Endoplasmic reticulum. Illustration of the structure of the endoplasmic reticulum (ER) membranes found in eukaryotic cells. These organelles form a network of tubules, vesicles, and cisternae to synthesise, store and transport chemicals. Rough endoplasmic reticula (top, sheets) are where protein synthesis occurs in ribosomes (yellow). Smooth endoplasmic reticula (lower right, filaments) synthesize lipids and steroids and metabolize carbohydrates. For this artwork with labels, see image C023/8739.