Important Cell Organelles
... a type of ribonucleic acid {RNA} called messenger RNA {mRNA}. The mRNA is sent from the nucleus to the ribosomes, where protein is manufactured. The Mitochondrion The mitochondria are small cellular structures or organelles found in the cytoplasm of eukaryotic cells {cells with a nucleus}. The mitochondria convert nutrients into energy-yielding molecule adenosine triphosphate {ATP} to fuel the cell’s activities. This function is known as aerobic respiration, it is also the reason why the mitochondria are frequently referred to as the powerhouse of the cell. The mitochondria are unusual organelles because the contain deoxyribonucleic acid {DNA} which is commonly found in the cell’s nucleus, and ribosomes, protein producing organelles abundant in the cytoplasm. The DNA within the mitochondria directs the ribosomes to produce proteins, many of which function as enzymes, or biological catalysts, in ATP production. The amount of mitochondria present in the cell depends on the cell’s function. Cells with large energy requirements, such as muscle cells, contain more mitochondria then others. A mitochondrion is usually long and slender, but it can also appear to be bean-shaped or oval shaped under the electron microscope. The mitochondria range in size from 0.5 micrometer to 1 micrometer in length. A mitochondrion has a double membrane which forms a sac within a sac. The smooth outer membrane holds numerous transport proteins, which shuttle materials in and out of the mitochondrion. The region between the outer and inner membranes that is filled with liquid is known as the outer compartment. The numerous folds of the inner membrane are called cristae. ATP synthesis takes place at the cristae. The cristae’s folded structure greatly increases the surface area where ATP synthesis occurs. Transport proteins and enzymes that synthesize ATP are among the molecules embedded in the cristae. The cristae surround a liquid-filled region known as the inner compartment, or matrix, which contains a large number of enzymes that are used in the process of aerobic respiration. The chief function of the mitochondria is to create energy for cellular activity by the process of aerobic respiration. In this process, glucose is broken down in the cell's cytoplasm to form pyruvic acid, which is transported into the mitochondrion. In a series of reactions, part of which is called the citric acid cycle or Krebs cycle, the pyruvic acid reacts with water to produce carbon dioxide and ten hydrogen atoms. These hydrogen atoms are transported on special carrier molecules called coenzymes to the cristae, where they are donated to the electron transport chain. The electron transport chain separates the electron and proton in each of the ten hydrogen atoms. The ten electrons are sent through the electron transport chain and some eventually combine with oxygen and the protons to form water. Energy is released as the electrons flow from the coenzymes down the electron transport chain to the oxygen atoms, and this energy is trapped by the components of the electron transport chain. As the electrons flow from one component to another, the components pump random protons from the matrix to the outer compartment. The protons cannot return to the matrix except by one pathway—through the enzyme ATPase, which is embedded in the inner membrane. As the protons flow back into the matrix, ATPase adds a phosphate group to a molecule in the matrix, adenosine diphosphate (ADP). The addition of a phosphate group to ADP forms ATP. Aerobic respiration is an ongoing process, and mitochondria can produce hundreds of thousands of ATP molecules each minute in a typical cell. The ATP is transported to the c...