Question

how is energy flow connected to food web

Answer 1

The closer you are to the bottom of the food web, the more energy there is. As you go higher and higher up the food chain there's less of that energy available.

Think about the energy flow of the following situation: a blade of grass captures energy from the sun and produces its own sugars through carbon fixation. An entire field of grass is going to contain a lot of energy from every single blade of grass undergoing photosynthesis. The grass is essentially at the bottom of the food chain since it's not a predator of anything and captures its own energy through photosynthesis.

A cow then comes by and eats some of the grass. Much of that energy is going to be absorbed by the cow, but the cow does need to expend some energy to get it. The cow needs to walk to wherever there's grass, bend down, and then chew it.

Now we have a hungry person who's going to eat the cow. The cow has already used a bunch of energy it acquired to process the grass it ate, but it's also used the energy from the grass for it's own metabolism and reproduction. There's a lot of energy in the cow that it got from the grass, but not all of it. Since the cow has used some of the energy it absorbed from the grass, there's less energy available for the human who is eating the cow.