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9.7 The Biochemistry of Movement: 8. Type 1 muscle cells

Syllabus reference (October 2002 version)
8. Gentle exercise uses type 1 muscles and involves aerobic respiration	Students learn to: describe the tricarboxylic acid (TCA) cycle as another multi-enzyme system involved in respiration outline the TCA cycle as oxidative decarboxylation with the addition of acetyl CoA as the energy source in each cycle identify the products of the TCA cycle and explain the role of oxidation and reduction in the cycle summarise the role of the cytochrome chain and identify the location of the chain of enzymes involved within the mitochondrion describe the role of oxygen in respiration	Students: process information from a simplified flow chart of biochemical pathways to produce a flow chart summarising the steps in aerobic respiration process information from a simplified flow chart of biochemical pathways to analyse the total energy output from glycolysis and compare it with the energy output from the TCA cycle

Extract from Chemistry Stage 6 Syllabus (Amended October 2002) © Board of Studies, NSW.
[Edit 27 Jun 08]

Background: We have seen that ATP is the compound that supplies energy directly to the contracting muscle proteins, myosin and actin. ATP is required for all cellular activities needing energy. ATP comes from the process of respiration carried out in all cells. Muscle cells store glycogen in the form of granules in the cytoplasm for use as a fuel during contraction. Muscle cells also have a supply of glucose and fatty acids from the blood to use as fuels to produce ATP.

process information from a simplified flow chart of biochemical pathways to produce a flow chart summarising the steps in aerobic respiration

• Use the Biochemical Pathways Chart to summarise the three main processes in aerobic respiration: G__________, O________ d____________ and O________ p_______________.
  
  
• There are two main types of muscle cells, found together in different proportions in skeletal muscles. These are type 1 (called red fibres because of many red blood capillaries) and type 2 (white fibres). Process this information to decide which type of muscle cell is best able to carry out aerobic respiration, respiration which requires a good supply of oxygen.

describe the tricarboxylic acid (TCA) cycle as another multi-enzyme system involved in respiration

• Use the Biochemical Pathways Chart to describe the tricarboxylic acid (TCA) cycle. Note that addition of the 2-carbon acetyl group to a 4-carbon compound forms 6-carbon citric acid which has three carboxyl groups. Hence the name tricarboxylic acid cycle. This cycle involves a number of enzyme catalysed steps, many of which release energy. Use the Biochemical Pathways flowchart to follow (but do not memorise) the changes in the TCA cycle. Work out why there are two turns of the TCA cycle for each glucose molecule.

outline the TCA cycle as oxidative decarboxylation with the addition of acetyl CoA as the energy source in each cycle

• Decarboxylation means removal of carbon dioxide. Production of carbon dioxide from carbon compounds requires oxygen. Hence the TCA cycle is called oxidative decarboxylation.
  
  
• Oxidation of the two carbons in the acetyl group input to the TCA cycle is what releases energy. The energy is not released suddenly like it is in the burning of carbon compounds in oxygen. Instead energy is released slowly in small amounts in the form of ATP and compounds such as NADH and FADH₂ which can later produce ATP.

Cellular oxidation of sugar vs. burning of sugar Access Excellence, national health museum, USA

identify the products of the TCA cycle and explain the role of oxidation and reduction in the cycle

• In the TCA cycle it is simplest to picture oxidation as the addition of oxygen or the removal of hydrogen from an organic molecule. Oxidation and reduction occur together so if oxygen oxidises an organic molecule, at the same time the oxygen itself is reduced. Similarly if hydrogen is removed from an organic molecule then the organic molecule is oxidised while the species receiving the hydrogen, for example NAD⁺, is reduced, for example to NADH.
  
  
• You should be able to identify the gaseous oxidised carbon product and the reduced products NADH and FADH₂. Explain the role of oxidation of carbon compounds containing hydrogen in producing reduced NADH and FADH₂ to ultimately provide the cell with ATP.

summarise the role of the cytochrome chain and identify the location of the chain of enzymes involved within the mitochondrion

• Your summary should result in inclusion of the following points:
  • The reduced forms NADH and FADH₂ are oxidised by oxygen in a series of reactions to recycle NAD⁺ and FAD.
  • These reactions are catalysed by a series of enzymes called the cytochrome chain found inside mitochondria.
  • Water is the main product and energy released is in the form of ATP.

describe the role of oxygen in respiration

• Describe where oxygen is needed to allow glycolysis, oxidative decarboxylation and oxidative phosphorylation.
  
