Green algae

For other uses, see Algae (disambiguation) and Alga (disambiguation).

Algae (pronounced /ˈældʒiː/ or /ˈælɡiː/; singular alga /ˈælɡə/, Latin for "seaweed") are a large and diverse group of simple, typicallyautotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They arephotosynthetic like plants, and "simple" because their tissues are not organized into the many distinct organs found in land plants. The largest and most complex marine forms are called seaweeds.

Though the prokaryotic cyanobacteria (commonly referred to as blue-green algae) were traditionally included as "algae" in older textbooks, many modern sources regard this as outdated as they are now considered to be bacteria. The term algae is now restricted to eukaryotic organisms. All true algae therefore have a nucleus enclosed within a membrane and plastids bound in one or more membranes. Algae constitute a paraphyletic and polyphyletic group, as they do not include all the descendants of the last universal ancestor nor do they all descend from a common algal ancestor, although their plastids seem to have a single origin.Diatoms are also examples of algae.

Algae are found in the fossil record dating back to approximately 3 billion years in the Precambrian. They exhibit a wide range of reproductive strategies, from simple, asexual cell division to complex forms of sexual reproduction.

Algae lack the various structures that characterize land plants, such as phyllids (leaves) and rhizoids in nonvascular plants, or leaves,roots, and other organs that are found in tracheophytes (vascular plants). Many are photoautotrophic, although some groups contain members that are mixotrophic, deriving energy both from photosynthesis and uptake of organic carbon either by osmotrophy,myzotrophy, or phagotrophy. Some unicellular species rely entirely on external energy sources and have limited or no photosynthetic apparatus.

Nearly all algae have photosynthetic machinery ultimately derived from the Cyanobacteria, and so produce oxygen as a by-product of photosynthesis, unlike other photosynthetic bacteria such as purple and green sulfur bacteria. Fossilized filamentous algae from the Vindhya basin have been dated back to 1.6 to 1.7 billion years ago.The singular alga is the Latin word for a particular seaweed and retains that meaning in English.The etymology is obscure. Although some speculate that it is related to Latin algēre, "be cold",there is no known reason to associate seaweed with temperature. A more likely source is alliga, "binding, entwining." Since Algae has become a biological classification, alga can also mean one classification under Algae, parallel to a fungus being a species of fungi, a plant being a species of plant, and so on.

The ancient Greek word for seaweed was φῦκος (fūkos or phykos), which could mean either the seaweed, probably Red Algae, or a red dye derived from it. The Latinization, fūcus, meant primarily the cosmetic rouge. The etymology is uncertain, but a strong candidate has long been some word related to the Biblical פוך (pūk), "paint" (if not that word itself), a cosmetic eye-shadow used by the ancient Egyptians and other inhabitants of the eastern Mediterranean. It could be any color: black, red, green, blue.

Accordingly the modern study of marine and freshwater algae is called either phycology or algology. The name Fucus appears in a number of taxa.The singular form is alga.

Relationship to higher plants

The first plants on earth evolved from shallow freshwater algae much like Chara some 400 million years ago. These probably had an isomorphic alternation of generations and were probably filamentous. Fossils of isolated land plant spores suggest land plants may have been around as long as 475 million years ago.

Paramecium

Paramecium is a group of unicellular ciliate protozoa, which are commonly studied as a representative of the ciliate group, and range from about 50 to 350 μm in length. Simple cilia cover the body, which allow the cell to move with a synchronous motion (like a caterpillar) at speeds of approximately 2,700μm/second (12 body lengths per second). There is also a deep oral groove containing inconspicuous compound oral cilia (as found in other peniculids) used to draw food inside. They generally feed on bacteria and other small cells. Osmoregulation is carried out by a pair of contractile vacuoles, which actively expel water from the cell absorbed by osmosis from their surroundings.

Paramecia are widespread in freshwater environments, and are especially common in scums. Recently, some new species of Paramecia have been discovered in the oceans.

Certain single-celled eukaryotes, such as Paramecium, are examples for exceptions to the universality of the genetic code: in their translation systems a few codons differ from the standard ones.

Physiology

The paramecium approximates a prolate spheroid, rounded at the front and pointed at the back. The pellicle is a stiff but elastic membrane that gives the paramecium its definite shape. Covering the outer edge are hairlike structures, called cilia. On the side, beginning near the front end continuing down half way, is the oral groove, which collects food until it is swept into the cell mouth. There is an opening near the back end called the anal pore. The contractile vacuole and its radiating canals — referred to previously for osmoregulation of the organism, are also found on the outside of a paramecium. The paramecium is very commonly mistaken for a blepharisma.

The paramecium contains cytoplasm, trichocysts, the gullet, food vacuoles, the macronucleus, and the micronucleus.

Movement

Cilia are the locomotive structures of the paramecium. In order for the paramecium to move forward, its cilia beat at an angle, backwards in unison (i.e., the cilium wiggles from tip-to-base). This means that the paramecium moves by spiraling through the water on an invisible axis. The paramecium can also move backwards when the cilia beat forward at an angle in unison (i.e., the cilia go backward. The paramecium turns slightly and goes forward again. If it runs into the solid object again it will repeat this process until it can get past the object.

Gathering food

Paramecia feed on microorganisms like bacteria, algae, and yeasts. To gather its food, the paramecium uses its cilia to sweep up food along with some water into the cell mouth after it falls into the oral groove. The food goes through the cell mouth into the gullet. When enough food has accumulated at the gullet base, it forms a food vacuole in the cytoplasm, and travels through the cell, through the back end first. As it moves along, enzymes from the cytoplasm enter the vacuole to digest the contents, digested nutrients then going into the cytoplasm, and the vacuole shrinks. When the vacuole reaches the anal pore, it ruptures, expelling its waste contents to the exterior.

