Energy Systems

ATP in Energy Production   

          

Foods are primarily composed of carbon, hydrogen, oxygen and in the case of protein, nitrogen. The bonds in these foods are relatively weak and therefore do not give much energy when broken, so cannot be directly used for providing energy, The energy stored in the food molecules bonds are chemically released and then stored in the form of adenosine triphosphate (ATP).

When energy is required to perform exercise, it is supplied from the breakdown of ATP to ADP (adenosine diphosphate) and a free phosphate. This is the only substance the body can use and is often referred to as our ‘energy currency’ as it powers all forms of biological reactions:

ATP    -->    ADP + P  + energy

The breakdown of ATP releases energy and is an exothermic reaction. The body has a store of about 85g of ATP, which would be used up very quickly if it did not have a way of resynthesising it. This requires an endothermic reaction:

ADP + P  + energy    -->    ATP

Many of the chemical reactions occurring in the body are linked, with those energy-generating reactions being coupled with energy-requiring reactions. These are referred to as ‘coupled reactions’.

Energy Systems

The resynthesis or ATP is achieved through three energy systems:

• ATP-PC
• Lactic Acid
• Aerobic

They all operate at the same time but the duration and intensity of exercise will determine which system predominantly provides energy.

ATP-PC System

This is an anaerobic and exothermic system that occurs in the muscle sarcoplasm; the location of phosphocreatine (PC). It provides a quick source of energy and lasts for 8 to 10 seconds; yielding 1 ATP molecule.

The breakdown of ATP to ADP + inorganic phosphate during exercise causes Creatine Kinase to detect the rising levels of ADP, resulting in it catalysing the breakdown of PC:

Phosphocreatine à Creatine + inorganic phosphate + energy

This energy causes ATP to resynthesise from ADP + inorganic phosphate. This system is for high intensities such as sprinting flat out in a football game.

The Lactic Acid System (Anaerobic Glycolysis)

In the absence of oxygen, the body resorts to the Lactic Acid System and the use of glycogen or glucose. It produces by-products such as lactic acid that are detrimental to muscle function, limiting the activity to approximately 2 minutes, however this can be improved through training. It is an anaerobic, exothermic system that is short-lasting and normally kicks in at 30 seconds lasting for just over 2 minutes, yielding 2 molecules of ATP.

Glycogen is the synthesised form of glucose (glucose molecules formed together) and is an efficient way of storing our carbohydrate in our muscle and liver. The Lactic Acid system involves a series of ten chemical reactions that break down glycogen that has been converted into glucose, or glucose directly, into pyruvic acid. These chemical reactions release enough energy to resynthesise two ATP molecules. These 10 reactions are known as glycolysis and from glucose they produce 2 x ATP, H   and 2mmols pyruvic acid.

Glycolysis takes place in the muscle sarcoplasm and is regulated by various enzymes, the most important being phosphofrucktokinase (PFK).

A by-product of glycolysis is hydrogen and this needs to be removed, as otherwise a build up in hydrogen ions would make the muscle cell acidic and interfere with its functioning. It is thought that the hydrogen ions interfere with the binding site of calcium ions to the troponin-tropomysin complex so muscle contraction is affected.

This results in the hydrogen ions being removed by carrier molecules known as nicotinamide adenine dinucleotide (NAD). The NAD is reduced to NADH  (when it takes on the hydrogen) and deposits the hydrogen at the electron transport chain to be combined with oxygen. 

However, if there is not sufficient oxygen then the NADH  cannot offload the hydrogen ions, and this would result in them beginning to build up in the cell, again. So, to prevent a rise in acidity pyruvic acid accepts the hydrogen ions and it is this process that forms lactic acid.

Once lactic acid is formed to remove it, it dissociates into lactate and hydrogen ions. Some of the lactate diffuses into the blood and takes the hydrogen ions with it to act as a mechanism for lowering the hydrogen ion concentration in the muscle cell.

