Mitochondria are organelles that break up food materials to make energy. Other important organelles are ribosomes, which make proteins, and endoplasmic reticulum. This is a structure, made of double membranes, that is linked to the nucleus and to the cell wall, so that chemicals can be carried around the cell. The cell wall itself is said to be semi-permeable. That means that some chemicals can pass through it into the cell but none can pass out.
Mitochondria are specialized structures unique to the cells of animals, plants and fungi. They serve as batteries, powering various functions of the cell and the organism as a whole. Though mitochondria are an integral part of the cell, evidence shows that they evolved from primitive bacteria.
All living organisms are built with one fundamental brick: the cell. In some cases, a single cell constitutes an entire organism. Cells contain genetic material (DNA and RNA), and they carry out essential functions, such as metabolism and protein synthesis. Cells are also capable of self-replicating. However, the level of organization varies within the cells of different organisms. Based on these differences, organisms are divided into two groups: eukaryotes and prokaryotes.
Plants, animals and fungi are all eukaryotes and have highly ordered cells. Their genetic material is packaged into a central nucleus. They also have specialized cellular components called organelles, each of which executes a specific task. Organelles such as the mitochondria, the rough endoplasmic reticulum and the Golgi serve respectively to generate energy, synthesize proteins and package proteins for transport to different parts of the cell and beyond. The nucleus, as well as most eukaryotic organelles, is bound by membranes that regulate the entry and exit of proteins, enzymes and other cellular material to and from the organelle.
Prokaryotes, on the other hand, are single-celled organisms such as bacteria and archaea. Prokaryotic cells are less structured than eukaryotic cells. They have no nucleus; instead their genetic material is free-floating within the cell. They also lack the many membrane-bound organelles found in eukaryotic cells. Thus, prokaryotes have no mitochondria.