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Protist Cell Structure

Instructor: Christie Spadafora

Christie has a B.S. and an M.S. in Biology. She teaches life and chemical science courses at college, high school, and middle school levels in MA.

Protists are an extremely diverse group of eukaryotic organisms. This lesson will provide an overview of various protist cells, including their structures, organelles, shapes, and sizes.

Protists Are Very Diverse

Think about all of the humans you've ever met. Although they are all similar, you can likely tell them all apart. They probably each have different faces, different personalities, and even different voices. Humans are surely diverse, but, protists are even more diverse than humans!

By definition, a protist is essentially any eukaryotic (has a nucleus) organism that is not an animal, plant or fungus. So... what exactly are protists? Some are more like plants, others are more like animals, and still others are more like fungi. Some are single-celled, while others are multicellular. Some protists are photosynthetic, meaning that they, like plants, can convert the energy from sunlight into foods like sugar. Most, but not all, cannot be seen without a microscope. Some protists reproduce asexually, yet others reproduce sexually. In short, protists are a very varied group of organisms, and so are the eukaryotic cells that make up protists.

Finding and Seeing Protist Cells

Head outside and collect a sample of pond water. Bring it back to a biology lab, and you'd be surprised at what you could see under a microscope. Just a tiny sample of pond water can reveal numerous different protists right before your very eyes.

Assorted protists (diatoms) as seen through a microscope.
Assorted diatoms

In fact, microscopist Antonie van Leeuwenhoek said ''No more pleasant sight has met my eye than this of so many thousands of living creatures in one small drop of water'' when he became the first human to visualize tiny protists with the help of a microscope in the 1600s. So, what exactly do these guys look like? Let's start with their innards.

Internal Structure of Protist Cells

Because protists are eukaryotes, each protist cell contains a nucleus. This nucleus protects the protist's DNA, which is the blueprint or code that runs every function of the protist cell. Protist cells, like other eukaryotic cells, have organelles, or tiny organs that each serve a different function within the cell. Ribosomes are the organelles responsible for synthesizing all the proteins the protist will need, while the endoplasmic reticulum and Golgi apparatus help to fold and sort these proteins.

Protists also have mitochondria, the organelles responsible for turning food into energy the cell can use. Photosynthetic protist cells also have chloroplasts, the organelles that are able to capture the energy from sunlight and turn it into sugars. All protist cells are surrounded by a cell membrane, a fluid, protective layer that helps to separate the protist cell from its external environment, keeping everything inside safe and sound. Some protists even have an eyespot, an organelle that helps them detect light, so they can head towards or away from light as they desire.

This protist, Euglena, is often used as a model organism to study biological concepts.
Euglena

Alveolates are protists that have membranous tiny sacs, called alveoli, that are found just below their cell membranes. The exact function of alveoli is unknown, but they may help with protection and regulation. Dinoflagellates are a group of alveolates that have two very interesting characteristics. They are capable of glowing in the dark, a phenomenon known as bioluminescence. When their populations experience sudden growth, dinoflagellates can also be responsible for what we refer to as red tide, or the reddish/brownish water you might see at coastlines from time to time. Dinoflagellates are often spherically shaped, usually single-celled, and, as their name might suggest, also have flagella.

Bioluminescence lighting up the ocean, thanks to Dinoflagellates.
Bioluminescence

Less spherical and more oblong, Plasmodium cells also contain alveoli. These protists, when transferred by certain mosquitoes, can cause malaria.

There are over 100,000 different species of diatoms. These protists are unicellular algae (or plant-like protists) and come in numerous different shapes. From cylindrical to triangle to oval to star, diatoms earn an A+ when it comes to cell shapes! You can find these protists in the ocean; grab a bucket of water and you may have collected millions of these microscopic cells!

External Structure of Protist Cells

As we head past the cell membrane, leaving the inside of the protist cell, things start to get even more diverse! Unlike humans, protists don't have feet or legs to help them move around, but protists have a variety of other ways to move from place to place. Some protists, for example, have a flagellum. Anchored into the cell membrane, flagella are whip-like structures that allow organisms to move through fluids. Protists aren't the only organisms with flagella. Human sperm cells use flagella too! This is how they swim up a female's reproductive tract to reach an egg.

One group of protists, called Trypanosoma, look like tiny squiggles, each with a flagellum at the end. These protists live in tsetse flies and can cause sleeping sickness!

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