Cell Exploration Flashcards
Green algae
What are algae?
The term ‘algae’ is used for some lower plants and many, often unrelated groups of microorganisms that are able to perform photosynthesis.
Photosynthesis (converting light energy into chemical energy) is performed in parts of the cell called chloroplasts. They can be found in different shapes and colours and in many different organisms. Not all these organisms are green. Diatoms, Chrysophytes and dinoflagellates have yellow to brown chloroplasts. There are brown algae (Phaeophyta), red algae (Rhodophyta) and many other groups of unicellular algae in many shades of green. The blue green Cyanobacteria also photosynthesize.
Bacteria
Bacteria are the most ancient life forms Most bacteria are so small that under a light microscope you can only see them as little dots. Some groups however grow to larger sizes and have spectacular shapes.
The top picture shows an example of the beauty of one of the more advanced groups of bacteria, the cyanobacteria, or blue green algae. For a long time they were regarded algae since they performed photosynthesis. But they are nowadays regarded bacteria: they lack a membrane bound nucleus. DNA is present in a loop in the cytoplasm.
Because they have a very distinct blue green colour a bloom of cyanobacteria is clearly visible when a pond or ditch becomes blue-ish green.
Ciliates
What are Ciliates?
Ciliates are unicellular protists that can be recognised by their hairlike ‘cilia’. They use them for locomotion and for feeding.
Some ciliates are very small, not much larger than the largest bacteria. Others like the ‘trumpet animalcule’ Stentor can reach a size of two millimetres so it can be seen with the naked eye. Paramecium does not become much larger than 0.3 mm.
Below the centre of the organism you see the feeding opening. The cilia make a current to sweep in bacteria and other food particles. At the base of the feeding opening the food has been enclosed by a vacuole.
Food vacuoles are used to transport the food through the cell.
Star-shaped contractile vacuoles or water expelling vesicles are used to balance the amount of water in the cell.
Two nuclei bear the genetic information.
Why cilia?
When you are less than a millimeter in body size, water acts like a thick syrup so swimming like a fish would not be an efficient method of locomotion. If you want to swim fast and with manouverability you need cilia, tiny hairs acting like paddles. Most ciliates are superb swimmers, some species are so fast that they are hardly visible when observed .
Trichocysts of a Paramecium
In multicellular organisms the many cells are specialised (differentiated). Different types of cells are able to perform all kinds of tasks (i.e. act as nerve cells, muscle cells, blood cells etc.)
Unicellular organisms possess organelles, special structures inside or on the cell that help to perform all kinds of tasks. Food vacuoles and the water expelling vesicles are such organelles. Many ciliates have developed all kinds of very special organelles.
Paramecium uses so-called trichocysts: tiny pointed filaments that can be fired at a predator when threatened.
Myonemes: they are fibers that act like muscles. We can see them in the stalks of bell animalcules like Vorticella and Carchesium. They are able to contract the stalk with exceptional speed. In the righthand image the myonemes are visible as the light wavy bands inside the stalks.
In many ciliates fused cilia can be seen. Groups of cilia are fused into sheet-like membranelles. As undulating sheets they sweep in food particles. They can be seen in the bell animalcules.
Other ciliates have thick round bundles of cilia called cirri which act like legs and enable the organism to actually walk over a surface. To coordinate these cirri independently they have nerve-like organelles called neurofibrils. The top image of Euplotes and Stylonychia shows both types of cilia.
FLAGELLATED PROTOZOA
The name ‘protozoa’ is used for the more animal-like single celled organisms like amoebas and ciliates. The term ‘algae’ is used for the more plant-like microorganisms. But the distinction is often vague. Dinobryon (top image) has chloroplasts for photosynthesis but it can also feed on organic matter. It is even able to swim. Such an organism can neither be animal nor plant. Nowadays, all these unicellular organisms that are neither animals, plants, bacteria or fungi are called ‘protists’
What are flagella?
A flagellum, also called undulipodium, is a whip-like structure used for locomotion, for feeding or other purposes. Almost all organisms have flagella (at some stage in their lives). We humans have them in our bodies. Even our own spermatozoa can be regarded as flagellates. All these flagella have a similar basic design.
