The structure of cells

structure of cells

Cells are essential elements of our organism. They constitute all the organs and tissues of it and ensure its proper functioning. Each cell, which varies in size, has the same structure, and communicates with each other. Focus on this fundamental microscopic unit.

Humans have around one million billion cells in their bodies.

The cell is a fundamental, structural, and functional unit of living organisms. It can fulfill all the functions of the organism, namely metabolism, movement, growth, reproduction, or even the transmission of genes. It is a living entity that functions autonomously while remaining coordinated with others. There are two types:

- Eukaryotic cells: They have a nucleus containing genetic material (example: humans, yeast).

- Prokaryotic cells: They have no nucleus, so their genetic material is free in the cell (example: bacteria).

Structure of the cell

1.1. The cell or plasma membrane

The cell is surrounded by a plasma membrane delimiting an interior space containing the cytoplasm and various organelles of exterior space.

1.1.1. Plasma membrane structure

The plasma or cell membrane is composed of phospholipids, which give it its basic structure, to which are added cholesterol molecules, which increase the impermeability and stiffness of the membrane. Phospholipids are amphiphilic, i.e., they have a hydrophilic pole (which likes water, the head) and a hydrophobic pole (which does not like water, the tail), thus forming a phospholipid bilayer.

The cell membrane is also made up of proteins that perform most of its specific functions. There are different forms of protein associations to the membrane: intrinsic proteins (part or all of the protein is integrated into the membrane) and extrinsic proteins (the protein is outside the membrane).

This membrane thus has several functions:

- protection of the cell from the outside environment;

- the individuality of the cell (the membrane surrounding the cell forms closed compartments separating it from other cells);

- Exchange of molecules between the extracellular and intracellular medium (the membrane thus controls the entry of nutrients and the rejection of waste.

- Recognition of certain products by means of receptors present in the membrane, and to which it will react

- Identification of the cell by the presence of specific antigens such as proteins of the HLA system, blood groups, and rhesus.

1.1.2. Membrane transport

A cell is a living unit. It feeds by drawing from the extracellular environment the nutrients necessary to maintain its activity and growth. It rejects unnecessary or toxic degradation products in this same medium. Due to the hydrophobic nature of its lipid layer, the plasma membrane of the cell does not allow the passage of polar molecules (electrolytes) and macromolecules. This is semi-permeable because the cell must regulate its ionic concentrations. All of the exchanges will thus take place across the plasma membrane or using transmembrane proteins. There are two modes of molecular transport: passive transport and active transport.

Passive transport

The molecules can cross the double lipid layer by a spontaneous movement towards equilibrium, without a supply of energy, in the direction of the concentration gradient.

- By diffusion:

Diffusion is the movement of molecules from an area where they are in high concentration to an area where they are in low concentration

  • Simple diffusion: When a small molecule (often fat-soluble) crosses the phospholipid bilayer (examples: oxygen, carbon dioxide, urea, fats) directly.
  • Facilitated diffusion: When the molecule must use a transmembrane protein to be transported (often water-soluble).

- By osmosis:

It is the process of the diffusion of water in various mediums. When there is a difference in concentration between the intracellular and extracellular medium, the water molecules move to dilute the medium most concentrated in solute. Until there is eventually a balance of concentrations i.e., osmotic pressure;

  • · If the extracellular and intracellular medium is of the same concentration, the solution is said to be isotonic.
  • · If the extracellular fluid is less concentrated than the cell, the medium is said to be hypotonic. The cell then becomes soaked in water (turgor phenomenon) and swells.
  • · If the extracellular fluid is more concentrated than the cell, the medium is said to be hypertonic. Water diffuses out of the cell (the phenomenon of plasmolysis) and shrinks.

These three cases are very important, especially when applying solutions to patients.

Active transportation

Active transport requires energy (ATP) to make the transport structure capable of operating against a concentration gradient. It is mainly used for the transport of macromolecules. This transport requires a protein transporter, often called a pump, located at the level of the plasma membrane and which fulfills the role of a ferryman.

Example of the Na + / K + / ATPase pump: the sodium concentration (Na +) is higher inside the cell, while the potassium concentration (K +) is higher in the extracellular medium. Due to the production of ATP, the antiport pump will be able to expel Na + ions outside at the same time to integrate K + ions inside the cell. Thus, at the cellular level, we speak of endocytosis when the cell captures elements, by invagination of its plasma membrane, to form vacuoles in its cytoplasm. Conversely, we speak of exocytosis when the cell pours substances trapped in vacuoles in the extracellular medium. The vacuole membrane then fuses with the plasma membrane.

