Hydrozoans, sponges, coral, and coelenterates are colonial invertebrates. Doliolidae, Synascidiaepyrosomata , and salps are lower chordates that form colonies. Multicellular colonial organisms are often called ramets, zooids or modules. Filamentous organisms form an end to end arrangement as they undergo binary fission. Actinomycetes are the filamentous form of bacteria.
Filamentous algae form visible thread-like structures. The intertwining of the filaments forms a mat-like structure that resembles a wet wool. These mat-like structures are either attached to a structure or float on the water surface. The floating large mats are called pond scums. Generally, filamentous algae are a type of primary producers in aquatic food chains.
Figure 2: Filamentous Algae. Filamentous fungi are multicellular organisms that show filamentous growth. The filamentous structures of fungi are called the hyphae. Keeping volume constant, a sphere has the minimum surface area of any shape, and departures from an isodiametric shape increases the surface area and thus the surface area: volume ratio Table 2.
To see how surface area and form are significant we will use the example of the flow of heat from a warmer environment to a cooler cell , but the same principle would apply to the heat flow from the cell to the environment or the movement of materials, e.
Because smaller objects have relatively more surface area than larger ones, smaller organisms heat up more quickly than larger ones. In fact, because of effective heat exchange between them and their environment, small organisms are always very close to the same temperature as their environment. Only large organisms, with a small surface area to volume ratio, can develop temperatures substantially different from their environment.
Considering shape, spher ical bodies, with the least surface area per unit volume, heat more slowly than any other shape when put in an environment that is hotter than it ; t he more deviation from a spher ical shape the faster it will gain heat. If you had three pieces of ice, one spherical, one filamentous and one disc-shaped, all of the same volume, the disk would melt first, then the filament and last the sphere. Assuming equal volumes for ice cubes, the best ice cubes, if you want them to last not melt , are spherical ones, the best ice cubes if you want them to cool the drink that they are in, are shapes that deviate the most from spheres.
An often-overlooked fact is that organisms change their environment around them. In the example just given, the transfer of heat to the cell results in a cooling of the environment adjacent to cell. Th e cooling of the environment next to the cell will reduce the gain of heat by the organism and diminish the significance of surface area to heat transfer. Because of this, a second characteristic related to form becomes important: the volume of the environment that is within some distance the distance depends characteristics of transfer of the organism.
Hence surface area by itself is not always the best measure of how much interaction an organism or object might have with its environment. A consequence of this is that form is important in influenc ing the transfer of materials between the organism and the environment in two ways: 1 by determining surface area of a given volume of organism, and 2 by influencing the volume of the environment that is in close contact with the cell.
The significance of how much of an environmental volume is explored depends on several factors including the rate at which heat or material is conducted through the environment and the rate at which heat or material can be transferred from the environment to the organism. If the environment transfers heat or material readily, or if the rate of transfer into the cell is slow, the importance of how much environment is explored is of less importance.
Consider another example of two cells with the same number of multiple outies, extensions outward, and the same surface areas. One has the outies close together, the other has them spaced out Fig. Some perhaps familiar situations demonstrate the importance of form and some of the complications related to it. The microvillae, small projections of small intestine that extend out into the gut track, are often cited as being important in the absorption of materials from the gut because they provide increased surface area.
This is certainly the case but it should also be pointed out that the movement of material through the gut track, a result of peristalsis, is what allows the additional surface area to be significant. Peristalsis changes the environment next to the microvillae. Root hairs, cylindrical extensions from the cells on the outside of roots, are another situation where increased surface area is cited as being significant to the water absorption function of roots.
This may not always be the case, especially when the root hairs are extremely dense and if water is abundant, which allows it to move more readily through the soil. Moreover, the conductivity of the soil to water and minerals is very strongly affected by how much water is present, a very dynamic property for most soils.
Root hairs probably do multiple things that are significant for absorption of water and minerals: 1 increase surface area, 2 increase the volume of soil in close proximity to the root, 3 improve contact between the root and the soil by preventing gaps air spaces which would drastically reduce absorption of water and nutrients, 4 perform metabolic functions that facilitate nutrient absorption, e.
While there are a wide variet y of shapes of organisms , three common forms are cylinders, sheets and spheres. Many organisms are composites of different shapes, i. M ost above-ground plants are composed of flattened sheets leaves attached to cylindrical stems. Both the above-ground and below-ground form of plants typically are filaments that branch repeatedly, a form that is also found in fungi.
Common forms for the organisms covered in this text are outlined in the Table 4. As mentioned above, spheres have a minimum surface area per unit volume. Assuming that there is a specialized boundary on the outside of the object, be it unicellular or multicellular, a sphere would require the minimum amount of boundary, which often is composed of relatively expensive materials. Spherical shapes are also more mobile in many situations owing to their reduced drag, which in general increases with surface area.
While there are a number of roughly spherical animals, spherical multicellular organisms from other groups, in particular, the groups we are covering, are uncommon.
However, the form is commonly found organisms that are unicellular: many bacteria, many unicellular green algae , dinoflagellates , cryptophytes and coccolithophore are roughly spherical in shape and the shape occasionally occurs in colonial organisms some green algae. Spheres are also common in dispersal units: pollen, seeds, spores , all of which are entities that might be considered organisms.
And spherical shapes are also a common shape for the structures organs that contain elements to be dispersed: sporangia spore containers , fruits seed containers , anthers pollen containers. The advantage s of spherical shapes no doubt varies on circumstances and may also reflect other constraints on development. In rare occasions, round shaped seeds, fruits and even whole plants may aid in the dispersal of propagules by wind and gravity.
A whole-plant example of the this is the tumbleweed, whose spherical shape promotes the dispersal by the wind of the seeds which are released from the rolling plant.
Spherical forms are typical of the usually underground storage organs of flowering plants: corms, bulbs and tubers and this is probably a consequence of surface area to volume considerations. Flattened structures are especially common in photosynthetic organisms, undoubtably because of the importance of intercepting light. What is significant is how much volume is required to produce a given area of light absorbing surface, and also how much total surface area is required.
Table 5 compares morphological characteristics for three different leaf shapes: cubes, sheets and filaments.
As can be seen the same amount of light could be intercepted by ten cuboidal leaves, each 10 x 10 x 10 cm in size, or ten filamentous leaves, each x 1 x 1 cm or by ten planar leaves 10 x 10 x 1 cm. The difference between filamentous leaves and planar leaves is that planar leaves reduce the total surface area required to produce a given amount of absorptive surface. It is probably these two factors that are significant in ensuring that light absorbing surfaces are generally planar: thick leaves require volume that cannot be used effectively for photosynthesis; filamentous leaves produce excess total surface area which may be costly in terms of other factors, e.
Interestingly, wings are primarily found in relatively large propagules, the seeds and fruits of seed plants, with dimensions greater than a few millimeters. Cylindrical structures are extremely common, both as parts of organism and as the whole organism. Cylindrical unicellular organisms are found in rod-shaped bacteria and archaea; filamentous colonial forms are represented by some bacteria, many cyanobacteria, colonial diatoms, many green algae and some red and brown algae.
For most of these colonial organisms, the filaments are all one cell thick, but, especially in the red and brown algae, the filaments may be thicker, often several cells thick. There are many coenocytic green algae that are filamentous. Such a form is also effective in providing anchorage because it allows for extensive interaction between the organism and its substrate.
While it is advantageous to live in a colony, if a cell were to break off, it would still be able to survive unlike in a multicellular organism. Back to other subjects. Introduction to The Organisation of Cells. Unicellular, Multicellular and Colonial Organisms This post will be an introduction to the organisation of cells topic and will explore unicellular, multicellular and colonial organisms for Prelim Biology.
0コメント