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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.