Cellular Respiration a real brain twister

Cellular Respiration the process by which plants and animals extract energy from glucose. takes place in plant and animal cells Respiration is the name we give to a process that involves almost a hundred chemical reactions. During respiration, cells extract the energy stored in the chemical bonds of glucose by breaking glucose down in a discreet number of controlled steps. All plant and all animal cells undergo respiration. There are two types of respiration: one type requires the presence of oxygen and is called aerobic respiration. The second type can occur without oxygen and is called anaerobic respiration.

Cellular Respiration Overview C6H12O6 Energy + H2O + CO2 Glycolysis The Three “Big Parts” Glycolysis Krebs Cycle Electron Transport Chain Krebs Cycle Electron Transport Chain O2 Respiration is broken down into 3 main sets of chemical reactions. Before respiration can even begins, complex carbohydrate molecules must be “cut up” so that individual glucose molecules can be freed up. We don’t explore this part in biology. We begin with glucose. Glucose is broken down, and the energy from its chemical bonds are transferred through a series of carefully controlled steps into the bonds of a high energy compound found in all cells – ATP (adenosine triphosphate). “breakdown” of glucose steps is grouped into 3 partsand we will discuss each one and where, in the cell, each set of reactions occurs. The three parts are named: Glycolysis The Krebs Cycle (aka TCA cycle) Electron Transport Chain (aka ETC)

Cellular Respiration – The Energy Players ADP/ATP - the energy currency of the cell NAD+/NADH - the shuttle bus of the cell FAD/FADH2 - the other shuttle bus of the cell

The Major Energy Players – ADP / ATP But before we can discuss the breakdown of glucose, we need to acquaint ourselves with the ADP and ATP molecules. ATP stands for adenosine tri-phosphate. ADP stands for adenosine diphosphate. These are very important molecules because chemical energy is transferred from the chemical bonds in glucose to the chemical bonds in ATP. ATP, now rich in energy captured during glucose metabolism, can move anywhere in the cell to drive other chemical reactions that require energy, and most do. It is a key player in active transport, among other actions. It’s useful to think of ATP as the cell’s money, that moves about to make other reactions happen. (adenosine triphosphate)

Shuttle Buses: NAD+ / NADH (nicotinamide adenine dinucleotide) NAD stands for nicotinamide adenine dinucleotide. NADH is NAD with an extra hydrogen atom. During cellular respiration, NAD captures free hydrogen ions and ferries them to the electron transport chain where they are used to drive other reactions, specifically the formation of ATP.

Another Shuttle Bus - FAD (flavin adenine dinucleotide) FAD stands for flavin adenine dinucleotide. FADH2 is FAD with two extra hydrogen atoms and 2 electrons. During cellular respiration, FAD captures free hydrogen ions and electrons and ferries them to the electron transport chain where they are used to drive other reactions, specifically the formation of ATP.

Krebs Cycle ETC Glycolysis Glucose 2C3 2 ATP 2 ADP 2 CO2 2 Pyruvate 10 NAD+ 10 NADH 2 FAD 2 FADH2 Here are the 3 big steps in cellular respiration again. The reactant for glycolysis is glucose and the product is pyruvate. During the process, a small amount of ATP is formed. Pyruvate is converted to acetylCoA that enters the Krebs cycle. NADH and FADH2 are produced along with 6 molecules of carbon dioxide. Oxygen, NADH and FADH2 enter the ETC. Water and ATP are produced. 2 CO2 ETC 2 CO2 32 ATP H2O

Glycolysis – in the cytoplasm Let’s look at the first set of steps in the breakdown of glucose. This first set of steps is called glycolysis. During these steps which are carefully controlled in our bodies, glucose is broken up into 2, three carbon long molecules called pyruvic acid. To get the reactions to happen, the cell must invest 2 ATP. As a result of this investment, the glucose breaks down and yields 4 ATP for a net gain of 2 ATP energy molecules. This set of reactions happens in the cell’s cytoplasm and is the oldest means by which glucose generates energy. In the absence of oxygen, this is as far as glucose is broken down. Glycolysis

Krebs Cycle – in the mitochondrial matrix If oxygen is present, pyruvate is converted to acetylCoA which then enters the Krebs cycle in the mitochondrial matrix. Two, three carbons sugars enter one after the other and are broken down further in the Krebs cycle, releasing 2 molecules of CO2 and creating another ATP energy molecule. NADH and FADH2 capture hydrogen ions and energy in the form of high energy electrons. They then transfer their captured energy to the third set of reactions also located in the mitochondria….. Krebs Cycle

Krebs Cycle Overview ETC Glucose 2C3 2 ATP 2 ADP 2 Pyruvate 2 CO2 10 NADH 10 NAD+ 2 FAD 2 FADH2 2 ADP 2 ATP In the Krebs Cycle, each three carbon pyruvate molecule loses 1 carbon in the form of carbon dioxide to form acetyl CoA. Each Acetyl CoA molecule is further broken down yielding two carbon dioxides. During these two sets of reactions, a small amount of ATP is formed. NAD and FAD pick up free hydrogen ions and excited electrons. They will shuttle the hydrogen ions and the excited electrons to the electron transport chain. 2 CO2 ETC 2 CO2 32 ATP H2O

Kreb’s Cycle

The Electron Transport Chain - Overview Glucose 2C3 2 Pyruvate 2 ATP 2 ADP 2 CO2 4 ADP ½ O2 4 ATP 10 NAD+ 10 NADH 2 FAD 2 FADH2 2 CO2 ETC 2 CO2 32 ATP H2O

The Electron Transport Chain – Part 1 NADH, H+ and FADH2 move to the inner mitochondrial membrane

The Electron Transport Chain – Part 2 NADH releases its H+ and excited electrons into the membrane. The H+ moves across the membrane into the intermembrane space. The electrons move along the ETC and their energy is used to move more H+ across the membrane

The Electron Transport Chain – Part 3 FADH2 also relinquishes its H+ and electrons driving even more H+ across the membrane.

The Electron Transport Chain – Part 4 Electrons are transferred to oxygen that combines with H+ to form water

The Electron Transport Chain – Part 5 The H+’s in the inter-membrane space move down their concentration gradient through an enzyme, ATP synthease that makes ATP from ADP and P. ETC

in Summary: In summary, the three sets of reactions are: Glycolysis that yields 2, three carbon molecules and a net sum of 2 ATP energy molecules Krebs Cycle that releases six molecules of CO2, 4 ATP energy molecules and “electron carriers”, and Electron Transport Chain that generates 6 H2O molecules, 32 ATP molecules and regenerates the electron carriers The balanced chemical equation is the dual of the photosynthesis equation. C6H12O6 + 6O2  6CO2 + 6 H2O + 38 ATP

in the absence of oxygen: