7 Chapter 7

Learning Objectives

  1. Describe reactions vital to cellular respiration
  2. Identify cellular location, requirements and products for the three processes of cellular respiration in eukaryotes
  3. Describe conditions of fermentation

Cellular Respiration

ATP functions as the energy currency for cells. It allows the cell to store energy briefly and transport it within the cell to support endergonic chemical reactions. ATP has the structure of an RNA nucleotide with three phosphates attached. ATP is used for energy by detaching a phosphate group or two, resulting in ADP (two phosphates) or AMP (one phosphate). Energy derived from glucose catabolism is used to convert ADP to ATP. When ATP is used in a reaction, the third phosphate is temporarily attached to a substrate in a process called phosphorylation. The two processes of ATP regeneration that are used in conjunction with glucose catabolism are substrate-level phosphorylation and oxidative phosphorylation through the process of chemiosmosis.
Aerobic Cellular Respiration of glucose occurs in the presence of oxygen and includes three main processes in eukaryotes.
  1. Glycolysis
  2. Citric Acid Cycle (CAC) — sometimes referred to as Krebs
  3. Electron Transport Chain (ETC)

Glycolysis is the first pathway used in the breakdown of glucose to extract energy. It was probably one of the earliest metabolic pathways to evolve and is used by nearly all of the organisms on Earth. Glycolysis consists of two parts. The first part prepares the six-carbon ring of glucose for cleavage into two three-carbon sugars. ATP is invested in the process during this half to energize the separation. The second half of glycolysis extracts ATP and high-energy electrons from hydrogen atoms and attaches them to NAD+. Two ATP molecules are invested in the first half and four ATP molecules are formed by substrate phosphorylation during the second half. This produces a net gain of two ATP and two NADH molecules for the cell.

Glycolysis occurs in the cytoplasm of prokaryotes and eukaryotes for glucose (C6H12O6) metabolism. Glucose contains six carbons and is converted to two, three-carbon molecules of pyruvate, in a ten-step process driven by enzymes. The process requires 2 ATP and yields 4 ATP, for a net gain of 2 ATP.

This illustration shows the steps in the first half of glycolysis. In step one, the enzyme hexokinase uses one A T P molecule in the phosphorylation of glucose. In step two, glucose dash 6 dash phosphate is rearranged to form fructose dash 6 dash phosphate by phosphoglucose isomerase. In step three, phosphofructokinase uses a second A T P molecule in the phosphorylation of the substrate, forming fructose dash 1, 6 dash bisphosphate. The enzyme fructose bisphosphate aldose splits the substrate into two, forming glyceraldeyde dash 3 dash phosphate and dihydroxyacetone-phosphate. In step 4, triose phosphate isomerase converts the dihydroxyacetone-phosphate into glyceraldehyde dash 3 dash phosphate.

This image shows the first half of glycolysis, which requires two ATP molecules. The phosphorylation of glucose is followed by the split into two three-carbon molecules.

There is one substep between glycolysis and CAC. Within the mitochondria, pyruvate is converted to acetyl-CoA in a three-step process to prepare for citric acid cycle. In the presence of oxygen, acetyl CoA can enter several pathways, but most often, the acetyl group is delivered to the citric acid cycle for further catabolism. During the conversion of pyruvate, a molecule of carbon dioxide and two high-energy electrons are removed. The carbon dioxide accounts for two of the six carbons from the original glucose molecule. The electrons are picked up by NAD+, and the NADH carries the electrons to a later pathway for ATP production. At this point, the glucose molecule that originally entered cellular respiration has been completely oxidized.

The citric acid cycle is a series of enzyme-driven reactions removing high-energy electrons and carbon dioxide. The electrons, temporarily stored in molecules of NADH and FADH2, are used to generate ATP in a subsequent pathway. Citric acid cycle (aka Krebs) converts Acetyl CoA to citric acid. Products replenish an ongoing cycle. Molecules of CO2 are released, hydrogens are carried by NAD+ and FAD to Electron Transport Chain (ETC), and one molecule of ATP is produced from each Acetyl group entering the cycle for a total of two per glucose.

This illustration shows the eight steps of the citric acid cycle. In the first step, the acetyl group from acetyl uppercase C lower case o upper case A is transferred to a four-carbon oxaloacetate molecule to form a six-carbon citrate molecule. In the second step, citrate is rearranged to form isocitrate. In the third step, isocitrate is oxidized to alpha-ketoglutarate. In the process, one N A D H is formed from N A D superscript plus sign baseline; and one carbon dioxide is released. In the fourth step, alpha-ketoglutarate is oxidized and upper C lower o upper A is added, forming succinyl upper C lower o upper A. In the process, another N A D H is formed and another carbon dioxide is released. In the fifth step, upper C lower o upper A is released from succinyl upper C lower o upper A, forming succinate. In the process, one G T P is formed, which is later converted into A T P. In the sixth step, succinate is oxidized to fumarate, and one F A D is reduced to F A D H subscript 2 baseline. In the seventh step, fumarate is converted into malate. In the eighth step, malate is oxidized to oxaloacetate, and another N A D H is formed.

