Euglena Cells

What Are Euglena?

Euglena are tiny protist organisms that are classified in the Eukaryota Domain and the genus Euglena. These single-celled eukaryotes have characteristics of both plant and animal cells. Like plant cells, some species are photoautotrophs (photo-, -auto, -troph) and have the ability to use light to produce nutrients through photosynthesis. Like animal cells, other species are heterotrophs (hetero-, -troph) and acquire nutrition from their environment by feeding on other organisms. There are thousands of species of Euglena that typically live in both fresh and saltwater aquatic environments. Euglena can be found in ponds, lakes, and streams, as well as in waterlogged land areas like marshes.

Euglena Taxonomy

Due to their unique characteristics, there has been some debate as to the phylum in which Euglena should be placed. Euglena have historically been classified by scientists in either the phylum Euglenozoa or the phylum Euglenophyta. Euglenids organized in the phylum Euglenophyta were grouped with algae because of the many chloroplasts within their cells. Chloroplasts are chlorophyll-containing organelles which enable photosynthesis. These euglenids get their green color from the green chlorophyll pigment. Scientists speculate that the chloroplasts within these cells were acquired as a result of endosymbiotic relationships with green algae. Since other Euglena do not have chloroplasts and the ones that do obtained them through endosymbiosis, some scientists contend that they should be placed taxonomically in the phylum Euglenozoa. In addition to photosynthetic euglenids, another major group of non-photosynthetic Euglena known as kinetoplastids are included in the Euglenozoa phylum. These organisms are parasites that can cause serious blood and tissue diseases in humans, such as African sleeping sickness and leishmaniasis (disfiguring skin infection). Both of these diseases are transmitted to humans by biting flies.

Euglena Cell Anatomy

Common features of photosynthetic Euglena cell anatomy include a nucleus, contractile vacuole, mitochondria, Golgi apparatus, endoplasmic reticulum, and typically two flagella (one short and one long). Unique characteristics of these cells include a flexible outer membrane called a pellicle that supports the plasma membrane. Some euglenoids also have an eyespot and a photoreceptor, which aid in the detection of light.

Euglena Cell Anatomy

Structures found in a typical photosynthetic Euglena cell include:

• Pellicle: a flexible membrane that supports the plasma membrane
• Plasma membrane: a thin, semi-permeable membrane that surrounds the cytoplasm of a cell, enclosing its contents
• Cytoplasm: gel-like, aqueous substance within the cell
• Chloroplasts: chlorophyll containing plastids that absorbs light energy for photosynthesis
• Contractile Vacuole: a structure that removes excess water from the cell
• Flagellum: cellular protrusion formed from specialized groupings of microtubules that aid in cell movement
• Eyespot: This area (typically red) contains pigmented granules that aid in the detection of light. It is sometimes called a stigma.
• Photoreceptor or Paraflagellar Body: This light-sensitive region detects light and is located near the flagellum. It assists in phototaxis (movement toward or away from light).
• Paramylon: This starch-like carbohydrate is composed of glucose produced during photosynthesis. It serves as a food reserve when photosynthesis is not possible.
• Nucleus: a membrane-bound structure that contains DNA
  • Nucleolus: structure within the nucleus that contains RNA and produces ribosomal RNA for the synthesis of ribosomes
• Mitochondria: organelles that generate energy for the cell
• Ribosomes: Consisting of RNA and proteins, ribosomes are responsible for protein assembly.
• Reservoir: inward pocket near the anterior of the cell where flagella arise and excess water is dispelled by the contractile vacuole
• Golgi Apparatus: manufactures, stores, and ships certain cellular molecules
• Endoplasmic Reticulum: This extensive network of membranes is composed of both regions with ribosomes (rough ER) and regions without ribosomes (smooth ER). It is involved in protein production.
• Lysosomes: sacs of enzymes that digest cellular macromolecules and detoxify the cell

Some species of Euglena possess organelles that can be found in both plant and animal cells. Euglena viridis and Euglena gracilis are examples of Euglena that contain chloroplasts as do plants. They also have flagella and do not have a cell wall, which are typical characteristics of animal cells. Most species of Euglena have no chloroplasts and must ingest food by phagocytosis. These organisms engulf and feed on other unicellular organisms in their surroundings such as bacteria and algae.

Euglena Reproduction

Most Euglena have a life cycle consisting of a free-swimming stage and a non-motile stage. In the free-swimming stage, Euglena reproduce rapidly by a type of asexual reproduction method known as binary fission. The euglenoid cell reproduces its organelles by mitosis and then splits longitudinally into two daughter cells. When environmental conditions become unfavorable and too difficult for Euglena to survive, they can enclose themselves within a thick-walled protective cyst. Protective cyst formation is characteristic of the non-motile stage.

In unfavorable conditions, some euglenids can also form reproductive cysts in what is known as the palmelloid stage of their life cycle. In the palmelloid stage, Euglena gather together (discarding their flagella) and become enveloped in a gelatinous, gummy substance. Individual euglenids form reproductive cysts in which binary fission occurs producing many (32 or more) daughter cells. When environmental conditions once again become favorable, these new daughter cells become flagellated and are released from the gelatinous mass.