Cellular Respiration

 ENERGY EXTRACTION

Photosynthesis provides plants and other photosynthetic organisms (autotrophs) with a long-term energy supply that they can draw on even when sunlight is not available. Plants store carbohydrates in their tissues or in special structures such as tubers in potatoes.

Other organisms (heterotrophs) rely on these energy-rich carbohydrates for their nourishment. They break down these complex polysaccharides through digestive processes into a simple compound called glucose whose high-energy bonds can be broken to release useful energy.  The extraction of energy from a molecule of glucose involves complex enzyme-controlled steps that occur in the cell cytoplasm and inside the mitochondrion, the energy factory of cells.

                               An overview of glycolysis and cellular respiration

In most organisms, the extraction of energy from glucose involves two processes. The first, called glycolysis, takes place in the cytoplasm of the cell in the absence of oxygen. The second process, called aerobic respiration or cellular respiration, occurs inside the mitochondria in the presence of oxygen. Glycolysis and cellular respiration are catabolic reactions in which a molecule of glucose is broken down gradually with the aid of enzymes to yield energy needed to drive the activities of organisms.  The overall reaction is represented by the following formula, which is essentially a reverse of the formula for photosynthesis.

 Glycolysis

Glycolysis or ‘glucose splitting’ is the first step in the catabolic degradation of the glucose molecule to release energy contained in its bonds. In nine enzyme-catalyzed reactions, one molecule of glucose is split into two molecules of a 3-carbon compound called pyruvic acid. The process produces two molecules of ATP by substrate-level phosphorylation ( substrate-level phosphorylation is the production of ATP using energy from other high-energy compounds but without the use of the electron transport system in the mitochondria) and reduces two molecules of nicotinamide adenine dinucleotide (NAD⁺) to two molecules of nicotinamide adenine dinucleotide hydrogen (NADH). Both ATP and NADH are rich-energy storage molecules. The cell can use the energy in ATP immediately but not that of NADH because electrons from NADH must first pass down the respiratory transport electron chain before they can yield ATP, the energy currency of cells.

 (Energy yield from glycolysis: 2ATP and 2NADH).

Fermentation

The fate of pyruvic acid depends on the environment of the organisms in which the reactions are taking place. In organisms that live in environments that lack oxygen, called anaerobes, such as yeast and some chemoautotrophic bacteria that dwell in deep-sea thermal vents, the pyruvic acid is broken down to alcohol with the release of CO₂. This is known as alcoholic fermentation. Other anaerobic bacteria and muscle cells deprived of oxygen especially after a strenuous exercise, covert pyruvic acid to lactic acid in another fermentative process called lactic acid fermentation.  Fermentation is not a very effective way of extracting energy.

 Aerobic Respiration

 The fact that glycolysis exists today in all organisms, supports the view that it was probably the first process by which the original inhabitants of the earth used to extract useful energy. Glycolysis must have evolved at a time when there was not much oxygen in the atmosphere. Aerobic respiration became important in energy extraction from carbohydrates after photosynthetic plants had appeared.

Although glycolysis occurs in all organisms, it releases only a fraction of the total energy contained in glucose. The rest of the energy remains trapped in the two molecules of pyruvic acid. Aerobic respiration is by far a more efficient method of dismantling the bonds of glucose than glycolysis.

Formation of Acetyl-coA

Glycolysis ends where the formation of acetylCoA starts. In the presence of oxygen, the pyruvic acid produced in glycolysis enters the ²mitochondrion and once inside each molecule of pyruvic acid loses a carbon dioxide, converts NAD⁺ to NADH and attaches an enzyme called coenzyme A to form a new molecule called acetyl CoA.

                                         Formation of acetylCoA

Acetyl CoA is a 2-carbon molecule important in metabolism. In addition to being produced from pyruvic acid, it is part of the catabolic pathways of fats and amino acids. The formation of acetyl CoA is a bridge between glycolysis and aerobic or respiration.

Briefly, let us recap what has happened so far. One molecule of glucose has been converted to two molecules of pyruvic acid, each of which has been converted to a molecule of acetyl CoA and two molecules of CO₂ have been released.

The Krebs cycle

Acetyl CoA now enters another pathway, the Krebs cycle, also known as the citric acid cycle. Energy is released as hydrogen atoms and CO₂ are removed from the intermediates of the Krebs cycle. The Krebs cycle results in the production of 1 ATP molecule and 3 NADH molecules. I molecule of another electron carrier, flavin adenine dinucleotide (FAD), is reduced to FADH₂.  However, the energy from each glucose molecule is twice this because each glucose molecule sends 2 molecules of acetyl CoA through the Krebs cycle.

                                                                  The Krebs cycle

(Energy yield of Krebs cycle: 2ATP, 6NADH, 2FADH₂).