Symbiosis

One of the most interesting known symbiotic relationships is that of Paramecium aurelia and its bacterial endosymbionts. See also the Chlorella symbiosis with Paramecium bursaria.

Genome

The paramecium genome has been sequenced (species: Paramecium tetraurelia), providing evidence for three whole-genome duplications.

In some ciliates, like Stylonychia and Paramecium, only UGA is decoded as a stop codon, while UAG and UAA are reassigned as sense codons.

Learning

The question of whether paramecia exhibit learning has been the object of a great deal of experimentation, yielding equivocal results. In one of the most recent experiments published,the authors, by using a voltage as a reinforcement, concluded that paramecium may indeed learn to discriminate between different brightness levels.

Amoeba (genus)

Amoeba (sometimes amœba or ameba, plural amoebae) is a genus of Protozoa.

Terminology

There are many closely related terms that can be the source of confusion:

• Amoeba is a genus that includes species such as Amoeba proteus
• Amoebidae is a family that includes the Amoeba genus, among others.
• Protista is a kingdom that includes the Amoebidae family, among others.
• Amoeboids are organisms that move by crawling. Many (but not all) amoeboids are Amoebozoa

History

The amoeba was first discovered by August Johann Rosel von Rosenhof in 1757. Early naturalists referred to Amoeba as the Proteus animalcule after the Greek god Proteus who could change his shape. The name "amibe" was given to it by Bory de Saint-Vincent, from the Greek amoibè (αμοιβή), meaning change.Dientamoeba fragili was first described in 1918, and was linked to harm in humans.

Anatomy

Anatomy of an amoeba

The cell's organelles and cytoplasm are enclosed by a cell membrane, obtaining its food through phagocytosis. Amoebae have a single large tubular pseudopod at the anterior end, and several secondary ones branching to the sides. The most famous species, Amoeba proteus, averages about 220-740 μm in length while moving, making it a giant among amoeboids. A few amoeboids belonging to different genera can grow larger, however, such as Gromia, Pelomyxa, and Chaos.

Amoebae's most recognizable features include one or more nuclei and a simple contractile vacuole to maintain osmotic equilibrim. Food enveloped by the amoeba is stored and digested in vacuoles. Amoebae, like other single-celled eukaryotic organisms, reproduce asexually via mitosis and cytokinesis, not to be confused with binary fission, which is how prokaryotes (bacteria) reproduce. In cases where the amoeba are forcibly divided, the portion that retains the nucleus will survive and form a new cell and cytoplasm, while the other portion dies. Amoebae also have no definite shape. equilibrium

Genome

The amoeba is remarkable for its very large genome. The species Amoeba proteus has 290 billion base pairs in its genome, while the related Polychaos dubium (formerly known asAmoeba dubia) has 670 billion base pairs. The human genome is small by contrast, with its count of 2.9 billion bases.

Osmoregulation

Like most other protists, amoebas have a contractile vacuole complex. Amoeba proteus, a free-living, freshwater species of amoeba, has one contractile vacuole (CV) which is a membrane-bound organelle. The CV slowly fills with water from the cytoplasm (diastole) and whilst fusing with the cell membrane, it quickly contracts releasing water to the outside (systole) by exocytosis. This process regulates the amount of water present in the cytoplasm of the amoeba; it is therefore a means of osmoregulation.

Immediately after the CV expels water, its membrane crumples, and soon afterwards, many small vacuoles or vesicles appear surrounding the membrane of the CV. It is suggested that these vesicles split from the CV membrane itself. The small vesicles gradually increase in size as they take in water and then they fuse with the CV, which grows in size as it fills with water. Therefore, the function of these numerous small vesicles is to collect excess cytoplasmic water and channel it to the central CV. The CV swells for a number of minutes and then contracts to expel the water outside. The cycle is then repeated again.

The membranes of the small vesicles as well as the membrane of the CV have aquaporin proteins embedded in them. These transmembrane proteins facilitate water passage through the membranes. The presence of aquaporin proteins in both CV and the small vesicles suggests that water collection occurs both through the CV membrane itself as well as through the function of the vesicles. However, the vesicles, being more numerous and smaller, would allow a faster water uptake due to the larger total surface area provided by the vesicles

The small vesicles also have another protein embedded in its membrane; Vacuolar-type H-ATPase or V-ATPase.This ATPase pumps H⁺ ions into the vesicle lumen, lowering its pH with respect to the cytosol. However, the pH of the CV in some amoebas is only mildly acidic, suggesting that the H⁺ ions are being removed from the CV or from the vesicles. It is thought that the electrochemical gradient generated by V-ATPase might be used for the transport of ions (probably K⁺ and Cl^-) into the vesicles. This builds an osmotic gradient across the vesicle membrane, leading to influx of water from the cytosol into the vesicles by osmosis, which is facilitated by aquaporins.

Since these vesicles fuse with the central contractile vacuole which expels the water out, ions end up being removed from the cell, which is not beneficial for a freshwater organism. The removal of ions with the water has to be compensated by some as yet unidentified mechanism.

Like most cells, amoebae are adversely affected by excessive osmotic pressure caused by extremely saline or dilute water. Amoebae will prevent the influx of salt in saline water, resulting in a net loss of water as the cell becomes isotonic with the environment, causing the cell to shrink. Placed into fresh water, amoebae will also attempt to match the concentration of the surrounding water, causing the cell to swell and sometimes burst if the water surrounding the amoeba is too dilute.