This document discusses respiratory substrates and their energy values. It notes that glucose is the primary substrate for some cells, while others can use lipids and amino acids. Lipids have the highest energy value per gram and are oxidized into acetyl CoA units. The energy released during aerobic respiration comes from hydrogen oxidation. The respiratory quotient is the ratio of carbon dioxide produced to oxygen consumed, and can indicate the substrate being used or if anaerobic respiration is occurring. Respirometers are used to measure oxygen consumption and respiratory quotients.

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
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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
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