C ELLULAR R ESPIRATION How do heterotrophs get energy?

Q UESTIONS What event begins the Kreb’s cycle? Where does glycolysis occur? Where does the Kreb’s Cycle take place? What are the products of cellular respiration? What are the differences between the products of aerobic and anaerobic respiration? What are the energy carrying molecules used? How many ATP are made and at what point are they generated at?

T HERE ARE THREE STEPS IN THE PROCESS OF CELLULAR RESPIRATION : Glycolysis Krebs cycle electron transport chain.

G LYCOLYSIS Glycolysis occurs in the cytoplasm of a cell where a 6 carbon glucose molecule (the broken down food that you ate earlier) is broken down by enzymes into a 3 carbon pyruvic acid. cytoplasm pyruvic acid The execution of this process requires 2 ATP, and produces a net gain of 2 ATP. The enzymes involved remove hydrogen from the glucose (oxidation) where they take these hydrogen atoms. In anaerobic respiration, this is where the process ends, glucose is split into 2 molecules of pyruvic acid. When oxygen is present, pyruvic is broken down into other carbon compounds in the Kreb's Cycle. When it is not present, the pyruvic acid is broken down into lactic acid (or carbon dioxide and ethanol).

T HE K REB ’ S C YCLE When oxygen is present, respiration can harness more ATP from a single unit of glucose. The pyruvic acid from the glycolysis stage diffuses into a cell organelle called a mitochondrion (pl. mitochondria ). These mitochondria are sausage shaped structures that host a large surface area for the respiration to occur on. pyruvic acid glycolysis mitochondria The pyruvic acid is then subject to more enzymes which break it down into a 2 carbon compound, as seen below. The diagram illustrates the Kreb's cycle, consisting of three main actions Kreb's cycle The carbon element is in an infinite cycle where the 2 carbon compound derived from pyruvic acid binds with the 4 carbon compound that is always present in the cycle. CO 2 is released, where the oxygen that is present in aerobic respiration combines with carbon from the carbon compounds which is released as CO 2. Hence the need for animals to breath out and expel this CO 2. aerobic respiration

L ACTIC A CID F ERMENTATION Lactic acid fermentation is done by some fungi, some bacteria like the Lactobacillus acidophilus. in yogurt, and sometimes by our muscles. Normally our muscles do cellular respiration like the rest of our bodies, using O 2 supplied by our lungs and blood. However, under greater exertion when the oxygen supplied by the lungs and blood system can’t get there fast enough to keep up with the muscles’ needs, our muscles can switch over and do lactic acid fermentation. In the process of lactic acid fermentation, the 3-carbon pyruvic acid molecules are turned into lactic acid. It is the presence of lactic acid in yogurt that gives it its sour taste, and it is the presence of lactic acid in our muscles “the morning after” that makes them so sore. Once our muscles form lactic acid, they can’t do anything else with it, so until it is gradually washed away by the blood stream and carried to the liver (which is able to get rid of it), our over-exerted muscles feel stiff and sore even if they haven’t been physically injured.

A LCOHOLIC F ERMENTATION Alcohol fermentation is done by yeast and some kinds of bacteria. The “waste” products of this process are ethanol and carbon dioxide (CO 2 ). Humans have long taken advantage of this process in making bread, beer, and wine. In bread making, it is the CO 2 which forms and is trapped between the gluten (a long protein in wheat) molecules that causes the bread to rise, and the ethanol evaporating that gives it its wonderful smell while baking. It is the CO 2 produced by the process of fermentation that gives the baked goods their appearance.