OK, let's look at some real cells! Click on the thumbnails below to enlarge these pictures of a single-celled
organism (an organism that is but a single cell) called an amoeba. This cell is rather like the model animal cell
we talked about on the previous page (Cells 1) but remember that cells can change shape, so instead of sitting
there looking like a ball, this cell has spread itself flat on the surface of a glass microscope slide.
Wow! See how it moves! The video was speeded up, the counter gives you seconds, in real life they still move
quite quickly, but in a graceful flowing manner. Notice how it moves like a blob of liquid or slime - remember
cells can change their consistency from a solid jelly into a watery liquid as need be. All those various bits inside
that jiggle around are its organelles - to see the structure of the organelles clearly you would need to employ
special microscopy, including the use of electron microscopes, the images above used a standard light
microscope. Remember that this organism is a machine more complex than any built by human hands!
Amoebae (singular amoeba) are sensitive creatures! This single cell can detect touch, heat, light, chemical
'smells' or 'tastes' and a whole lot of other things. It also has its own clock to measure the passing of time.
This amoeba was busy hunting for food which was mostly the small dots that you saw jiggling around outside
the cell, these were bacteria. The amoeba absorbs the bacteria - it surrounds them and simply absorbs them
and then breaks them down for food once they get inside. Imagine if certain species of bacteria were too tough
to avoid destruction, or if they even exploited the amoeba by letting them engulf it to get inside and do mischief.
Now imagine that the amoeba and these tough bacteria came to an agreement to help one another - that may
be how mitochondria evolved!
It is interesting to note that many human cells resemble the amoeba. Take a human white blood cell and look at
it down a microscope and it will spread out and crawl around in a similar way and hunt down bacteria to eat,
because that's what it does inside your body - gobbles up bacteria and garbage. Just like the amoeba, this cell
will also respond to touch and to chemical signals and to light. You are really a colony of many cells living
Below are some pictures of single celled organisms (all from the kingdom Protoctista) that Bot drew down his
microscope (all from life) click the thumbnails to enlarge:
The amoeba (click images to enlarge).
Left a type of large amoeba with
veil-like pseudopods containing clear
cytoplasm (ectoplasm). Note that the
cytoplasm occupying the bulk of the
cell (endoplasm) contains numerous
granules and vesicles and is fluid
(the arrow indicates the direction of
flow of the cytoplasm as the amoeba
crawls). These cells are about 100
micrometres in length (one tenth of a
Left: a fairly large type of amoeba,
similar to the one above, but bloated
with large vacuoles containing the
digesting remains of other
single-celled organisms that the
amoeba has absorbed. These
smaller organisms include algae and
Left: a type of amoeba with long
slender pseudopods, called
filopods. These filopods can retract
or grow out from any part of the cell
in seconds and they can also wave
from side-to-side, as shown by the
arrows. The bar indicates ten
micrometres, so these are small
It could be argued that the amoeba is not really a 'cell' at all, after all what is it a 'compartment' of? (A
cell is a compartment of protoplasm in a multicellular organism). It is a bit of misnomer to refer to these
organisms as 'single-celled organisms', however, these organisms have all the same basic structures
as a cell in a multicellular animal, and the ancestor of multicellular organisms was presumably a single
cell (or perhaps two single-cells that fused into a single organism, as recapitulated when a sperm
fertilises an egg). However, what is perhaps most remarkable about these single-celled organisms is
that the single cell must perform all the functions essential to life, functions which are divided between
the many specialised cells in a multicellular organism in a division of labour.
Below: a medium-sized type of amoeba with broad flattened lobe-like pseudopodia, called lobopodia.
Notice the clear ectoplasm that makes up these pseudopods. This type of pseudopod is also what is
known as an eruptive pseudopod - a new pseudopod can form very dramatically by breaching the
boundary of the cell and appearing to flow from the cell as if it was ruptured (arrow) and then resolidify.
Most probably the cell membrane remains intact throughout and it is the ectoplasm that ruptures. This
cell is changing direction by forming a new lobopod at its rear. The inset shows another type of
amoeba that was very small, containing just a couple of granules and capable of forming only a single
pseudopod (its body really was equivalent to a small pseudopod). These two amoebae are not
necessarily shown to the same scale, but are approximately so. The bar indicates ten micrometres.
Left: a medium-sized amoeba
capable of forming several lobopods
at once (of the non-eruptive type).
Each frame shows its shape at one
second intervals (as accurately as
Bot could draw such a rapidly
shape-shifting creature!). Initially
this amoeba was anchored to an
algal filament before detaching.
Far left: two species of ciliate. The
topmost one is attached to a solid
surface via a long (retractile) stalk. It
uses cilia at the free anterior end to
generate water vortices to suck in
prey which it ingests. Ingested prey
items sitting inside membrane-bound
vacuoles fill its body. Its surface has
underlying protein bands that confer
contractility upon the creature. Below
this is another type of ciliate which is
stalkless and free swimming.
Left: a flagellated golden-brown alga
with its strange mobile appendage.
The flagella were too fine and moving
too fast to visualise (they often
require special microscopical
Left: two types of the unicellular
flagellate, Euglena. A single flagella
allows this cell to swim about. It is also
capable of tremendous movements as
its body is very flexible, especially the
type on the right which executed what
are called euglenoid movements
(shown in the bottom inset is a 55
second sequence of frames showing
these shape-changes). The euglenoid
on the left has chloroplasts (green)
and so can photosynthesise and has
a more rigid body cap[able of flexing
movements, like an elastic rod. Note
the red eyespot, which is part of the
Interestingly, single-celled algae, like the Euglena above, can have quite sophisticated light sensors.
Some species even incorporate minute lenses, possibly the smallest optical lenses on Earth!
Once classed as an animal,
the amoeba is actually a
proto-animal or protozoan
and a member of the
Protoctistan Kingdom, which
also include proto-plants and
The compound light microscope (maximum useful magnification about 2500 x ). Many of the observations on this
page were made on a microscope a lot like this one, but the videos of the amoebae were made on a much more
state-of-the-art (and much more expensive!) model.