Only Bacteria have flagella of a totally different type.
The cilia of Ciliates are just shorter versions that act different. A cilium beats like a small pedal and generates a sideways motion. Most undulipodia act more like a propellor, wavy motions push or pull an organism in a forward direction.
This small flagellated Peranema has a long flagellum that only moves at the front end almost like a propellor.
An interesting flagellated protist to study is Euglena. Some of its many features are shown here.
Euglena can become so numerous it may turn the water of a pond bright green.
Apart from using their undulipodium Euglenids are also able to locomote by flexing their bodies and changing their body shape, so called Euglenoid motion.
Euglenids have the ability to lose their chloroplasts. If you would keep Euglenids in the dark they start feeding on organic matter and may loose their pigment. There are many species without chloroplasts.
A single nucleus contains the genetic material.
Bright green chloroplasts transform light into energy.
The energy is stored in storage grains.
A long flexible undulipodium used for locomotion.
Near base of the flagellum we find a round contractile vacuole. It helps the organisms to remove surplus water. Because living tissue is saline, organisms living in fresh water take up water.
A red eyespot is clearly visible. In fact the real light sensitive organ is the swelling near the base of the flagellum. The red area makes sure only light from one direction is detected. Since the light sensitive organ is directly connected to the flagellum, Euglena is perfectly able to swim towards a light source.
Most flagellated protists have two or even more flagella. Often we see one flagellum trailing behind like a kind of rudder. Flagella and cilia are transparent and thin so it is not easy to see them with a microscope. Special contrast techniques like dark field illumination or phase contrast help to make these structures visible.
Dinoflagellates (Dinophyta) are flagellated protists. Many species have a beautiful textured armor made of cellulose plates. One of their two flagella runs as a spiral through a groove along the cell. The other projects downwards. A spectacular species is Ceratium hirundinella
Click to see a drawing of a Dinoflagellate.
It is not just protists that use flagella for locomotion. Even our own spermatozoa have a flagellum.
Not all flagellated forms are free swimming. Many live a sessile life.
SUN ANIMALCULES
AND AMOEBAS
What are Amoebas?
Amoebas (Phylum Rhizopoda) are unicellular protists that are able to change their shape constantly. Each species has its own distinct repertoir of shapes.
How does an amoeba locomote?
Amoebas locomote by ways of cytoplasmic movement. (cytoplasm is the cell content around the nucleus of the cell) The amoeba forms pseudopods (false feet) with which they ‘flow’ over a surface. The cytoplasma not only flows it also changes from a fluid into a solid state.
These pseudopods are also used to capture prey, They simply engulf the food. They can detect the kind of prey and use different ‘engulfing tactics’.
The image shows several cell organelles. Left from the center we can see a spherical water expelling vesicle and just right of it, the single nucleus of this species can be seen. Other species may have many nuclei. The cell is full of brown food vacuoles and also contains small crystals.
Amoebas can grow to exceptionally large sizes for a unicellular organism. Some species are 5 millimetre long. They can reach such a size by having many nuclei within their single cell.
Find out much more about Amoeba in the Micscape article Amoebas are more than just ‘blobs’
Amoebas are not known to have any sexual way of reproduction. They only multiply by cell division.
Sun animalcules or Heliozoa, phylum Actinopoda (=ray footed) have axopodia, long stiff projections of cytoplasm. They cannot locomote that much with them but they are excellent for capturing prey. The prey, usually small protists, sticks to the axopods and will be covered by the cytoplasmic flow running along the axopods and transported towards the cell
Sometimes Heliozoa can be found feeding on the same prey. In this image 2 individuals of the species Actinophrys sol are feasting on a diatom.
Arcella, a common shelled amoeba
Some amoebas form a shell from found material that they glue together to form a protective covering. Often sand grains or other particles are used to make the construction, called a ‘test’ stronger.
You can see how this shelled amoeba hangs in its test keeping contact using thin pseudopods. Long pseudopods are extruded to capture prey.
