The word algae refers to a group of organisms that show a great variety in structure and size, ranging from microscopic to over 50 meters in length. In general, they are considered plant-like, as they are photosynthetic. They do not, however, have roots or vessels like plants, and are almost always aquatic. The different divisions, or phyla, of algae are largely based upon structural differences.
Algal Cell Structure
Algae are eukaryotic cells, or cells that contain a nucleus, which makes them slightly more complex than bacteria. They also contain chloroplasts, which are structures that generate energy for the cell through photosynthesis. Other structures that algae may have vary greatly. Some algae have silica exoskeletons, flagella for movement or other structures. The pigment used for photosynthesis can even vary, resulting in algae that appear green, red, or brown.
Algae are divided into two groups based on size. Microalgae are microscopic, like bacteria, and are too small to be seen with the naked eye. Algae can also grow in a group to form large, plant-like structures, called macroalgae. Kelp and seaweed are two well-known types of macroalgae. Some of the larger macroalgae show some degree of cell differentiation, such as leaves or root-like anchoring structures, which makes them appear very plant-like.
Types of Algae
Algae are divided into several different phyla based on the unique structures of the group. Diatoms are single-celled and have a silica shell. Euglenophyta are also single-celled and can create energy by photosynthesis and absorb food. Dinoflagellata have flagella and can move, are usually photosynthetic, are sometimes phosphorescent, and make up a significant portion of plankton. Chrysophyta usually have silica in their cell wall and are usually found in fresh water. The phylum Phaeophyta contains many familiar seaweeds. The phylum Rhodophyta is distinguished by having red pigments, and contains many types of seaweed.
Differences Between Algae and Plants
Although most algae use photosynthesis for fuel, like plants, and some even have roots and leaves like plants, they are not considered plants. The roots that some algae have are not true roots and are usually anchoring structures. Algae lack vascular structures, which are tubes that are found within plants to transport nutrients throughout the plant. Plants also have reproductive structures, whereas most algae reproduce asexually or by cell division.