Aerobic system
The aerobic system is used to resynthesise ATP during low or
moderate intensity activity lasting longer than 1-2 mins (aerobic
activity-under the anaerobic threshold).
There are 3 stages to the aerobic system.
1 Aerobic glycolysis
2 The Kreb’s cycle
3 Electron transport chain (ETC)
Description of process
Stage 1-Aerobic glycolysis (sarcoplasm)– the only difference
between this and anaerobic glycolysis is that because 02 is present
lactic acid is not produced and the system does not stop there.
Glycogen to (GPP)
Glucose to (PFK)
Pyruvic acid to
Acetyl CoA
2 ATP
Stage 2-Kreb’s cycle (matrix of mitochondria)
Acetyl CoA
Oxaloacetic acid
Citric acid
Hydrogen (only) left – enters the ETC
O2 present
CO2 produced and removed
2 ATP
Stage 3-Electron transport chain (ETC) (cristae of the
mitochondria)
Hydrogen enters
Combines with O2
Produces H2O
34 ATP
Total ATP yield = 38 ATP
Glucose molecule is completely broken down from C6 H12 O6
to CO2 and H2O.
Aerobic/anaerobic - aerobic
Intensity – low/moderate – below anaerobic threshold
Duration – over 1-2 mins (takes a while for system to get going)
Number of ATP resynthesised – 38 ATP (2ATP+2ATP+34ATP)
Where reaction takes place – stage 1-sarcoplasm stage 2+3mitochondria
Fuel(s) used – glycogen/glucose – fats(can be used after approx 20
mins of aerobic activity).
Enzyme(s) – GPP/PFK
By-product(s) – Kreb’s cycle –CO2 ETC – H2O
Sporting example – marathon running
Advantages of system
Large glycogen and fat stores
Efficient resynthesis of ATP – if O2 is present
Breaks down fats
Large ATP resynthesis – 38ATP from 1 mol of glucose – compared to
2ATP from LA system and 1 ATP from ATP/PC.
Provides energy for low/mod intensity for long duration activities.
No fatiguing by-products – CO2 and H2O easily removed.
Disadvantages of system
Slow resynthesis of ATP compared to other 2 systems.
Requires good O2 supply (15% more for breaking down fats)
Complex series of reactions
Con not begin resynthesis of ATP at start of exercise due to delay
in O2 supply to CV system.
No use during high intensity and short duration work.
Practice writing out each system using the structure used above.
Also practice drawing diagram we drew back in September- v. useful
for learning the structure of each system. Similar diagrams can be
found on pages 369-374 in the textbook.
As we have already said the aerobic system can use fats as a fuel as
well as glucose. Fats are stored as triglycerides before being broken
down by lipase enzymes into free fatty acids (FFA) and glycerol.
These are then used to provide energy to resynthesise ATP within
the aerobic system.
The FFAs are broken down into Acetyl CoA which enter the kreb’s
cycle as with glycogen.
FFAs produce more Acetyl CoA than glucose and consequently
produce lots more energy for resynthesis of ATP.
The down side is that this process requires approx 15% more O2 so
is not so good for higher intensity aerobic work –glycogen would be
preferable.
FFAs will only become the main energy source after approx 20 mins
of aerobic activity. After a period of aerobic training this can be
reduced to about 15 mins.
This is known as glycogen sparing, keeping the glycogen store for
later when maybe higher intensity aerobic, or even anaerobic
(remember the LA system uses glycogen) activity will take place- as
in a sprint at the end of a marathon.
Training adaptations
Increase in muscle and liver glycogen stores.
Increase in aerobic enzyme action.
Glycogen sparing – earlier use of FFAs as a fuel.
Raises the anaerobic threshold – so you can work at a greater
intensity within the aerobic system.
Delays fatigue resulting from onset of blood lactate (OBLA).
Removes lactate more efficiently during periods of recovery.