In the citric acid cycle, acetyl CoA is converted through a series of steps. Because the final product of the citric acid cycle is also the first reactant, the cycle runs continuously in the presence of sufficient reactants. Credit: Rao, A., Ryan, K., Tag, A., and Fletcher, S. Department of Biology, Texas A&M University.

The electron transport chain is the portion of aerobic respiration using free oxygen as the final electron acceptor of electrons removed from intermediate compounds in glucose catabolism. The electron transport chain is composed of four large, multiprotein complexes embedded in the inner mitochondrial membrane and two small diffusible electron carriers shuttling electrons between them. Electrons are passed through a series of redox reactions, with a small amount of free energy used at three points to transport hydrogen ions across a membrane. This process contributes to the gradient used in chemiosmosis. The electrons passing through the electron transport chain gradually lose energy. High-energy electrons donated to the chain by either NADH or FADH2 complete the chain, as low-energy electrons reduce oxygen molecules and form water.  Hydrogen ions [H+] are protons pumped by active transport through proteins into the intermembrane space. The primary passage for [H+] is through ATP synthase, a protein enzyme embedded in the membrane. Proton pumps [H+] power ATP production and leave ETC to form water, when oxygen is available. Since this process only occurs in the presence of oxygen, it is called aerobic respiration. Oxygen is the final electron acceptor. One glucose molecules will metabolize to yield approximately 36 ATP per molecule of glucose. A number of intermediate compounds of the citric acid cycle can be diverted into the anabolism of other biochemical molecules, such as nonessential amino acids, sugars, and lipids. These same molecules can serve as energy sources for the glucose pathways.

This illustration shows the electron transport chain, the A T P synthase enzyme embedded in the inner mitochondrial membrane, and chemiosmosis occurring in the mitochondrial matrix. The electron transport chain oxidizes substrates and, in the process, pumps protons into the intermembrane space. A T P synthase allows protons to leak back into the matrix and synthesizes A T P in chemiosmosis.

The electron transport chain is a series of electron transporters embedded in the inner mitochondrial membrane shuttling electrons from NADH and FADH2 to molecular oxygen. In the process, protons are pumped from the mitochondrial matrix to the intermembrane space, and oxygen is reduced to form water. In oxidative phosphorylation, the pH gradient formed by the electron transport chain is used by ATP synthase to form ATP. Credit: Rao, A., Ryan, K., Fletcher, S. and Tag, A. Department of Biology, Texas A&M University.

In aerobic respiration, the final electron acceptor is an oxygen molecule.  The fermentation method used by animals and certain bacteria is lactic acid fermentation. This type of fermentation is used routinely in skeletal muscle with insufficient oxygen supply for aerobic respiration. This would be muscles used to the point of fatigue. Another familiar fermentation process is alcohol fermentation, which produces ethanol. The fermentation of pyruvic acid by yeast produces the ethanol found in alcoholic beverages. Other fermentation methods take place in bacteria and many prokaryotes are facultatively anaerobic. This means that they can switch between aerobic respiration and fermentation, depending on the availability of free oxygen. 

Glucose is not the only macromolecule to be catabolized, but it is the most efficient. Proteins, other carbohydrates and fats can all enter the process at different points. Each contribute to ATP production less efficiently than glucose.

This illustration shows that glycogen, fats, and proteins can be catabolized via aerobic respiration. Glycogen is broken down into glucose, which feeds into glycolysis at the start. Fats are broken down into glycerol, which is processed by glycolysis, and fatty acids are converted into acetyl CoA. Proteins are broken down into amino acids, which are processed at various stages of aerobic respiration, including glycolysis, acetyl CoA formation, and the citric acid cycle.

Glycogen from the liver and muscles, as well as other carbohydrates, hydrolyzed into glucose-1-phosphate, together with fats and proteins, can feed into the catabolic pathways for carbohydrates.

Cellular respiration is controlled by a variety of means. The entry of glucose into a cell is controlled by the transport proteins that aid glucose passage through the cell membrane. Most respiration control is accomplished through control of specific enzymes in the pathways. This type of negative feedback mechanism turns enzymes off. Other pathway intermediates affect certain enzymes in the systems.

Exercises

Reading Quiz

 

Key Takeaways

  1. Cellular respiration involves several pathways.
  2. Glycolysis occurs in the cytoplasm and yields two, three-carbon pyruvate molecules.
  3. Eukaryotes convert pyruvate to enter CAC and ETC within the mitochondria when oxygen is available.
  4. Fermentation can occur when oxygen is not available.
Biology-2e. (2018). Houston, RX: website: OpenStax Book title: Biology 2e .
Access for free at https://openstax.org/books/biology-2e/pages/1-introduction

License

Icon for the Creative Commons Attribution 4.0 International License

Introductory Biology Copyright © 2023 by Mona Easterling is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

Share This Book