A generic chloroplast-based model for the carbon concentratingmechanism (CCM) in eukaryotic algae is presented. The finestructure of chloroplasts is represented by separate compartments:marginal and bulk stroma, pyrenoid, girdle lamella, bulk thylakoids,and central lamella traversing the pyrenoid. The roles of theindividual structural elements of the chloroplast with respect tothe CCM and the effect of carbonic anhydrase activity in variouscompartments are analyzed. Hypothetical HCO3- transport into theacidic thylakoid lumen is adjusted by imposing an optimizationprinciple: a given CO2 at the site of Rubisco is achievedwith minimum energy costs for the CCM. Our model is highlyefficient in terms of saturation of Rubisco carboxylase activityand the affinity of the chloroplast for CO2, if either a girdlelamella or a pyrenoid is present. The highest efficiency isachieved with a pyrenoid. A eukaryotic CCM is not necessarilyassociated with accumulation of dissolved inorganic carbon (DIC)as in cyanobacteria.Chloroplasts are categorized into four types corresponding tomorphological characteristics of all major algal classes with regardto the presence of pyrenoids, girdle lamellae, and the distributionof CA activity.