Answer and Explanation: 
Available Data:
- NPP = GPP – Respiration = 120 grams/m2/day 
- NPP= Net primary productivity of a system 
- GPP= Gross productivity of the producers 
- Respiration loss 25% of NPP 
- Insolation energy is 800 calories/cm2/day 
- % Efficiency of Photosynthesis = (NPP/Insolation Energy) X 100 
1) Find the gross productivity of the sugar beets. 
We need to calculate the GPP, which equals NPP - Respiration rate. The respiration loss is 25% of the NPP. This is: 
100% NPP--------------- 120 g/m²/day 
25% NPP ----------------X = (25 x 120)/100 = 30 g/m²/day 
Now that we know the values of NPP and respiration rate, we can calculate the GPP. 
GPP = NPP - Respiration 
GPP = 120 (g/m²/day) - 30 (g/m²/day) 
GPP = 90 g/m²/day 
GPP = 0.009 g/cm²/day
 2) Using the NPP, calculate the efficiency of photosynthesis for the sugar beets. 
% Efficiency of Photosynthesis = (NPP/Insolation Energy) X 100 
We know the NPP, but this value is calculated in g/m²/day, while the Insolation energy is in g/cm²/day. We need to calculate the % Efficiency of Photosynthesis in cm. To do that, we must transform the information. 
So if 1 m² = 10,000 cm², then: 
10,000cm²----------120 grams/m²/day NPP 
1 cm²---------------- (1 x 120) / 10,000 = 0.012 grams/cm²/day NPP 
We also know that 1 gr beets = 2,000 cal, and that the Insolation energy is 800 calories/cm²/day 
So we need to transform Insolation energy from calories/cm²/day to grams/cm²/day, so: 
2,000 calories ----------------- 1 gr beet 
800 calories/cm²/day---------X = 0.4 gr beet. 
% Efficiency of Photosynthesis = (NPP/Insolation Energy) X 100 
% Efficiency of Photosynthesis = (0.012 (gr/cm2/day) /0.4 (gr/cm2/day)) X 100 
% Efficiency of Photosynthesis = 3 gr/cm2/day