CBSE Class 10 Science 2017 · Paper 31/2/C · Question 6

Example: grass $\to$ grasshopper $\to$ frog $\to$ snake. The grass captures about $1\%$ of the solar energy falling on its leaves and stores it as food. This energy passes to the grasshopper when it eats the grass, then to the frog and then to the snake - always in the direction of the arrows - and only about $10\%$ of it reaches each next level (for example $10000$ J $\to$ $1000$ J $\to$ $100$ J $\to$ $10$ J); the rest is used in life processes and lost as heat. The energy captured by the grass does not go back to the Sun, and the energy that has passed to the grasshopper never comes back to the grass: within the chain energy passes only from the eaten to the eater, and the heat lost at each step cannot be used by the plants, which need sunlight to make food. So, as energy moves from one trophic level to the next, it is no longer available to the previous level: the flow of energy is unidirectional.