Seaweeds - more complex algae. Algae - building bodies from balls, chains, sheets and tubes. Volvox - ball-shaped colony. Building bodies - filaments Many algae are filaments - chains of cells that form when the plane of cell division is fixed resulting in a '1D' colony. Many bacterial and fungal forms are filamentous and one of the advantages is that it allows organisms to reach above the substratum surface and Scenedesmus asexual reproduction fission the boundary layer a layer of almost motionless fluid that forms around all solid objects in a fluid and obtain a good supply of nutrients in the flowing water and to shed spores into the faster moving air or water.
Some filamentous cyanobacteria photosynthetic bacteria that are sometimes called the blue-green algae live inside slime tubes that they secrete for themselves and they can glide up an down inside this tube, positioning themselves higher in the water column when needed. See the pdf book chapter for more advantages of being filamentous to bacteria. An advantage of being multicellular is that different cells can specialise to perform different functions.
Building bodies - spheres Algae demonstrate well some of the different approaches to building multicellular bodies - there are a number of ways to construct a 3D body from cells. Cells can be joined together to form chains essentially 1D organisms or more precisely remain joined together when reproduce by cell Scenedesmus asexual reproduction fission. If all the cells divide in the same plane, then a chain will result.
By dividing sometimes in a second plane at right angles to the first, a 2D sheet of cells may be produced see Ulva and Porphyra below. Volvox is a green alga that consists of a hollow ball of cells, which is essentially a 2D colony or sheet folded around and joined together. Each ball, or coenobiumconsists of a single layer of superficial cells - it is a colony of cells. Each cell is surrounded by a thick mucilaginous wall, forming an enclosing cell. In some species, these mucilaginous walls may extend toward the centre of the ball and almost completely fill it.
Each cell sits on the surface of the sphere and bears two flagella which protrude into the surrounding water and beat to propel the whole colony through the water. Each cell has a red eyespot stigma which forms part of a photosensor. Volvoxlike other algae, is photosynthetic and so it has to keep itself illuminated and so it will swim toward the light or away from very bright lights that may damage its chlorophyll.
This immediately poses a problem - if each cell beats its flagella independently of the others then the colony will move nowhere, rather they must coordinate themselves so as to beat in unison. To achieve this coordination, all the cells are connected to their nearest neighbours by protoplasmic bridgessuch that all the cell cytoplasms are continuous with one another through these bridges.
This allows waves of electric charge to travel throughout the colony, triggering flagella motion in an Scenedesmus asexual reproduction fission and controlled manner. The number of cells present in Volvox and related genera which form a group called the Volvocales ; Gonium forms flat plates with 4 or 16 cells, Pandorina forms a sphere of 16 cells. The number of cells in the larger Volvox is always a power of two, and may number several thousand, a result of coordinated cell division with all cells dividing and doubling in unison.
The Volvox ball has a preferred front-end and cells in this part of the sphere have larger eyespots than the rest. Chlamydomonas is an example of an alga comprising a single cell that resembles a cell from a Volvox colony.
Reproduction in Volvox Volvox can reproduce asexually by forming daughter colonies. Daughter colonies form as hollow balls of cells inside the parental colony. The daughter colonies form from enlarged cells in the surface of the parent colony, called gonidiaat the posterior end of the colony. Each gonidium divides repeatedly in a plane at right-angles to the surface of the parent coenobium, forming a cup-shaped plate of cells.
Divisions continue and the plate forms a small spherical daughter colony inside the parent colony and suspended from its surface. The daughter colonies are originally formed inside-out, with their flagella pointing inwards, and invaginate or invert, often at around the time that they "Scenedesmus asexual reproduction fission" from the parent colony which ruptures and dies, releasing the daughters which are miniature versions of the parent and grow by cell division.
Daughter colonies may contain small granddaughter colonies upon hatching. Some Scenedesmus asexual reproduction fission are monoecious hermaphroditic whilst others are dioecious with two separate sexes. In dioecious forms, sexual reproduction begins when some male colonies appear and secrete pheromones that induce the gonidia of other colonies to undergo sexual rather than asexual reproduction. The gonidia of colonies of female clones descended or cloned by asexual reproduction from a mother colony are stimulated to develop into female daughter colonies at the posterior of the parent colony.
