How Cells Release Chemical Energy Chapter 6

How do we derive energy from food? ATP is the most important energy storage molecule Potential energy from the breakdown of food is used to drive the endergonic reaction to make ATP ATP has the energy that can be released at any time (breaking off the last P group) to do cellular work ALL organisms must be able to convert the energy stored in the covalent bonds of a macromolecule (food) into ATP ATP IS THE ONLY MOLECULE A CELL CAN USE TO DO WORK!!

Electrons from the macromolecules that make up food carry the energy needed to make ATP. Electrons from food (glucose) “run downhill” and release energy. Energy released is used to make ATP!

Aerobic Respiration Formation of ATP (energy) Requires oxygen and will produce the greatest yield of ATP Glucose + 6O2 + 36 P + 36 ADP → 6CO2 + 6H2O + 36 ATP

Stages of Cellular Respiration Glycolysis Kreb’s Cycle

Stages of Cellular Respiration Glycolysis “Sugar splitting” Series of 10 enzyme-catalyzed reactions This stage begins the breakdown of glucose and produces small amounts of ATP Does NOT require oxygen = ANAEROBIC Location: cytosol of the cell Breakdown refers to the removal of hydrogen which contains an electron (and thus the removal of electrons). These electrons are donated to a carrier molecule (NAD+). Input = 1 glucose molecule (one 6-Carbon molecule), 2 ATP Output = 2 pyruvate molecules (each is a 3-Carbon molecule), 2 protons AND 2 ATP MOLECULES!!!!! In the absence of O2, this is the only way organisms can get energy from food

What are pyruvate’s options? With Oxygen Continues on to further stages of Cellular Respiration (Krebs cycle) Without Oxygen Lactate fermentation Alcoholic fermentation

Fermentation Type of ANAEROBIC respiration Some organisms can only derive energy from these reactions. For others, when oxygen is NOT in sufficient quantities OR when quick bursts of energy are needed, these reactions are used.

Fermentation, cont’d Fermentation begins with glycolysis Alcoholic fermentation is conducted by plants and yeasts Glucose → ethanol + 2CO2 + 2 ATP Used to make beer and wine Lactic acid fermentation is conducted by animals and bacteria Glucose → lactic acid + 2CO2 + 2 ATP Used to make soy sauce and yogurt

More on Fermentation Does NOT harvest energy that was in the glucose. Some extra energy is released in the form of heat, but most of the energy is stored in one of the products (lactic acid or ethanol).

If oxygen is present (and in sufficient quantities), pyruvate is oxidized before it enters the Kreb’s Cycle

Oxidation of Pyruvate The 3 molecules of pyruvate are oxidized (electrons are lost) Location = Intermembrane space of mitochondria IF your body needs energy, the Acetyl CoA can be channeled into ATP production IF your body DOES NOT need energy, the Acetyl CoA is channeled into fat synthesis Input = 3 molecules pyruvate Output = 1 molecule of Acetyl CoA

With Oxygen, Pyruvate continues on to form Acetyl CoA

Stages of Cellular Respiration, cont’d 2. Krebs Cycle Completes the breakdown of glucose (cycle goes through twice for every one glucose molecule). Harvests electrons from chemical bonds and uses their energy to power production of ATP Requires oxygen = Aerobic Location = mitochondrial matrix Input = 2 molecules Acetyl CoA Output = 1 molecule ATP, 2 CO2, 4 protons

Electron Transport Chain Produces most ATP (32 or 34) Location = mitochondrial cristae Electron carriers (NADH and FADH2 that collected electrons and hydrogens from glycolysis and Kreb’s) carry their electrons to the mitochondrial inner membrane Electrons and protons are passed to cytochromes. Cytochromes (proton acceptors) line the cristae and pass electrons. This forms a gradient across the mitochondrial membrane. ATP is produced. Input = 12 protons + 6 O2 + 17 ADP + 17 P ions Output = 17 ATP + H20

Complete breakdown of glucose 36 ATP molecules of are produced!! In compliance with the Second Law of Thermodynamics, 60% of the stored energy in glucose will end up in heat. Note. Cellular respiration also yields electrons hat then go into other cycles to produce ATP.

Cellular respiration and macromolecules Fats, proteins and sugars (other than glucose) can enter the pathway and be converted to ATP Food eaten in excess of caloric demands can be converted from amino acids, fatty acids and sugars into proteins, fats and carbohydrates, respectively, for building structures for long term energy storage.

Fig. 6.27

Link between Cellular Respiration and Photosynthesis Raw materials from the products of photosynthesis become the reactants for cellular respiration. Products of cellular respiration (i.e. CO2) become the reactants for photosynthesis. Ultimately, the energy from the sun is converted to glucose which is converted to ATP. REMEMBER, ENERGY CANNOT BE CREATED!!