1. (1) ATP ATP is the only form of usable energy in the body.
Energy is released from ATP when it is broken down into ADP + Pi.
Stores of ATP last for only 3 seconds.
When ATP stores are depleted, they need to be replenished immediately.

2. (2) Energy systems
There are three energy systems that can regenerate ATP:
the ATP–PC system (anaerobic)
the lactic acid system (anaerobic)
the aerobic system

3. (3) Energy systems
The use of each of these systems depends on the intensity and duration of the activity:
If the activity is short duration (less than 10 seconds) and high intensity, we use the ATP–PC system.
If the activity is longer than 10 seconds and up to 3 minutes at high intensity, we use the lactic acid system
If the activity is long duration and submaximal pace, we use the aerobic system.

4. (4) The energy continuum
Sometimes we need to use all three systems to regenerate ATP because the demands of an activity are varied. For example, in rugby:
a short sprint to tackle a player uses the ATP–PC system
a long sprint the length of the pitch to score a try uses the lactic acid system
positional play will use the aerobic system

5. (5) What you need to know about energy systems
Type of reaction
The chemical or food fuel used
Where the reaction occurs
The energy yield (how many ATP molecules)
Specific stages in a system
The by-products
When the system is predominant

6. (6) The ATP–PC system Type of reaction — anaerobic coupled
Fuel used — phosphocreatine
The reaction occurs — in the sarcoplasm
Energy yield — 1 ATP
Specific stages in the system — PC is stored in the muscle and broken down to release energy; this energy is used to regenerate ATP
By-products — none
The system is predominant in high-intensity activity lasting less than 10 s, e.g. 100 m sprint or a slam dunk

7. (7) The ATP–PC system Advantages
Phosphocreatine stores can be regenerated quickly (50% replenishment in 30 s; 100% in 3 mins)
No fatiguing by-products
Creatine supplementation extends the time that the ATP–PC system can be utilised

8. (8) The ATP–PC system Disadvantages
There is a limited supply of phosphocreatine in the muscle cells, i.e. it can only last for 10 s
Only 1 molecule of ATP can be regenerated for every molecule of PC
PC regeneration can only take place in the presence of oxygen (i.e. when the intensity of the exercise is reduced)

9. (9) The lactic acid system
Type of reaction — anaerobic coupled
Fuel used — glycogen
The reaction occurs — in the sarcoplasm
Energy yield — 2 ATP
Specific stages in the system — glycogen is broken down into glucose and in the absence of oxygen forms pyruvic acid
By-product — lactic acid
The system is predominant — in high-intensity activity lasting between 10 s and 3 min, e.g. a succession of tackles in rugby, or a full court press followed by a fast break in basketball

10. (10) The lactic acid system
Advantages
ATP can be regenerated quite quickly because few chemical reactions are involved.
In the presence of oxygen, lactic acid can be converted back into liver glycogen, or used as a fuel by oxidation into carbon dioxide and water.
It can be used for a sprint finish (i.e. to produce an extra burst of energy).

11. (11) The lactic acid system
Disadvantages
Lactic acid is the by-product! The accumulation of acid in the body denatures enzymes and prevents them increasing the rate at which chemical reactions take place.
Only a small amount of energy (5%) can be released from glycogen under anaerobic conditions (as opposed to 95% under aerobic conditions).

12. (12) The aerobic system Type of reaction — anaerobic coupled
Fuel used — glycogen/fats
The reaction occurs — stage 1 (glycolysis) in the sarcoplasm; stage 2 (Kreb’s cycle) in the matrix of the mitochondria; stage 3 (electron transport) in the cristae of the mitochondria
Energy yield — stage 1: 2 ATP; stage 2: 2 ATP; stage 3: 34 ATP

13. (13) The aerobic system
Specific stages in the system — stage 1 glycolysis; stage 2 Kreb’s cycle; stage 3 electron transport
By-products — carbon dioxide (stage 2); water (stage 3)
The system is predominant in sub-maximal exercise such as keeping up with play

14. (14) The aerobic system Advantages
More ATP can be produced — 38 ATP from the complete breakdown of one glucose molecule.
There are no fatiguing by-products (only carbon dioxide and water).
Stores of of glycogen and triglyceride are plentiful, so exercise can last for a long time.

15. (15) The aerobic system Disadvantages
This is a complicated system so it cannot be used immediately. It takes time for enough oxygen to become available to meet the demands of the activity and ensure glycogen and fatty acids are completely broken down.
Fatty acid transportation to muscles is low and fatty acids require 15% more oxygen to break them down than glycogen.

16. (16) Food fuels
Food is the basic form of energy for ATP regeneration. The main energy foods are:
carbohydrates — stored as glycogen and converted into glucose during exercise
glycogen — a complex sugar supplied from muscle or liver stores
glucose — a simple sugar supplied from the blood
fats — stored as triglycerides in adipose tissue under the skin and converted by the enzyme lipase to free fatty acids when required

17. (17) When are these fuels used during exercise?
The intensity and duration of exercise play a huge a role in determining whether fats or carbohydrates are used.
The breakdown of fats to free fatty acids requires more oxygen than that required to breakdown glycogen. It is also a much slower process.
Therefore, during high-intensity exercise when oxygen is in limited supply, glycogen will be the preferred source of energy.