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Protists are a group of organisms placed in a single kingdom because they do not quite fit into any of the other eukaryotic kingdoms. Plant-like protists resemble plants but have unique characteristics and reproductive options.

What Are Protists?

Protists are an eclectic group of organisms that are eukaryotic, which means they have a nucleus, organelles, and a cell membrane. But these guys are special–they don’t really belong to the plant, animal, or fungi club, but they still share many characteristics with other eukaryotic life forms. What makes a protist a protist is that it cannot be placed into any of the other kingdoms physically or genetically.

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For instance, the plant-like protists aren’t enough like plants to be one.

Plant-Like Protists

The characteristic that makes algal protists (algae) plant-like is the ability to do photosynthesis. Like plants, plant-like protists have chloroplasts that contain the pigment chlorophyll that collects and converts light into energy. As you might suspect, algal protists can be green, but they can also be red, brown, or gold; their colors come from pigments that mask the green of chlorophyll.It kind of makes sense that microscopic, swimming organisms would not be invited to sit at the plant table because, well, when was the last time you saw a tiny swimming tree? But wait, what about seaweed? Seaweeds have leaves, stems, and roots, but they are still not allowed in the plant club.

No matter how much plant-like protists resemble plants, they are just protists in disguise; a closer look reveals that these organisms only resemble plants on the outside. As shown in the images below, pine needles have many different types of cells. For instance, plants have exterior cells that provide protection just as our skin protects us. Algal protists are collections of cells encased in a layer of slime called an extra-cellular matrix (ECM) as observed below. Because they do not have specialized cells, algae are just wannabe plants.

Pine needles (A) have differentiated cells (B). Seaweeds (C) look like plants, but are actually cells in an ECM as indicated in D and E.


Unicellular Plant-like Protists:

Pond water is filled with billions of microscopic creatures, including many unicellular plant-like protists such as ‘Chlamydomonas’ and ‘Euglena’ swimming around in it. These and other microscopic algae don’t look anything like plants so it’s easy to get why they aren’t part of the plant club. But down in all of that mud and muck you might also find ginormous unicellular protists such as the seaweed ‘Caulerpa.

‘ These big guys can grow up to two feet (60 cm) in height, but they are still one single cell. What’s even cooler is that when ‘Caulerpa’ start to grow, each organism can make up to 5,000 leaves that are still a part of the same cell. Unfortunately, no matter how much ‘Caulerpa’ looks like a plant, it is a single cell; for that reason, it isn’t a plant and it can’t sit at the plant table.

Caulerpa, a kind of seaweed, is a large, single cell with many nuclei seen as white dots in the figure.
Unicellular Algae

Multicellular Plant-Like Protists

Multicellular algae are groups of two or more cells that act as a single organism.

If you have seen pond scum growing in water, it is likely that there are multicellular algae growing in there. Similar to unicellular plant-like protists, multicellular algae can be enormous or miniscule. Colonial algae are groups of algae embedded in an ECM that can form a variety of shapes, including structures that resemble leaves, roots, and stems.

‘Volvox’ is an example of microscopic colonial algae that resemble a ball. ‘Ulva’ may look like a plant, but no matter how hard ‘Ulva’ try, they are just a big group of algal cells stuck in slime, just like the ‘Volvox.’

Volvox, a type of colonial plant-like protists.
Klebsormidium, filamentous plant-like protists.
Multicellular Algae

How Do Plant-Like Algae Reproduce?

When the algae aren’t cosplaying plants or being pond scum, they are busy reproducing. Some plant-like algae are able to switch between sexual and asexual reproduction, while others can only reproduce in one way–it just depends on the species.

Sexual reproduction in plant-like protists is similar to sexual reproduction in other eukaryotes: it involves gametes, or cells produced for the purpose of sexual reproduction, such as human eggs and sperm. Gametes are haploid, which means they have one copy of each chromosome. The gametes fuse to form a zygote that will grow into an adult. Just remember that haploid kind of sounds like half and it takes two to tango (or make a zygote).

The zygote is diploid, so it has two copies of every chromosome. The individual organism created by sexual reproduction is unique with different chromosomes from each parent.Asexual reproduction involves making an exact copy of the parent. Some plant-like protists replicate using binary fission, which is just dividing in two. Still others reproduce by making asexual spores, which spread out and form new individuals. Finally, fragmentation is yet another way of asexual reproduction in plant-like protists.

Imagine if a new you formed every time you cut your nails–that is how fragmentation works; a part of the parent algae breaks off and forms a new individual.

Plant-like protists reproduce asexually (A) or sexually (B) when haploid gametes (C) fuse to form a diploid zygote (D).
algae reproduction

Lesson Summary

Plant-like protists are photosynthetic like plants.

A lot of them are unicellular, but many others are multicellular forming colonies and long filaments. Many of these organisms resemble plants, but they do not actually have leaves, stems, or roots because they are unicellular organisms in disguise. Plant-like protists can reproduce sexually or asexually; some of them can even do both!

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