Hornwort

The plant body of a hornwort is a haploid gametophyte stage. This stage usually grows as a thin rosette or ribbon-like thallus between one and five centimeters in diameter. Each cell of the thallus usually contains just one chloroplast. In most species, this chloroplast is fused with other organelles to form a large pyrenoid that both manufactures and stores food. This particular feature is very unusual in land plants, but is common among algae.
Many hornworts develop internal mucilage-filled cavities when groups of cells break down. These cavities are invaded by photosynthetic cyanobacteria, especially species of Nostoc. Such colonies of bacteria growing inside the thallus give the hornwort a distinctive blue-green color. There may also be small slime pores on the underside of the thallus. These pores superficially resemble the stomata of other plants.
The horn-shaped sporophyte grows from an archegonium embedded deep in the gametophyte. The sporophyte of a hornwort is unusual in that it grows from a meristem near its base, instead of from its tip the way other plants do. Unlike liverworts, most hornworts have true stomata on their sporophyte as mosses do. The exceptions are the genera Notothylas and Megaceros, which do not have stomata.
When the sporophyte is mature, it has a multicellular outer layer, a central rod-like columella running up the center, and a layer of tissue in between that produces spores and pseudo-elaters. The pseudo-elaters are multi-cellular, unlike the elaters of liverworts. They have helical thickenings that change shape in response to drying out; they twist and thereby help to disperse the spores. Hornwort spores are relatively large for bryophytes, measuring between 30 and 80 µm in diameter or more. The spores are polar, usually with a distinctive Y-shaped tri-radiate ridge on the proximal surface, and with a distal surface ornamented with bumps or spines.

Life cycle

The life of a hornwort starts from a haploid spore. In most species, there is a single cell inside the spore, and a slender extension of this cell called the germ tube germinates from the proximal side of the spore.^[2] The tip of the germ tube divides to form an octant of cells, and the first rhizoid grows as an extension of the original germ cell. The tip continues to divide new cells, which produces a thalloid protonema. By contrast, species of the family Dendrocerotaceae may begin dividing within the spore, becoming multicellular and even photosynthetic before the spore germinates.^[2] In either case, the protonema is a transitory stage in the life of a hornwort.

Life cycle of a typical hornwort Phaeoceros. Click on the image to enlarge.

From the protonema grows the adult gametophyte, which is the persistent and independent stage in the life cycle. This stage usually grows as a thin rosette or ribbon-like thallus between one and five centimeters in diameter, and several layers of cells in thickness. It is green or yellow-green from the chlorophyll in its cells, or bluish-green when colonies of cyanobacteria grow inside the plant.
When the gametophyte has grown to its adult size, it produces the sex organs of the hornwort. Most plants are monoicous, with both sex organs on the same plant, but some plants (even within the same species) are dioicous, with separate male and female gametophytes. The female organs are known as archegonia (singular archegonium) and the male organs are known as antheridia (singular antheridium). Both kinds of organs develop just below the surface of the plant and are only later exposed by disintegration of the overlying cells.
The biflagellate sperm must swim from the antheridia, or else be splashed to the archegonia. When this happens, the sperm and egg cell fuse to form a zygote, the cell from which the sporophyte stage of the life cycle will develop. Unlike all other bryophytes, the first cell division of the zygote is longitudinal. Further divisions produce three basic regions of the sporophyte.
At the bottom of the sporophyte (closest to the interior of the gametophyte), is a foot. This is a globular group of cells that receives nutrients from the parent gametophyte, on which the sporophyte will spend its entire existence. In the middle of the sporophyte (just above the foot), is a meristem that will continue to divide and produce new cells for the third region. This third region is the capsule. Both the central and surface cells of the capsule are sterile, but between them is a layer of cells that will divide to produce pseudo-elaters and spores. These are released from the capsule when it splits lengthwise from the tip.

Source :http://en.wikipedia.org/wiki/Hornwort 

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Marchantiophyta

Most liverworts are small, usually from 2–20 millimetres (0.08–0.8 in) wide with individual plants less than 10 centimetres (4 in) long,^[5] so they are often overlooked. The most familiar liverworts consist of a prostrate, flattened, ribbon-like or branching structure called a thallus (plant body); these liverworts are termed thallose liverworts. However, most liverworts produce flattened stems with overlapping scales or leaves in two or more ranks, the middle rank is often conspicuously different from the outer ranks; these are called leafy liverworts or scale liverworts.^[6][7] (See the gallery below for examples.)

A thallose liverwort, Lunularia cruciata

Liverworts can most reliably be distinguished from the apparently similar mosses by their single-celled rhizoids.^[8] Other differences are not universal for all mosses and all liverworts;^[7] but the lack of clearly differentiated stem and leaves in thallose species, or in leafy species the presence of deeply lobed or segmented leaves and the presence of leaves arranged in three ranks, all point to the plant being a liverwort.^[9][10] In addition, 90% of liverworts contain oil bodies in at least some of their cells, and these cellular structures are absent from most other bryophytes and from all vascular plants.^[11] The overall physical similarity of some mosses and leafy liverworts means that confirmation of the identification of some groups can be performed with certainty only with the aid of microscopy or an experienced bryologist.
Liverworts have a gametophyte-dominant life cycle, with the sporophyte dependent on the gametophyte.^[11] Cells in a typical liverwort plant each contain only a single set of genetic information, so the plant's cells are haploid for the majority of its life cycle. This contrasts sharply with the pattern exhibited by nearly all animals and by most other plants. In the more familiar seed plants, the haploid generation is represented only by the tiny pollen and the ovule, while the diploid generation is the familiar tree or other plant.^[12] Another unusual feature of the liverwort life cycle is that sporophytes (i.e. the diploid body) are very short-lived, withering away not long after releasing spores.^[13] Even in other bryophytes, the sporophyte is persistent and disperses spores over an extended period.

Life cycle

Life cycle of a typical liverwort

The life of a liverwort starts from the germination of a haploid spore to produce a protonema, which is either a mass of thread-like filaments or else a flattened thallus.^[14][15] The protonema is a transitory stage in the life of a liverwort, from which will grow the mature gametophore ("gamete-bearer") plant that produces the sex organs. The male organs are known as antheridia (singular: antheridium) and produce the sperm cells. Clusters of antheridia are enclosed by a protective layer of cells called the perigonium (plural: perigonia). As in other land plants, the female organs are known as archegonia (singular: archegonium) and are protected by the thin surrounding perichaetum (plural: perichaeta).^[7] Each archegonium has a slender hollow tube, the "neck", down which the sperm swim to reach the egg cell.
Liverwort species may be either dioicous or monoicous. In dioicious liverworts, female and male sex organs are borne on different and separate gametophyte plants. In monoicious liverworts, the two kinds of reproductive structures are borne on different branches of the same plant.^[16] In either case, the sperm must move from the antheridia where they are produced to the archegonium where the eggs are held. The sperm of liverworts is biflagellate, i.e. they have two tail-like flagellae that enable them to swim short distances,^[17] provided that at least a thin film of water is present. Their journey may be assisted by the splashing of raindrops. In 2008, Japanese researchers discovered that some liverworts are able to fire sperm-containing water up to 15 cm in the air, enabling them to fertilize female plants growing more than a metre from the nearest male.^[18]
When sperm reach the archegonia, fertilisation occurs, leading to the production of a diploid sporophyte. After fertilisation, the immature sporophyte within the archegonium develops three distinct regions: (1) a foot, which both anchors the sporophyte in place and receives nutrients from its "mother" plant, (2) a spherical or ellipsoidal capsule, inside which the spores will be produced for dispersing to new locations, and (3) a seta (stalk) which lies between the other two regions and connects them.^[17] When the sporophyte has developed all three regions, the seta elongates, pushing its way out of the archegonium and rupturing it. While the foot remains anchored within the parent plant, the capsule is forced out by the seta and is extended away from the plant and into the air. Within the capsule, cells divide to produce both elater cells and spore-producing cells. The elaters are spring-like, and will push open the wall of the capsule to scatter themselves when the capsule bursts. The spore-producing cells will undergo meiosis to form haploid spores to disperse, upon which point the life cycle can start again.

Source : http://en.wikipedia.org/wiki/Marchantiophyta

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Impulse and Momentum

Momentum

• The momentum of a body is equal to its mass multiplied by its velocity.

Momentum is measured in N s. Note that momentum is a vector quantity, in other words the direction is important.

Impulse

The impulse of a force (also measured in N s) is equal to the change in momentum of a body which a force causes. This is also equal to the magnitude of the force multiplied by the length of time the force is applied.

• Impulse = change in momentum = force × time

Conservation of Momentum

When there is a collision between two objects, Newton's Third Law states that the force on one of the bodies is equal and opposite to the force on the other body.
Therefore, if no other forces act on the bodies (in the direction of collision), then the total momentum of the two bodies will be unchanged. Hence the total momentum before collision in a particular direction = total momentum after in a particular direction.
This can be used to solve problems involving colliding spheres (see also: restitution).

Example

We have the following scenario (a ball of mass 3kg is moving to the right with velocity 3m/s and a ball of mass 1kg is moving to the left with velocity 2m/s):

Suppose we are told that after the collision, the ball of mass 1kg moves away with velocity 2m/s, then we can use the principle of conservation of momentum to determine the velocity of the other ball after the collision.

Initial momentum = 3.3 - 2.1 = 7 [the minus sign is important: it is there because the velocity of the 1kg body is in the opposite direction to the velocity of the 3kg body].
Final momentum = 2 - 3x
Hence 7 = 2 - 3x (since momentum is conserved)
x = -5/3
Which means that my arrow was pointing in the wrong direction (because of the minus sign), hence the velocity of the 3kg body after the collision is 5/3 ms^-1 to the right.

Source : https://www.google.co.id/#hl=id&tbo=d&sclient=psy-ab&q=impulse+and+momentum&oq=impuls+a&gs_l=hp.3.1.0j0i10j0j0i30.8101.11798.0.14648.8.7.0.1.1.0.701.3326.2-2j1j0j3j1.7.0...0.0...1c.1.CRKn8wZ7Fyw&pbx=1&bav=on.2,or.r_gc.r_pw.r_cp.r_qf.&fp=44ad8154c426a17d&bpcl=39314241&biw=1366&bih=625

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Brittle Star (Ophiuroidea)

Brittle stars or ophiuroids are echinoderms in the class Ophiuroidea closely related to starfish. They crawl across the seafloor using their flexible arms for locomotion. The ophiuroids generally have five long slender, whip-like arms which may reach up to 60 centimetres (24 in) in length on the largest specimens. They are also known as serpent stars.
Ophiuroidea contains two large clades, Ophiurida (brittle stars) and Euryalida (basket stars). Many of the ophiuroids are rarely encountered in the relatively shallow depths normally visited by humans, but they are a diverse group.
There are some 1,500 species of brittle stars living today, and they are largely found in deep waters more than 500 metres (1,650 feet) down.

Range

Brittle star in Kona, Hawaii

Fossil brittle star Palaeocoma egertoni from the Jurassic of England

The ophiuroids diverged in the Early Ordovician, about 500 million years ago. Ophiuroids can be found today in all of the major marine provinces, from the poles to the tropics. In fact, crinoids, holothurians, and ophiuroids live at depths from 16–35 m, all over the world. Basket stars are usually confined to the deeper parts of this range. Ophiuroids are known even from abyssal (>6000 m) depths. However brittle stars are also common, if cryptic, members of reef communities, where they hide under rocks and even within other living organisms. A few ophiuroid species can even tolerate brackish water, an ability otherwise almost unknown among echinoderms. A brittle star's skeleton is made up of embedded ossicles.

Taxonomy

There are roughly 1900 extant species in 230 genera, grouped in the three orders currently living: Oegophiurida, Phrynophiurida, and Ophiurida. There is also a Paleozoic order, the Stenurida.
The relationships among ophiuroids and all other echinoderms provide an enduring problem in invertebrate evolution. Developmental and other studies based on modern organisms imply that asteroids and ophiuroids are not closely related within the echinoderms. Stenurid morphology, in contrast, suggests a close common ancestry for the two; the nature of the ambulacral plates is important, but even their general form is transitional.

Stenurida (extinct)

This is a Paleozoic (Ordovician–Devonian) order, bearing a double row of plates (ambulacra) that abut across the arm axis either directly opposite one another or slightly offset. In contrast, modern ophiuroids have a single series of axial arm plates termed vertebrae. In stenurids, as in modern ophiuroids, lateral plates are present at the sides of ambulacrals, and prominent lateral spines are typical. Stenurids lack the dorsal and ventral arm shields that are found in most ophiuroids. Proximal ambulacral pairs can be partially separated, forming a buccal slit, an expansion of the mouth frame. The arms of some stenurids are slender and flexible, but those of others are broad and comparatively stiff. The central disk varies from little larger than the juncture of the arms to an expansion that extends most of the length of the arms. The content of the order is poorly established, and fewer than 10 genera are known.

Anatomy

Asteriacites, a trace fossil of an ophiuroid; Carmel Formation, near Gunlock, Utah; scale bar is 10 mm.

Of all echinoderms, the Ophiuroidea may have the strongest tendency toward 5-segment radial (pentaradial) symmetry. The body outline is similar to that of starfish, in that ophiuroids have five arms joined to a central body disk. However, in ophiuroids, the central body disk is sharply marked off from the arms.^[1]
The disk contains all of the viscera. That is, the internal organs of digestion and reproduction never enter the arms, as they do in the Asteroidea. The underside of the disk contains the mouth, which has five toothed jaws formed from skeletal plates. The madreporite is usually located within one of the jaw plates, and not on the upper side of the animal as it is in starfish.^[1]
The ophiuroid coelom is strongly reduced, particularly in comparison to other echinoderms.

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Sickle Cell Anemia

Sickle-cell disease (SCD), or sickle-cell anaemia (or anemia, SCA) or drepanocytosis, is an autosomal recessive genetic blood disorder with overdominance, characterized by red blood cells that assume an abnormal, rigid, sickle shape. Sickling decreases the cells' flexibility and results in a risk of various complications. The sickling occurs because of a mutation in the hemoglobin gene. Life expectancy is shortened. In 1994, in the US, the average life expectancy of persons with this condition was estimated to be 42 years in males and 48 years in females,^[1] but today, thanks to better management of the disease, patients can live into their 50s or beyond.^[2]
Sickle-cell disease occurs more commonly in people (or their descendants) from parts of tropical and sub-tropical regions where malaria is or was common. In areas where malaria is common, there is a fitness benefit in carrying only a single sickle-cell gene (sickle cell trait). Those with only one of the two alleles of the sickle-cell disease, while not totally resistant, are more tolerant to the infection and thus show less severe symptoms when infected.^[3]
Sickle-cell anaemia is the name of a specific form of sickle-cell disease in which there is homozygosity for the mutation that causes HbS. Sickle-cell anaemia is also referred to as "HbSS", "SS disease", "haemoglobin S" or permutations thereof. In heterozygous people, who have only one sickle gene and one normal adult haemoglobin gene, it is referred to as "HbAS" or "sickle cell trait". Other, rarer forms of sickle-cell disease include sickle-haemoglobin C disease (HbSC), sickle beta-plus-thalassaemia (HbS/β⁺) and sickle beta-zero-thalassaemia (HbS/β⁰). These other forms of sickle-cell disease are compound heterozygous states in which the person has only one copy of the mutation that causes HbS and one copy of another abnormal haemoglobin allele.
The term disease is applied, because the inherited abnormality causes a pathological condition that can lead to death and severe complications. Not all inherited variants of haemoglobin are detrimental, a concept known as genetic polymorphism.

Source :http://en.wikipedia.org/wiki/Sickle-cell_disease 

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