Cell: Definition, Functions and Structure of These Fundamental Units for Life

It is the fundamental unit of all organisms, be they animals or plants.

Whatever its behavior and shape, the cell has all the tools to survive in a constantly changing environment. The human body contains trillions of cells. There are more than 200 types that differ in form, structure, and function.

The cell is composed of 4 basic elements: carbon, nitrogen, hydrogen, oxygen and also less abundant elements, trace elements that are of essential importance for the fulfillment of certain functions.

Elements such as calcium, sodium, and potassium in their ionic form can carry an electrical charge and are called electrolytes .

Cell functions

Human cells are the most fundamental structural and functional units of our body, and their functions vary according to their type.

A basic function of all human cells is to work together to form tissues, which in turn work together to form organs and organ systems.

Each type of cell in the body has a specific function.

There are many types of cells, including blood cells, skin cells, bone cells, reproductive cells, hair cells, and stem cells.

Some cells, especially glandular cells, have a specific function of producing chemical compounds, hormones, or enzymes.

Insulin makes pancreatic cells and cells in the lungs make mucus.

Other cells, such as nerve cells, only transfer electrical impulses, and blood cells do not transfer nutrients to our blood.

Cells like nerve and muscle cells have reactions in their mitochondria responsible for breaking down glucose in the body and converting it into usable energy, also known as glycosis.

Cell structure

Cell types differ somewhat from each other, but all cells share several structural and functional characteristics.

Cells most often comprise three main regions, the nucleus generally being in the center of the cell surrounded by a semi-liquid cytoplasm, all enclosed in the plasma membrane.

The nucleus

The nucleus is the regulatory center of the cell.

Its nucleus can be spherical or oval in shape and is located near the center of the cell.

The nucleus contains the genes that are the structure with the hereditary information.

That is, it contains the guidelines for the development of proteins and is essential for cell reproduction.

It is made up of three different structures such as: the nuclear envelope, the nucleoli and the chromatin.

  1. The nuclear envelope : it is the envelope of the nucleus. It consists of two membranes identical to the plasma membrane. It has the nuclear pores that allow the passage of molecules in a controlled way between the interior and the exterior of the nucleus, which allows a selective permeability.
  2. Nucleoli: are made of deoxyribonucleic acid and ribonucleic acid proteins. This is the meeting place for ribosomes.
  3. Chromatin: it is a fine network of filaments that consists of proteins and deoxyribonucleic acid. At the time of cell division, these filaments condense to form chromosomes.

The plasma membrane

It is the outer limit of the cell. It separates the intracellular medium from the extracellular medium. Its particular structure ensures a dynamic function in many cellular activities.

The plasma membrane is a barrier and a filter. The plasma membrane is composed of two layers of lipids whose molecules are contiguous from tail to tail and in which protein molecules float.

The lipid part is mainly made up of phospholipids with a large amount of cholesterol and sometimes protein.

The polar heads of phospholipid molecules are hydrophilic and are attracted to water, the main constituent of intracellular and interstitial fluids.

Nonpolar “tails” are hydrophobic, therefore they turn inward into the membrane and form a relatively impermeable barrier to most water-soluble molecules.

Phospholipids have no link to each other and provide some membrane flexibility.

Cholesterol has a stabilizing effect and helps maintain the fluidity of the membrane.

Membrane proteins, some of which are enzymes, perform more specialized membrane functions.

Many are receptors for chemical messages, such as hormones, or have binding sites that provide structural stability to the cell.

Proteins can serve as transporters through the cell or form pores allowing the passage of certain substances.


The cytoplasm is the cell’s contents between the nucleus and the plasma membrane, where most of the cell’s activities take place.

Suspended organelles and inclusions are found in this fluid.

Cytoplasmic organelles are specialized intracellular compartments, each of which provides a specific function to keep the cell alive.

  • Mitochondria: Mitochondria are made up of a permeable outer membrane and an inner membrane that forms many folds. These are the power plants of the cell. They break down glucose molecules or other nutrients and convert the released energy into heat and adenosine triphosphate. This chemical reaction is multi-stage and part of aerobic cellular respiration in the presence of oxygen.
  • Ribosomes: These are small structures, made of ribonucleic acid proteins. They are the site of protein synthesis in the cell from amino acids using ribonucleic acid as a template. The association of ribosomes with the membrane forms the rough endoplasmic reticulum.
  • The endoplasmic reticulum: it is a type of circulatory system because its channels transport substances, mainly proteins, between the various parts of the cell.
  • The granular endoplasmic reticulum: It is so named because it is packed with ribosomes. It is the seat of proteins destined to be transformed and transported outside the cell or isolated in vesicles (lysosomes).
  • The smooth endoplasmic reticulum: intervenes in the metabolism of lipids (synthesis and degradation of fats and cholesterol and their derivatives).
  • The Golgi apparatus: it is made up of membranous sacs stacked on top of each other and associated with vesicles. Its function is to provide proteins and lipids recently synthesized by the endoplasmic reticulum. Directs macromolecules to their appropriate intracellular destination.
  • Lysosomes: These are large vesicles formed by the rough endoplasmic reticulum and packed by the Golgi apparatus. They contain hydrolytic and proteolytic digestive enzymes. It is the “digestive tract” of the cell, they use their enzymes to recycle organelles and digest extracellular debris for tissue repair.
  • The cytoskeleton: it is a network of protein filaments. Provides shape and support to the cell.
  • Centrioles: These consist of fine microtubules. During cell division, the centrioles are responsible for the mitotic spindle that separates the chromosomes evenly between the two daughter cells.
  • Peroxisomes: These are vesicles surrounded by a bilayer lipid membrane. They have a detoxifying role by degrading molecules. They contain enzymes that will use oxygen to neutralize certain toxic substances such as alcohol, free radicals, among others.
  • The eyelashes and flagella: they are cellular extensions that some cells have. Cytoplasm formed, surrounded by plasma membranes and with dynamic properties through a network of microtubules. If they are short they are called eyelashes, like the ciliated cells of the respiratory system and if they are long, they are called flagella, like sperm cells.
  • Cytoplasmic inclusions: they are not delimited by a membrane. These are different types of substances made or stored by the cell that vary depending on the type of cell.