the inside, that is, they take Na + out of the interior of the cell and introduce K + from the outside, which goes against the osmotic balance. This is very important in order to keep the Na + and K + conditions stationary, but not in equilibrium.

Cytoplasm of eukaryotic cells

The cytoplasm contains the cellular organelles , made up of a substance called cytosol . This is formed by protein filaments that make up the skeleton or cytoskeleton. The cytoskeletal fibers extend from the plasma membrane to the nucleus. Here we can distinguish three types of filaments: microtubules, microfilaments, intermediate filaments.

Nucleus of eukaryotic cells

The nucleus is in the center of the cell and is the center of genetic information. Here is the DNA of the chromosomes. The nucleus is bounded by two nuclear membranes, internal and external. The whole is called a nuclear envelope. The nuclear envelope has many small-sized openings, called pores, through which water-soluble substances are exchanged.  Inside the nucleus is the nucleolus , site of protein synthesis, responsible for the subunits of ribosomes.

Mitochondria and chloroplasts

The study of eukaryotic organelles begins with the mitochondria . It is made up of two membranes, one internal and one external. Here most of the chemical reactions involved in the oxidation of sugar take place.

The next organelle that we find is the chloroplast . It has three membranes, which are shaped like a flattened saccule, called a stromal thylakoid . Chloroplasts are known for photosynthesis. Other plant organelles are plastids and they have different functions in plant cells. The cromoplastos have pigments responsible for the color of flowers while Amyloplasts have the function of storing starch.

Endoplasmic reticulum

Extending through the cytoplasm it reaches the  endoplasmic reticulum (ER) . It consists of a system of tubular membranes and flattened bags. The internal space of the ER is called the lumen . The ER can be rough or smooth . It is called rough, because it is dotted with ribosomes. These ribosomes synthesize proteins. These proteins insert and are transported across the reticulum membrane. Later they are directed towards the cell surface in a complex process, which not only involves the rough endoplasmic reticulum but also the Golgi apparatus and secretory vesicles. The smooth endoplasmic reticulum does not intervene in protein synthesis and is very abundant in cells that produce steroids and lipids.

Golgi apparatus

The Golgi apparatus is made up of flattened vesicles. It plays an important role in the processing and packaging of secretory proteins and in the synthesis of complex polysaccharides. These proteins are transported through secretory vesicles, which we will see later. Here the proteins are modified until they have their final appearance and it is when they are sent to intracellular as well as extracellular behaviors. The secretory vesicles  are responsible for the transport of these proteins. Once processed in the Golgi apparatus, secretory proteins are packaged into secretory vesicles. For example, the cells of the pancreas contain many of these vesicles, since this gland is responsible for the synthesis of many digestive enzymes. The enzymes are synthesized in the rough endoplasmic reticulum, packaged in the Golgi apparatus, and then released from the cell as secretory vesicles.

Lysosomes and peroxisomes

Lysosomes are organelles used by the cell as the storage site for hydrolases, enzymes capable of digesting biological molecules, such as proteins, carbohydrates or lipids.

Other very common cytoplasmic organelles are peroxisomesThey are vesicle-shaped and contain oxidases and catalases. Peroxisomes are made up of a membrane, which contains a lipid double layer made up of various proteins. Inside there is a peroxisomal matrix, which contains proteins of enzymatic function. These enzymes catalyze many metabolic synthesis and degradation reactions of compounds. Peroxisomes are found in all tissues, but predominate in the liver, kidney, and brain during the period of myelin formation (the material that covers nerve fibers and forms the white matter of the brain). They have an essential role in lipid metabolism, especially in the shortening of very long chain fatty acids, for their complete oxidation in the mitochondria, and in the oxidation of the cholesterol side chain, necessary for the synthesis of bile acids; it is also involved in the synthesis of lipids such as glycerol (phospholipids and triglycerides) and isoprenoids.


Vacuoles are cytoplasmic organelles surrounded by a membrane and with a high water content, in which various substances accumulate. Depending on their function, vacuoles can be vegetable, contractile and digestive. The latter, related to endocytosis processes. Plant cells have a large vacuole, the membrane of which is called the tonoplast, with a liquid content of variable nature.

The functions of the tonoplast can be varied:

  1. It contributes to the maintenance of cell turgor and increases the cell surface, and therefore, the exchange capacity with the outside.
  2. It serves as a reserve store for various ions, carbohydrates, amino acids, proteins, pigments and other plant substances, as well as for toxic and waste products.
  3. Contains lysosomal enzymes

The contractile vacuole is a membranous organelle with aqueous content, present in a large number of protists, whose function is the expulsion of water that enters the interior of the cell by osmosis.


Last but not least are ribosomes . They are intracytoplasmic cellular organelles composed of RNA and proteins, which participate in protein synthesis. These organelles are made up of two subunits: a large subunit, with 2-3 RNA molecules and proteins, and a small subunit, with only one type of RNA associated with proteins. Both units form a groove, to which the protein that is being synthesized associates, and a second groove, in which the messenger RNA is housed. Although their structure and function are very similar, the ribosomes of prokaryotic and eukaryotic cells differ by their sedimentation coefficient, as indicated in the following diagram, fig. 2.

* 70S ribosomes (prokaryotes) * 80S ribosomes (eukaryotes)
Large 50S subunit : 5 S RNA + 23 S RNA + 34 proteins Large 60S subunit : 28S RNA + 5.8S RNA + 5S RNA + 49 proteins
30S small subunit: 16 S RNA + 21 proteins 40S small subunit: 18 S RNA + 33 proteins

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