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18.2: Overview of Cellular Respiration - Biology

18.2: Overview of Cellular Respiration - Biology


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Learning Objectives

  1. Define catabolism and anabolism and state which is exergonic and which is endergonic.
  2. Define precursor metabolites and state their functions in metabolism.
  3. Define the following:
    1. cellular respiration
    2. aerobic
    3. anaerobic
  4. Name one aerobic and two anaerobic forms of cellular respiration.

As mentioned previously, to grow, function, and reproduce, cells must synthesize new cellular components such as cell walls, cell membranes, nucleic acids, ribosomes, proteins, flagella, etc., and harvest energy and convert it into a form that is usable to do cellular work.

Catabolism refers to the exergonic process by which energy released by the breakdown of organic compounds such as glucose can be used to synthesize ATP, the form of energy required to do cellular work. Anabolism is the endergonic process that uses the energy stored in ATP to synthesize the building blocks of the macromolecules that make up the cell. As can be seen, these two metabolic processes are closely linked. Another factor that links catabolic and anabolic pathways is the generation of precursor metabolites. Precursor metabolites are intermediate molecules in catabolic and anabolic pathways that can be either oxidized to generate ATP or can be used to synthesize macromolecular subunits such as amino acids, lipids, and nucleotides.

In this section we will concentrate primarily on harvesting energy and converting it to energy stored in ATP through the process of cellular respiration, but we will also look at some of the key precursor metabolites that are produced during this process.

Cellular respiration is the process cells use to convert the energy in the chemical bonds of nutrients to ATP energy. Depending on the organism, cellular respiration can be aerobic, anaerobic, or both. Aerobic respiration is an exergonic pathway that requires molecular oxygen (O2). Anaerobic exergonic pathways do not require oxygen and include anaerobic respiration and fermentation. We will now look at these three pathways.

Summary

  1. Catabolism refers to the exergonic process by which energy released by the breakdown of organic compounds such as glucose can be used to synthesize ATP, the form of energy required to do cellular work.
  2. Anabolism is the endergonic process that uses the energy stored in ATP to synthesize the building blocks of the macromolecules that make up the cell.
  3. Precursor metabolites are intermediate molecules in catabolic and anabolic pathways that can be either oxidized to generate ATP or can be used to synthesize macromolecular subunits such as amino acids, lipids, and nucleotides.
  4. Cellular respiration is the process cells use to convert the energy in the chemical bonds of nutrients to ATP energy.
  5. Aerobic respiration is an exergonic pathway that requires molecular oxygen (O2).
  6. Anaerobic exergonic pathways do not require oxygen and include anaerobic respiration and fermentation.

Introduction

The electrical energy plant in Figure 7.1 converts energy from one form to another form that can be more easily used. This type of generating plant starts with underground thermal energy (heat) and transforms it into electrical energy that will be transported to homes and factories. Like a generating plant, plants and animals also must take in energy from the environment and convert it into a form that their cells can use. Mass and its stored energy enter an organism’s body in one form and are converted into another form that can fuel the organism’s life functions. In the process of photosynthesis, plants and other photosynthetic producers take in energy in the form of light (solar energy) and convert it into chemical energy in the form of glucose, which stores this energy in its chemical bonds. Then, a series of metabolic pathways, collectively called cellular respiration, extracts the energy from the bonds in glucose and converts it into a form that all living things can use.

