Energy systems – towards a clearer understanding of the requirements for the exam

Energy systems loom large in the understanding of the PHED 3 specification.  In simple terms, the majority of the physiology content is underpinned by energy systems, to the extent that without a clear understanding of energy systems, the detailed physiology of PHED 3 can be difficult to comprehend.  That’s why I always recommend that the PHED 3 physiology content should always be started by talking about energy systems.

When we exercise our body is constantly working to supply muscles with enough energy to keep going, but the way energy is made available to muscles changes depending on the specific intensity and duration of exercise.  There is no clear answer to the question – ‘which energy system is being used when?’.   It all depends, so the exam question tends to follow the generalisation – ‘what are the main energy systems being used?’.  A simple analogy is ‘what gear did you use on that car journey?’.  Answer – well I used every gear and it depends on what part of the journey you are talking about, but I mainly used….

ATP – The Energy Source for Muscle Contraction

Muscle cells need energy to be able to contract during exercise. The energy for muscle contractions comes from food.  A complex chemical process converts the energy stored in food into a form that is used by our muscles – a chemical called ATP.

The energy stored within an ATP molecule is released for our muscles to use when chemical bonds within ATP are broken. Breaking this bond reduces the ATP molecule to ADP and energy is available for muscle contraction.  There are lots of ways of drawing this concept, and student recall images better than words.

ADP  can be restored back to ATP by adding the missing phosphate group. There is always ATP available for muscle contraction, so some texts talk about ‘stored ATP’ as one way of supplying energy.

Three Exercise Energy Systems

The processes that convert food energy into ATP depend on the availability of oxygen. For the vast majority of people’s existence, oxygen is readily available and ATP can be supplied aerobically.  Just occasionally, exercise limits oxygen supply and ATP has to be generated without sufficient oxygen (anaerobic).

The Aerobic Energy System

During low  intensity (aerobic) activity, sufficient oxygen is made available to the contracting muscles. This continuous supply of oxygen allows a reduced intensity exercise to be maintained for a long period of time. The aerobic system uses carbohydrates and fats as its primary energy sources.  Once the carbohydrates and fats have been prepared, most of the action takes place in specialised organelles called mitochondria.  Carbohydrates, mainly in the form of glucose, but also glycogen, are prepared by being broken down in a series of chemical reactions to pyruvate (pyruvic acid).  This breakdown is called glycolysis and releases sufficient energy to enable the resynthesis of some ATP (some because the number of molecules of ATP resynthesised varies between glucose and glycogen).

To enable better understanding, I get my students to draw this process using the appropriate terms and arrows (pointing down) to show breakdown.  Start with glucose in the top left hand corner, with an arrow drawn diagonally towards the centre of an A4 sheet where pyruvate can be added and the arrow labelled as glycolysis.  A branch off this arrow can then be added to show (some) ATP being resynthesised. Students will then add to this elementary diagram as their knowledge develops until they can eventually draw a complete guide to the energy systems from memory.

The pyruvate then forms acetyl-CoA which enters the mitochondria.  Once in the mitochondria, the acetyl group joins in with Kreb’s cycle, which is simply a mechanism for removing H atoms from the complex chemicals involved, leaving carbon and oxygen to form carbon dioxide.  The H atoms are then carried to the Electron Transport Chain where further chemical reactions add oxygen to the hydrogen forming water.  During these chemical changes large numbers of ATP molecules are resynthesised.

Students should then add this mitochondria/Kreb’s cycle part to their diagram – mitochondria = a box with Kreb’s cycle and the electron transport chain inside, with pyruvate entering as acetyl Co-enzyme A.  Arrows leaving the mitochondria should show the CO2 and water formed, and the ‘lots’ of ATP resynthesised.  Another arrow should show the oxygen needed entering the mitochondria.  The ‘bit’ in the mitochondria is the aerobic part.

Fats can also be used in Kreb’s cycle.  Free fatty acids, are transported to the mitochondria, where the carbon atoms are used to produce acetyl-CoA (a process called beta-oxidation) for entry into Kreb’s cycle and the Electron Transport Chain.

Students should add this process to their diagram.

As long as there is a continuous supply of oxygen this process continues – glucose/glycogen and fat breakdown; mitochondria and Kreb’s cycle – water and CO2 as waste products and lots of ATP resynthesised for muscle contraction.

The lactate (Lactic Acid)  Anaerobic System

If the need for energy increases, then the process simply speeds up – I use the analogy of water flowing out of a tap into a sink and through the plughole.  Need more water (energy) simply open the tap and increase the flow.  Let’s keep the analogy.  If you kept increasing the flow, eventually the sink will start to fill with water as not enough water can get through the plug hole.  This happens during exercise as we increase the intensity.  Biologically speaking, there are insufficient enzymes to transfer all the pyruvate into the mitochondria and not enough oxygen to keep up with the flow of Kreb’s cycle.  In our muscles, the extra pyruvate is immediately converted into another substance called lactate (lactic acid).  In our analogy the excess water (lactate) simply floods the sink and eventually overflows onto the floor.  Lactate, as all students know (hopefully!), causes fatigue, mainly because of the localised acidity that limits the muscle’s ability to contract.

This idea can be added to the diagram – a downward arrow from pyruvate to lactate.  Notice that on the diagram, and in a practical sense, the breakdown of glucose to pyruvate to lactate produces resynthesised ATP during the glycolysis stage, and does not require oxygen (only used in the mitochondria); it is anaerobic.

The ATP-PC (Phospagen) System

During short-term, intense activities, a large amount of energy needs to be produced very quickly by the muscles, creating a high demand for ATP.  The muscle use the ATP-PC system for this type of activity.  Muscles have a store of phosphocreatine (PC) which can be readily broken down into creatine and phosphate.  This breakdown releases sufficient energy to resynthesise one molecule of ATP.  No oxygen is involved and so it is anaerobic.  Phosphocreatine stores are limited, so the ability to resynthesise ATP from PC breakdown is strictly limited to flat out exercise lasting about 5-8 seconds (depends on fitness).  The idea of PC breakdown to permit ATP resynthesis should be added to the diagram, but make sure it goes on the left-hand/anaerobic side, as no oxygen is involved.

As a final analogy to explain how the systems work together during an activity, students might like to consider driving a car.  Many year 13 students will be learning to drive at this time.   But this car only has three gears.  So on a reasonably long journey, which gear is used most – answer – top/third gear – corresponding to the aerobic system for long continuous exercise.  The car may start off in first gear for a few seconds, corresponding to the ATP-PC system, but soon gets into second gear (lactate anaerobic system) and then into top gear.  You might use the second gear/ lactate anaerobic system  when going up hills for example.  And if you were in a mad rush to cover a short distance as quickly as possible you might use first gear/ATP-PC system for most of the journey.

So in an activity, which system is being used depends on the intensity and duration of the exercise.  Examination questions will tend to relate to a specific intensity/duration and so demand that rather than re-write all the previous 100 words or so carefully explaining all the energy systems, students will be asked to apply their knowledge and describe only the main system being used, as in the following examples.


A sprint cycle race involves cycling four laps of a 250 metre track, with the final lap being completed as fast as possible.  Elite performers cover the final lap in times of between 10 and 11 seconds.

Name the main energy system being used in the final sprint to the finishing line and explain how this system provides energy for the working muscles.                             


Elite swimmers can complete a 200-metes free-style race in just under 2 minutes.  Describe how the majority of energy will be produced for this type of race.


Games often last for over an hour and performers have to cope with high levels of energy expenditure.  How is the majority of energy required by games players produced?

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