Oxygen

• • Cyanobacteria are aquatic and photosynthetic
  • small and usually unicellular
  • one of the largest and most important groups of bacteria on earth
  • produce oxygen during photosynthesis as they fix carbon dioxide dissolved in water
• • until great oxidation event most respiration was anoxic, similar to anaerobic respiration
  • 16 times less efficient than aerobic respiration
  • evolution of protobacteria occurred that managed to use the oxygen and therefore gained major energy boost
  • this allowed aerobic bacteria to multiply, leaving anoxic bacteria in oxygen-deprived areas
  • some oxygen-using bacteria were swallowed by larger cells who used them as specialised intracellular breathing compartments (mitochondria)
• At standard temperature and pressure oxygen is a colourless, odourless and tasteless gas with molecular formula O2 referred to as dioxygen.
• Oxygen makes up 21% of the atmosphere by volume. The element and its compounds make up 49.2% by mass of the Earth's crust, and about two-thirds of the human body.
• As dioxygen, two oxygen atoms are chemically bound to each other. The bond can be described as a covalent double bond that results from the filling of molecular orbitals of each atom.
• oxygen is a highly reactive element and is capable of combining with most other elements.
• Adenosine triphosphate is the energy currency of the cell. ATP is made of adenine and ribose bonded to three phosphate groups through phosphate ester bonds. If you remove one of the phosphate groups from the end the molecule is more stable.
• The conversion from ATP to ADP is an extremely crucial reaction for the supplying of energy for life processes. Cleavage of one bond with accompanying rearrangement is sufficient to liberate 7.3 kcal per mole (30.6 kJ per mole)
• ATP is used for cell functions:

  • moving substances across cell membranes
  • muscle contraction - heart muscle and skeletal muscle, chromosomes and flagella
  • synthesis of macromolecules
• basal metabolic rate is the energy expenditure per unit time by endothermic animals at rest. It can also be defined as the total energy conversion rate of a person at rest, divided among various systems in the body. BMR is a measure of an animal's metabolic rate when it is quiet, not stressed out or excited, and not doing anything active.
• About 70% of a human's total energy expenditure is due to basal life processes in organs of the body. About 20% comes from physical activity and the other 10% from thermogenesis or digestion of food. These processes all require oxygen and coenzymes and expel carbon dioxide.
• The liver is a very active organ that performs different vital functions. One of its most important functions is the maintenance of blood glucose. Liver cells proliferate rapidly.
• The brain is the second largest source of energy consumption - 20% of total oxygen metabolism. Neurons consume 75-80% of energy produced in the brain. This energy is primarily utilised at the synapse with a large proportion spent in restoration of neuronal membrane potentials following depolarisation.
• Oxidation and reduction reactions are important for generating ATP. An oxidation-reduction (redox) reaction is a type of chemical reaction that involves a transfer of electrons between two species. A redox reaction is any chemical reaction in which the oxidation number of a molecule, atom or ion changes by gaining or losing an electron.
• Redox reactions are common and vital to some basic life functions including photosynthesis, respiration, combustion and corrosion or rusting.
• A chemical that supplies electrons is called a reducing agent (or a reductant) and a chemical that accepts electrons is called an oxidising agent (or oxidant).
• Respiration is the process of breaking down organic molecules to harvest chemical energy. Glucose is the most important source of energy in all organisms as it is one of the most common monosaccharides broken down from disaccharides such as sucrose and lactose. During cellular respiration a glucose molecule is completely oxidised, releasing high energy electrons.
• Glucose transporters are a group of membrane proteins that facilitate the transport of glucose across the plasma membrane. Because glucose is a vital source of energy for life, these transporters are present in all phyla.
• Virtually all mammalian cells use blood glucose as the major source of cellular energy and most express GLUT1, a plasma membrane uniporter that catalyses movement of glucose down its concentration gradient. GLUT1 alternates between two conformational states: in one, a glucose-binding site faces the outside of the membrane; in the other, a glucose-binding site faces the inside.
• Electron carriers are small organic molecules that play key roles in cellular respiration, where they shuttle electrons between molecules. Two types of electron carriers are important in cellular respiration:

