Eukaryotic cell structure. Coloured scanning electron micrograph (SEM) of endoplasmic reticulum (ER, red and blue) and microtubules (yellow) inside a cell. ER, a membrane-bound organelle, is the site of lipid synthesis and the production of membrane-bound proteins. Microtubules are protein filaments that form part of the cell cytoskeleton. The cytoskeleton gives the cell its shape and the capacity for directed movement.