Releasing Chemical Energy Chapter 7

Some History… The first cells we know of appeared on Earth ~ 3.4 billion years ago They didn’t use sunlight and were anaerobic (don’t need oxygen) Eventually, the cyclic pathway of photosynthesis evolved Sunlight offered an unlimited supply of energy When O2 began to accumulate in the atmosphere, cells that couldn’t cope with it began to die off Aerobic organisms were capable of living in the presence of oxygen and many evolved metabolic pathways that became known as aerobic respiration Requires oxygen to break down sugars to make ATP

Cellular Respiration In order to use the energy stored in sugars, cells must first transfer it to ATP Energy transfer occurs when the bonds of a sugar’s carbon backbone are broken, driving ATP synthesis Cellular Respiration: a chemical process in which glucose molecules are broken down to release energy (ATP) for cellular functions  CATABOLIC Cellular respiration has both aerobic and anaerobic reactions

Cellular Respiration Cellular respiration is the opposite of photosynthesis The products of photosynthesis are the reactants for cellular respiration, and the products of cellular respiration are the reactants for photosynthesis

Cellular Respiration There are two main mechanisms that organisms use to make ATP Aerobic Respiration: Main energy-releasing pathway in nearly all eukaryotes and some bacteria, remember, REQUIRES oxygen Fermentation (anaerobic) Whether aerobic or anaerobic, both begin with glycolysis

2 Pyruvate & 2 ATP/glucose (Used in the Krebs Cycle) Cellular Respiration Glycolysis: series of reactions that begin the sugar breakdown Happens in cytoplasm, anaerobic Glucose (sugar) is broken apart in the cytoplasm into two molecules called pyruvate Products 2 Pyruvate & 2 ATP/glucose (Used in the Krebs Cycle)

Aerobic Respiration Krebs Cycle (AKA Citric Acid Cycle) Happens in mitochondria, aerobic Breaks down pyruvate Pyruvate enters intermembrane space of mitochondria. Pyruvate is changed into Acetyl CoA by an enzyme called Coenzyme A, which causes the release of carbon dioxide (diffuses out of cell).

Aerobic Respiration Acetyl CoA combines with a molecule called oxaloacetate to make citric acid. Citric acid is turned into many other molecules to make ATP and NADH The cycle ends when oxaloacetate is regenerated Products 2 ATP (Used in the Electron Transport Chain) NADH (Used in the Electron Transport Chain) FADH2 (Used in the Electron Transport Chain) CO2 (Exits the cell)

Aerobic Respiration Electron Transport Happens in mitochondria, aerobic The NADH and FADH2 that were produced deliver electrons and hydrogen ions to electron transfer chains As electrons move through the chains, they give up energy little by little Hydrogen ions are actively transported across the inner membrane of the mitochondria Resulting H+ gradient causes the ions to flow through the ATP synthase, driving the formation of ATP

Aerobic Respiration Oxygen accepts electrons at the end of mitochondrial electron transfer chains and water is formed as a by- product Products NAD (Goes back to the Krebs Cycle to get energized) FAD (Goes back to the Krebs Cycle to get energized) 32 ATP (Used to perform cellular work) H2O (stays/exits cell)

NET 36 ATP

An Important Adaptation The thylakoid membrane of the chloroplast and the inner membrane of the mitochondria both have a similar adaptation that makes them more efficient. Both are heavily folded which allows for more surface area in a compact area This allows for more copies of the electron transport chain to happen at once.

Fermentation Like aerobic respiration, fermentation begins with glycolysis in the cytoplasm In fermentation, pyruvate is not fully broken down to CO2 Electrons do not move through electron transfer chains, so no additional ATP forms NAD+ is regenerated, allowing glycolysis to continue The net yield is 2 ATP

Fermentation Lactate Fermentation: anaerobic sugar breakdown pathway that produces ATP and lactate [animal cells] Done by your muscles when the demand for ATP is high, but you are low in oxygen (EXERCISE!). Can cause soreness in the muscles. Produces 2 ATP Once oxygen becomes available the cell will use Lactic Acid instead of pyruvate to restart the Krebs Cycle.

Fermentation Alcoholic Fermentation: anaerobic sugar breakdown pathway that produces ATP, CO2, and ethanol [yeast, bacteria] CO2 produced can be used to allow bread to rise Produces 2 ATP Once oxygen becomes available the cell will use ethanol instead of pyruvate to restart the Krebs Cycle.

Energy Aerobic respiration generates a lot of ATP by fully oxidizing glucose, completely dismantling it carbon by carbon Cells also dismantle other organic molecules by oxidizing them Complex carbohydrates, fats, and proteins in food can be converted to molecules that enter glycolysis or the citric acid cycle

Mitochondrial Malfunction Sometimes when oxygen enters an electron transfer chain, it escapes as a free radical Free radicals cause damage by oxidizing biological molecules and breaking carbon backbones A genetic disorder or encounter with a toxin can result in a missing antioxidant or defective electron transfer chain Free radicals accumulate and destroy the function of mitochondria, then the cell This tissue damage is called oxidative stress Hundreds of incurable disorders are associated with such defects Cancer, hypertension, Alzheimer’s disease, and Parkinson’s disease