HESI A2 Biology Practice Test 2026 - Free HESI A2 Biology Practice Questions and Study Guide

Photosynthesis occurs in specialized organelles called chloroplasts within plant cells. Chloroplasts contain structures known as thylakoids, where the light-dependent reactions of photosynthesis take place. These thylakoids contain chlorophyll, the pigment that captures light energy, and they form stacks called grana.

The stroma, another component of chloroplasts, is the fluid-filled interior space where the light-independent reactions, commonly known as the Calvin cycle, occur. While the stroma plays a crucial role in the overall process of photosynthesis, it is the chloroplasts, specifically the thylakoid membranes within them, that are directly involved in capturing light energy.

Mitochondria are involved in cellular respiration and energy production, but they are not the site of photosynthesis. Therefore, the correct answer highlights the specific part of the chloroplast where the light reactions take place, underscoring the essential role this organelle plays in the process of converting light energy into chemical energy.