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MUSCLES 5. Muscle cells cause movement by contraction along their length 5.1 describe the generalised structure of a skeletal muscle cell The body has three different types of muscle tissue: skeletal, cardiac and smooth. These three types of muscle tissue perform different roles and so have different structures. Skeletal muscle is by far the most common in our bodies and is primarily concerned with voluntary movement. Skeletal muscle is composed of fibre-like cells, each cell having many nuclei. Cardiac muscle, as the name suggests, is heart muscle. The cells in cardiac muscle have a branched structure to allow the whole heart to contract simultaneously. In cardiac muscle, like skeletal muscle, each cell has many nuclei. Smooth muscle is the muscle of the digestive and circulatory systems. Smooth muscle cells usually have only one nucleus and are slender in shape. SKELETAL MUSCLE STRUCTURE Skeletal muscle is also called striated muscle because of its banded appearance. The structure of skeletal muscle is perfectly suited to its primary movements of contraction and relaxation. As shown in Figure 30.2, skeletal muscle consists of bundles of fibres . Each fibre represents one long thin multi-nucleated cell and is made from a bundle of about one thousand fibrils. These fibrils, in turn, consist of alternating sections of thick and thin filaments. The striated nature of skeletal muscle is due to the way these protein filaments are arranged in alternating bands. Thus the regions of thin filaments within a fibre appear light in colour and the regions of thick filaments appear darker. 5.5 analyse information from secondary sources to describe the appearance of type 1 and type 2 skeletal muscle cells There are two types of muscle cells and their differences imply that different fuels are needed and different strategies are used during contraction and relaxation Type 1 (slow twitch) muscle cells Muscles designed to contract slowly and steadily, such as the flight muscles of migratory birds, have a predominance of type 1 muscle cells. They have a rich blood supply and therefore adequate oxygen for use in aerobic respiration. These cells have many mitochondria and obtain most of their ATP by the process of oxidative phosphorylation. The filament structure of these muscle fibres shows them to have fewer contractile filaments. Type 1 muscle cells are useful in light endurance exercise such as long-distance running. The top distance runners have a highly developed ability to produce ATP aerobically. Type 1 cells having adequate oxygen supply can gain the maximum 38 molecules of high energy ATP from complete aerobic respiration, including oxidative phosphorylation. They utilise a variety of fuels such as glucose, fatty acids and amino acids. Identify the characteristics of type 1 muscle cells as: 1. contracts relatively slowly 2. many mitochondria 3. well-supplied with blood 4. fewer contractile filaments 5. carries out aerobic respiration 6. used for light, endurance exercise Type 2 (fast twitch) muscle cells Type 2 muscle cells contract relatively rapidly. They contain less mitochondria and have a reduced supply of blood and therefore oxygen. As a result they mostly respire anaerobically. Type 2 or fast-twitch muscle cells are used in high intensity athletic events such as the sprint events in swimming and running. Type 2 muscle cells are primarily anaerobic, using the available glucose and stored glycogen as their energy sources. Some glycogen, up to about 2% of muscle tissue by mass, is stored in small granules in muscle tissue. This glycogen can readily be broken down to glucose-6-phosphate and then to glucose which is available for glycolysis. The action of type 2 muscle cells requires maximum energy in a short time, and is supplied by anaerobic glycolysis. The formation of acid and the resulting drop in pH associated with this process causes muscle fatigue and cramps. To avoid this, an athlete maximises their body’s ability to take in and utilise oxygen by a training program. Athletes also have a carbohydrate-rich diet, eating large quantities of pasta, bread and fruits leading up to their events. Identify the characteristics of type 2 muscle cells as: 1. contracts relatively rapidly 2. few mitochondria 3. poor blood supply 4. many contractile filaments 5. carries out mostly anaerobic respiration 6. used for heavy and sprinting-style exercise Construct your own table to summarise the main features of type 1 and type 2 muscles 5.2 identify actin and myosin as the long parallel bundles of protein fibres which form the contractile filaments in skeletal muscle Thick filaments Thick filaments consist of the protein myosin. Myosin is a large protein molecule consisting of three pairs of polypeptide chains with a total molecular mass of about 540 000. Myosin has the properties of both globular and fibrous proteins, having two globular protein ‘heads’ and a fibrous helical tail (Fig. 30.4). Thick filaments consist of several hundred of these myosin molecules bonded together. Myosin performs the dual roles of structural protein and enzyme. It catalyses the hydrolysis of ATP and facilitates the release of energy to fuel muscle contraction. Myosin comprises between 60% and 70% of muscle protein. The structure of the thick filaments is shown below Thin filaments The major constituent of thin filaments is the protein actin, making up between 20% and 25% of the total protein content of muscle fibres. Actin can exist in different structural forms at the tertiary level of protein structure. In thin muscle filaments it exists in its fibrous form, F-actin. Two other proteins combine with actin to form thin filaments. These are tropomyosin and troponin. These two proteins play an important role in muscle contraction, by preventing the interaction between myosin and actin when the muscle is in a relaxed state. The repeating structural unit of a muscle fibril is called a sarcomere. These are similar in concept to a wavelength, in that they represent one complete unit of a muscle fibril. A sarcomere is a longitudinal section in a muscle fibril between two dark ‘Z discs’ (the thin dark lines in Figure 30.2). A sarcomere therefore contains segments of thick and thin filaments together with the regions where these segments overlap. 