The structure of cell: a research on the bricks of the human body!

We are made up of 100,000 billion cells. Understanding the basic composition and functioning of the cell can help us understand what happens at more complex levels of organs. The first phase is called chemical evolution, from very simple molecules such as hydrogen, ammonia, methane, monoxide, and carbon dioxide, by the action of ultraviolet radiation (sun), heat, and electric discharges (lightning). They are very reactive intermediate molecules formed such as acetaldehyde, hydrogen cyanide, formaldehyde, and perhaps even some simple fatty acids such as acetic acid and some simple amino acids such as aniline.

A second phase followed, called biological evolution, in which more complex molecules of acids, amino acids, proteins, and especially nucleic acids were formed.

- The first prokaryotic cell (plant cell) appeared about 3.5-3.0 billion years ago.

- The first eukaryotic cell (animal cell) appeared much later, around 0.9 billion years ago. 

- Homo sapiens made his appearance 400,000 years ago.

Hypothesis on the origin of the cell

According to Jacques Monod and other authors, the gradually increasing order structures formed in the cell are determined by the genetic information contained in the DNA from the primary sequence of a polypeptide chain. There are gradually the secondary and tertiary structures of the proteins, then the formation of compounds oligomeric, and finally, by the interaction between proteins, lipids, and nucleic acids, the formation of the first simple plant cells.

Probably, only with the appearance of the genetic code was it possible to determine the formation of the first heterotrophic prokaryotic cell (which feeds on organic molecules) and anaerobic (which lives in the absence of oxygen). Subsequently, autotrophic prokaryotes appeared (using the food synthesized inside), obviously provided with photosynthetic pigments.

With the appearance and accumulation of oxygen, the first autotrophic and aerobic prokaryotic cells are born. In turn, this type of cell opens the way for the formation of the first eukaryotic cell, which appears after 2.6-3.1 billion years after that of the prokaryotic cell.

The structure of a eukaryotic cell

The cells have uniform morphological characters only in the simplest organisms; in others, the cells differ in shape, size, and tasks.

There are two types of cells

1. Eukaryotic cell (animals and humans), which contains a cell nucleus delimited by a nuclear membrane and separate organelles;

2. Prokaryotic (plant) cells: These cells are instead free of the nuclear membrane and intracellular organelles, with the exception of ribosomes.

In cells, we can distinguish:

Cell membrane

The typical structure of the cell membrane consists of a double phospholipidic layer between two protein layers located at the level of the separation surfaces between the internal and external phases of the cell. Water enters and exits easily, while for many other substances, this passage is not so easy.

This ability of the membrane to allow or not the passage of substances is called “selective capacity.”

Core

It constitutes the most important part of the cell by controlling all its activities. For example, the nucleus brings hereditary information to the cell for the construction of a "unique" type of organism. It directs the cell's activities, ensuring that the complex molecules that the cell requires for the realization of the various cellular activities and for the formation of organelles and other structures. They are of the necessary number and type; decides when the right time for cellular reproduction is and controls all phases.

The nucleus is wrapped in the nuclear membrane that separates it from the cytoplasm. The nuclear membrane is sprinkled with numerous small holes called nuclear pores, due to which various substances pass from the cytoplasm into the nucleus.

The nucleus contains DNA combined with proteins (chromatin). Chromosomal DNA performs two types of activity, i.e., auto synthetic and all-synthetic; in the first case, the DNA molecule is replicated through a semi-conservative process; in the second case, it synthesizes the three types of RNA. When the cell divides, chromosomes form from the chromatin. The cell genome is inherent in the chromosomes. Inside the nucleus, there is a very dense body, the nucleolus, also formed, like chromosomes, by DNA and proteins; a particular type of RNA, ribosomal RNA, is also formed in the nucleolus.

Cytoplasm

It is the viscous substance between the cell membrane and the nuclear envelope. The majority of cellular activities take place inside the cytoplasm, and the energy necessary for the life of the cell is also produced. In it, there are organelles used for the various functions that the cell must perform. They are:

Mitochondria

Microscopic power plants, where the formation of ATP (energy necessary for cells) takes place through biochemical reactions

RIBOSOMES

They are of fundamental importance because protein synthesis occurs in them. Ribosomes are the most numerous cell organelles; they consist of two subunits, a major and a minor, which dissociate at the end of each protein synthesis cycle. The RNA sequence is translated into the corresponding sequence of amino acids assembled to form the protein. The ribosome is, therefore, an apparatus for synthesizing proteins, capable of bringing together in the appropriate arrangement the molecules necessary for the synthesis reaction.

LYSOSOMES

Intracellular vesicular organelles delimited by a single membrane, containing various enzymes, and localizable around the nucleus It can be considered the digestive system of the eukaryotic cell, for the action, carried out by the various enzymes with degradation. For example, of glycoproteins and glycolipids by means of lysosomal hydrolases, and degradation of senescent components of the same cell, as well as recycling and transport of degraded molecules to the sectors where they are necessary for the synthesis of new products, with consequent energy savings.

ENDOPLASMATIC RETICLE

Dynamic structure, which increases or decreases according to cellular activity. In cells, we have the rough endoplasmic reticulum and the smooth endoplasmic reticulum. The first one is ribosomes, which are attached and involved in the synthesis and the transport of proteins out of the cell. The second, which physically is a portion of the same rough endoplasmic reticulum, but is free of ribosomes, is important in the synthesis of lipids.

DNA and the genetic code

The following concepts serve to deepen how the DNA of a cell, for example, establishes which proteins are needed by our body and when they are needed.

DNA is present in all cells capable of reproducing. Even if it does not take part directly in protein synthesis, DNA contains the code to build all the proteins that are needed starting from the 20 amino acids, most of which are also synthesized by cells, except for some amino acids which must necessarily be introduced with the diet, and which therefore are called essential Deoxyribonucleic or deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic information necessary for the biosynthesis of RNA and proteins. 

DNA structure

DNA is an organic polymer made up of monomers called nucleotides.

All nucleotides are made up of three basic components:

1. a phosphate group;

2. deoxyribose (pentose sugar);

3. A nitrogenous base that binds to deoxyribose with an N-glycosidic bond.

Four nitrogen bases can be used in the formation of nucleotides and incorporated in the DNA molecule, i.e., adenine (A), guanine (G), cytosine (C), and thymine (T).

The genetic code

To preserve and exchange genetic information, in over 3 billion years, nature has built the genetic code. The DNA contains adenine, thymine, guanine, and cytosine. Three base pairs form a codon, which identifies either a specific amino acid to be used for protein synthesis, or a signal to stop the synthesis itself. Almost all living things use the same genetic code, called the standard genetic code.

Author: Vicki Lezama