What differentiates a plant from an algae?

Plants and algae are two types of organisms capable of photosynthesis. Thanks to the energy they absorb from solar radiation, both can synthesize organic molecules by combining carbon dioxide (CO2) and water (H20).

Algae generally live in an aquatic environment and plants in a terrestrial environment, although there are species of terrestrial algae and also aquatic plants.

The cellulose cell wall, a characteristic typically associated with plants, can also be found in various groups of algae, as well as chloroplasts containing chlorophyll also typically associated with plants.

And it’s more. In modern taxonomic classifications, the Plantar Kingdom includes eukaryotic and photosynthetic organisms, not only plants but also some types of algae (Ruggiero et al. 2015):

Kingdom to plant subkingdom biliphyta border Glaucophyta: small group of unicellular freshwater algae edge rhodophyta: known as red algae subkingdom Viridiplantae (the “green plants”) edge Chlorophyta subphylum Chlorophytin: a type of green algae, some multicellular, characterized by the presence of phycoplasts. subphylum Prasinophytina: a type of single-celled flagellated marine green algae underking Streptophyta: super sharp Charophyta: another group of green, paraphyletic algae, which includes the closest relatives of land plants super sharp Embryophytes: also known as “land plants«; is the group popularly known by the word “plant” and is the vegetation that covers most of the emerged terrestrial surface.

In addition to these groups, there are algae in other taxonomic clades, such as diatoms and some dinoflagellates, both from the Chromista Kingdom.

So what’s the difference?

To understand the difference between algae and plants, the first thing to do is accept that neither term refers to valid taxonomic clades.

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In a broad sense, the word “plant” may refer to the Plantar Kingdom, which would include plants and some groups of algae, but in popular usage it corresponds almost exclusively to embryophytes, a monophyletic group of organisms that evolved from a group of green algae.

Embryophytes include all those called «land plants»: bryophytes (liverworts, hornworts and mosses), mosses, ferns and seed plants (gymnosperms and angiosperms).

Algae, however, do not correspond to any monophyletic clade. A widespread definition of algae is a eukaryotic and photosynthetic organism that is not an embryophyte. Previously, some prokaryotes were considered algae; for example, cyanobacteria were known as blue-green algae.

Following this definition, among algae there can be unicellular or multicellular organisms. On the contrary, all embryophytes are multicellular.

But the main difference between plants and algae is that embryophyte embryos go through a diploid multicellular sporophyte stage, something that no algae does. Along with this multicellular embryonic stage, embryophytes have specialized reproductive structures, such as flowers, that are not present in algae.

Another notable difference is in the photosynthetic pigments. Chlorophyll and carotenes are common in plants and algae, but phycobilins are found only in some algae, specifically in rhodophyta, Cryptophyta S Glaucophyta, and in cyanobacteria.

In a technical sense, calling embryophytes “land plants” would be incorrect. Although embryophytes have colonized the Earth’s surface far superior to algae, there are algae unrelated to embryophytes that have also adapted to life on land.

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