1. Respiratory substrates
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2. • Glucose is the essential respiratory substrate
for some cells such as neurones in the
brains, red blood cells and lymphocytes
• Other cells can oxidise lipids and amino acids
• When lipids are respired, C atoms are
removed in pairs, as acetyl CoA, from the
fatty acid chains and fed into the Krebs cycle
• The carbon-hydrogen skeletons of amino
acids are converted into pyruvate or into
acetyl CoA
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3. Energy values of respiratory substrates
• Most of the energy released in aerobic respiration comes
from oxidation of hydrogen to H2O when NADH and
FADH are passed to the ETC
• The greater the number of hydrogen in the structure of
the substrate molecule the greater the energy value
• Lipids have a greater energy value per unit mass
(energy density) than carbohydrates and proteins
• The energy value of a substrate is determined by
burning a known mass of the substrate in oxygen in a
calorimeter
• Energy released by oxidising substrate can be
determined from the rise in temperature of a known
mass of water in the calorimeter
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4. Calorimeter
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5. Energy values
Food component Energy density
kcal/g kJ/g
Fat 9 37
Ethanol (alcohol) 7 29
Proteins 4 17
Carbohydrates 4 17
Organic acids 3 13
Sugar alcohols (sweeteners) 2.4 10
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6. Respiratory quotient (RQ)
• The overall equation for the aerobic
respiration of glucose shows that the
number of molecules, and hence the
volumes, of oxygen used and carbon
dioxide produced are the same:
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
• So the ratio of O2 taken in and CO2
released is 1:1
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7. • However, when other substrates are
respired, the ratio of the volumes of
O2 used and CO2 given off differ
• Measuring the ratio (RQ) shows what
substrate is being used in respiration
• It can also show whether or not
anaerobic respiration is occurring
• RQ = volume of CO2 given out in unit time
volume of O2 taken in in unit time
• RQ = moles or molecules of CO2 given out
moles or molecules of O2 taken in
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8. • For the aerobic respiration
– Glucose RQ = 1.0
– Lipid RQ = 0.7
– Protein RQ = 0.9
• For anaerobic respiration
– Alcoholic fermentation of glucose, RQ = ∞
• High values of RQ indicate anaerobic respiration
is occurring
• No RQ can be calculated for muscle cells using
lactate pathway since no CO2 is produced
glucose (C6H12O6) → 2 lactic acid (C3H6O3) +
energy
• O2 uptake during respiration can be measured
using respirometer
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9. Respirometer
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10. • CO2 produced in respiration is absorbed (by
soda-lime/potassium hydroxide/sodium
hydroxide)
• Decrease in volume of surrounding air results
from organisms’ O2 consumption
• O2 consumption in unit time can be measured by
reading the level of the manometer fluid against
the scale
• Once measurements have been taken at a series
of temperatures, a graph can be plotted of O2
consumption against temperature
– Temperature of the surroundings must be kept
constant whilst readings are taken
– Presence of a control tube containing an equal
volume of inert material to the volume of the
organisms used helps to compensate for the changes
in atmospheric pressure
ALBIO9700/2006JK

11. • RQ of an organism can be measured using the same
apparatus:
– O2 consumption at a particular temperature is found (x cm3min-1)
– Then respirometer is set up with the same organism at the same
temperature but with no chemical to absorb CO2
– Manometer scale will show whether the volumes of O2 absorbed
and CO2 produced are the same
– When volumes same, level of manometer fluid will not change
and RQ = 1
– When more CO2 produced than O2 absorbed, scale will show
increase in the volume of air in the respirometer (by y cm3min-1)
– RQ = CO2 = x + y
O2 x
– When less CO2 produced than O2 absorbed, volume of air in the
respirometer will decrease (by z cm3 min-1)
– RQ = CO2 = x – z
O2 x
ALBIO9700/2006JK