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    Glycolysis

    Glycolysis is a metabolic pathway that takes place in the cytosol of cells in all living organisms. This pathway can function with or without the presence of oxygen. Aerobic conditions produce pyruvate and anaerobic conditions produce lactate. In aerobic conditions, the process converts one molecule of glucose into two molecules of pyruvate (pyruvic acid), generating energy in the form of two net molecules of ATP. Four molecules of ATP per glucose are actually produced however, two are consumed as part of the preparatory phase. The initial phosphorylation of glucose is required to increase the reactivity (decrease its stability) in order for the molecule to be cleaved into two pyruvate molecules by the enzyme Aldolase. During the pay-off phase of glycolysis, four phosphate groups are transferred to ADP by substrate-level phosphorylation to make four ATP, and two NADH are produced when the pyruvate is oxidized. The overall reaction can be expressed this way:

    Glucose + 2 NAD+ + 2 Pi + 2 ADP → 2 pyruvate + 2 NADH + 2ATP + 2 H+ + 2 H2O + heat

    Starting with glucose, 1 ATP is used to donate a phosphate to glucose to produce glucose 6-phosphate. Glycogen can change into glucose 6-phosphate as well with the help of glycogen phosphorylase. During Energy metabolism, glucose 6-phosphate turns into fructose 6-phosphate. An additional ATP is used to phosphorylate fructose 6-phosphate into fructose 1,6-disphosphate by the help of phosphofructokinase. Fructose 1,6 diphosphate then splits into two phosphorylated molecules with three carbon chains that later degrades into pyruvate.

    Out of the cytoplasm, it goes into the Krebs cycle where the acetyl CoA. It then mixes with CO2 and makes 2 ATP, NADH, and FADH. From there the NADH and FADH go into the NADH reductase, which produces the enzyme. The NADH pulls the enzyme's electrons to send through the electron transport chain. The electron transport chain pulls H+ ions through the chain. From the electron transport chain, the released hydrogen ions make ADP for an end result of 32 ATP. 02 attracts itself to the leftover electron to make water. Lastly, ATP leaves through the ATP channel and out of the mitochondria.

    What happens to the energy stored in glucose during photosynthesis? How do living things make use of this stored energy? The answer is cellular respiration. This process releases the energy in glucose to make ATP (adenosine triphosphate), the molecule that powers all the work of cells.

    Stages of Cellular Respiration

    Cellular respiration involves many chemical reactions. The reactions can be summed up in this equation:

    The reactions of cellular respiration can be grouped into three stages: glycolysis (stage 1), the Krebs cycle, also called the citric acid cycle (stage 2), and electron transport (stage 3). Figure below gives an overview of these three stages, which are further discussed in the concepts that follow. Glycolysis occurs in the cytoplasm of the cell and does not require oxygen, whereas the Krebs cycle and electron transport occur in the mitochondria and do require oxygen.

    Cellular respiration takes place in the stages shown here. The process begins with a molecule of glucose, which has six carbon atoms. What happens to each of these atoms of carbon?

    Structure of the Mitochondrion: Key to Aerobic Respiration

    The structure of the mitochondrion is key to the process of aerobic (in the presence of oxygen) cellular respiration, especially the Krebs cycle and electron transport. A diagram of a mitochondrion is shown in Figure below.

    The structure of a mitochondrion is defined by an inner and outer membrane. This structure plays an important role in aerobic respiration.

    As you can see from Figure above, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the intermembrane space. The space enclosed by the inner membrane is called the matrix. The second stage of cellular respiration, the Krebs cycle, takes place in the matrix. The third stage, electron transport, takes place on the inner membrane.


    The first step in cellular respiration in all living cells is glycolysis, which can take place without the presence of molecular oxygen. If oxygen is present in the cell, then the cell can subsequently take advantage of aerobic respiration via the TCA cycle to produce much more usable energy in the form of ATP than any anaerobic pathway. Nevertheless, the anaerobic pathways are important and are the sole source of ATP for many anaerobic bacteria. Eukaryotic cells also resort to anaerobic pathways if their oxygen supply is low. For example, when muscle cells are working very hard and exhaust their oxygen supply, they utilize the anaerobic pathway to lactic acid to continue to provide ATP for cell function.

    Glycolysis itself yields two ATP molecules, so it is the first step of anaerobic respiration. Pyruvate, the product of glycolysis, can be used in fermentation to produce ethanol and NAD + or for the production of lactate and NAD + . The production of NAD + is crucial because glycolysis requires it and would cease when its supply was exhausted, resulting in cell death. A general sketch of the anaerobic steps is shown below. It follows Karp's organization.

