Source of energy for muscle contraction

 Chemical composition of muscle

• Skeletal Muscle

1. Water: ≈75% the muscle weight

2. Muscle proteins: ≈20% of the muscle mass

• contractile proteins (actin, myosin, troponin and tropomyosin)

• Myogen, myoglobin.

3. Organic substances:

• Carbohydrates: glycogen and hexaphosphate

• Lipids: neutral fat, cholesterol, lecithin and steroids

• Nitrogenous substances: ATP, adenylic acid, creatine,

phosphocreatine, urea, uric acid, xanthine and hypoxanthine.

4. Inorganic substances:

• Cations: potassium, sodium, calcium, magnesium and

• Anions: chloride, phosphate and sulphate.

Energy source for muscle contraction

• Muscle contraction requires lot of energy

• “a machine for converting chemical energy into mechanical work”

• Immediate source of energy: ATP

• Ultimate source: intermediary metabolism of carbohydrate and lipids.

Adenosine Triphosphate

• Adenosine-PO3 ~ PO3 ~ PO3 (HIGH ENERGY COME DUE TO PHSPHATE BOND BREAK DOWN)

• 7300 calories of energy per mole of ATP

• When one phosphate radical is removed, - 7300 calories of energy

• When the second phosphate radical is removed- 7300 calories

• Removal of the first phosphate adenosine diphosphate (ADP)

• Removal of the second converts this ADP into adenosine

monophosphate (AMP)

Hydrolysis of ATP

• Hydrolysis of ATP provides energy for muscle contraction

• ATP is stored in the muscle

• Consumed after a few twitches

• In about 3 seconds, all the ATP stored in the muscle cell is depleted.

• Thus, there is need for resynthesis of ATP

1. Phosphorylation of ADP by creatine phosphate
(Phosphocreatine-Creatine System)

2. Glycolysis (Glycogen Lactic Acid System)

3. Oxidative metabolism (Aerobic System)

Resynthesis of ATP

1. Phosphorylation of ADP by creatine phosphate.

• ATP is regenerated using the energy released by the dephosphorylation of
creatine phosphate (reserves of the muscle fibre)

• Lohman’s reaction: rapid and requires only single enzyme (creatine kinase)

• The amount of creatine phosphate is limited, the amount of ATP formed by
this mechanism is only sufficient for contraction of the muscle for next
about 5 s

• [At rest, the muscle contains large quantities of ATP, therefore, the reaction
proceeds from right to left forming creatine phosphate, and thus the store
is built up]

2. Glycolysis

• Glycogen stored in the muscles

• used to reconstitute both ATP and phosphocreatine by the process

of glycolysis which can sustain muscle contraction for about 1 min

• Each molecule of glycogen: produces two molecules of pyruvic
acid and two molecules of ATP

• Further changes in pyruvic acid depend upon the availability of
oxygen

• Oxygen available: Kreb’s cycle 38 molecules ATP
• Oxygen unavailable: Cori’s cycle

• Importance of glycolysis:
• occurs even in the absence of oxygen, so that muscle contraction
can be sustained for a short time even when oxygen is not
available

• Rate of formation of ATP by glycolysis process is about 2½ times as
rapid as ATP formation when the cellular foodstuffs react with
oxygen

3. Oxidative metabolism.

• combining of oxygen with various cellular foodstuffs to liberate
ATP is the final source of energy during muscle contraction

• Contributes more than 95% of all energy used by the muscles for
sustained long-term contraction

• Foodstuffs used: fats, carbohydrates and proteins

• Fatty acids: used for resynthesis of most of the ATP during
prolonged muscle contraction lasting over a period of many
hours

• Glycogen : contributes about half of the energy required for
muscle contraction lasting for 2–4 h.

• During intense but short lasting exercise, e.g. in a 100m race that
takes 10 s, 85% of the energy consumed is derived anaerobically

• During moderate intensity exercise, e.g. in a 3 km race that takes 16
min, 20% of the energy consumed is derived anaerobically.

• During mild intensity prolonged exercise, e.g. in a long distance race
that takes about an hour, only 5% of the energy comes from
anaerobic metabolism

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