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Algae
Other algae obtain buoyancy from liquids of lower specific gravity than seawater or freshwater
in a way similar to a bathyscaphe. Liquid-filled floats have the advantage of being virtually incompressible; but because of their higher density, they must comprise a much greater proportion of the
organism’s overall volume than is necessary with gas-filled floats if they are to give equivalent lifts.
The large central vacuole of diatoms contains cell sap of reduced density, obtained by the selective
accumulation of K + and Na + , which replace the heavier divalent ions, conferring some buoyancy.
In young, fast-growing cultures, diatom cells often remain suspended, or sink only very slowly,
although in older cultures they usually sink more rapidly. Studies of the distribution of diatoms
in the sea suggest that some species undergo diurnal changes of depth, usually rising nearer the
surface during daylight and sinking lower in darkness, possibly due to slight alterations of their
overall density effected by changes in specific gravity of the cell sap, or in some cases by formation
or disappearance of gas vacuoles in the cytoplasm. The dinoflagellate Noctiluca also gain buoyancy
from a high concentration of NH 4
+ ions in its large vacuoles, exclusion of relatively heavy divalent
ions, especially sulfate, and a high intracellular content of Na + ions relative to K + . As a result, the
density of the cell sap in the vacuoles is less than that of seawater, and the cells can therefore be
positively buoyant and float.
When buoyancy control is not possible by these mechanisms, algae can keep afloat and regulate
their orientation and depth through adaptations reducing sinking rates. The rate at which a small
object sinks in water varies with the amount by which its weight exceeds that of the water it displaces, and inversely with the viscous forces between the surface of the object and the water. The
viscous forces opposing the motion are approximately proportional to the surface area, and therefore, other things being equal, the greater the surface area, the slower the sinking rate.
There are a number of structural features of planktonic organisms, which increase their surface
area and must certainly assist in keeping them afloat. The majority of planktons are of small size,
and therefore have a large surface-to-volume ratio. In many cases, modifications of the body surface increase its area with very little increase in weight. These modifications generally take two
forms: a flattening of the body, or an expansion of the body surface into spines, bristles, knobs,
wings, or fins.
A great range of flattened or elaborately ornamented shapes occur in diatoms such as Chaetoceros
sp. In dinoflagellates, also, the cell wall is in some cases prolonged into spines (Ceratium) or wings
(Dinophysis). Among the Chlorophyceae, the wall of the peripheral cells of Pediastrum colonies
may bear clusters of very long and delicate chitinous bristles regarded as buoyancy devices. In
Scenedesmus also, the cells are clothed by large number of bristles with a complex structure, which
seem to help keep the cells in suspension.
Reduction of the sinking rate is also obtained by an increase in lipid content, which has a density
of about 0.86 g cm −3 . Oil droplets are common inclusions in the cytoplasm of algae; lipids stored in
this form are present in the Chrysophyceae and Phaeophyceae (Ochrophyta), and in the Haptophyta,
Cryptophyta, and Dinophyceae. The thermal expansion of these compounds may be of some significance in effecting diurnal depth changes, through reduction of cell density, but without producing
neutral buoyancy.
How a Flagellum Is Built: The Intraflagellar Transport
The mechanisms that determine and preserve the size and function of cellular organelles represent
a fundamental question in cell biology up to now only partially understood, and flagella have provided a handy model system to investigate organelles’ size-control analysis. It was discovered that
flagella are dynamic structures and that flagellar length is regulated by a process called intraflagellar transport (IFT). IFT is a motile process within flagella in which large protein complexes move
from one end of the flagellum to the other, and flagellar length is regulated by a balance between
continuous assembly of tubulin at the tip of the flagellum, counterbalanced by continuous disassembly. According to Iomini et al. (2001), the IFT cycle consists of four phases. In phase I, which takes
place in the basal body region of the flagellum, anterograde particles are assembled from retrograde
Algae
Other algae obtain buoyancy from liquids of lower specific gravity than seawater or freshwater
in a way similar to a bathyscaphe. Liquid-filled floats have the advantage of being virtually incompressible; but because of their higher density, they must comprise a much greater proportion of the
organism’s overall volume than is necessary with gas-filled floats if they are to give equivalent lifts.
The large central vacuole of diatoms contains cell sap of reduced density, obtained by the selective
accumulation of K + and Na + , which replace the heavier divalent ions, conferring some buoyancy.
In young, fast-growing cultures, diatom cells often remain suspended, or sink only very slowly,
although in older cultures they usually sink more rapidly. Studies of the distribution of diatoms
in the sea suggest that some species undergo diurnal changes of depth, usually rising nearer the
surface during daylight and sinking lower in darkness, possibly due to slight alterations of their
overall density effected by changes in specific gravity of the cell sap, or in some cases by formation
or disappearance of gas vacuoles in the cytoplasm. The dinoflagellate Noctiluca also gain buoyancy
from a high concentration of NH 4
+ ions in its large vacuoles, exclusion of relatively heavy divalent
ions, especially sulfate, and a high intracellular content of Na + ions relative to K + . As a result, the
density of the cell sap in the vacuoles is less than that of seawater, and the cells can therefore be
positively buoyant and float.
When buoyancy control is not possible by these mechanisms, algae can keep afloat and regulate
their orientation and depth through adaptations reducing sinking rates. The rate at which a small
object sinks in water varies with the amount by which its weight exceeds that of the water it displaces, and inversely with the viscous forces between the surface of the object and the water. The
viscous forces opposing the motion are approximately proportional to the surface area, and therefore, other things being equal, the greater the surface area, the slower the sinking rate.
There are a number of structural features of planktonic organisms, which increase their surface
area and must certainly assist in keeping them afloat. The majority of planktons are of small size,
and therefore have a large surface-to-volume ratio. In many cases, modifications of the body surface increase its area with very little increase in weight. These modifications generally take two
forms: a flattening of the body, or an expansion of the body surface into spines, bristles, knobs,
wings, or fins.
A great range of flattened or elaborately ornamented shapes occur in diatoms such as Chaetoceros
sp. In dinoflagellates, also, the cell wall is in some cases prolonged into spines (Ceratium) or wings
(Dinophysis). Among the Chlorophyceae, the wall of the peripheral cells of Pediastrum colonies
may bear clusters of very long and delicate chitinous bristles regarded as buoyancy devices. In
Scenedesmus also, the cells are clothed by large number of bristles with a complex structure, which
seem to help keep the cells in suspension.
Reduction of the sinking rate is also obtained by an increase in lipid content, which has a density
of about 0.86 g cm −3 . Oil droplets are common inclusions in the cytoplasm of algae; lipids stored in
this form are present in the Chrysophyceae and Phaeophyceae (Ochrophyta), and in the Haptophyta,
Cryptophyta, and Dinophyceae. The thermal expansion of these compounds may be of some significance in effecting diurnal depth changes, through reduction of cell density, but without producing
neutral buoyancy.
How a Flagellum Is Built: The Intraflagellar Transport
The mechanisms that determine and preserve the size and function of cellular organelles represent
a fundamental question in cell biology up to now only partially understood, and flagella have provided a handy model system to investigate organelles’ size-control analysis. It was discovered that
flagella are dynamic structures and that flagellar length is regulated by a process called intraflagellar transport (IFT). IFT is a motile process within flagella in which large protein complexes move
from one end of the flagellum to the other, and flagellar length is regulated by a balance between
continuous assembly of tubulin at the tip of the flagellum, counterbalanced by continuous disassembly. According to Iomini et al. (2001), the IFT cycle consists of four phases. In phase I, which takes
place in the basal body region of the flagellum, anterograde particles are assembled from retrograde