Biosynthesis

Some of the ATP produced by cellular respiration goes into biosynthesis: the process of building biomass.

Biomass is the mass of living organisms (which can include plants, animals, and microbes) in a given area or ecosystem. Biomass may be expressed in terms of the total mass or as the average mass per unit area. It may be measured for a certain species, group of species (e.g., producers), or all the species in a community.

On average, only about 0.5%–2% of the solar energy that reaches Earth’s surface is converted to biomass. Since biomass is part of the foods we eat, we typically think of this energy in terms of kilocalories (kcal).

A kilocalorie, or kcal, (also called a Calorie with a capital “C”) is equal to 1,000 calories. One calorie is the amount of energy it takes to heat 1 gram of water by 1°C, which equals about 4.19 joules. The “calories” shown on food packaging are actually kilocalories instead of calories!

Question 5

Complete the table calculations. Assume that 1.25% of the solar energy at the surface is converted into producer biomass at all locations. You will also need to know that 1 W = 7,568.6 kcal/yr. Select the numbers in the first row to show example calculations.

You can derive this relationship using the following information:
  • 1 watt (W) = 1 joule/second (J/s)
  • 1 joule (J) = 0.00024 kilocalories (kcal)
  • 1 year (yr) = 31,536,000 seconds (s)
Putting this all together:
1 W = 1 J/s
 = 0.00024 kcal/s
 = (0.00024 × 31,536,000) kcal/year
 = 7,568.6 kcal/yr
A table comparing how much energy (represented in terms of flux) producers convert into biomass at the five locations from Question 2.
Location Solar energy at Earth’s surface (W/m2) Energy converted to producer biomass (W/m2)
This is 1.25% of the solar energy at Earth’s surface. Round to the nearest two decimal places.
Energy converted to producer biomass in one year (kcal/m2)
Given that 1 W = 7,568.6 kcal/yr, this is the energy converted to producer biomass in W/m2 multiplied by 7,568.6. Round to the nearest whole number.
Arctic Tundra 103 1.29
103 W/m2 × 0.0125 = 1.29 W/m2
9763
1.29 W/m2 = (1.29 × 7,568.6) (kcal/yr)/m2
= 9,763 kcal/m2 in one year
Gulf of Mexico 156
Temperate Forest 125 1.56
Tropical Rainforest 195
18467
Tropical Savanna 225
21268