    Anaerobic respiration (both glycolysis and fermentation) takes place in the fluid portion of the cytoplasm whereas the bulk of the energy yield of aerobic respiration takes place in the mitochondria. Anaerobic respiration leaves a lot of energy in the ethanol or lactate molecules that the muscle cells cannot use and must excrete. A portion of the lactate will reach the liver through the bloodstream and may be converted back to glucose through the Cori cycle. The ethanol can be metabolized by the liver, but is a poor precursor for gluconeogenesis and may lead to hypoglycemia.


    Section Summary

    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 of the control of the respiration processes is accomplished through the control of specific enzymes in the pathways. This is a type of negative feedback mechanism, turning the enzymes off. The enzymes respond most often to the levels of the available nucleosides ATP, ADP, AMP, NAD + , and FAD. Other intermediates of the pathway also affect certain enzymes in the systems.


    Methods in Methane Metabolism, Part B: Methanotrophy

    Michael C. Konopka , . Mary E. Lidstrom , in Methods in Enzymology , 2011

    2.3.1 Overview

    Recently, a system was developed for measuring single-cell respiration rates for eukaryotic cells on a microscope ( Molter et al., 2009 ). It features an array of microwells into which single cells are seeded, each well containing a platinum porphyrin that is used to measure the oxygen concentration by phosphorescence lifetime determinations. The wells are diffusionally sealed with a lid under load, and the consumption of oxygen is measured over time ( Fig. 10.5 ). Adapting the wells and procedures to account for the smaller size and respiration rates of bacteria provides another method to utilize respiration in the analysis of single cells. The actual consumption of oxygen by a single cell is directly measured, as opposed to an indirect measurement in bulk cultures. Many of the experimental details have been described for the eukaryotic system, so here we highlight a few of the main differences for the system used for bacteria, as well as results from cells grown in pure culture.

    Figure 10.5 . Microobservation chamber for respiration detection. (A) The Microobservation chamber insert is part of a stage plate that can sit in any platen designed for microwell plates. A lid attached to a piston will lower to seal the wells on the chip with pressure. (B and C) The stage plate (1) has a window (6) that allows for observation with a microscope objective. The microobservation chamber (2) is centered above the window. A holder (5) helps to keep the chip (4) in place when the lid (3) comes down to seal the array of microwells. (D) Each chip has 16 arrays in a 4 × 4 arrangement. Each array is a 4 × 4 arrangement of microwells on a plateau raised above the rest of the chip to help in sealing the microwells. Each microwell is approximately 2 pL in volume and contains the platinum porphyrin that acts as the oxygen sensor. (E) Example of Methylomonas sp. LW13 cells labeled with FM1-43 to show wells containing single cells.


    Biology 171

    By the end of this section, you will be able to do the following:

    • Describe how feedback inhibition would affect the production of an intermediate or product in a pathway
    • Identify the mechanism that controls the rate of the transport of electrons through the electron transport chain

    Cellular respiration must be regulated in order to provide balanced amounts of energy in the form of ATP. The cell also must generate a number of intermediate compounds that are used in the anabolism and catabolism of macromolecules. Without controls, metabolic reactions would quickly come to a standstill as the forward and backward reactions reached a state of equilibrium. Resources would be used inappropriately. A cell does not need the maximum amount of ATP that it can make all the time: At times, the cell needs to shunt some of the intermediates to pathways for amino acid, protein, glycogen, lipid, and nucleic acid production. In short, the cell needs to control its metabolism.

    Regulatory Mechanisms

    A variety of mechanisms is used to control cellular respiration. Some type of control exists at each stage of glucose metabolism. Access of glucose to the cell can be regulated using the GLUT (glucose transporter) proteins that transport glucose ((Figure)). Different forms of the GLUT protein control passage of glucose into the cells of specific tissues.


