Chapter 8 How Cells Release Chemical Energy

Overview of Energy Releasing Pathways All organisms release chemical bond energy from glucose and other organic compounds to drive ATP formation. The main energy releasing pathways all start in the cytoplasm. Only aerobic respiration, which uses O, ends in the mitochondria. It has the greatest energy production. 2

Cont. Plants make ATP during photosynthesis and use it to synthesize glucose and other carbohydrates. But all organisms, plants included, can make ATP by breaking down carbohydrates, lipids, and proteins. 3

Comparison of the main types of energy releasing pathways ATP can be made by aerobic or anaerobic mechanisms (with or without oxygen). The first organisms on earth would have used anaerobic respiration since O was not readily available. Aerobic respiration is an efficient way to covert glucose to ATP The pathways starts in the cytoplasm called glycolysis 4

cont Glycolysis converts glucose to 2 pyruvate compounds If oxygen is present then aerobic respiration proceeds - the pyruvate enters the mitochondria. If no oxygen is present the process stays in the cytoplasm. 5

Overview of Aerobic Respiration More ATP is generated by aerobic respiration – yields 36 or more Anaerobic respiration (without oxygen) has a very low yield and only yields 2 ATP molecules 6

Aerobic respiration steps: Step 1--Glycolysis in cytoplasm, take 2 ATP to start then process; produces 2 pyruvates , electrons, hydrogen, and produces 4 ATP – a net of 2 ATP 7

Step 2--Krebs cycle breaks down pyruvate to carbon dioxide, e- & H+, and 2 ATP. The Krebs cycle takes place in the mitochondria.

Step 3—Electron transfer phosphorlation; membrane proteins generate 32 ATP, water by using oxygen as last e- acceptor. Takes place across membranes in the mitochondria.

Summary Equation for Aerobic Respiration C6H1206 + 6O2 6CO2 + 6H20 glucose oxygen carbon water dioxide 10 10

Glucose A simple sugar (C6H12O6) Atoms held together by covalent bonds 11 11

Transfer Phosphorylation Typical Energy Yield: 36 ATP CYTOPLASM glucose ATP 2 ATP 4 Glycolysis e- + H+ (2 ATP net) 2 NADH 2 pyruvate e- + H+ 2 CO2 2 NADH e- + H+ 8 NADH 4 CO2 KrebsCycle e- + H+ 2 ATP 2 FADH2 e- Electron Transfer Phosphorylation 32 ATP H+ water e- + oxygen Typical Energy Yield: 36 ATP 12 12

Glycolysis – Glucose Breakdown Starts Glycolysis is the first stage of aerobic respiration. It is also the first stage in anaerobic respiration, such as alcoholic and lactate (lactic acid) fermentation. 13

Importance of Oxygen Electron transport phosphorylation requires the presence of oxygen Oxygen withdraws spent electrons from the electron transfer chain, then combines with H+ to form water 14 14

15

Fermentation Pathways Without Oxygen or do not use oxygen – anaerobic Step 1 – Glycolysis in cytoplasm 2 pyruvates 2 Net ATP 2 NADH Step 2 – Fermentation No more ATP is made only new end products 16

Alcoholic fermentation This pyruvic acid is then converted to CO2, ethanol, and energy for the cell. Humans have long taken advantage of this process in making bread, beer, and wine. Yeast : in wheat & grains– sugar is converted into alcohol and carbon dioxide. The carbon dioxide will form bubbles, which will be trapped by the gluten of the wheat causing the bread to rise. small amounts of alcohol are formed, most of which will be evaporated during the bread baking process. Therefore, you won’t get drunk by eating bread! 17

Cont. In Grapes – produces ethanol at about 10% it kills yeast; Saccharomyces is responsible for the alcohol fermentation of wines. Grape juice contains naturally high levels of sugars. These sugars are converted into alcohol and carbon dioxide. 18

Lactate fermentation Ex. Food – Lactobacillus & other bacteria spoil food, some help make milk products; pickling –(sauerkraut) 19

Ex. Muscles – ATP produced quickly, short term   DURING EXERCISE, BREATHING CANNOT PROVIDE YOUR BODY WITH ALL THE OXYGEN IT NEEDS FOR AEROBIC RESPIRATION.  WHEN MUSCLES RUN OUT OF OXYGEN, THE CELLS SWITCH TO LACTIC ACID FERMENTATION. Blood removes Lactic Acid from muscles, but if it is not removed fast enough, the side effects of Lactic Acid Fermentation is Muscle Fatigue, Pain, Cramps, and Soreness. Most Lactic Acid made in the muscles diffuse into the bloodstream, then to the LIVER, where it is converted back to PYRUVIC ACID When Oxygen becomes Available. 20

Lactate Fermentation GLYCOLYSIS C6H12O6 2 ATP energy input 2 ADP 2 NAD+ 2 NADH 4 ATP 2 pyruvate energy output 2 ATP net LACTATE FORMATION electrons, hydrogen from NADH 2 lactate 21 21

Alterative Energy Sources in the Body When required, molecules other than glucose can enter the aerobic pathways as alternative energy sources. Cells must deal with too much or too little glucose. 22

Fate of Glucose at mealtime & between Meals Glucose  enter cell (at prompting of insulin) glucose-6-phosphate– trapped in cell If cells are not using up glucose rapidly it is converted to glycogen (polysaccharide) in the liver & muscles of animals Between meal glucose levels drop  pancreas secrete glucagon converts glycogen in liver to glucose. 23

Processes Are Linked sunlight energy water + sugar molecules PHOTOSYNTHESIS water + carbon dioxide sugar molecules oxygen AEROBIC RESPIRATION 24 24