In light of yesterday’s post, I thought why not just
write a few posts briefly describing the main groups of organisms that I am
familiar with? So, the first one is about the unicells.
This is not
where you end up if you fail university. Unicells is just an (informal)
shortening of unicellular organisms,
living things that only consist of a single cell that is capable of surviving
on its own. This includes bacteria, archea and protists.
On a very large
scale, all life forms are divided into three domains: Bacteria, Archaea and Eukaryota. Bacteria and achaea are both
prokaryotic, meaning they have no
nucleus, which is the core of a eukaryotic cell. We humans, all animals,
plants and fungi, belong to the Eukaryota, together with the protists, which
are unicellular eukaryotes, i.e. single-celled organisms that have a nucleus.
Bacteria and archaea are virtually indistinguishable to anyone who doesn’t know them in detail. They are defined based on complex genetics, biochemistry and structure. But they are both everywhere, in enormous quantities. There are many times more bacteria and archaea in a handful of soil than there are humans on this entire planet. Archaea also tend to be extremophiles, i.e. organisms that thrive in extreme environments, such as near-boiling temperature, extremely acidic or saline water. This can be highly advantageous, as there is little competition for space and nutrients in such unfriendly environments.
Bacteria and archaea are virtually indistinguishable to anyone who doesn’t know them in detail. They are defined based on complex genetics, biochemistry and structure. But they are both everywhere, in enormous quantities. There are many times more bacteria and archaea in a handful of soil than there are humans on this entire planet. Archaea also tend to be extremophiles, i.e. organisms that thrive in extreme environments, such as near-boiling temperature, extremely acidic or saline water. This can be highly advantageous, as there is little competition for space and nutrients in such unfriendly environments.
Prokaryotes,
i.e. bacteria and archaea, have no nucleus, which is where all DNA is kept in
eukaryotes; their DNA is instead floating around freely inside the cell, in
circles. Prokaryotes also basically have no organelles, which are the ‘organs’
of eukaryote cells, such as the mitochondria (where energy is ‘produced’) and
the lysosome (where ‘dead’ parts are broken down). This means that prokaryotes
are generally more simple organisms than eukaryotes. Finally, eukaryotes are
typically ten times larger than bacteria and archaea.
Now to the
unicellular eukaryotes, previously classified as Protista, alongside Animalia,
Plantae and Fungi (animals, plants and fungi, respectively). However, the
eukaryote classification has changed since, and is now rather complicated, even
at a basic level. But I will try to guide you through as best as I can.
Hopefully, you’ll come across odder cells than you ever heard of!
There are five
big groups of eukaryotes: Unikonta, Archaeplastida, Rhizaria, Excavata, and
Chromalveolata. I don’t know these very well, but based on notes from last
year’s module on unicellular organisms, there are some cool things to say about
each.
Unikonta includes animals
and fungi (yes, fungi are mong the closest relatives of animals, insane isn’t
it!), which are multicellular. The main unicellular unikonts are the amoebozoans, slimy-looking goos with a
very flexible cell membrane that can reshape very easily, and even extend as pseudopodia (singular: pseudopod),
false limbs, to move around or grab and engulf food or prey. Yes, some
amoebozoans are predators, which feed on bacteria and/or other protists.
Amoebozoa. Image from:
http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artsep01/amoeba.html
http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artsep01/amoeba.html
Archaeplastida includes
land plants (multicellular), red algae (mostly multicellular), and green algae.
Land plants (Embryophyta) and green algae (Chlorophyta) together form the group
Plantae, which is all plants. However, since we are discussing unicellular
organisms here, let us consider the green algae alone. Chlorophytes have a
rigid cell wall outside their cell
membrane, made out of cellulose a
type of large sugar molecule, and, most importantly, are capable of photosynthesis, the familiar set of
chemical reactions that produce organic molecules out of sunlight, water and
carbon dioxide, with oxygen as a byproduct (oxygen is actually a waste gas for
plants, much like a fart, seriously!).
Chlorophyta (green algae). Image from http://www.chdiagnostic.com/H_Photo%20Gallery.htm
Rhodophytes, red
algae, are primarily multicellular, so I might discuss them with plants in a
later post.
Rhizaria includes mainly the foraminiferans and radiolarians, typically enclosed in secreted shells, called tests. Foraminiferans have porous (hole-y) tests with multiple chambers, and thin pseudopodia radiating out from the holes, used to grab food particles from the surroundings. Radiolarians also have pseudopodia sticking out from their single-chambered test, but they are used to engulf other microorganisms, like those of the amoebozoans. The foraminiferan test is typically made up of calcite, while the radiolarians make them out of silica.
