Biomolecules II Cellular Respiration

Cellular Respiration is the name we give to the process by which plants and animals extract energy from glucose. takes place in all plant and animal cells are of two types: aerobic and anaerobic cellular respiration 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 it 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 Sugar Glycolysis Krebs Cycle Electron Transport Chain H2O + CO2 The Three “Biggies” Glycolysis the Krebs Cycle the Electron Transport Chain Respiration is broken down into 3 main sets of chemical reactions. Before respiration can even begin, complex carbohydrate molecules must be “cut up” so that individual glucose molecules can be freed up. Glucose is then broken down, and the energy from its chemical bonds are extracted through a series of carefully controlled steps. These steps are grouped into 3 sets and we will discuss each one and where, in the cell, these sets of reactions occur. Glycolysis The Krebs Cycle Electron Transport Chain

A Major Player - ATP (adenosine triphosphate) the Cell’s Energy Currency = ATP P = Phosphate But before we can discuss the breakdown of glucose, we need to revisit the ATP molecule. ATP stands for adenosine tri-phosphate or. It is a very important molecule because it captures the energy that is released when the chemical bonds in glucose are broken apart. 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’s useful to think of ATP as the cell’s money, that moves about to make other reactions happen. Adenosine P P P ~ ~ ~ High Energy Bonds

Stylized Cell ID cytoplasm mitochondria cell membrane

Glycolysis – in the cytoplasm break! energize! 2 ATP 2 ADP 2 ATP 2 ADP C C C C C C C C C C C C C C C C C C 1 Glucose molecule 2 Pyruvic Acid molecules 2 NAD+ 2 NADH Now 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. to the ETC

No Oxygen! anaerobic fermentation in animals and plants 2 ATP 2 ADP C C C C C C C C C C C C C C C C C C Glucose 2 Pyruvic Acid 2 Lactic Acid 2 NAD+ 2 NADH 2 NADH 2 NAD+ If no oxygen is available, pyruvic acid is converted to lactic acid and that is all the energy that the body is able to extract from glucose Animal Bacteria muscles cheese yogurt

No Oxygen! anaerobic fermentation in yeast 2 Pyruvic Acid Alcohol 2 NADH 2 NAD+ CO2 If no oxygen is available or more specifically excluded from the environment, the pyruvic acid is converted to alcohol and CO2 – hence the term anaerobic fermentation. Yeast: Bread, Wine, Beer, Root Beer

Stylized Cell ID cytoplasm mitochondria cell membrane

Krebs Cycle – in mitochondria Glycolysis (cytoplasm) ETC (membrane) Krebs Cycle (matrix) If oxygen is present, the next set of steps in the extraction of energy from glucose occurs through a series of chemical reactions called - the Krebs cycle. Two, three carbons sugars enter one after the other and are broken down further, releasing 2 molecules of CO2 and creating 2 more ATP energy molecules. There are two other types of molecules present in this cycle that also capture energy in the form of high energy electrons, (NADH and FADH2). They then transfer their captured energy to the third set of reactions also located in the mitochondria…..

Acetyl CoA formation – in the cytoplasm 2 Pyruvic Acid molecules C 2 carbon dioxide 2 acetyl coA to Krebs Cycle

Krebs Cycle – in mitochondrial matrix each Acetyl-CoA enters Krebs cycle in turn each Acetyl-CoA releases 2 CO2 molecules forms 1 ATP molecules converts 3 NAD+ to NADH converts FAD to FADH2 The next set of steps occurs in the Krebs cycle. Two, three carbons sugars enter one after the other and are broken down further, releasing 2 molecules of CO2 and creating 2 more ATP energy molecules. There are two other types of molecules present in this cycle that also capture energy in the form of high energy electrons, (NADH and FADH2). They then transfer their captured energy to the third set of reactions also located in the mitochondria…..

the Electron Transport Chain The bulk of energy from breaking down glucose – 32 ATP - is generated in the ETC along the inner membrane of mitochondria oxygen combines with hydrogen ions to form water the H+ ‘s drive the formation of ATP In the final steps of glucose breakdown, the energy captured by the molecules in the Krebs cycle is used to form 32 ATP molecules and is used to make oxygen combine with hydrogen (carried by those other energy molecules) to form H20.

Electron Transport Chain in the membrane This diagram shows how carefully organized the transfer of energy is in the mitochondrial membrane.

in Summary: In summary, the three sets of reactions are: Glycolysis that yield 2, three carbon molecules and a net sum of 2 ATP energy molecules Krebs Cycle that releases six molecules of CO2, 2 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 + 36 ATP

Alternative Biomolecule Pathways Not only does glucose provide energy for cellular metabolism, fats and amino acids also enter the glycolytic and Krebs cycle pathways. Like glucose, the chemical bonds in fats and proteins yield energy to form high energy ATP molecules, leaving behind smaller molecules. Fats break down into glycerol and fatty acids. Proteins break down into ammonia and other smaller “waste product” molecules.