Organelles of energy production - week 16

• Metabolism: Sum total of chemical processes occurring in cells by which energy is released from chemical bonds of food molecules (by CATABOLISM) or converted into useful forms of energy for synthesis of new molecules (by ANABOLISM).
• Life is maintained by a balance between the rates of catabolic and anabolic processes.
• Enzyme catalysed steps in metabolic pathways to produce the end product (metabolite).
• Unique energy-storing molecule, ATP, is involved in these processes.
• In oxidative phosphorylation, which occurs in mitochondria, an electron-transport system uses energy derived from the oxidation of food to generate a proton (H+) gradient across a membrane.
• In photosynthesis, which occurs in chloroplasts, an electron-transport system uses energy derived from the sun to generate a proton gradient across a membrane.
• In both cases, this proton gradient is then used to drive ATP synthesis.
• In the first stage, a proton pump harnesses the energy of electron transfer to pump protons (H+) across a membrane, creating a proton gradient.
• These high-energy electrons can come from organic or inorganic molecules, or they can be produced by the action of light on special molecules such as chlorophyll.
• The protons are derived from water, which is ubiquitous in the aqueous environment of the cell.
• Proton gradient serves as a versatile energy store that can be used to drive a variety of energy-requiring reactions in e.g mitochondria, chloroplasts—most importantly, the synthesis of ATP by ATP synthase.
• Both organelles contain their own DNA-based genome and the machinery to replicate this DNA and to make RNA and protein.
• Inner compartments of these organelles—the mitochondrial matrix and the chloroplast stroma—contain DNA and a special set of ribosomes.
• Membranes in both organelles—the mitochondrial inner membrane and the chloroplast thylakoid membrane—contain the protein complexes involved in ATP production.
• High energy bonds in ATP are the usual source of chemical energy for cellular growth and metabolism.
• Principal sources of ATP in non-photosynthetic cells are glucose and fatty acids.
• Aerobic degradation of a single glucose molecule may generate ~32 molecules of ATP during cellular respiration.
• Glycolysis has a net production of 2 molecules of ATP and 2 NADH per glucose unit.
• Although no molecular O2 is involved, oxidation occurs as electrons are removed and transferred to NAD+ (producing NADH).
• Small amount of ATP formed in glycolysis is by substrate-level phosphorylation transfer of phosphate to ADP from an organic molecule in catabolism of glucose.
• Glycolysis occurs whether O2 is present or not.
• If O2 is present, pyruvate moves to the Krebs cycle and the energy stored in NADH can be converted to ATP by the electron transport system and oxidative phosphorylation.
• In the kreb's cycle, each turn of the cycle produces 3 NADH, 1 ATP, 1 FADH2 and releases 2 CO2.
• The Kreb’s cycle takes place at the cristae of inner mitochondrial membrane.
• Electron carriers NADH and FADH2 transfer their high-energy electrons (gained from glycolysis and the Kreb’s cycle) to the electron transport chain in the inner mitochondrial membrane (cristae) and finally combine with O2.
• Electron carrier complexes pump protons from matrix into intermembrane space (forming a proton motive force)
• Energy released from these electron transfers is harnessed as protons flow back from intermembrane space to the matrix to add phosphate to ADP
• ATP synthase, a membrane enzyme, adds phosphate to ADP to form ATP.
• Oxidative phosphorylation: ATP synthesis is powered by redox reactions that transfer electrons from food to oxygen.
• The electrochemical H+ gradient across the inner mitochondrial membrane includes a large force due to the membrane potential and a smaller force due to the H+ concentration gradient—that is, the pH gradient.
• The intermembrane space is slightly more acidic than the matrix because the higher the concentration of protons.
• Both the membrane potential and the pH gradient combine to generate the proton-motive force, which pulls H+ back into the mitochondrial matrix.
• Stage 1: Photolysis of H2O with the production of O2, ATP and NADPH (electron carrier).
• Light capturing systems (light absorbing chlorophyll, photosystem protein complexes, electron-transport chains, and ATP synthase) located on thylakoid membrane.
• Stage 2: ATP and NADPH produced by the light reactions are energy source and reducing power to reduce CO2 with the production of sugar.
• Enzymes involved in the light reactions are located in the stroma.
• Return ADP, inorganic phosphate, and NADP to the light reactions.
• Water is split by photosystem II on the side of the membrane facing the thylakoid space.
• The diffusion of H+ from the thylakoid space back to the stroma powers ATP synthase.
• Chemiosmosis: mechanism that uses the energy stored in a transmembrane proton gradient to drive an energy requiring process, such as synthesis of ATP.