Onset of Blood Lactate Accumulation

The lactate threshold is the exercise intensity at which lactic acid starts to accumulate in the blood stream. This happens when it is produced faster than it can be removed, and is sometimes referred to as the anaerobic threshold or the onset of blood lactate accumulation (OBLA). The normal pH of the muscle cell is 7.1 but if the build up of H  continues and pH is reduced to around 6.5 then muscle contraction may be impaired and the low pH will stimulate the free nerve endings resulting in the perception of pain. This is the point that is measured as OBLA.

Glycolysis

This is the first part of the aerobic energy system otherwise known as respiration. It occurs in the cytoplasm of the cell.

Firstly glucose is phosphorylated, meaning that a phosphate group is attached to the molecule which can activate or deactivate the protein by changing the overall shape of the molecule. The phosphorylated glucose then splits into two triose molecules, which is oxidised to pyruvate giving a small yield of ATP and reduced NAD.

The Link Reaction

This is the second stage in aerobic respiration, whereby the pyruvate from glycolysis goes to the matrix of the mitochondria where it is dehydrogenated and decarboxylated. Acetyl combines with coenzyme A to form acetyl coenzyme A, in aerobic conditions.

The Krebs Cycle

This is the third stage of aerobic respiration and is also known as the citric acid cycle. It occurs in the matrix of the mitochondria.

The Krebs cycle produces:

·         2 molecules of ATP

·         Carbon dioxide

·         H₂ (removed by NAD and FAD)

The Krebs cycle turns twice for each glucose molecule as only one pyruvate is used at a time. It is a continuous reaction of low intensity.

The Electron Transport Chain

This is the last part of aerobic respiration:

• ·    NADH  and FADH  move to the inner mitochondrial membrane where they are oxidised.
• ·    The hydrogen splits into H  and e  .
• ·    The electrons (e  ) pass through a series of oxidation and reduction reactions and move down the electron carriers.
• ·    This creates energy to pump the hydrogen ions (H  ) into the Intermembranal space.
• ·    This creates an imbalance of hydrogen ions resulting in them diffusing back across the inner membrane through stalked particles.
• ·    The energy created promotes the resynthesis of ATP (catalysed by ATPase).
• ·    Oxygen acts as a terminal electron acceptor and combines with hydrogen to form water.

By the end of the process, enough energy is produced to resynthesis 34 ATP at a low intensity.

Overview of the Aerobic System

This exothermic system is made up of the following stages:

·         Glycolysis and the Link Reaction

·         The Krebs Cycle

·         The Electron Transport Chain

It occurs in the cytoplasm and mitochondria of the cell where glucose is used to produce:

·         38 ATP (2 from glycolysis, 2 from the Krebs Cycle and 34 from the Electron Transport Chain)

·         Hydrogen

·         Carbon dioxide

·         NAD and FAD

·         Water

It is catalysed mainly by ATPase but also PFK. It has a duration of 2 minutes + and works at a low intensity.

Fat Metabolism

We store 95% of our fat as triglyceride in our adipose tissue. We also store small amount in our muscle and liver. When required as an energy source the triglyceride is broken down into glycerol and free fatty acids (FFA) by the process of lipolysis. This is catalysed by the enzyme lipase.

These substances are then released into circulation and it is the FFA that can be utilised by skeletal muscle. When entering the muscle, the process of beta oxidation breaks down the FFA molecules into acetyl CoA, which can then progress through the Kreb’s cycle and the electro transport chain. The FFA molecules are made up of a chain of carbon atoms that can be broken down into the two carbon acetyl CoA.

It is essential that oxaloacetic acid is present in the Kreb’s cycle for acetyl CoA to combine with, Oxaloacetic acid comes from carbohydrate breakdown and so if we run short of glycogen or glucose then it affects our ability to oxidise fat as a fuel. It is often said that ‘fat burns in a carbohydrate flame’.

Protein Metabolism

Only in situation of extreme exhaustion, sustained prolonged exercise or starvation does the body metabolise protein as an energy source. It is this basic unit, the amino acid, which is converted into acetyl CoA. Protein can provide 5 to 10 % of energy requirements during prolonged exercise and may be relied on more as glycogen stores are depleted.