The Respiratory Electron Transport Chain

As is now familiar to us, glucose is an energy rich compound and its breakdown releases energy that is stored as ATP, which is the energy currency of cells. Our examination of the first three stages of glucose catabolism shows a net gain of only 4 ATP molecules (2 in glycolysis and 2 in Krebs cycle). These represent a fraction of the energy originally present in a molecule of glucose. Of the remainder, much is liberated during the first 3 stages (mostly as heat, which is necessary for the reaction to proceed); the rest is stored in the high-energy intermediaries, NADH and FADH.    

In the final stage of extraction, the energy contained in NADH and FADH2 is used to convert ADP to ATP. This takes place through a series of electron accepting enzymes that are embedded in the inner mitochondrial membrane forming a respiratory chain. Electrons are transferred from NADH and FADH₂ to electron carrier molecules in the respiratory chain, losing energy as they move from one carrier molecule to the next in the chain. Some of the released energy is captured to make ATP. Each molecule of NADH is used to convert 3 molecules of ADP to ATP, while the energy from 1 molecule of FADH₂ is used to convert 2 molecules of ADP to ATP.  Oxygen, the final acceptor molecule of the respiratory chain, combines with hydrogen to form water.

Summary of the Energy generated from 1 molecule of glucose

Cellular respiration step	location	Number of ATP produced	Function
Glycolysis	cytoplasm	4 gross, net 2 ATP	Begins catabolism of glucose; reduces glucose to ATP & NADH
Krebs cycle	matrix	2 ATP	Completes the degradation of glucose; converts glucose energy to ATP, NADH and FADH2
Electron transport	cristae	32 ATP	Converts energy stored in NADH and FADH2 into ATP

The ATP output from the Krebs cycle is therefore 2+18+4, or 24. A total ATP accounting (6 from glycolysis, 6 from acetyl CoA production, and 24 from the Krebs cycle) indicates that 36 ATP are generated from the complete breakdown of 1 glucose molecule.

   ¹Yeast is used in bread baking and in wine making.  When it is added to bread dough, yeast is in an aerobic environment. As the yeast cells grow and multiply, they use up oxygen and give off carbon dioxide. The carbon dioxide bubbles cause the bread to rise. When the bread is baked, the heat kills yeast cells.

In wine making, yeast is added to solution in sealed vats. The yeast cells then consume all the available oxygen as they multiply making the environment in the vats increasingly anaerobic. The yeast cells then turn to the sugars in the wine solution for energy. The sugars are broken down to yield energy, carbon dioxide and alcohol. This type of energy pathway is called alcoholic fermentation.  The yeast cells are killed when the alcohol content of the wine reaches 12-16 % due poisoning by their own wastes.

 Some anaerobic bacteria and animal cells that are temporarily deprived of oxygen, convert the pyruvic acid of glycolysis to a three-carbon compound called lactic acid. Lactic acid fermentation, as this pathway is called, occurs in human muscle cells when they are exercising so strenuously that the consumption of pyruvic acid exceeds their supply of oxygen. In this condition of ‘oxygen debt’, the cells revert to anaerobic respiration causing an accumulation of lactic acid in the muscles. This lowers the pH and causes muscle fatigue and soreness. When the oxygen supply is restored, lactic acid is converted to pyruvic acid by the liver.

²The mitochondrion
Mitochondria are rod-shaped structures that are enclosed within two membranes - the outer membrane and the inner membrane.  The outer membrane is smooth and completely permeable to nutrient molecules, ions, ATP and ADP. The inner membrane is more complex than the outer membrane as it contains the complexes of electron transport chain and ATP synthetase complex. The mitochondrion  is permeable to oxygen, CO₂ and water and consists of a large number of proteins that take part in the production of ATP. The inner membrane has infoldings called the cristae that increase the surface area for the complexes and proteins that aid in the production of ATP. The matrix is a complex mixture of enzymes that are important for synthesis of ATP molecules,

After glycolysis occurs, the two pyruvic acid molecules enter the mitochondria and diffuse to the matrix where the remaining hydrogen atoms and their energy rich electron are removed. One turn of the Krebs cycle yields one ATP, one  FADH₂ and 3 NADH with the release of 2 CO₂ . There are 2 molecules of pyruvic acid. Therefore, there are 2 turns of the Krebs cycle, yielding 2 ATP, 2 FADH₂, and 6 NADH with 4 molecules of carbon dioxide released.

The FADH₂ and 3 NADH proceed to the cristae where they provide energy for Electron Transport (also known as oxidative phosphorylation).The energy provided by all the NADH and FADH₂ is used to pump H ions into the outer compartment. This creates   a charge imbalance across the cristae (high potential energy) and as H ions diffuse back to the matrix, they pass through a protein channel with an enzyme, ATP synthase, which takes the energy released by the H ions and uses it to create ATP (from ADP + P). Oxygen bonds with 2 hydrogen ions (removing them so aerobic respiration can continue) to form water.