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# AQA unit 4 bio question! help!! watch

1. 3.74 KJx10^6 year-1 of energy is used in the production of new tissue in a cow. It has been estimated that an area of 8100m2 of grassland is needed to keep one cow.The productivity of grass is 21 135 kJm–2 year–1. What percentage of the energy in the grass is used in the production of new tissue in one cow? Show your working.

the answer is 2.18% but i dont know how to get it HELP SOMEONE PLEASE
2. Hi,

we know that the productivity is 21135 kJm-2year-1 which means that in this area the total gross productivity will be 21135 x 8100 because there are 8100 metres squared and the productivity given is per metre squared.

next we know the net productivity and so the calculation will be as follows:

net productivity/gross productivity x 100 = 2.18

3.74 x 10^6 / (21135 x 8100) = 2.18%

i hope that makes sense.
3. (Original post by AdotK)
Hi,

we know that the productivity is 21135 kJm-2year-1 which means that in this area the total gross productivity will be 21135 x 8100 because there are 8100 metres squared and the productivity given is per metre squared.

next we know the net productivity and so the calculation will be as follows:

net productivity/gross productivity x 100 = 2.18

3.74 x 10^6 / (21135 x 8100) = 2.18%

i hope that makes sense.

where did you get this equation from : net productivity/gross productivity x 100 and how did you know to use it ??
4. (Original post by klgyal)
where did you get this equation from : net productivity/gross productivity x 100 and how did you know to use it ??

oh im not sure if its an actual equation in the textbook.. i just used that equation to explain what i was doing...... erm it seemed right/like common sense to approach the question like that.

If i explain the equation:

if you're trying to work out the productivity/ energy efficiency then you would do gross productivity-respiratory losses/gross productivity because gross productivity is the total energy available at the start if that makes sense... so total energy - energy loss/total energy x 100 = % of efficiency

edit: note that net productivity = gross productivity - respiratory losses
sorry if im confusing you!
5. (Original post by AdotK)
oh im not sure if its an actual equation in the textbook.. i just used that equation to explain what i was doing...... erm it seemed right/like common sense to approach the question like that.

If i explain the equation:

if you're trying to work out the productivity/ energy efficiency then you would do gross productivity-respiratory losses/gross productivity because gross productivity is the total energy available at the start if that makes sense... so total energy - energy loss/total energy x 100 = % of efficiency

edit: note that net productivity = gross productivity - respiratory losses
sorry if im confusing you!
hmm..i think im understanding it a bit better now. can you explain to me what net production and net productivity means because i get so confused when i read the aqa neslon book please thanks x
6. (Original post by klgyal)
hmm..i think im understanding it a bit better now. can you explain to me what net production and net productivity means because i get so confused when i read the aqa neslon book please thanks x
well basically gross productivity is the total amount of light energy which is converted to chemical energy by the producers i.e. photosynthesising plants. some of this energy is used for respiration so the total amount of energy STORED is less than the total amount of light energy converted.

the total amount of energy stored i.e. the rate at which it is stored is known as the net productivity hence:

net productivity = gross productivity - respiratory losses
7. (Original post by AdotK)
well basically gross productivity is the total amount of light energy which is converted to chemical energy by the producers i.e. photosynthesising plants. some of this energy is used for respiration so the total amount of energy STORED is less than the total amount of light energy converted.

the total amount of energy stored i.e. the rate at which it is stored is known as the net productivity hence:

net productivity = gross productivity - respiratory losses
wow i so get it now thank you soo much x
...oh but what is net production then ??
8. (Original post by klgyal)
wow i so get it now thank you soo much x
...oh but what is net production then ??

well im not 100% sure but to me it seems as though net productivity describes the rate of energy storage i.e. its expressed in kJm-2year-1 whereas net production is the total amount of energy stored i.e. its expressed in kJ...
9. (Original post by AdotK)
well im not 100% sure but to me it seems as though net productivity describes the rate of energy storage i.e. its expressed in kJm-2year-1 whereas net production is the total amount of energy stored i.e. its expressed in kJ...
thanks

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