9.3 Getting Energy to Make ATP

Cellular Respiration Cellular respiration is when mitochondria break down food molecules to produce ATP. 3 stages: glycolysis, the citric acid cycle, and the electron transport chain. Electron carriers NAD+, NADH, FAD and FADH2 are used. The first stage (glycolysis) is anaerobic, which means it requires no oxygen. The last two stages (citric acid cycle and electron transport chain) are aerobic, which means it requires oxygen to be completed.

Stage 1: Glycolysis Glycolysis is a series of chemical reactions that break down glucose into two molecules of pyruvic acid within the cytoplasm of a cell and uses NAD+ and NADH. Glycolysis is not very effective because it uses 2 ATP molecules to start the process and produces only 4 ATP molecules (2 ATP for each glucose molecule broken down). After glycolysis the pyruvic acid molecules move into the mitochondria to be transformed into energy through a series of reactions. Pyruvic acid gives off a molecule of CO2 and it combines with a molecule called coenzyme A to form acetyl-CoA and produces NADH and H+.

Stage 2: The Citric Acid Cycle (Krebs Cycle) The citric acid cycle is a series of chemical reactions similar to the Calvin cycle in that the molecule used in the first reaction is also one of the end products. At the end of the citric acid cycle 1 molecule of ATP and 2 molecules of carbon dioxide are produced and 2 electron carriers, NAD+ and FAD, are used. Three NADH, three H+ ions, and one FADH2 are formed.

The Electron Transport Chain NADH and FADH2 each deliver two energized electrons at the top of the electron transport chain in the inner membrane of the mitochondrion. This is similar to the chains in the thylakoid membrane of chloroplasts in plant cells during photosynthesis. The electrons are passed from protein to protein along the chain within the membrane slowly releasing their energy. Some of that energy is used to form ATP and some is used by an enzyme to pump H+ ions into the center of the mitochondrion.

Electron Transport Chain continued… The last electron acceptor at the end of the chain is oxygen and reacts with four hydrogen ions, which forms two molecules of water. This is why oxygen is so important in our bodies. Without oxygen proteins in the electron transport chain cannot pass along the electrons and therefore cannot accept more electrons causing a block and ATP production to stop. The electron transport chain adds 32 ATP molecules to the 4 already produced by glycolysis, which makes the aerobic process of ATP production much more effective. However in the absence of oxygen the anaerobic process can produce small amounts of ATP to keep the cell from dying.

Fermentation Your cells can go short periods of time without oxygen, like during exercise. When this happens an anaerobic process called fermentation follows glycolysis and allows ATP production to continue until oxygen is available again. Two major types of fermentation are lactic acid fermentation and alcoholic fermentation.

Lactic Acid Fermentation Under anaerobic conditions the citric acid cycle and glycolysis cannot continue without a steady supply of NAD+ and FAD. NAD+ can be replaced through lactic acid fermentation in the absence of oxygen. Two molecules of pyruvic acid produced in glycolysis use NADH to form two molecules of lactic acid. This releases NAD+ to be used in glycolysis again allowing 2 ATP molecules to form for each glucose molecule. The lactic acid is transferred from muscle cells where it is produced from strenuous exercise, to the liver which converts it back to pyruvic acid. This lactic acid results in muscle fatigue.

Alcoholic Fermentation Alcoholic fermentation is used by yeast cells and some bacteria to produce CO2 and ethyl alcohol. Example: when making bread yeast cells produce carbon dioxide, which forms bubbles in the dough causing it to rise. Once it is baked the heat of the oven kills the yeast and the bubbles leave holes in the bread making it lighter.

Comparing Photosynthesis and Cellular Respiration Both use electron carriers, a cycle of chemical reactions to form ATP, electron transport chains to form ATP, and a concentration gradient of H+ within the cell to form ATP by chemiosmosis. Each accomplishes different tasks. Photosynthesis produces high-energy carbohydrates and oxygen from the sun’s energy, whereas cellular respiration uses oxygen to break down carbohydrates to form ATP and compounds that provide less energy.

Comparing Photosynthesis and Cellular Respiration Food synthesized Food broken down Energy from sun stored in glucose Energy of glucose released Carbon dioxide taken in Carbon dioxide given off Oxygen given off Oxygen taken in Produces sugars from PGAL Produces CO2 and H2O Requires light Does not require light Occurs only in presences of chlorophyll Occurs in all living cells