Module 6 Energy Centers - BMSC6010E Fundamental Cell Biology Fall 2023 PDF

10/9/23, 2:46 PM VBDI 4997E: Pre-Veterinary Histology VBDI 6010E: Fundamental Cell biology Module 6 Energy Centers 6.1. Introduction The mitochondria is one of the other locations in the cell that contains DNA besides the nucleus. It produces most of the energy that is required for the cell’s va...

10/9/23, 2:46 PM VBDI 4997E: Pre-Veterinary Histology VBDI 6010E: Fundamental Cell biology Module 6 Energy Centers 6.1. Introduction The mitochondria is one of the other locations in the cell that contains DNA besides the nucleus. It produces most of the energy that is required for the cell’s various functions, and for this reason, it is called the powerhouse of the cell. Within the mitochondria, there are several important reactions that take place such as the electron transport chain and the Krebs cycle. https://uga.view.usg.edu/d2l/le/content/2926415/viewContent/47443580/View 1/10 10/9/23, 2:46 PM VBDI 4997E: Pre-Veterinary Histology 6.2. The Endosymbiotic Theory The mitochondria and other energy centers (such as chloroplasts) are thought to have originally been prokaryotes that were engulfed by eukaryotes. This would explain the fact that they have their own DNA. The endosymbiotic theory states that when these prokaryotes were “swallowed” by larger eukaryotic cells, both of the organisms received an advantage. The eukaryotic cells no longer needed to only rely on anaerobic metabolism (i.e: glycolysis), and the prokaryotic cells now had a safe home in which they could stay and not be destroyed. https://uga.view.usg.edu/d2l/le/content/2926415/viewContent/47443580/View 2/10 10/9/23, 2:46 PM VBDI 4997E: Pre-Veterinary Histology 6.3. The Structure and Function of the Mitochondria The mitochondria is often illustrated as a cylindrical organelle that contains an outer membrane, inner membrane, and two intermembrane spaces. The two membranes contain a variety of proteins that are necessary for their different functions. The inner membrane contains many infoldings called cristae. Between the outer membrane and the inner membrane is the intermembrane space, and separated from the rest of the organelle by the inner membrane is the matrix. A very important feature of the mitochondrial membranes is that they t i i l li id ll d https://uga.view.usg.edu/d2l/le/content/2926415/viewContent/47443580/View di li i Thi li id i i ll 3/10 10/9/23, 2:46 PM VBDI 4997E: Pre-Veterinary Histology contain a special lipid called cardiolipin. This lipid is especially prominent in the inner membrane. Besides this lipid, there are also many transport proteins that make both membranes selectively permeable to specific molecules like fatty acids, carnitine, etc. The inner mitochondrial membrane also contains the proteins that are necessary for the electron transport chain. One of the main functions of the mitochondria is the conduction of the Krebs cycle and the electron transport chain. The Krebs cycle utilizes the pyruvate generated from glycolysis and turns it into acetyl CoA, which can be used in the Kre bs cycle to generate ATP and the electron carriers, NADH and FADH2. These electron carriers are then used in the electron transport chain which uses a protein gradient and electron transfer between the different complexes of the chain in order to generate energy for the cell through oxidative phosphorylation (generation of ATP through the use of oxidized electron carriers). The electron transport chain (ETC) requires complexes 1, 2, 3, and 4 https://uga.view.usg.edu/d2l/le/content/2926415/viewContent/47443580/View 4/10 p 10/9/23, 2:46 PM ( ) q p VBDI 4997E: Pre-Veterinary Histology , , , (ATP Synthase) in order to produce ATP from the electron carriers generated from the Krebs cycle (NADH and FADH2). Each of these complexes contains cytochromes — a family of colorful proteins which contain an iron atom that can accept electrons — that aid in the transfer of electrons from NADH and FADH2 to lower energy levels in order to generate energy. In the process, some of these complexes will also pump H+ into the intermembrane space, creating a proton gradient between the mitochondrial matrix and the intermembrane space. This proton gradient is then used at the end of the electron transport by ATP synthase to form ATP. As compared to glycolysis and anaerobic respiration, the ETC is extremely efficient, generating a large amount of ATP for only a few As the cell divides, the number of mitochondria also increases. The same occurs with chloroplasts due to organelle division. molecules lost. Fig 6.4. Fig courtesy: Science with the Amoeba sisters. Difference between cellular respiration (breaks down glucose) and photosynthesis (makes glucose). https://uga.view.usg.edu/d2l/le/content/2926415/viewContent/47443580/View 5/10 10/9/23, 2:46 PM VBDI 4997E: Pre-Veterinary Histology 6.4. The Chloroplast The chloroplast, similar