  
• Overview of all types of respiration: During muscle contraction, glucose can be broken down in two ways, either completely to carbon dioxide and water or partially to lactic acid. One of these ways requires oxygen (AEROBIC) while the other does not (ANAEROBIC).
  
  
• Glycolysis is the first step where glucose is converted to pyruvic acid in the cytoplasm. This is a rapid process, however, it produces only a small amount of ATP. How many ATP per molecule of glucose? (Answer 1)
  
  
• Pyruvic acid is then converted either to acetyl Co A, if oxygen is readily available for oxidation to CO₂ and H₂O, or to lactic acid if oxygen is unavailable.

  

• If oxygen supply can meet demand, the pyruvic acid will be converted to acetylCoA and on to carbon dioxide and water. This is known as aerobic respiration since oxygen is required for this series of reactions. Aerobic respiration is slow but releases a lot of ATP.
  
  
• However, if the supply of oxygen cannot keep up with demand for ATP, the pyruvic acid will be converted to lactic acid. Since oxygen is not required for glucose conversion to lactic acid, this is termed anaerobic respiration. Anaerobic respiration is very fast, does not require oxygen, but releases little ATP. If two pyruvic acid are converted to two lactic acid, another two pyruvic acid can be formed from a glucose molecule thus releasing (Answer 2) ATP
  
  
• If lactic acid is the end product, the lactic acid will build up in the cell and the pH will begin to fall. Therefore anaerobic respiration cannot continue for long.
  
  
• Analyse and process information above to decide which type of muscle cell mainly uses aerobic respiration (Answer 3) and which type mainly uses anaerobic respiration (Answer 4.)

process information from a simplified flow chart of biochemical pathways to analyse the total energy output from glycolysis and compare it with the energy output from the TCA cycle

• Processing information from the Biochemical Pathways Flowchart to analyse the total energy output requires you to count the ATP released and calculate the ATP that can be obtained from NADH + H⁺ and FADH₂. This should be done for conversion of one glucose molecule to two pyruvic acid in glycolysis and for the conversion of two acetyl groups in the TCA cycle.
  
  
• Compare the quantity of energy output by fast glycolysis with the energy output by the slower TCA cycle.
  
  
• If the supply of oxygen can meet the rate of demand for ATP, the acetyl CoA will continue to be broken down via the TCA cycle to carbon dioxide and via oxidative phosphorylation to water. The total amount of ATP produced per glucose molecule is 38 ATP in contrast to a net 2 ATP per glucose when lactic acid is produced.
  
  

  Features	Glucose to lactic acid	Glucose to CO2 and H2O
  Net production ATP per glucose	2	38
  Requirement for oxygen	None	ready supply oxygen required
  Where reactions take place in cell	cytoplasm	Cytoplasm and mitochondria
  Ability to be sustained	Short term only	Indefinitely, as long as glucose and oxygen are supplied

  
  
  
• The lactic acid produced during anaerobic respiration can cause muscle contraction to stop until the lactic acid is metabolised and the pH restored to about 7.4.
  
  
• You can now deduce that type 1 muscles will be able to process glucose → carbon dioxide + water more readily than type 2 because of the better blood supply (hence oxygen supply) and therefore greater capacity for aerobic respiration. This means they will be able to supply a greater quantity of ATP, but will not be able to supply ATP as quickly.
  
  
• In contrast, type 2 muscle fibres are better equipped to process glucose → lactic acid to supply ATP rapidly during periods of rapid contraction. Type 2 fibres contract more rapidly, do not require such a rich blood supply and mostly carry out anaerobic respiration during contraction.
  
  
• • Discuss the different roles of type 1 and type 2 fibres.
    Which fibres are best suited to light, endurance exercise such as long distance events like marathons? (Answer 5)
    
    
  • Which fibres are best suited to sprints, short bursts of heavy exercise lasting much less than a minute? (Answer 6)
    
    
  • Skeletal muscles contain both types of fibres. Training can alter the relative proportions of each type. A distance runner will have a greater percentage of type 1 and a sprinter will develop more type 2 as a result of different training regimes.