Unicellular (Single-Celled) Protists – Eukaryotic Protozoan
Euglena
Euglena usually live in freshwater ponds. These organisms are heterotrophic (must consume
food) and autotrophic (can make its own food). When light is not available to photosynthesize, the
Euglena will absorb nutrients across their cell membrane hence the term heterotrophic.
Chloroplasts within the euglena trap sunlight required for photosynthesis and can be seen as
several rod-like structures though out the cell. Because the Euglena can undergo photosynthesis,
they detect light via eyespot and move toward it; a process known as phototaxis. When an organism
responds to light, a stimus (plural, stimuli), they move either toward or away from light. These guys
move using flagellum (plural ‚ flagella), which is a long whip-like structure.
In the center of the cell is the nucleus, which contains the cell's DNA and controls the cell's
metabolism (cell activities). The interior of the cell contains cytoplasm, a jelly-like fluid substance.
Toward the posterior of the cell is a star-like structure, the contractile vacuole. This organelle helps
the cell remove excess water. Without it, the euglena could take in so much water via osmosis that
the cell would burst.
The euglena reproduces asexually via longitudinal cell division (see below).
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Amoeba
The amoeba is typically found on decaying bottom vegetation of freshwater streams and
ponds. This organism creeps along by stretching its cytoplasm into fingerlike extensions called
pseudopodia. The word "pseudopodia" means "false foot". Their cell membrane is very flexible and
allows for the amoeba to change shape.
Amoeba can sense their prey through a process called chemotaxis where it is attracted to
chemicals given off. By pushing the endoplasm toward the cell membrane, the amoeba extends its
body and slowly creeps. The amoeba uses it’s pseudopodia to engulf its food, such as bacteria, algae
and other protozoa (endocytosis). As the food is taken in, a food vacuole is formed in which
digestive enzymes break down its prey. After digestion, the waste is expelled through the process of
exocytosis.
Also visible in the amoeba is the nucleus, which contains the amoeba's DNA. In order to
reproduce asexually, the amoeba goes through a process called binary fission. The nucleus duplicates
its genetic material and the cytoplasm splits into two new daughter cells, each identical to the
original parent. Another structure easily seen in the amoeba is the contractile vacuole. Like the
euglena, this organelle pumps out excess water so that the amoeba does not burst or lyse.
The amoeba was once thought to only reproduce asexually during a process called binary
fission. A mature amoeba will split in half creating two identical amoebas. However, they may also
reproduce sexually, exchanging genetic information (see below).
End of binary fission
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Conjugation
Paramecium
The Genus Paramecium are ciliates and can be found in either fresh or marine water
containing bacteria and decaying organic matter. The outside of the cell is covered with a tough
pellicle and is covered with many hundreds of minute hair-like projections called cilia that allow the
paramecium to move as well as sweep prey into its oral groove.
Freshwater paramecia usually have two, sometimes three, contractile vacuoles (typically one
at each end). In order to maintain homeostasis via osmoregulation, the organelle pumps out excess
water that tends to accumulate inside the cytoplasm due to osmotic pressure. These structures are
absent in marine Paramecium because they live in a hypertonic environment. They need to retain
water rather than pump it out. Food vacuoles, which are round in shape, contain enzymes to digest
the other smaller protozoan that the paramecium feeds on. These vacuoles can be seen at the
mouth where the food is loaded into them for digestion. Undigested food leaves through the anal
pore.
Paramecia have two nuclei; the macronucleus and micronucleus. Although both contain
genetic information, they have different jobs. The macronucleus is the center of all cell activity
whereas the micronucleus contains genetic information for sexual reproduction. They may also
reproduce asexually via binary fission
Binary Fission
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Conjugation