In colonies of male lineages descended or cloned by asexual reproduction from a father colony the gonidia are stimulated to develop into male colonies. Female colonies produce specialised egg cells - enlarged superficial cells at the posterior of the colony and which lack flagella and remain attached to neighbouring cells by the protoplasmic bridges.
Male colonies produce packets of spermatozoidsalso at the posterior of the colony. These form much like daughter colonies in asexual reproduction - a superficial cell divides to produce a hollow ball of cells hanging down from the wall into the interior of the coenobium. These cells are naked lack cell walls and flagellated and are originally inside-out but invert so that their flagella point outwards.
These cells detach from the parent colony and swim towards egg cells, which are immotile and remain bound to the female colonies. This is oogamy - large immotile egg cells are formed, toward which the sperm swim.
The gametes are produced by mitosis, so the parent coenobia are haploid. When a spermatozoid fertilises an egg cell, a thick-walled hypnozygote is formed. A hypnozygote is a diploid cell formed by fertilisation zygotes which enters a mandatory dormant stage. It has a thick spiny wall to protect the zygote within. The possession of dormant stages is particularly important to freshwater organisms that live in ephemeral ponds. If a pond dries or freezes, then Scenedesmus asexual reproduction fission dormant stages can survive until better conditions for growth return.
After dormancy, the Volvox hypnozygote undergoes meiosis and germinates to produce a haploid zoid which undergoes mitosis to form a new coenobium. The zygote is the only diploid stage, Scenedesmus asexual reproduction fission Volvox an example of what is called a haplon t. Chlamydomonas with a detailed version on the right and simpler versions thumbnailed on the left.
One of the morphologically simplest single-celled algae is Chlorellawhich grows on walls and tree bark. Bacteria - study more about bacteria, including cyanobacteria. Filamentous bacteria are covered in more detail here. Spirogyraillustrated above in a 3D mathematical computer Scenedesmus asexual reproduction fission Pov-Ray is a green alga which forms filamentous chains of cells, which float freely in ponds and other bodies of still water.
Each cell has a distinctive characteristic spiral green chloroplast. Many cells will typically be joined in long filamentous chains. Spirogyra lives in freshwater ponds and wet drainage ditches and the like. Spirogyralike other algae, needs light for photosynthesis. Simply relying on passive means of reaching the light like flotation can be problematic and many algae can also move toward the light.
Spirogyra has no flagella and cannot swim quickly as can Chlamydomonasbut it is nevertheless capable of limited movement. Blue light will trigger positive phototaxis or movement toward the light. First the filaments align, pointing toward the light source and then they bind together into bundles this bundling is essential for the following movements. First the anterior filaments in the bundles, nearest the light, curve toward the light and then those at the rear also roll up toward the light forming open hoops and then the filaments stretch out "Scenedesmus asexual reproduction fission." Repeated rolling and stretching allows the mat of filaments to move toward the light in mass, albeit quite slowly at about 1 millimetre per minute.
Filaments are also capable of slowly gliding along, possibly as a result of mucilage secretion, and will move up the sides of glass containers, generally moving faster in the dark. It really is amazing teh tyricks that apparently simple organisms have evolved to enhance their Scenedesmus asexual reproduction fission The end cells on each filament of Spirogyra are also capable of adhesion.
Blue or UV light will rapidly stimulate these anchor cells to adhere reversibly to glass, probably by secretion of cementing mucoproteins. An increase in temperature and shaking the culture in the dark can also trigger this rapid and reversible adhesion. Red light also trigger a slow adhesion takes about 1 hour to develop which is apparently irreversible.
Filaments of Spirogyra grow by division binary fission of cells throughout the filament except perhaps the end cells and asexual reproduction comes about when filaments fragment and each sub-filament continues growing.
This fragmentation is not necessarily passive - there are changes in the joining walls that weaken the filament at certain breakage points and then one cell may swell more than Scenedesmus asexual reproduction fission neighbour as it takes up water and stretches under turgor pressure pushing against the weakened join and breaking it.
Sexual reproduction occurs by conjugation not to be confused with conjugation in bacteria which is a different process that serves a very different "Scenedesmus asexual reproduction fission." The parent vegetative filaments are haploid n.