5.3 identify the cause of muscle cell contraction as the release of calcium ions after a nerve impulse activates the muscle cell membrane Teacher notes 5.4 identify that the cause of the contraction movement is the formation of temporary bonds between the actin and myosin fibres and explain why ATP is consumed in this process Perform experiment to model the mechanism of muscle contraction. Write up your method below 10. Sprinting involves muscles contracting powerfully and rapidly and utilises type 2 muscle cells 10.1 outline the problems associated with the supply and use of fuels during sprinting and relate this to the sprinting muscles’ reliance on non-oxygen/non-mitochondrial based ATP production Problem – Energy is needed in a very short period of time. Sprinting and weight lifting both require type 2 muscles and undergo anaerobic respiration. There is a limited supply of glucose in the blood but additional glucose comes from the breakdown of glycogen to meet the needs of short term intense activities such as sprinting. The production of high concentrations of glucose by the breakdown of glycogen followed by glycolysis and anaerobic respiration results in the production of ATP at a rate faster than can be achieved by the TCA cycle and the cytochrome chain. Anerobic respiration can supply muscles with high levels of ATP in a very short time period but only in short bursts eg in the 100m sprint or weightlifting. These athletes have large skeletal muscles which store glycogen for quick conversion to glucose How does this compare with aerobic exercise? As we have seen energy can be derived from the metabolism of carbohydrates, fats and proteins. Aerobic respiration involves glycolysis (in which glucose is converted to pyruvate), the TCA cycle (which utilises acetyl– CoA) and oxidative phosphorylation (which utilises the high energy compounds NADH and FADH2 to produce ATP). Glucose, fatty acids and proteins all lead to the formation of acetyl–CoA and therefore provide the initial fuel for the TCA cycle and accompanying oxidative phosphorylation. The advantage of aerobic respiration is that it provides the maximum amount of energy, as ATP, that can be obtained from these oxidation processes. These athletes are long distance runners, very slim and not large skeletal muscle. Their fat supply is constantly being used up during their aerobic exercise, 10.2 explain the relationship between the production of 2–hydroxypropanoic (lactic) acid during anaerobic respiration and the impairment of muscle contractions by changes in cellular pH See large flow chart, where lactic acid produced in muscles lowers the pH and can cause the denaturing of enzymes which leads to muscle fatigue, soreness, cramps.Excess lactic acid is oxidised to pyruvic acid as oxygen becomes available and can be oxidized into glucose in the liver. Under anaerobic conditions, the pyruvate produced in glycolysis is reduced to lactic acid (2-hydroxypropanoic acid) or the lactate ion. Their structures are illustrated in Figure 30.10. The pyruvate produced as a result of glycolysis oxidises NADH to produce lactate and NAD+. Much of the resulting lactate is transported to the liver where it is converted to glucose. Anaerobic glycolysis, produces hydrogen ions resulting in a lowering of the pH of the muscle tissue. This inhibits the activity of some enzymes, in particular (PFK), the enzyme responsible for one of the stages in glycolysis. This lowering of pH is the cause of muscle fatigue and cramps. It is believed to be a protective mechanism to prevent muscles completely exhausting their supply of ATP. When oxygen levels increase, the cells revert to aerobic respiration. Figure 30.10 Structure of (a) lactic acid and (b) lactate ion. 10.3 solve problems and process information from a simplified flow chart of biochemical pathways to summarise the steps in anaerobic glycolysis and analyse the total energy output from this process See large flow chart. Already covered when we studied glycolysis 10.4 use available evidence and process information from a simplified flow chart of biochemical pathways to trace the path of lactic acid formation and compare this with the process of fermentation See large flow chart . Already covered when we studied glycolysis 10.5 solve problems and process information to discuss the use of multiple naming systems in chemistry using lactic acid (2-hydroxypropanoic acid or 2-hydroxypropionic acid) as an example What view do you take? Naming of organic compounds should be systematic and uniform across all countries. The use of alternative chemical names should not be allowed. Chemists should strive for uniformity in naming. IUPAC names The international Union of Pure and Applied Chemistry provides a system for the clear communication of chemical nomenclature with an explicit or implied relationship to the structure of compounds. Semi-systematic or trivial names also exist, such as methane, propanol, styrene and cholesterol which are so familiar that few chemists realise that they are not fully systematic. They are retained, and indeed, in some cases there are no better systematic alternatives.