Mitochondria and endoplasmic reticulum. Coloured high resolution scanning electron micrograph of mitochondria and endoplasmic reticulum (ER) in an olfactory bulb mitral cell. Mitochondria (red) are the powerhouses" of the cell. They oxidise sugars and fats to produce energy. ER (grey and green) is a network of folded membranes forming sheets or tubes in the cytoplasm. On the surface of some of the ER membranes are protein-synthesizing ribosomes (small dots). They give ER a granular appearance, hence the name "rough" ER. Other ER membranes have no ribosomes and are the "smooth" ER involved in fat metabolism."