Unicellular and Multicellular Organisms: Cell Structure and Nucleic Acids

Unicellular Organisms

Unicellular organisms are formed by a single cell, which carries out all vital functions. Each cell must continuously ensure access to its nutrients, metabolism, and energy, which allows it to manufacture its own structures and even perform other functions such as movement, from prokaryotes to eukaryotes, and reproduction.

Colonies

Cells can be grouped into colonies through orderly division, forming systems in which cells remain linked together. Each cell in a colony performs all the functions of a living being, and independent colonies also exist.

In other cases, there is a principle of division of labour, since some cells can specialise through cell differentiation and lose their reproductive capacity.

Volvox

Volvox is a colonial organism whose body is formed by numerous unicellular algae.

Multicellular Organisms

Multicellular organisms are constituted by cells that originate from a single initial cell. These cells do not function independently; they are coordinated and interconnected, acting as a whole body. These organisms have cell differentiation, so cells group together to form more complex structures called tissues, which in turn form organs, and organs are structured into organ systems.

Multicellular Organisation Models

• Fungi: organised into cell tissues forming a thallus.
• Cormophytes: organised as a cormus. Their organs are root, stem and leaves.
• Reeds: organised with cell tissues but without defined organs.
• Animals: most animals have a greater level of complexity, with organs, apparatus and systems.

Nucleic Acids

Nucleic acids are long molecules made up of carbon, hydrogen, oxygen, nitrogen and phosphorus atoms. The simplest molecules that form them are called nucleotides, which are composed of:

• A nitrogenous base. There are five bases: adenine, cytosine, guanine, thymine and uracil.
• A sugar with five carbon atoms. These sugars are called pentoses; in this case there are two: deoxyribose and ribose.
• A phosphate group, which is a derivative of phosphoric acid.

Types of Nucleic Acids

RNA (ribonucleic acid): found in the cytoplasm and nucleus of the cell. Its nucleotides contain ribose, and its nitrogenous bases are adenine, cytosine, guanine and uracil.

DNA (deoxyribonucleic acid): located in the nucleus of eukaryotic cells. It contains the genetic information that governs the cell's functions and the development of the individual. In eukaryotic cells, DNA is found as linear molecules forming chromatin. The nucleotides of DNA contain deoxyribose and the nitrogenous bases adenine, guanine, cytosine and thymine.

Levels of Biological Organisation

Tissue: a grouping formed by cells with a similar form that perform the same function.

Organ: a structure with a concrete form, made up of several tissues that perform a particular function.

Organ system: formed by a set of organs that collaborate in a common activity.