    Some reactions are controlled by having two different enzymes—one each for the two directions of a reversible reaction. Reactions that are catalyzed by only one enzyme can go to equilibrium, stalling the reaction. In contrast, if two different enzymes (each specific for a given direction) are necessary for a reversible reaction, the opportunity to control the rate of the reaction increases, and equilibrium is not reached.

    A number of enzymes involved in each of the pathways—in particular, the enzyme catalyzing the first committed reaction of the pathway—are controlled by attachment of a molecule to an allosteric site on the protein. The molecules most commonly used in this capacity are the nucleotides ATP, ADP, AMP, NAD + , and NADH. These regulators—allosteric effectors—may increase or decrease enzyme activity, depending on the prevailing conditions. The allosteric effector alters the steric structure of the enzyme, usually affecting the configuration of the active site. This alteration of the protein’s (the enzyme’s) structure either increases or decreases its affinity for its substrate, with the effect of increasing or decreasing the rate of the reaction. The attachment signals to the enzyme. This binding can increase or decrease the enzyme’s activity, providing a feedback mechanism. This feedback type of control is effective as long as the chemical affecting it is attached to the enzyme. Once the overall concentration of the chemical decreases, it will diffuse away from the protein, and the control is relaxed.

    Control of Catabolic Pathways

    Enzymes, proteins, electron carriers, and pumps that play roles in glycolysis, the citric acid cycle, and the electron transport chain tend to catalyze nonreversible reactions. In other words, if the initial reaction takes place, the pathway is committed to proceeding with the remaining reactions. Whether a particular enzyme activity is released depends upon the energy needs of the cell (as reflected by the levels of ATP, ADP, and AMP).

    Glycolysis

    The control of glycolysis begins with the first enzyme in the pathway, hexokinase ((Figure)). This enzyme catalyzes the phosphorylation of glucose, which helps to prepare the compound for cleavage in a later step. The presence of the negatively charged phosphate in the molecule also prevents the sugar from leaving the cell. When hexokinase is inhibited, glucose diffuses out of the cell and does not become a substrate for the respiration pathways in that tissue. The product of the hexokinase reaction is glucose-6-phosphate, which accumulates when a later enzyme, phosphofructokinase, is inhibited.


    Phosphofructokinase is the main enzyme controlled in glycolysis. High levels of ATP or citrate or a lower, more acidic pH decreases the enzyme’s activity. An increase in citrate concentration can occur because of a blockage in the citric acid cycle. Fermentation, with its production of organic acids such as lactic acid, frequently accounts for the increased acidity in a cell however, the products of fermentation do not typically accumulate in cells.

    The last step in glycolysis is catalyzed by pyruvate kinase. The pyruvate produced can proceed to be catabolized or converted into the amino acid alanine. If no more energy is needed and alanine is in adequate supply, the enzyme is inhibited. The enzyme’s activity is increased when fructose-1,6-bisphosphate levels increase. (Recall that fructose-1,6-bisphosphate is an intermediate in the first half of glycolysis.) The regulation of pyruvate kinase involves phosphorylation by a kinase (pyruvate kinase), resulting in a less-active enzyme. Dephosphorylation by a phosphatase reactivates it. Pyruvate kinase is also regulated by ATP (a negative allosteric effect).

    If more energy is needed, more pyruvate will be converted into acetyl CoA through the action of pyruvate dehydrogenase. If either acetyl groups or NADH accumulates, there is less need for the reaction, and the rate decreases. Pyruvate dehydrogenase is also regulated by phosphorylation: a kinase phosphorylates it to form an inactive enzyme, and a phosphatase reactivates it. The kinase and the phosphatase are also regulated.

