|MoBio||Cytoplasmic Organelles||Chapter 1|
By definition, organelles are the membrane-bound structures in a cell. The nucleus is an example. Other organelles are located in the cytoplasm such as mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, peroxisomes, lysosomes, vacuoles and glyoxisomes.
An eukaryotic cell contains many mitochondria, occupying up to a quarter of the cytoplasmic volume. The size of a mitochondrion is about 1.5-2 μm in length, 0.5-1 μm in diameter, approximately the same as E. coli. It has two membranes: outer membrane and inner membrane. Mitochondria also have their own DNA (represented as mtDNA), which encodes some of the proteins and RNAs in mitochondria. However, most proteins operating in mitochondria still originate from nuclear DNA.
The major role of mitochondria is to produce ATP (adenosine triphosphate), which carries high energy to power most cellular processes. Such energy is stored in the phosphoanhydride bonds of ATP. During ATP hydrolysis, the bond is broken, releasing 7.3 kcal/mole of energy. Many cellular processes can utilize the released energy by coupling with the ATP hydrolysis.
In animal cells, the major sources for the synthesis of ATP are fatty acids and glucose. Oxidation of an 18-carbon fatty acid can make 146 ATP molecules. By contrast, oxidation of one glucose molecule (6 carbons) can generate only 36 ATP molecules.
The generation of ATP involves a series of electron transport. Inevitably, electrons may leak from the electron transport chain, producing free radicals. This has been suggested to be the major mechanism involved in the aging process. See "Mitochondria, Apoptosis and Aging".
Like mitochondria, a chloroplast also contains both outer and inner membranes on its surface. Inside the chloroplast, there are many thylakoids, each is enclosed by a membrane. Chlorophylls are located on the thylakoid membrane to absorb light for photosynthesis.
In the first step of photosynthesis, light energy is used to split water into hydrogen ions and oxygen molecules. The generated hydrogen ions will create a concentration gradient across the thylakoid membrane. Movement of hydrogen ions through the membrane is coupled to ATP synthesis. The overall reactions can be written as
Like mitochondria, chloroplasts also have their own DNA, but most chloroplast proteins are still encoded by nuclear DNA.
Endoplasmic reticulum (ER) can be divided into rough ER and smooth ER. The major role of rough ER is to process the newly synthesized peptides from ribosomes. Therefore, the surface of rough ER is usually associated with ribosomes and thus appears "rough". Smooth ER is involved in the synthesis and metabolism of lipids. Hepatocytes are abundant in smooth ER.
Golgi apparatus is a major site for sorting and modifications of proteins and lipids. After proteins are sorted at rough ER, they are enclosed in transport vesicles and carried to the Golgi apparatus. Some proteins could be modified into glycoproteins and then transported to other destinations.
Peroxisomes contain enzymes for degrading amino acids and fatty acids. These reactions produce harmful hydrogen peroxide. Hence, peroxisomes also contain catalase to convert hydrogen peroxide into water and oxygen:
The major function of lysosomes is to degrade various macromolecules in the cell. They contain nuclease for degrading DNA and RNA, protease for degrading proteins and other enzymes for degrading polysacchrides and lipids. Lysosomes exist only in animal cells. Although plant cells do not have lysosomes, their vacuoles are also capable of degrading macromolecules.
Vacuoles store small molecules such as water, ions, sucrose and amino acids. They can also hold waste products which will be slowly degraded. They typically occupy more than 30% of the cell volume, but may expand up to 90%.
Glyoxisomes are found mainly in plant seeds. Their major function is to convert fatty acids into acetyl CoA for the glyoxylate cycle where two acetyl-CoA molecules are converted to a 4-carbon dicarboxylic acid. Peroxisomes and glyoxisomes are also called microbodies.