Energy is transferred between trophic levels in a food chain through the process of consumption.

In a food chain, organisms are categorized into different trophic levels based on their feeding behavior. The first trophic level consists of primary producers, typically plants, which capture energy from the sun through photosynthesis. They convert this solar energy into chemical energy, primarily stored in the form of glucose. When primary consumers, usually herbivores, consume these plants, the energy is then transferred to the next trophic level.

When a herbivore ingests a plant, it digests the plant material and breaks down the glucose, releasing the stored energy. This energy is utilized by the herbivore for growth, reproduction, and various life processes. However, a portion of this energy is lost as heat due to metabolic activities. The remaining energy is stored in the herbivore’s body tissues.

This stored energy is subsequently transferred to the next trophic level when a secondary consumer, typically a carnivore, consumes the herbivore. Similar to herbivores, carnivores utilize some of this energy for their own life functions, lose a portion as heat, and store the remainder in their body tissues. This process of energy transfer continues up the food chain, moving from one trophic level to the next through consumption.

It is crucial to understand that not all energy is effectively transferred between trophic levels. In fact, approximately $10\%$ of the energy is passed on, while the remainder is either lost as heat or utilized by the organism for its own metabolic processes. This phenomenon is referred to as the 10% rule in ecology. Consequently, food chains typically consist of only four or five trophic levels, as there is insufficient energy to sustain additional levels.

In summary, energy is transferred between trophic levels in a food chain through consumption. However, only a small fraction of this energy is passed on to the next level, while the majority is either utilized by the organism or lost as heat. This energy transfer is fundamental to the flow of life in ecosystems, influencing everything from the smallest microorganisms to the largest predators.