    Citric Acid Cycle

    The citric acid cycle is controlled through the enzymes that catalyze the reactions that make the first two molecules of NADH (Review). These enzymes are isocitrate dehydrogenase and α-ketoglutarate dehydrogenase. When adequate ATP and NADH levels are available, the rates of these reactions decrease. When more ATP is needed, as reflected in rising ADP levels, the rate increases. Alpha-ketoglutarate dehydrogenase will also be affected by the levels of succinyl CoA—a subsequent intermediate in the cycle—causing a decrease in activity. A decrease in the rate of operation of the pathway at this point is not necessarily negative, as the increased levels of the α-ketoglutarate not used by the citric acid cycle can be used by the cell for amino acid (glutamate) synthesis.

    Electron Transport Chain

    Specific enzymes of the electron transport chain are unaffected by feedback inhibition, but the rate of electron transport through the pathway is affected by the levels of ADP and ATP. Greater ATP consumption by a cell is indicated by a buildup of ADP. As ATP usage decreases, the concentration of ADP decreases, and now, ATP begins to build up in the cell. This change in the relative concentration of ADP to ATP triggers the cell to slow down the electron transport chain.

    View more about the electron transport chain and ATP synthesis by watching Electron Transport Chain: The Movie (Flash interactive, video)

    For a summary of feedback controls in cellular respiration, see (Figure).

    Summary of Feedback Controls in Cellular Respiration
    Pathway Enzyme affected Elevated levels of effector Effect on pathway activity
    glycolysis hexokinase glucose-6-phosphate decrease
    phosphofructokinase low-energy charge (ATP, AMP), fructose-6-phosphate via fructose-2,6-bisphosphate increase
    high-energy charge (ATP, AMP), citrate, acidic pH decrease
    pyruvate kinase fructose-1,6-bisphosphate increase
    high-energy charge (ATP, AMP), alanine decrease
    pyruvate to acetyl CoA conversion pyruvate dehydrogenase ADP, pyruvate increase
    acetyl CoA, ATP, NADH decrease
    citric acid cycle isocitrate dehydrogenase ADP increase
    ATP, NADH decrease
    α-ketoglutarate dehydrogenase calcium ions, ADP increase
    ATP, NADH, succinyl CoA decrease
    electron transport chain ADP increase
    ATP decrease

    Section Summary

    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 of the control of the respiration processes is accomplished through the control of specific enzymes in the pathways. This is a type of negative feedback mechanism, turning the enzymes off. The enzymes respond most often to the levels of the available nucleosides ATP, ADP, AMP, NAD + , and FAD. Other intermediates of the pathway also affect certain enzymes in the systems.

    Free Response

    How does citrate from the citric acid cycle affect glycolysis?

    Citrate can inhibit phosphofructokinase by feedback regulation.

    Why might negative feedback mechanisms be more common than positive feedback mechanisms in living cells?

    Negative feedback mechanisms actually control a process it can turn it off, whereas positive feedback accelerates the process, allowing the cell no control over it. Negative feedback naturally maintains homeostasis, whereas positive feedback drives the system away from equilibrium.

    Glossary


    Anaerobic Respiration

    Anaeribic Respiration (Source: Wikimedia) This process occurs just like the typical cellular reaction (same glycolytic and Krebs cycle pathway) but only differs because it is used by organisms like bacteria and archaea where oxygen is not the final electron acceptor. Rather, these organisms use sulfates or nitrates instead.
    • It is important to note that while both fermentation and anaerobic happen in the absence of oxygen, the former is only an alternative and extends glycolysis to produce energy whereas the latter uses other molecules to complete the cycle as the organism will die in the presence of oxygen.
    • Unlike aerobic respiration that occurs in the mitochondria, aerobic respiration happens in the cytosol.
    • The process of anaerobic respiration generates only 2 ATP per glucose molecule.

    Looking back at the overall process, it will be apparent that living things should produce ATP, which in turn powers every metabolic and activity of organisms. Also, the whole pathway of the cellular respiration equation is so precise that it cannot proceed if a single molecule or enzyme is missing. Just imagine the metabolic confusion if they are not so.


    Watch the video: Cellular Respiration (November 2022).