Cellular Respiration

Energy Review: As an open system , cells require a constant source of energy to carry out their life functions. The main source of energy for most living systems is the sun.

Photosynthetic organisms capture sunlight and transform it into a useable source of energy via the chemical bonds in the organic compounds it produces.

Cells use some of this chemical bond energy to make ATP, the energy source for cellular work. Much of this energy is released as unusable heat. The products of respiration are the raw materials of photosynthesis. 

Chemical elements essential for life are recycled but energy is not. In this first half of the unit, we are going to examine the cellular respiratory process.

Catabolism: Chemical pathways that break down materials and release energy.

Fermentation. An ATP producing pathway in which both electron donors and acceptors are organic compounds. The process can be anaerobic and results in partial degradation of sugars.

Cellular Respiration. An ATP producing process in which the ultimate electron acceptor is an inorganic molecule, such as oxygen. It is the most prevalent and efficient catabolic process. It is an exergonic process producing a delta G of -686 kcal/mol.  

The catabolic process of respiration transfers the energy stored in food molecules to ATP. If the energy of sugar molecules was released at one time the body could not use it efficiently.  

Organisms use ATP molecules to capture and release small amounts of energy to fuel various bodily functions. ATP is unique in its function of transferring energy. 

ATP review: The molecule contains the nitrogenous base adenine connected to three molecules of phosphorous. The bonds between the last 2 phosphates are high energy bonds.

When ATP releases the terminal (end) phosphate, energy is released while forming a new compound ADP. ADP can be refitted with another phosphate (phosphorylation) to form ATP again.A-P-P-P---> A-P-P + Pi +energy

Cellular respiration contains the steps to allow this process to occur. In order for the most efficient production of ATP to occur the cell must transfer this energy from the chemical bonds of the organic compounds (like glucose) to the ATP molecule with minimal loss.  

The process involved is found in basic inorganic chemistry, the REDOX reactions. Redox is short for oxidation-reduction which involves partial or complete transfer of electrons from one reactant to another. Oxidation = Partial or complete loss of electrons. Reduction = Partial or complete gain of electrons.  

Basic rule to remember: Electrons lose potential energy when they shift toward more electronegative atoms, redox reactions that move electrons closer to oxygen release energy.

Cellular respiration is a redox process that transfers hydrogen atoms from sugar to oxygen.  

In order to allow these hydrogen atoms to be transfered, an intermediate organic compound is called into use by the cell. This coenzyme is called NAD+

short for nicotinamide adenine dinucleotide.  

During the oxidation of glucose, NAD+ functions as an oxidation agent by trapping energy rich electrons from glucose or food. These reactions are catalyzed by enzymes called dehydrogenases.

The end results are the removal of 2 hydrogen atoms from the substrate (glucose) with the delivery of one to NAD+ forming NADH and the other into solution.

The hydrogen attached to the NADH will be passed down a series of protein receptors in the inner mitochondrial membrane (electron transport chain) to produce an abundant supply of ATP.

Cellular respiration can be divided up into 3 stages: Glycolysis Krebs Cycle (Citric Acid Cycle) Electron Transport Chain (ETC) and oxidative phosphorylation  

Glycolysis: harvests chemical energy by oxidizing glucose to pyruvate.

 

Glycolysis is a catabolic pathway during which six-carbon glucose is split into 2 three- carbon sugars,which are then oxidized and rearranged to produce two pyruvate molecules.

Each reaction is catalyzed by specific enzymes dissolved in the cytosol. No carbon dioxide is released as glucose is oxidized to pyruvate. All the carbon can be accounted for in the 2 molecules of pyruvate.  

It occurs under aerobic or anaerobic conditions. The process occurs in two phases: The energy investment phase and the energy yielding phase. The key part is that it releases 2 pyruvate molecules that are later shuttled into the mitochondrial membrane.

The Steps of Glycolysis: Taken from the Univ. of Virginia web Page.

   

The Krebs Cycle: completes the energy yielding oxidation of organic molecules. Most of the chemical energy originally stores in glucose still remains in the two pyruvate molecules produced by glycolysis.

The fate of pyruvate depends upon the presence or absence of oxygen. If oxygen is present, pyruvate enters the mitochondrion where it is completely oxidized by a series of enzyme-controlled reactions.

The junction between glycolysis and the Krebs Cycle is the formation of Acetyl-CoA. The Acetyl-CoA combines with oxaclacetic acid to begin the cycle. This process occurs in the mitochondrial matrix.  

The Electron Transport Chain: is made of electron carrier molecules embedded in the inner mitochondrial membrane.

Steps of the Electron Transport Chain:    

Each successive carrier in the chain has a higher electronegativity than the carrier before it, so the electrons are pulled down hill toward the oxygen. Except for ubiquinone (Q), most of the carriers are protein containing a non -protein cofactor.

The cofactors alternate between an oxidized and reduced state as they accept an d donate electrons.   Protein Electron CarriersCoFactor GroupsFlavoproteinsflavin mononucleotide (FMN)iron-sulfur proteinsiron and sulfurcytochromesheme group 

Fermentation: a cell process that can produce ATP without the presence of oxygen. Fermentation recycles NAD+

from NADH. The two most common forms of fermentation are: 1) alcoholic and 2). lactic acid fermentation.  

Alcohol Frementation (plants cells).    

Lactic Acid Fermentation (animal cells).