Aerobic and Anaerobic Metabolism

Movement can be categorized into

  • Burst exercise

    • Rapid, well-coordinated escape

      • Sudden burst of exercise

        • Requires muscles to immediately and dramatically use ATP

      • E.g. startle response in cockroaches, tail-flip response in crayfishes

  • Sustained exercise

    • Steady rate over long time

      • Muscles need to be continuously supplied with ATP for long periods

      • E.g. migratory birds, cruising fish

Each cell produces its own ATP

  • ATP is NOT transported between cells

Energy from foods (carbohydrates, lipids etc.) is converted into bonds of ATP and are released during physiological work.

The conversion from food into ATP occurs via 2 major catabolic biochemical pathways

  • Aerobic pathways

    • Require oxygen

    • Typically for sustained exercise

      • Due to pathways being more slowly activated

  • Anaerobic pathways

    • Do not require oxygen

    • Typically used for burst exercise

      • Due to pathways being rapidly activated

Two other mechanism to produce ATP:

  • From phosphagens

    • In absence of oxygen

  • From stores of oxygen in tissues

Aerobic catabolism

  • Carbohydrates:

    • Glycolysis

    • The Krebs cycle

    • The electron-transport chain

    • Oxidative phosphorylation

  • 38 molecules of ATP is produced per molecule of glucose. 60-70% of the energy is contained from glucose

Anaerobic catabolism

  • Primarily vertebrate skeletal muscles are capable of anaerobic glycolysis

  • ATP and lactic acid are end products

  • 2 molecules of ATP are produced per molecule of glucose.

    • 7% of energy is contained from glucose, rest is transferred to lactic acid

      • High-energy molecule, but cannot be directly used for physiological work.

      • Lactic acid is prevented from being excreted, but is removed metabolically in the presence of O2.

        • Can either be converted back to glucose using ATP or oxidated in the Krebs cycle and electron-transport chain.

          • Latter produces 36 ATP molecules per pair of lactic acid molecules

Phosphagens

  • Serve as temporary stores of high-energy phosphate bonds

  • Found in skeletal muscles of vertebrates and muscles of many invertebrates

  • E.g. creatine phosphate (vertebrates), arginine phosphate (invertebrates)

  • In presence of kinases, the molecules can transfer their phosphates to ADP to create ATP

    • Reaction is reversible

    • Does not require presence of O2

Internal O2 stores

  • Apart from aerobic metabolism, O2 is also stored in certain tissues (e.g. bound to myoglobin in skeletal muscles)

  • Can only support physiological work for a limited period of time

When a person performs exercise, they are capable of a certain maximal rate of O2 consumption.

  • This exercise is referred to as maximal aerobic exercise

  • Exercise requiring less is called submaximal exercise

  • Exercise requiring more is called supramaximal exercise

If exercise starts abruptly, the pulmonary and circulatory systems increase delivery of O2 to the tissues gradually

  • In other words, the supply of O2 is less than the theoretical demand

    • This difference is called oxygen deficit

  • Aerobic catabolism thus cannot meet the full ATP energy required. Instead, ATP is supplied via anaerobic catabolism + phosphagens + internal O2 stores

In sub-maximal exercise (80%), the pulmonary and circulatory systems increase delivery of O2 to the tissues sufficiently to meet the full O2 demand of the exercise

  • Aerobic catabolism works

At an abrupt end of the exercise, the sustained rate of O2 uptake does not instantly fall to zero.

  • Declines gradually, remaining above resting levels for several minutes

  • This elevated rate of oxygen uptake is called excess post-exercise oxygen consumption (EPOC)

    • = breathing hard after exercise

On the other hand, in light sub-maximal exercise (50-60% of animal’s maximal rate of O2 consumption), the anaerobic etc pathways needed for sudden onset of exercise are unpronounced, unlike in heavy sub-maximal exercise (>50-60%).

In heavy supramaximal exercise, ATP demands are greater than can ever be achieved by steady-state aerobic catabolism

  • Anaerobic catabolism must supply all ATP needed

    • Increased oxygen deficit and build-up of lactic acid

      • Lactic acid accumulates until massive fatigue is inevitable

Lack of environmental O2 is also a reason to use anaerobic mechanisms

  • Hypoxia

    • Low O2 in tissues

    • Animals become metabolically depressed

      • Regulate reduction in ATP needs

  • Anoxia

    • No O2 in tissues

Vertebrate brains are obligatorily aerobic, which may cause issues for deep-diving mammals and birds, that operate in hypoxic conditions.

  • Much ATP is produced anaerobically for long dives

  • Many diving mammals and birds can partition their bodies metabolically

    • O2 stores are reserved for aerobic catabolism in the brain

    • Anaerobic catabolism is used in other parts of the body