When the term protoplasm was first used in 1835, it referred to the clear, semi-fluid substance within the cell membrane of all living things. At that time, scientists believed that there was only one type of liquid within the cell, and that it was responsible for all functions. This concept has been revised since the development of scientific equipment has allowed researchers to discover the incredible complexity of the cell. Now, the term no longer has any specific technical significance, but is used to describe all of the matter within the walls of the cell, including the cytoplasm, nucleus and various organelles.
Protoplasm is composed of 90% water, mineral salts, gases such as oxygen and carbon dioxide, proteins, lipids, or fats, carbohydrates, nucleic acids and enzymes. Within this complex unit are numerous small bodies referred to as organelles, structures which have distinct purposes. The first organelle to be discovered was the nucleus. Others include the mitochondria, endoplasmic reticulum (ER), Golgi apparatus, chloroplasts and plastids. The clear, jelly-like liquid which fills the cell is now referred to as the cytoplasm.
The brain of the cell, the nucleus, controls all of the cell’s activities, and is responsible for the transmission of hereditary characteristics through a reproductive process of cell division called mitosis. Within the nucleus is the nucleolus, the center for RNA, and the chromatin network, which is formed of DNA and synthesizes proteins which form the chromosomes. It also controls the enzymes which govern the functions of other parts of the cell.
The powerhouses of the cell are the mitochondria that contain the enzymes needed for cellular respiration, the process through which a cell produces ATP and releases waste. ATP, which is made up of adenine, ribose and three phosphates, is the primary energy source for all other cellular reactions. The number of mitochondria present in cells varies, depending upon the type of cell and the energy requirements it has. Some cells may only have one mitochondrion, while other cells may have thousands.
Another organelle located in the protoplasm is the Golgi apparatus, or Golgi bodies, named after the Italian scientist Carmillo Golgi, who discovered them. The Golgi apparatus is composed of folded membrane-covered sacs which store and eventually transport the proteins produced by the endoplasmic reticulum (ER) of the cell. Prior to releasing the protein for use elsewhere in the cell, the Golgi body makes several modifications. In addition, this important structure produces complex sugars and lysosomes, molecules that act like garbage disposals, digesting dead or unneeded components within the cell.
ER within the protoplasm is responsible for producing most of the protein and lipids used by other organelles within the cell. There are two types of ER; rough ER, which has ribosomes attached, and smooth ER, which does not have ribosomes. The rough ER produces the proteins and sends them to the Golgi apparatus. The smooth ER is a storage facility which stores ions and creates and stores steroids for future use.
Some organelles are only found in the protoplasm of plant cells. These include chloroplasts and plastids. Chloroplasts contain the plant’s chlorophyll and are the center for the process of photosynthesis. Plastids are plant organelles that are involved in a number of important functions, including pollination, photosynthesis, and the synthesis of fats, starches and proteins.
