Chloroplast. Illustration of the internal structure of a chloroplast, the organelle in plant cells responsible for photosynthesis. This sectioned view (top) shows the outer (light green) and inner (dark green) membranes enclosing a fluid called stroma. Internal membrane structures called thylakoids (green) form both grana (stacks) and lamellae (flat planes). It is the thykaloids that are the site of photosynthesis, the capturing of energy from sunlight using the green pigment chlorophyll. Also shown is a granule of starch (orange), a metabolic product of photosynthesis. For this artwork with labels, see image C023/8758.