  • NAD+
  • FAD

  Both are derived from B vitamins.
• Reduced forms of NAD+ and FAD (NADH and FADH2) are produced during earlier stages of cellular respiration (glycolysis, pyruvate oxidation, and the citric acid cycle). When they drop off the electron they go back to their original form.
• Reactions in which NAD+ or FAD gain or lose electrons are examples of redox reactions.
• Electron carriers provide a controlled flow of electrons that enables the production of ATP. Without them, the cell would cease to function.
• Glucose can enter the cell relatively easily and can be converted to glucose-6-phosphate in the cytoplasm of the cells. Glucose-6-phosphate is then converted to glycogen, which is our main form of storage of glucose.
• Liver glycogen serves as a glucose reserve for blood and muscle. Glycogen supplies glucose to exercising muscle tissue. Glycogen stores are limited; fasting or strenuous exercise can deplete them rapidly. The brain, blood and adipose tissue store little glucose compared to liver and muscle tissues.
• Glycogenesis converts glucose-6-phosphate to glycogen.
• When the body needs glucose glycogenolysis breaks down the glycogen into glucose.
• Glucose-6-phosphate can lead to the formation of ribose (essential for nucleic acid synthesis) via the pentose phosphate pathway.
• Generation of most energy occurs in the mitochondria membrane from acetyl CoA in the electron transport chain. When we get to this stage we also produce citrate which goes on to the production of fatty acids. We can generate energy, fatty acids and nucleic acids all from glucose.
• Four stages to aerobic respiration:

  • glycolysis
  • link reaction
  • Krebs cycle
  • oxidative phosphorylation
• In cells which can become oxygen starved, such as muscle cells during intensive activity, we still need to produce ATP. Cells switch to anaerobic respiration - does not need much oxygen to produce ATP but produces much less.
• Anaerobic respiration only produces about 10% of the energy released in the complete oxidation of glucose. Cell converts pyruvate to lactate using lactate dehydrogenase - lactic acid fermentation
• Lactate produced during fermentation is of no further use to cells in terms of energy generation. Therefore it is transported out of the cells and carried in the blood to the liver. Excess lactate gets shuttled to the liver to undergo gluconeogenesis (production of new glucose)
• Gluconeogenesis is a metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates such as lactate, glycerol and glycogenic amino acids.
• Lactic acid is produced in physiologically normal processes and is a common finding in disease states. When increased production is combined with decreased clearance, the severity of clinical course escalates. The effects of severely elevated levels of lactic acid can have profound haemodynamic consequences and can lead to death.
• Serum lactate levels can be both marker for risk as well as a therapeutic target. The higher the level and the longer the time to normalisation of elevated serum lactate, the greater the risk of death.
• Lactic acidosis usually occurs in the presence of inadequate tissue perfusion, abnormalities in carbohydrate metabolism and with the use of certain medications.
• Glycolysis is the first pathway in the breakdown of glucose. It takes place in the cytoplasm of both prokaryotic and eukaryotic cells. Does not use oxygen (anaerobic)
• 1. phosphate + glucose = glucose-6-phosphate
  2. rearrangement = fructose-6-phosphate
  3. + phosphate = fructose-1,6-bisphosphate
  4. split = 2 glyceraldehyde 3-phosphate
  5. oxidation + phosphorylation = 1,3-bisphosphoglycerate + NADH
  6. substrate-level phosphorylation + ADP = ATP + 3-phosphoglycerate
  7. oxidation = phosphenolpyruvate
  8. substrate-level phosphorylation + ADP = ATP + pyruvate
• When oxygen is present, pyruvate and NADH enter the mitochondria matrix where it is oxidised and converted into Acetyl-CoA

  In this process of pyruvate oxidation, electrons are transferred to NAD+, making NADH, and a carbon is lose, forming carbon dioxide.