Rhizaria includes mainly the foraminiferans and radiolarians, typically enclosed in secreted shells, called tests. Foraminiferans have porous (hole-y) tests with multiple chambers, and thin pseudopodia radiating out from the holes, used to grab food particles from the surroundings. Radiolarians also have pseudopodia sticking out from their single-chambered test, but they are used to engulf other microorganisms, like those of the amoebozoans. The foraminiferan test is typically made up of calcite, while the radiolarians make them out of silica.
Froaminifera. Image from
Foraminifera with extended pseudopodia. Epic image
from
Radiolarian. Image from http://livelikedirt.blogspot.co.uk/2012/04/microfossils-i-alveolates-foraminifera.html
Excavata are characterised
by lacking a cell wall, unlike most single-celled organisms (bacteria and I’m
pretty sure archaea also have cell walls). The group includes diplomonads, which are dikaryotic, i.e. have two nuclei
(plural of nucleus), parabasalids,
with shrunk mitochondria, and euglenozoans,
which posses a curious crystal rod inside their flagella (organelle used for
swimming around) of unknown function.
Diplomonad. Image from http://faculty.uca.edu/johnc/heterotinvert.htm
Parabasalid. Image from
Euglenozoan. Image from http://protist.i.hosei.ac.jp/pdb/images/mastigophora/euglena/limnophila/limnophila_1a.html
Chromalveolata includes
brown algae (Phaeophyta, mainly multicellular), ciliates, apicomplexans,
diatoms and dinoflagellates. Brown algae, basically seaweed, are primarily
multicellular, so they will be discussed in a later post. Ciliates are dikaryotic, like the diplomonads, except their two
nuclei are of different sizes; they also use cilia (the things that look a bit
like hair, on many unicellular organisms) diligently to move and feed. Apicomplexans are nasty animal
parasites, with an apical complex of organelles specially designed for
penetrating into the host. Plasmodium,
the pathogen behind malaria, is an apicomplexan.
Diatoms have cell walls
made of silica, full of pores, and divided into two halves, one always slightly
smaller than the other. Diatoms are photosynthetic, and together produce about
20% of all oxygen every year. Plants are not the only organisms that can
photosynthesise. Another important group of photosynthetic organisms are the
cyanobacteria (which are bacteria), also known as the blue-green algae.
Dinoflagellates are as cool as they sound. They are among the most bizarre-looking cells, with heavily modified cell walls to look like alien space ships or I don’t know what. The cell wall is divided in two, just like for the diatoms, and contains armour plates made out of cellulose. The two flagella that stick out of the grooves in this armour is a main defining feature of the dinoflagellates. Some dinoflagellates are photosynthetic, many of which live in symbiosis with corals, giving them their magnificent colours, and others are predatory. One dinoflagellate that has blown the minds of experts is Pfiesteria piscicida, the ‘cell from hell’, which has about 24 different life stages and is highly toxic to fish, and humans. I can imagine the nickname came both from the toxicity and the hell the experts must have gone through trying to classify it!
Dinoflagellates are as cool as they sound. They are among the most bizarre-looking cells, with heavily modified cell walls to look like alien space ships or I don’t know what. The cell wall is divided in two, just like for the diatoms, and contains armour plates made out of cellulose. The two flagella that stick out of the grooves in this armour is a main defining feature of the dinoflagellates. Some dinoflagellates are photosynthetic, many of which live in symbiosis with corals, giving them their magnificent colours, and others are predatory. One dinoflagellate that has blown the minds of experts is Pfiesteria piscicida, the ‘cell from hell’, which has about 24 different life stages and is highly toxic to fish, and humans. I can imagine the nickname came both from the toxicity and the hell the experts must have gone through trying to classify it!
Ciliate. Image from
Artist’s interpretation of an apicomplexan.
Diatom shells. Image from http://www.calacademy.org/science_now/archive/academy_research/sarah_spaulding.php
Dinoflagellate. Image from http://www.studiovandam.com/rational-noctiluca.html
That is what I
had to say this time, except one last note: remember that unicellular organisms
require water to live in, even those in soils, etc., they need
water as a medium to move around, and also as a medium where their chemical
reactions can occur.
I misstakedly forgot that many fungi are unicellular, such as yeasts, so they should have been adressed here as well. However, they will now be part of Part 3, on fungi.
I misstakedly forgot that many fungi are unicellular, such as yeasts, so they should have been adressed here as well. However, they will now be part of Part 3, on fungi.
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