Unseen Prose Passages — quick quiz

Read the passage and answer the question. Photosynthesis is the process by which green plants convert sunlight into chemical energy stored in glucose. This process takes place mainly in the leaves, where cells contain chloroplasts filled with the green pigment chlorophyll. Chlorophyll absorbs light energy, primarily from the red and blue parts of the spectrum, and uses it to convert carbon dioxide from the air and water from the soil into glucose and oxygen. The oxygen is released into the atmosphere as a by-product, which is essential for the survival of most living organisms. Photosynthesis is the foundation of the food chain, as it provides energy for plants, which in turn are consumed by herbivores and then carnivores. Without photosynthesis, life on Earth as we know it would not be possible. Question: The word 'essential' in the passage is closest in meaning to: