a 3D computer model of Chlamydomonas, a single-celled green
alga found in freshwater and soil. The study of such organisms
really should not need justification. They are, like all living
things, highly sophisticated machines and to ignore the inner
workings of such natural machines would be truly ignorant of
humanity. They deserve to be studied on aesthetics alone, but as
people increasingly need financial reasons to do anything at all on
planet Earth (except, paradoxically to waste vast amounts of money
and resources on their own petty pleasures!), it can be pointed out
that these organisms make ideal 'model organisms'. Animal cells can
be very difficult to work with and there is much that can be learned
about the fundamental process of life itself by studying such
organisms. They do also have several potential technological uses.
A schematic of the detailed internal structure (shown in
section) is given below:
are many different species of Chlamydomonas and the details differ.
All possess the usual eukaryotic cell organelles (nucleus (G), endoplasmic
reticulum (not shown), Golgi apparatus (H) (usually x 1-4 arrayed
around the nucleus), vesicles (F), lipid droplets and mitochondria
(A)). The mitochondria are often branched, and probably divide and
move around the cell. They can be found inside the cup of the
chloroplast, at the front of the cell, and squeezed between the
chloroplast and the cell-surface membrane. A single cup-shaped
chloroplast (I) fills about the posterior two-thirds of the cell and
this contains one or two pyrenoids. This fixed carbon is converted
into starch, which is stored as starch grains in the chloroplast.
Starch grains are distributed throughout the chloroplast, but are
concentrated around the pyrenoid.
Pyrenoid(s): contain the enzyme RuBisCO (Ribulose bisphosphate carboxylase/oxygenase) which is responsible
for fixing carbon into carbohydrate during photosynthesis.
Contractile vacuoles: one pair of contractile vacuoles are situated near the front-end of the cell (topmost in the diagram). These expel excess water which enters the cell by osmosis when the surrounding solution is more dilute than the cell cytoplasm (that is when the surrounding water has a higher water potential).
Flagella (E): one pair of (smooth) flagella project from the anterior or apical end of the cell. These enable the cell to swim by executing breast-stroke like movements. Each flagellum crosses the cell wall through a collar and is rooted in the cytoplasm by a basal body. Two long rootlets of microtubule-bundles extend from each basal body into the cell (not shown), one bundle contains two microtubules, the other four. Thus there are four bundles altogether, which meet in a cross beneath the basal bodies, with the microtubule doublets opposite one-another (and the quadruple bundles also opposite one another) in a 4-2-4-2 cruciate (cross) arrangement. A pair of contractile bundles (rhizoplasts) also connect the two basal bodies together, maintaining them at an angle of about 80 degrees. These contractile bundles contain the protein centrin, which contracts by supercoiling. Contraction of the rhizoplasts, moves the nucleus forward and detaches the antenna and is a shock response (and as one of its functions may allow the cell to escape from predators which have grabbed hold of its flagella?). Another contractile bundle also joins the basal bodies to the nucleus. If the cells are grown on solid surfaces like agar (in which the cells cannot swim) flagella are not synthesised. During mitosis (cell division occurring in asexual reproduction) the
basal bodies detach from the flagella (which are absorbed and so disappear) and resume their function as centrioles, organising the microtubule spindle which separates the chromatid pairs to each daughter cell.
Eye-spot (stigma). The highly reflective red eyespot, found only on one side of the cell inside the chloroplast is constructed of either a flat or parabolic plate of granules spaced at a quarter of the wavelength of the light they reflect. They, therefore, appear to behave as a quarter-wave plate which reflects light of a certain wavelength in such a way that the waves combine by constructive interference, intensifying the light. A parabolic plate (shaped like a satellite-receiver dish antenna) makes an ideal reflector, since it has the property of focusing the reflected light. Certain colours of light are thus reflected and focused onto the light sensor itself, which is situated in the cell surface membrane above the stigma. This receptor is tuned to match the wavelength of light reflected by the stigma. Occurring only one side of the cell enables the cell to detect the direction of a light source. This is assisted by the fact that the cells rotate as they swim, tracing out helical paths. Thus, the light sensor will be periodically exposed to a higher light intensity if the cell is swimming at an angle to the light source, but will remain evenly illuminated if the cell swims towards it. A powerful light microscope will reveal the eyespot very clearly glinting as the cell rotates in the light.
Cell Wall. Chlamydomonas is surrounded by a rigid cell wall, but unlike plant cells in which the cell wall is made of cellulose (a glucose polymer), the Chlamydomonas wall is made of fibrous glycoproteins (in most algae it consists of carbohydrate polymers) and is triple-layered. The apical papilla is a small hemispherical or flattened projection of the cell wall between the two flagella.
Genetics. Microscopy usually reveals 8 chromosomes in the haploid nucleus, however, genetic studies suggest that there are 16-17 genetic units or so, which suggests that each visible chromosome contains two molecules of DNA, instead of the more usual one.
The mature vegetative cells are haploid (n). When nutrients especially nitrogen (ammonia) are depleted and present in low concentrations, Chlamydomonas activates its sexual cycle and the cells differentiate into gametes, each vegetative cell becoming a single gamete. Some species and strains also require blue light as a stimulus for gamete development. The gametes are present in two genetic polarities, plus and minus, and fertilisation can only occur between two gametes of opposite polarity. The gametes shed their cell walls, which are dissolved by an autolysin enzyme. The gametes have sticky glycoprotein projections on their flagella (visible under the electron microscope as tiny stalked projections with globular heads). Plus cells have a glycoprotein which will interact with and stick to the complementary glycoprotein on minus cells (rather like velcro). The flagellae thus adhere along their length, expressing more of these glycoprotein adhesins (which is more concentrated toward the flagella tips) to strengthen the bond. Some species are homothallic - meaning that gametes descended from the same parent can fuse (self-fertilisation) whilst others are heterothallic (meaning that at least two parental stocks are needed for cross-fertilisation). Compatible gametes apparently bump into one another and adhere by chance, as apparently no sex pheromones are produced to attract gametes of the opposite polarity.
At least in Chlamydomonas eugamatos, the plus gametes pushes back the flagella of the minus gamete. In all forms a cytoplasmic bridge (copulation tube) grows between the two gametes, joining their front (apical) ends together.
The adherent flagella disengage from one another. Depending on species, these coupled cells may swim around for several hours connected in this manner and in Chlamydomonas eugamatos only the plus-cell
flagella remain active during this phase. Other gametes may join them, adhering to the complementary adhesins on their flagella, and a clump of cells may form.
The two adherent gametes, one plus and one minus, will later undergo cell fusion or plasmogamy (cytogamy) which takes a few minutes. Light is a necessary stimulus to initiate plasmogamy, and in the darkness the joined gametes will keep swimming until they die. In Chlamydomonas eugamatos this fusion occurs between the two front ends, but in some forms it occurs side-on. This fused quadriflagellate cell, called a plasmozygote, continues swimming for some time.
The two nuclei fuse, first of all their outer membranes fuse together, and then the inner membranes break and join together. The cell now has a diploid (2n) nucleus. The flagella disintegrate from their tips down. The,
now non-motile, zygote becomes invested in a thick warty wall and becomes filled with starch and lipid reserves and enters a dormant stage. This kind of zygote is called a hypnozygote.
After a few days the hypnozygote spore is able to germinate once suitable conditions of light and sufficient nitrogen supply return. Germination begins with a meiotic reduction division, forming four haploid daughter cells. The spore wall breaks down and the four daughter cells escape as mature vegetative cells and the cycle is complete.
In the species usually considered, the plus and minus gametes look identical (isogamy) but in some species the plus gametes (male gametes) are much smaller than the minus gametes (female gametes) and these are said to be anisogamous. In the soil-dwelling Chlamydomonas zimbabwaensis a single vegetative cell will divide to form many small plus gametes, contained within the parental cell wall. When these male gametes escape they release a sex pheromone which induces vegetative cells to escape from their walls and form (much larger) minus or female gametes. In some species, the minus gametes are immotile (eggs) and these forms are said to be oogamous.
Chlamydomonas can also reproduce asexually as illustrated below:
4, 8, 16 or more cells from a single parent. Mitosis is closed,
meaning that the nuclear membrane does not break down. Instead
microtubules of the mitotic spindle cross the nuclear envelope
through pores in either end of the nucleus. Chlamydomonas is
haploid and only the zygote is diploid. Such an organism is
described as haplontic. (In contrast, human
beings are diplontic - they are diploid and only the gametes are
Like many motile algae, Chlamydomonas will swim toward the surface to intercept the light during the day and then swim deeper down at night (probably in order to disperse and escape predators). The proteins it needs for photosynthesis, UV protection and nitrogen assimilation whilst nourishing itself in the day also undergo a daily cycle as more of these proteins are synthesized during the daytime. These cycles are not simply passive responses to light, rather the cells have internal biological clocks, enabling them to anticipate the dawn and so prepare and make the most of the available light. This clock is constantly reset by brief exposure to light, allowing the clock to remain correct throughout the year and at different latitudes.