1. Metabolism and Survival
Key Area 3a
Measuring Metabolic Rate

2. Learning Intentions By the end of this topic you should be able to:
Understand how the rate of metabolism can be measured;

3. You should already know:
The pathway of blood through the human heart, lungs and body
the structure of the human heart including the right and left atria and ventricles;
red blood cells contain haemoglobin and are specialised to carry oxygen.

4. Metabolic Rate Glucose + Oxygen ---- > ATP + carbon dioxide + water
Metabolic rate is the quantity of energy used by the body over a given time. It is measured in kilojoules (or kilocalories).
Metabolic rate can be measured as
Respirometers
Oxygen consumption per unit time
CO2 production per unit time
Energy production (heat) per unit time
Glucose + Oxygen ---- > ATP + carbon dioxide + water

5. Measuring metabolic rate
There are different ways to measure metabolic rate.
Respirometers and Calorimeters are two different pieces of equipment that do this

6. Respirometers
Respirometers measure the volume of oxygen uptake by an organism.
If a man consumes 350L of oxygen on average per day this equates to 1700Kcal or 7000Kj

7. How a respirometer works
Chemical that removes CO2 from air
Air is drawn from tube, pulling up liquid which can be measured on scale
Air used to zero scale
The organism uses the O2 from the air during respiration. The CO2 it produces is absorbed by the chemical.
As the air in the chamber is used up, more is pulled up from the capillary causing the liquid to move up the scale.
The distance moved is equivalent to the O2 consumed
This is sometimes called indirect calorimetry

8. Calorimeters
Calorimeters measure the heat generated by an organism and calculates the metabolic rate from the results
This experiment shows the heat produced by germinating peas. The right flask is the control. There should be no temperature rise in the control flask.

9. Direct Calorimeter
A calorimeter measures the heat generated by an organism by comparing the temperature of water entering and leaving a well-insulated container.

10. Metabolic rate
The energy expended by an organism can be broken down into three main parts
The BMR (Basal metabolic rate) – the minimum energy required by an organism at rest
Physical Activity – the more activity, the higher the energy cost
Dietary-induced thermogenesis – cost of digesting and processing food

11. Basal metabolic rate (BMR)
At rest, the metabolic rate is known as the basal metabolic rate (BMR). The BMR is low compared to the metabolic rate when the body is undergoing activities like exercise.
During this resting state the body only needs to use energy to keep vital organs such as the heart, lungs and brain functioning properly.

12. Different organisms will have different BMR.
Animal
Volume of oxygen consumed (mm3 g⁻1 body mass h¯1)
Sea anemone 13
Octopus 80
Eel
128
Frog
150
Human
200
Mouse
1500
Hummingbird
3500

13. Comparative metabolic rate
As a general rule, the greater the mass of an organism the higher that organism’s metabolic rate is.
However, BMR is higher per unit of body mass in small animals compared to larger ones. This is because the higher metabolic rate of small animals needs a greater delivery of oxygen to tissues around the body.
Also, the smaller animals have a greater surface area to volume ratio, so more heat is lost