2.5The Division of Roles in the Cell—Subcellular Organelles
Cellular functions are highly diverse and complex. In some cases, each cell performs a function completely different from that of the others. Organisms have "specialist" organelles (Fig. 2-6) that enable them to perform such cellular functions efficiently and uniformly while preserving their diversity. Organelles can be classified into ones that are surrounded by double membranes (nucleus, mitochondria, and chloroplasts), ones that are surrounded by a single membrane (e.g., endoplasmic reticulum, Golgi apparatus, endosomes, and lysosomes), and ones that are not surrounded by any membrane (e.g., ribosomes and cytoskeletons). Each of these organelles performs a unique function (Table 2-3), and cellular function is expressed as a combination of these functions. Let us now delve further into the cell and examine the functions of its main organelles.
The nucleus is the command center of the cell and the location for replication of genetic information, i.e., DNA. DNA is not something that stays exposed, but binds to proteins called histones in order to form structures called "nucleosomes" (see Chapter 4, Column Fig. 4-3). In the nucleus, messenger RNA is transcribed from DNA, and genetic information is read from the messenger RNA (see Chapter 3). The reading of genetic information is tightly controlled by developmental expression, intercellular interactions, and the external environment. The key proteins that help read this genetic information move in and out of the nucleus through pores in the nuclear membrane.
Subcellular Organelles that Have their Own DNA
Fig. 2-7. Endosymbiotic theory
Mitochondria originated in ancient times from a primitive aerobic bacterium that entered an anaerobic eukaryotic cell.
Although the entire cell is controlled by information from DNA in the nucleus, mitochondria and chloroplasts have their own DNA and systems of protein synthesis, and make their own proteins. However, the genes of most proteins that form mitochondria and chloroplasts are contained in the nuclear DNA. Although mitochondria and chloroplasts have their own DNA, their proliferation and function are controlled by the nucleus. Mitochondria and chloroplasts are thought to have originated from a primitive aerobic bacterium and a primitive cyanobacterium, respectively, that entered a primitive eukaryotic cell at least two billion years ago (Fig. 2-7). It is believed that much of the DNA of these bacteria migrated to the nuclei of primitive eukaryotic cells during evolution, and that the bacteria thus came under the control of their host primitive eukaryotic cells.
Mitochondria produce energy and synthesize large amounts of ATP by aerobic respiration. This process requires cooperation between two energy production pathways—the citric acid cycle in the soluble portion of the mitochondria and the electron transport chain on the inner membrane. The number of mitochondria in the cell and the degree of development of the inner membrane differ from cell to cell. In liver cells, which require a high amount of energy, many mitochondria are present, and the inner membrane is well developed. Moreover, since energy production is important for the activities of life, abnormalities in mitochondria often cause serious diseases (see Column below).
Chloroplasts photosynthesize and are unique to plants. Two chemical reactions occur inside the chloroplast: light (dependent) reactions and dark reactions (light independent reactions). Light reactions, in which the sun's energy is captured and converted into chemical energy, occur on the membranes inside the chloroplast. Dark reactions, in which chemical energy created by the light reactions converts carbon dioxide into organic substances, occur in the soluble portion of the chloroplast. Chloroplasts are considered to be extremely efficient apparatuses for converting energy from sunlight. Organic substances formed by this process are used for building living bodies.
The organic substances are then broken back down by a reaction called glycolysis (see Chapter 8) and by reactions in the mitochondria, leading to release of chemical energy. Many organisms on the earth consume the organic materials produced by plants and break these organic materials down to produce energy.
Mitochondria divide their own DNA during binary fission (Column Fig. 2-1). This DNA contains genes for many proteins that function in mitochondria. Mutations of portions of these genes often cause "mitochondrial diseases." Mitochondria are indispensible apparatuses for producing energy in cells. Mitochondrial diseases occur when abnormalities in mitochondria cause malfunction of energy-demanding cells such as muscle cells, nerve cells, and kidney cells. Symptoms of mitochondrial diseases include skeletal muscle symptoms such as muscle weakness and atrophy, various neurological symptoms such as decreased intelligence, convulsions, hearing loss, and extraocular muscle paralysis, and occasionally other symptoms such as enlargement of the heart. Mitochondria are inherited maternally; therefore, these diseases are not transmitted from fathers to their children.
Vesicular Transport System
The membrane proteins, secretory proteins, and polysaccharides in the cells are carried to their final destinations by vesicles inside the cells. The synthesis, transport, and breakdown of these compounds are performed by the endoplasmic reticulum, the Golgi apparatus, endosomes, and lysosomes.
Synthesis of membrane proteins and secretory proteins occurs on ribosomes attached to the rough endoplasmic reticulum. Functions such as phospholipid synthesis, glycogen metabolism, and regulation of intracellular calcium ions are also performed on the endoplasmic reticulum. The Golgi apparatus accurately distributes proteins sent from the endoplasmic reticulum to various locations within the cell, and acts as the location for modification of sugar chains of glycoproteins.
Molecules such as proteins that have entered the cell from outside are sorted by endosomes and transported to the organelles such as lysosomes. Lysosomes engulf and break down unnecessary large molecules in the cell. The insides of lysosomes are acidic and contain large amounts of hydrolytic enzymes such as proteases and ribonucleases. The fact that lysosome abnormalities cause many diseases shows that intracellular decomposition mechanisms are very important for cell function (see Column at the bottom).
The peroxisomes contain oxidases (oxidation enzymes) such as catalase, D-amino acid oxidase, and uricase. Peroxisomes also participate in fatty acid metabolism, amino acid metabolism, and synthesis of substances such as cholesterol and bile acid.
The cytoskeleton is a network that stretches around the entire cell and is involved in cell motion, maintenance of cell shape, and transport of substances within the cell. Cytoskeletons are filaments made of proteins. There are three types of filaments—microtubules, actin filaments, and intermediate filaments. Each of these filaments can perform various functions by interacting with other proteins.
Abnormalities in Intracellular Transport
Lysosomes contain large numbers of acid hydrolases, which play a role in breaking down proteins, polysaccharides, lipids, etc. Genetic abnormalities in the genes for these enzymes will prevent them from functioning properly and lead to accumulation of substances necessary to be broken down in the lysosomes. This condition is called "lysosomal disease." In addition to abnormalities in the enzymes themselves, lysosome malfunctions caused by inability to accurately transport enzymes are also classified as lysosomal diseases. For example, special sugars attach to these enzymes as labels indicating their destination to enable accurate transfer from the Golgi apparatus to the lysosomes (Column Fig. 2-2). Lysosomal diseases are caused by abnormalities in the enzymes that attach these labels. More than 30 types of lysosomal diseases are currently known. These diseases cause various symptoms, such as delayed psychomotor development, distorted facial features, bone abnormalities, and enlarged liver and spleen.