Cellular Respiration 9 th grade – Biology Miss Alexandra Martínez GCI

We will learn how organic compounds are broken down in ATP, and the basic events of Cellular Respiration (Glycolisis, Krebs Cycle, Electron Transport Chain); as well as alternate energy pathways that take place in the absence of oxygen. What will we learn?

Cellular respiration is the process by which the energy in organic compounds, specially glucose, is stored in ATP. Just as our cars need fuel to function, we need energy for our everyday activities. For these activities, our bodies need to breakdown food molecules and extract that energy. So we relay on this process “Cellular Respiration” to live. We harvest energy by breaking down “energy-rich” molecules (such as glucose), and as this happens, energy is captured in the bonds of ATP molecules. Cellular Energy

INPUT OUTPUT Glucose + Oxygen Carbon Dioxide + Water + ATP C 6 H 12 O 6 + O 6 CO 2 + H 2 O + ATP Cellular Respiration

Glycolisis is a series of chemical reactions inside the cytoplasm in which glucose is essentially splited in halves, and where a little amount of energy is released. Stage 1: Glycolisis

1.Two molecules of ATP are added to the glucose, attaching two phosphates to each side, making the molecule unstable, and vulnerable to break down. 2.Three chemical reactions yield the production of energy:  Each half is added to another phosphate group and each half donates High Energy Electrons to “electron carrier molecules” called NAD +, and protons (hydrogen ions H + ) forming two NADH molecules.  As glycolisis continues, two phosphate bonds from the two halves of glucose are broken, and the energy released is quickly recaptured by ADP molecules, making two ATP molecules. Stage 1: Glycolisis

Two molecules of water are also produced during glycolisis. And by the end two ADP molecules take the remaining phosphate groups, forming two additional ATP molecules. 2.By the end of glycolisis, a molecule of glucose has been broken down into two molecules of Pyruvate.  The final output of glycosis results in the production of 2 NADH, and 4 ATP molecules. However, two ATP molecules were originally invested at the beginning of the process, so the net yield is 2 ATP and 2 NADH. Stage 1: Glycolisis

Stage 2: Krebs Cycle  The two pyruvate molecules move on to the inner sack of the mitochondria, where the Krebs Cycle takes place.  To prepare for the Krebs Cycle, the pyruvate is modified in three quick steps : 1.With oxygen present, a molecule of CO 2 is removed from the pyruvate, and eventually exhaled from the body. 2.A coenzyme present in the mitochondria (Coenzyme-A) is attached to each pyruvate, releasing electrons and protons, which are donated to NAD +, making two more NADH molecules. 3.The two pyruvates become Acetyl-CoA

Stage 2: Krebs Cycle AFTER THE 3 QUICK MODIFICATIONS

Stage 2: Krebs Cycle  The steps of the Krebs Cycle are as follows: 1.The first step of this pathway is when Acetyl-CoA combines with water and a 4-carbon compound (oxaloacetate), to be rearranged into a 6-carbon molecule, releasing the Coenzyme- A. 2.This resulting molecule of 6-carbons is reorganized by the addition and then removal of water, giving away electrons and protons, forming two NADH and releasing two CO2 3.Energy is drawn from the remaining molecule, to join ADP and a phosphate to produce a molecule of ATP. 4.With the addition of water, this remaining molecules donates more electrons and protons to another energy carrier molecule FADH+, becoming FADH2, and another molecule of NADH is also formed.

Stage 2: Krebs Cycle  The final outcome of the Krebs Cycle is the re-formation of the 4-carbon molecule (oxaloacetate).  The entire cycle happens again with the other Acetyl-CoA.  As the second round continues, more NADH, more CO2, more ATP and more FADH2 is formed.  Considering the molecules formed during the modifications of pyruvates, we are able to realize that the 6-carbons contained in the original glucose molecule, are all released as CO2 molecules.  The final outcome of this cycle is then: 6 CO2, 6 NADH, 2 ATP, and 2 FADH2

Stage 2: Krebs Cycle Although a small amount of ATP has been formed in the Krebs Cycle, it is the NADH, and FADH2, that represents the most energy for the cell. Because these molecules are going to be used in the next stage to produce a large amount of ATP.

Stage 3: Electron Transport Chain  The last step in cellular respiration, the Electron Transport Chain (ETC), takes place in the membrane of the inner sack of the mitochondria.  This step is essential to produce enough ATP for animals and many other living organisms to survive.  Through the ETC the cell can now use the energy temporarily stored in NADH and FADH2, to produce ATP. These molecules donate their electrons to the ETC.  As electrons move from one protein to another in the ETC, they transfer their energy to these proteins to pump H+ across the membrane.

Stage 3: Electron Transport Chain  With each transfer, electrons loose energy, which is used to pump more protons.  The process of electron transfers, results in a difference of concentration of H+, on the two sides of the membrane, creating a concentration gradient.  The oxygen that we breath is essential to ETC. This oxygen grabs the electrons at the end of the ETC, and together with H+, form molecules of water.  A complex in the membrane, provides the pathway for H+, to move out of the membrane and produce ATP.  The H+ will tend to flow in this direction until the concentration gradient disappears

Stage 3: Electron Transport Chain

Oxygen grabs the electrons

Stage 3: Electron Transport Chain

Fermentation  If oxygen disappears, the Kreb Cycle and the ETC, will shut down.  Fermentation is an alternative pathway when oxygen is not present, and it follows glycolisis.  There are two types of fermentation: 1.Lactic Fermentation 2.Alcoholic Fermentation

Lactic Acid Fermentation  Under anaerobic (absence of oxygen), the Krebs Cycle and the ETC cannot happen.  So each of the two molecules of pyruvates produced during glycolisis use a molecule of NADH to form two molecules of Lactic Acid.  This chemical reaction causes the NADH, become again NAD+, which is recycled to glycolisis again.  Lactic acid builds up in muscles, causing a burning pain, blood pH drops causing muscle fatigue.  At rest, lactate, is converted back to pyruvate.  So we end up, with only the two molecules of ATP, produced during glycolisis.

Alcoholic Fermentation  In other organisms, the 3-carbon pyruvate is broken down to ethanol, through alcoholic fermentation.  First, the pyruvate is converted to a 2-carbon compound, releasing CO2.  Second, electrons and H+ are transferred from NADH to the 2-carbon compound to form Ethanol (Ethyl Alcohol).  The molecule of NAD+ is recycled back to glycolisis.  The outcome of this process will then be: CO2 and Ethyl Alcohol, and yielding only the production of two molecules of ATP, produced during glycolisis.  Alcoholic fermentation by yeast, a fungus, has been used in the preparation of many foods and beverages.

Production of ATP Glucose Glycolisis Pyruvate Krebs Cycle 2 ATP With O2 ETC Aerobic Respiration 34 ATP Fermentation Without O2 Lactate Ethanol and CO2 Anaerobic Processes