We all know plants such as pines, oaks or beeches. We all know seaweed such as sea lettuce, wakame or others used in Japanese cuisine. But do we really know what plants and algae are? Well, as we might suppose the answer is no. All the organisms mentioned above evolutionarily are related and belong to the same group, that of the plants. However, another group of organisms less known and less evolutionarily related are also called algae, the cyanobacteria, belonging to the kingdom of bacteria. If you want to know what the similarities and differences between algae and plants are, read on because we will reveal the answer.
Evolutionary history to learn more about plants and algae
Molecular analysis of the ribosomal RNA of living beings in the late 70’s allowed its division into three major domains: Bacteria, Archaea and Eukarya. The first two, Bacteria and Archaea, are formed by prokaryotic organisms and the third by eukaryotic organisms. The Eukarya domain hosts a great biological diversity that includes unicellular and pluricellular organisms, autotrophs and heterotrophs .
The first eukaryotic organisms according to the Theory of endosymbiosis (Lynn Margulis, 1967) come from the fusion of two prokaryotic organisms , one of which had the ability to breathe oxygen, which gave rise to the mitochondria. Later, this primitive eukaryote would phagocytose a photosynthetic cyanobacterium, which was integrated into the cell and gave rise to the current chloroplasts of plant cells .
The knowledge of the origin of the plant cells and the subsequent phylogenetic analyzes have allowed us to know the kinship that exists between the plant species and how and when they have been diversifying. These analyzes suggest that green algae and terrestrial plants (chlorobites), red algae (rhodophytes) and a small group of freshwater unicellular algae (glaucophytes) descend from a common ancestor . This common ancestor would be the first eukaryote to acquire chloroplasts about 1,500 million years ago. This monophyletic group is called Plantae or Archaeplastida.
What is a plant and what is an alga
The first of all is to explain a series of concepts widely used in evolutionary biology and taxonomy that help us better understand kinship relationships between species: monophyletic group, paraphyletic and polyphyletic. A monophyletic group is one that includes a common ancestor and all its descendants. In a paraphyletic group the common ancestor is included but not all the descendants and a polyphyletic group includes species with different ancestry.
Understood this we will see what is a plant and what an algae . In order for the plants to be a monophyletic group, the terrestrial plants, known as EmbriFitas, the Glaucophytes, the Rhodophytes and the Chlorophytes must be included. The terrestrial plants or embriofitasinclude the vascular as ferns nonvascular plants and bryophytes (liverworts and mosses), and and seed plants (gymnosperms and angiosperms). However, terrestrial plants would be a paraphyletic group because they do not include all the descendants of the ancestor that originated them.
On the contrary, the concept of “algae” broadly includes all photosynthetic eukaryotic organisms other than terrestrial plants. In this way the algae would form a paraphyletic group. But they are also algae, or rather we call them that, the cyanobacteria , prokaryotic photosynthetic organisms, which have a very different origin from the rest of the other algae (Glaucophytes, Rhodophytes and Chlorophytes). In this way what we call algae would correspond to a polyphyletic group without any evolutionary or taxonomic sense.
Therefore, we could conclude that the plants or the monophyletic group Plantae includes all terrestrial and aquatic plants (Embri-phytas), and some algae (Chlorophytes, Rhodophytes, Glaucophytes). And on the other hand, that the group of algae consists of several photosynthetic eukaryotic descendants of the original ancestor of the Plantae group and by the cyanobacteria, photosynthetic prokaryotic organisms.
Well, once we know that plants and algae are going to see their similarities and differences. In this article, eukaryotic algae will be treated, since cyanobacteria are less related organisms and for this we should better talk about similarities and differences between prokaryotic and eukaryotic organisms.
Similarities between plants and algae
We start with similar points between these two types of living beings. These are the main similarities between plants and algae :
- They have chloroplasts with two membranes. The existence of the two membranes suggests that in this group the organelles that enable photosynthesis evolved from an endosymbiotic event between a primitive eukaryotic ancestor and photosynthetic cyanobacteria. The chloroplasts of plant cells have chlorophyll.
- Chlorophytes, Rhodophytes, Glaucophytes and Embryophytes store starch as a reserve carbohydrate.
- The mitochondria of the cells usually have flattened ridges. The mitochondria are the organelles where cellular respiration is performed , a process by which the cell consumes oxygen and organic matter in exchange for energy.
- The cell walls are constituted by cellulose polysaccharides.
- They perform photosynthesis . Thanks to solar energy, they fix CO2 and produce oxygen and organic matter that they will need to carry out cellular respiration and obtain energy.
- They are autotrophic , that is, they make their organic matter from inorganic. Concept related to photosynthesis.
- Both algae and plants can live in aquatic environments and terrestrial environments .
Differences between plants and algae
Finally, we indicate the main differences between plants and algae . Before, we re-emphasize what has been explained in the definition. The terrestrial plants , understood as Embrihitas, would form a paraphyletic group while the algae , including the cyanobacteria, would form a polyphyletic group. Therefore, we are going to see the differences between the EmbriFitas (Terrestrial Plants) and the algae.
- Differences in the level of structural complexity . All embryophytes are multicellular while algae can be multicellular or unicellular , such as glaucophytes.
- Embryophytes, as their name suggests, go through an embryonic stage in some of the phases of their life while in algae this does not occur. In embryophytes, during their development, the embryo develops and gives rise to a diploid multicellular sporophyte.
- Embryophytes can develop specialized reproductive structures , such as flowers .
- Although algae and embryophytes share photosynthetic pigments, some such as phycobilins are exclusive to algae such as Rhodophytes and Glaucophytes, although these can also be found in cyanobacteria.
- Although we have commented that both algae and embryophytes share the same habitats , if that is true, that algae usually inhabit aquatic environments and plants have adapted much better to the terrestrial environment.
- The algae do not have true tissues , meaning tissue as a group of specialized cells that perform a specific function. In the case of cells, the tissues of the leaves, stems and roots are different.