Transport proteins

Certain transports require the intervention of proteins present in the cell membrane. These proteins pierce through the membrane and function as doors. There are three different types:

- Unipart: transport of a single substance in one direction;

- Symport: transport of 2 different molecules in the same direction;

- Antiport: transport of 2 different molecules in opposite directions.

1.2. The cytoplasm

The cytoplasm is the content of a living cell. More precisely, it is all of the cellular material delimited by the plasma membrane.

1.2.1. The cytosol

The intracellular medium is made up of a liquid called cytosol. It consists of an average of 85% water, and its pH is neutral. It is in this environment that all of the cell's organelles are bathed and that the main cellular activities take place.

1.2.2. The cytoskeleton

This fibrous network of protein nature constitutes, at the same time, a skeleton and musculature for the cells. It is used to maintain their shape and is involved in internal movements, movements, as well as during cell division.

The cytoskeleton contains centrosomes, tubular elements involved in cell division. Each centrosome is composed of two centrioles; these are intra-cytoplasmic cellular structures made up of 9 triplets of 3 tubules. Each cell, therefore, contains two centrioles (perpendicular and not touching), and the whole forms the centrosome, which is always close to the nucleus. Their function is to direct, like magnets, the direction of cell division.

1.2.3. Organelles

The endoplasmic reticulum

The endoplasmic reticulums (ERs) are organelles with a double intracellular membrane and resemble a mass of folds forming cavities, called "cisterns." They are in continuity with the membrane of the nucleus. The granular endoplasmic reticulum (REG) or rough endoplasmic reticulum (RER) has its surface covered with ribosomes that assemble amino acids into proteins according to information coming from the nucleus. The smooth endoplasmic reticulum (REL) does not. It is involved in the synthesis of lipids (phospholipids, fatty acids, etc.), the detoxification of cells, and the storage of calcium.

Ribosomes

These spheres, free or associated with REG, participate in protein synthesis from RNA. Their function is to synthesize protein molecules from amino acids. They use the orders given by the nucleus.

The Golgi apparatus

It consists of bags flattened on top of each other. Its role is to store proteins from REG, to complete their maturation, to check their quality, and to secrete them. The proteins to be secreted are then concentrated in vesicles from the ends of the Golgi apparatus. These vesicles are discharged into the extracellular medium by exocytosis.

Lysosomes

These are vesicles containing hydrolytic enzymes that come from the ER or the Golgi apparatus. These enzymes are used to digest unusable macromolecules such as destroyed or damaged organelles, toxic substances; it is cellular digestion.

The mitochondria

Mitochondria are very small bean-shaped organelles. They look like small rods and have a double intracellular membrane. The inner membrane forms fold called ridges, which overlap in a substance called a matrix. There are many enzymes responsible for the breakdown of nutrients in a simple form, such as glucose. These degradations take place in the presence of oxygen and are called cellular respiration. They allow the mitochondria to form ATP, the cell's energy source. ATP is used for all of the cell's synthesis activities as well as for active transport.

The number of mitochondria in a cell depends on the intensity of its activity: a muscle cell, for example, has many. These organelles have their own genetic material, mitochondrial DNA, inherited by the mother. They can synthesize around 10% of their own proteins thanks to the dozen genes in their DNA, the other mitochondrial proteins coming from the synthesis work carried out by ribosomes.

The vacuoles

These are spherical and mobile cavities that contain substances stored by the cells or waste to be eliminated.

The core

It is visible in the cell when it does not divide (interphase). It is limited by the nuclear envelope and contains:

- The chromatin that consists of uncondensed DNA, the cell of the genetic support; 

- The nucleolus, made up of RNA, which, associated with proteinsstructure of cells, will synthesize ribosomes.

The nucleus has a diameter varying from 10 to 20 µm (the largest of the organelles) and is surrounded by a double membrane: the nuclear membrane. This nuclear membrane contains pores allowing nucleocytoplasmic exchanges in both directions. The nucleoplasm is the liquid in which the elements contained in the nucleus bathe. It has two main functions: to control the chemical reactions of the cytoplasm and to store the information necessary for cell division.

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Jul 01, 2020

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