Here are the solutions for the crossword clues provided:

1. **Plankton** (Small fish and whales eat these very small animals)
2. **Two** (and five)
3. **Food chain** (Animals eat plants, and other animals eat those animals, etc.)
4. **Energy** (All plants and animals need this. Plants get it from the sun, animals get it from food)
5. **Orca** (This big animal eats penguins)

Feel free to ask if you need more help!