Questions to consider

I am taking a hiatus after the multiple postings. "huff 2 puff 2"

Consider these random questions first which came to mind and students seemed to have trouble grasping during the course of teaching or hearsay (will post them in comments)

1) What are the purposes/functions of mRNA? (students on remedial should know)
2) What is the neutral theory about?

mmm......until more random snippets of thoughts self-generate or diffuse through from the school community....zzz......

Hope holidays have been good

3 comments:

Anonymous said...: Hey Mr Chan..
got wad you asked from me =)

Shell Facts: The shell is secreted by a thin, specialised tissue called the mantle. A shell is made mostly of calcium carbonate and shell material is added to both the outer edge as well as existing shell so that a shell gets both bigger and thicker with age. The outer surface of a shell is usually covered with a tough protein layer. Pigment cells in the mantle create the beautiful colours and patterns of the shell. The shell protects a snail from drying out as well as from predators. They come in a a wide range of shapes, textures and sizes. Some have spikes to keep off predators, large lips to protect them as they forage for food.

Slugs are mostly gastropods that have lost their shells. Instead of shells, these creatures have developed chemical and other defences.

(more info from http://www.wildsingapore.com/chekjawa/text/p401.htm); 3:41 PM, September 09, 2006
CJWD said...: Thanks JH ;)
Will add to that:

http://www.andrewgray.com/essays/molluscs.htm

Gastropods are the only molluscs to have successfully colonised the land, and their ability to withdraw into protective, waterproof shells undoubtedly played a part in this achievement. However, the construction of a thick shell requires calcium, and this vital element is often scarce in terrestrial ecosystems. Land snails therefore have very thin shells by comparison with their marine relatives. The total shell loss that is seen in terrestrial slugs may be an adaptation to cope with a lack of calcium, and there is evidence that the original distribution of slugs was confined to low-calcium environments. Slugs are largely restricted to humid environments, too, since without shells they are extremely vulnerable to dehydration, in spite of water-conserving adaptations such as nocturnal behaviour and a remarkable physiological ability to tolerate desiccation. Slugs do, nonetheless, have several major advantages over snails: in the absence of shells, not only do they economise on calcium, but they can crawl into tighter spaces, burrow more easily in search of prey, and distend their bodies to swallow large meals.

There is always plenty of calcium in the sea, so the fact that many marine gastropods have also lost their shells cannot be explained by mineral deficiency. Instead, the slug-like body form of such molluscs may be an adaptation to a burrowing or swimming lifestyle, in which shell reduction is necessary for streamlining, and (in the case of pelagic forms) reducing the weight of the animal. In some sea slugs, exposing the whole body to sunlight (rather than covering it in a shell) confers a more bizarre advantage: the ability to photosynthesise! These species either contain chloroplasts (absorbed from algae eaten by the slug), or harbour entire colonies of symbiotic algae (zooxanthellae), allowing them to utilise energy directly from the sun.

Loss of the shell obviously leaves a mollusc vulnerable to attack, but sea slugs have evolved a variety of alternative means to deter predators. Some secrete toxins (or sequester toxins from their diet), some gain protection by mimicking unpalatable species, some are well-camouflaged, some are nocturnal and hide in crevices during the day (a tactic that is made far easier by the absence of a cumbersome shell), some can swim rapidly to escape danger, and certain species even have the remarkable ability to absorb intact stinging cells (nematocysts) from the cnidarians upon which they feed, and incorporate them into the slugs' own defences! In some sea slugs, secondary gills have evolved as outgrowths from the skin, since the mantle cavity, and the original gills (ctenidia) that it housed, were lost along with the shell.

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Mollusks make their shells from calcium they derive from their environment, either the food they eat or the water they dwell in. When a tiny mollusk hatches from its egg, it comes into the world equipped with a tiny shell. This shell is actually a part of the animal, growing as it grows, accommodating its needs. Each different species of mollusk makes a shell that is, in most cases, unique to it alone. Indeed, this uniqueness of form is partly what allows amateur shell collectors (conchologists) and professional scientists who study mollusks (malacologists) to determine a mollusk's species. Each species is destined genetically to develop the same type of shell its progenitors did. But, just as with humans, there are many distinct differences. Food, climate, environment, accident, and the mollusk's particular heredity all play their parts in making each shell an individual.; 5:32 PM, September 09, 2006
CJWD said...: ha! It came as an afterthought but I realised that I can edit the msg directly so I will post the stuff on neutral theory and mRNA on the page itself. NO need to check the comment page. Makes better sense. Muahaha; 11:36 PM, September 12, 2006

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