Every living thing is made up of microscopic Cells are tiny, separate entities. There are two major groups of cells: the Protocyten without cell nucleus and the eucytes with nucleus. Humans and most animals and plants are called multicellular organisms. They consist of billions of cells with a nucleus. Among these, there are many different types, each specialized in a determined task. The cells of a type merge to form associations. Thus, an association is called tissue. When multiple tissues are responsible for a specific task, they together form an organ. All the cells are filled with a gel-like substance, the so-called cytoplasm. The cytoplasm is a type of transport medium for nutrients, proteins and enzymes. Plant cells differ from animal cells in that they have additional organelles, such as chloroplasts. In these, the photosynthesis takes place.