Introduction to the Euglenoids

This section describes what euglenoids are, where they live, how they reproduce, and how the internal components of the cell are organized.

Cell Surface:

The euglenoid cell surface, called the pellicle, is unique among eukaryotes. A cross-section through the cell shows how the pellicle is organized as a series of ridges and grooves. The pellicle consists of the plasma membrane, a series of proteinaceous strips underneath the plasma membrane, and groups of microtubules associated with each strip.

Some euglenoids have pellicles with few, longitudinally arranged pellicle strips. These organisms are usually rigid and do not exhibit metaboly. Other euglenoids have many helically arranged strips. These organisms often exhibit a wriggling motion known as metaboly.

Longitudinal vs. Helical Pellicle

To see metaboly in action, view the Quicktime movie.

The Endomembrane System:

Euglenoids have an extensive endomembrane system. The endoplasmic reticulum (ER) originates from the nuclear envelope, extends throughout the cytoplasm, and forms a large network at the periphery of the cell under the pellicle strips. 

The ER also serves as the major source of membrane for the Golgi apparatus. Euglenoids have some of the largest dictyosomes found among eukaryotes, with large numbers of individual cisternae making up each Golgi stack.

Mitochondria and Chloroplasts:

Numerous mitochondrial profiles are found throughout the euglenoid cell. Each mitochondrion is surrounded by two membranes. The inner membrane folds into the matrix and forms the cristae. In euglenoids, the cristae arepaddle-shaped. When viewed from the surface the cristae appear disc-shaped and appear as flattened sacks when viewed from the side. The short "handle" of the paddle connects the disc with the inner membrane of the mitochondrion.

The number of chloroplasts present in photosynthetic euglenoids varies greatly, however lens-shaped chloroplasts are common. The photosynthetic membranes (thylakoids) within the plastids are stacked in groups of three. The grana stacks, typical of higher plant chloroplasts are not present. Euglenoids are believed to have acquired their chloroplasts by the ingestion and subsequent endosymbiosis of a green algal cell. The chloroplast is surrounded by three membranes. The outermost membrane is believed to represent the plasma membrane of the original host cell which engulfed the green alga.