to the mitochondria, is also thought to have come from another prokaryotic organism called a cyanobacteria. These organisms engage in photosynthetic activity for energy. The chloroplast is the photosynthesis center of the cell. They use electrons from water and the energy from sunlight to convert CO2 from the environment into carbohydrates for the cell to use. Another end product of this reaction is the release of oxygen (O2) into the atmosphere. The cells typically use chloroplasts during the day to generate energy for the plant, while at night they use mitochondria. https://uga.view.usg.edu/d2l/le/content/2926415/viewContent/47443580/View 6/10 10/9/23, 2:46 PM VBDI 4997E: Pre-Veterinary Histology 6.5. Structure and Function of the Chloroplast The structure of the chloroplast is very similar to the mitochondria except that they have an extra compartment within them called the thylakoid space. The main difference between the mitochondria and the chloroplast is that the inner membrane of the chloroplast is not folded into Cristae. Rather, it creates one large compartment called the stroma, which is analogous to the mitochondrial matrix. One important feature of the chloroplast is that it actually has three membranes. The third and innermost membrane creates thylakoids — flattened disc-like sacs. Within these thylakoids are the thylakoid spaces. The chloroplast is responsible for two main reactions: one is the generation of ATP using light, and the other is conversion of CO2 into a carbohydrate. In the light-dependent reactions, the electrons will excite chlorophyll, the pigment within plant cells, so that electrons from chlorophyll can move along an oxidation chain similar to the electron transport chain. The dark reactions are the reactions that are used to convert CO2 to a carbohydrate (also known as carbon fixation). These reactions use the ATP and NADPH that was produced in the light reactions to turn CO2 into a carbohydrate. https://uga.view.usg.edu/d2l/le/content/2926415/viewContent/47443580/View 7/10 10/9/23, 2:46 PM VBDI 4997E: Pre-Veterinary Histology The main reaction of carbon fixation occurs in the stroma of the chloroplast and is catalyzed by the enzyme ribulose bisphosphate carboxylase, which is the world’s most abundant enzyme. This enzyme catalyzes the conversion of CO2 into 3-phosphoglycerate. Eventually, this molecule is converted to glyceraldehyde-3phosphate which can then be used to produce sugars, fats, and amino acids. So then how do plants adapt to different environments such as hot, humid, or dry environments? Sometimes, ribulose bisphosphate carboxylase uses oxygen instead of CO2. This results in the consumption of O2 and the release of CO2. This is called photorespiration. This process is used especially in hot and dry climates. Besides these main reactions that have been mentioned, the chloroplast also carries out other biosynthetic functions. For example, all of a plant cell’s fatty acids are made by enzymes that are located within the chloroplast stroma. Another important reaction that occurs here is the reduction of nitrite (NO2-) to ammonia (NH3), which is managed by light-activated electrons. The ammonia that is generated during this process is used by the cell to make nucleic acids and amino acids. 6.6. A Brief Summary of the Evolution of the ETC The earliest known cells used to make ATP via fermentation, aka anaerobic respiration. This was possible because these cells were https://uga.view.usg.edu/d2l/le/content/2926415/viewContent/47443580/View 8/10 10/9/23, 2:46 PM p p VBDI 4997E: Pre-Veterinary Histology in an environment that lacked oxygen. However, due to changes in the environment, these cells were able to adapt and use the electron transport chain as an aerobic method of energy production. 6.7. Inheritance and the role of mitochondria and chloroplasts Mitochondria are maternally inherited. This means that when a sperm and an oocyte fuse to form the zygote, the mitochondria is transferred from the oocyte and not the sperm. This phenomenon is called cytoplasmic inheritance, a type of non-Mendelian inheritance. https://uga.view.usg.edu/d2l/le/content/2926415/viewContent/47443580/View 9/10 10/9/23, 2:46 PM VBDI 4997E: Pre-Veterinary Histology Clinical Correlate: Mitochondrial Disorders Some individuals are affected by diseases that are caused due to mitochondrial inheritance. When different genetic disorders are examined, a pedigree chart is typically drawn that will help to reveal typical Mendelian inheritance patterns. One identifying feature of the pedigree of a disease caused by mitochondrial inheritance is that all of the offspring from the affected mother will have the disease. Some examples of mitochondrial disorders are: Leber Hereditary Optic Neuropathy, Kearns-Sayre syndrome, and Myoclonic Epilepsy with Ragged Red Fibers. https://uga.view.usg.edu/d2l/le/content/2926415/viewContent/47443580/View 10/10