Energy systems in muscle cells.

During strenuous muscle activity the cell rapidly breaks down its reserves of ATP to release energy. Muscle cells have an additional source of energy in creatine phosphate that can be used to replenish ATP pools during rigorous bouts of exercise. Creatine phosphate breaks down to release energy and phosphate that is used to convert ADP to ATP at a fast rate. Creatine phosphate During intense muscle activity ATP is broken down to ADP + Pi to release energy. However muscle cells store a limited quantity of ATP and the cell quickly breaks down its ATP reserves to release energy during strenuous exercise. Much of the energy for repeated muscle contraction comes from a chemical called creatine phosphate. During strenuous muscle contraction, creatine phosphate in muscle cells breaks down releasing energy and phosphate to convert ADP to ATP. energy Phosphorylation of ADP ATP ADP creatine Creatine phosphate

This system can only support strenuous muscle activity for around 10 seconds, when the creatine phosphate supply runs out. When muscle energy demand is low, ATP from cellular respiration is used to restore the levels creatine phosphate. energy ATP ADP creatine Creatine phosphate Phosphorylation of creatine The ATP formed is used to maintain muscle contraction for a few more seconds. In this way enough energy is released for short bursts of energy needed, e.g. for a sprint or to lift a heavy weight. When energy demands are low, ATP produced by respiration replenishes the cell’s ATP store and is also used to restore levels of creatine phosphate

Lactic acid metabolism.

Oxygen deficiency, conversion of pyruvate to lactic acid, muscle fatigue, oxygen debt. During vigorous exercise, the muscle cells do not get sufficient oxygen to support the electron transport chain. Under these conditions, pyruvate is converted to lactic acid. This conversion involves the transfer of hydrogen from the NADH produced during glycolysis to pyruvic acid to produce lactic acid. This regenerates the NAD needed to maintain ATP production through glycolysis. Lactic acid metabolism During vigorous exercise, the small store of ATP lasts for a few seconds, ATP produced using creatine phosphate provides energy for a few seconds more then the muscle cells have to respire anaerobically because they do not receive enough oxygen to allow the electron transport chain to operate and aerobic respiration to occur. When this happens, only glycolysis provides additional ATP (since oxygen is not needed for glycolysis). Glycolysis produces 2 ATP for each glucose molecule broken down. It also produces 2 NADH. In anaerobic conditions (no oxygen present), NADH transfers hydrogen to pyruvate (pyruvic acid), converting it to lactic acid and regenerating NAD to be used again during glycolysis.

Lactic acid accumulates in muscle causing fatigue. Oxygen debt repaid when exercise is complete allows respiration to provide the energy to convert lactic acid back to pyruvic acid and glucose in the liver. Muscle fatigue Build up of lactic acid in the muscles causes muscle fatigue Oxygen debt During anaerobic respiration an oxygen debt builds up. When exercise has finished, energy from aerobic respiration is used to convert lactic acid to pyruvic acid (which is transported in the blood to the liver) and glucose.

Types of skeletal muscle fibres

Slow twitch (Type 1) muscle fibres contract more slowly, but can sustain contractions for longer and so are good for endurance activities like long distance running, cycling or cross-country skiing. Slow twitch muscle fibres rely on aerobic respiration to generate ATP and have many mitochondria, a large blood supply and a high concentration of the oxygen storing protein myoglobin. The major storage fuel of slow twitch muscles fibres is fats. Fast twitch (Type 2) muscle fibres contract more quickly, over short periods, so are good for bursts of activity like sprinting or weightlifting. Fast twitch muscle fibres can generate ATP through glycolysis only and have few mitochondria and a lower blood supply than slow twitch muscle fibres. The major storage fuels of fast twitch muscles fibres are glycogen and creatine phosphate. Most human muscle tissue contains a mixture of both slow and fast twitch muscle fibres. Athletes show distinct patterns of muscle fibres that reflect their sporting activities. Types of skeletal muscle fibres Myoglobin Myoglobin is a protein in muscles that binds with oxygen. It has a higher affinity for oxygen than haemoglobin and can therefore extract oxygen from the blood for use by muscle cells. Slow twitch and fast twitch muscle fibres There are two types of skeletal muscle fibres:  Type 1 – slow twitch muscle fibres  Type 2 – fast twitch muscle fibres

Feature of muscle fibreType of muscle fibre Slow twitchFast twitch Speed and length of contractionSlow and over longer periodFast and over shorter period Length of time contraction lastslongshort Process used to produce ATPaerobic respirationglycolysis only Number of mitochondria in cellslargesmall Blood supply / number of capillaries present higherlower Concentration of myoglobin in cellshighlow Major storage fuel in the fibresfatsglycogen and creatine phosphate Type of activity that use the fibres Those requiring endurance, e.g. long distance running Those requiring bursts of activity, e.g. sprinting, weight lifting Most human muscle tissue contains a mixture of both slow and fast twitch muscle fibres. Athletes show distinct patterns of muscle fibres that reflect their sporting activities, e.g. sprinters have a higher proportion of fast twitch fibres while long distance runners have a higher amount of slow twitch muscle fibres.