With the exception of blue green algae which are treated as Cyanobacteria, all algae have eukaryotic cell organization. The cell wall is made up of cellulose and pectin. There is a well defined nucleus and membrane bound organelles are found.
Three types of Photosynthetic pigments are seen in algae.
They are 1. Chlorphylls 2. Carotenoids 3. Biliproteins. While chlorophyll a is universal in all algal classes, chlorophyll b,c,d,e are restricted to some classes of algae.
The yellow, orange or red coloured pigments are called carotenoids. It includes the caroteins and the Xanthophylls. The water soluble biliproteins called phycoerythrin (red) and phycocyanin (blue) occur generally in the Rhodophyceae and Cyanophyceae and the latter is now called cyanobacteria. These pigments absorb sunlight at different wavelengths mainly in blue and red range and help in photosynthesis. Pigmentation in algae is an important criterion for classification.
The colour of the algae is mainly due to the dominance of some of the pigments. For example in red algae(class Rhodophyceae) the red pigment phycoerythrin is dominant over the others. The pigments are located in the membranes of chloroplasts. In each chloroplast one or few spherical bodies called pyrenoids are present. They are the centres of starch formation.
Algae are autotrophic in their mode of nutrition. The carbohydrate reserves of algae are various forms of starch in different classes of Algae. For example, in Chlorophyceae, the reserve food is starch and in Rhodophyceae it is Floridean starch, in Phaeophyceae it is laminarian starch while in Euglenophyceae it is paramylon. Members of Phaeophyceae storemannitol in addition to carbohydrate. Members of Xanthophyceae and Bacillariophyceae store fats, oils and lipids. The nature of reserve food material is also another important criterion used in classification.