Drag and Drop Exercise (Mollusca)

You can do the interactive exercise in this page itself. If you want to have a larger view, Click on the following link to open the same in a pop-up window
Click Here to open in a Popup Window



Post your comments. To learn how to post comments click here.

The Origin of Oxygen in the Atmosphere

Cyanobacteria Bloom: Thanks to algal blooms like this one, the Earth's atmosphere is 21 percent oxygen.

It's hard to keep oxygen molecules around, despite the fact that it's the third-most abundant element in the universe, forged in the superhot, superdense core of stars. That's because oxygen wants to react; it can form compounds with nearly every other element on the periodic table. So how did Earth end up with an atmosphere made up of roughly 21 percent of the stuff?

The answer is tiny organisms known as cyanobacteria, or blue-green algae. These microbes conduct photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and, yes, oxygen. In fact, all the plants on Earth incorporate symbiotic cyanobacteria (known as chloroplasts) to do their photosynthesis for them down to this day.

For some untold eons (eon is the longest division of geologic time, containing two or more eras) prior to the evolution of these cyanobacteria, during the Archean eon, more primitive microbes lived the real old-fashioned way: anaerobically. These ancient organisms—and their "extremophile" descendants today—thrived in the absence of oxygen, relying on sulfate for their energy needs.

But roughly 2.45 billion years ago, the isotopic ratio of sulfur transformed, indicating that for the first time oxygen was becoming a significant component of Earth's atmosphere, according to a 2000 paper in Science. At roughly the same time (and for eons thereafter), oxidized iron began to appear in ancient soils and bands of iron were deposited on the seafloor, a product of reactions with oxygen in the seawater.

"What it looks like is that oxygen was first produced somewhere around 2.7 billion to 2.8 billon years ago. It took up residence in atmosphere around 2.45 billion years ago," says geochemist Dick Holland, a visiting scholar at the University of Pennsylvania. "It looks as if there's a significant time interval between the appearance of oxygen-producing organisms and the actual oxygenation of the atmosphere."

So a date and a culprit can be fixed for what scientists refer to as the Great Oxidation Event, but mysteries remain. What occurred 2.45 billion years ago that enabled cyanobacteria to take over? What were oxygen levels at that time? Why did it take another one billion years—dubbed the "boring billion" by scientists—for oxygen levels to rise high enough to enable the evolution of animals?

Most important, how did the amount of atmospheric oxygen reach its present level? "It's not that easy why it should balance at 21 percent rather than 10 or 40 percent," notes geoscientist James Kasting of Pennsylvania State University. "We don't understand the modern oxygen control system that well."

Climate, volcanism, plate tectonics all played a key role in regulating the oxygen level during various time periods. Yet no one has come up with a rock-solid test to determine the precise oxygen content of the atmosphere at any given time from the geologic record. But one thing is clear—the origins of oxygen in Earth's atmosphere derive from one thing: life.

Anti-ageing pill activates telomerase

Peter Pan stayed forever young in Neverland. In real life, some scientists are looking at telomeres, or regions of repetitive DNA at the ends of our chromosomes, to try to arrive at something like a real version of this story.

Telomeres consist of up to 3,300 repeats of the DNA sequence TTAGGG. They protect chromosome ends from being mistaken for broken pieces of DNA that would otherwise be fixed by cellular repair machinery. But every time our cells divide, the telomeres shrink. When they get short enough, our cells no longer divide and our body stops making those cells. Over time, this leads to aging and death.

New York-based T.A. Sciences claims to be the only company in the world manufacturing a supplement in a pill form that has been lab tested and shown to stop telomeres from shortening, in hopes of halting the aging process. The product, TA-65, comes from extracts of the Chinese herb astragalus, which has been used for medicinal purposes for more than 1,000 years, says Noel Patton, chief executive officer of the company.

TA-65 is produced at very low levels in the astragalus plant, but the company purifies and concentrates the substance, which is thought to "turn on" the enzyme telomerase (hTERT) that acts to maintain or lengthen telomeres. hTERT is usually "off" in adult cells, except in immune, egg and sperm cells, and in malignant cancer-forming cells.

Telomerase is the same enzyme that allows cancer cells to stop aging or to become immortal, so there is a chance that TA-65 could keep alive cancer cells that would otherwise die.

However, telomerase activation should keep all telomeres longer in the first place, and that actually reduces the chances of cells becoming cancerous. The enzyme should keep immune cells, which can fight off most cancerous cells, alive longer.

Appendix is not a vestigial organ!

Duke University Medical Center researchers said that the supposedly useless appendix is actually where good gut bacteria safely hide out during some unpleasant intestinal conditions.

Now the research team has looked at the appendix over evolutionary history. They found that animals have had appendixes for about 80 million years. And the organ has evolved separately at least twice, once among the weird Australian marsupials and another time in the regular old mammal lineage that we belong to.

Darwin thought that only a few animals have an appendix and that the human version was what was left of a digestive organ called the cecum. But the new study found that 70 percent of rodent and primate groups have species with an appendix. And some living animals have a cecum and an appendix. If Darwin had known about species that had both organs, he probably would have revised his views of the appendix, the researchers note.

Ironically, it’s natural selection that keeps the human appendix from shrinking away completely. Because smaller ones are more likely to become infected. And keep your genes out of the pool.

—Steve Mirsky

Endocrine System Animations

Continue reading fullpost to view animations in endocrine system

Enzyme Amplification

Hormones are very potent chemicals even at very low concentrations due to enzyme
amplification. Even the slightest amount of hormone acting on the cell surface can initiate a powerful cascading activating force for the entire cell. View the animation below

Positive and Negative Feedback

The secretion of TSH, FSH, LH and ACTH from pituitary decreases when blood levels of their target gland hormones rise. This is called negative feedback. The other type of feed back control is positive feed back. View the animation below to understand positive and negative feedback mechanisms.

Mechanism of Hormone Action:
Lipid-soluble Hormones

Lipid-soluble hormones such as steroid hormones diffuse through lipid bilayer of the plasma membrane. They bind to intracellular hormone receptors. The activated
receptor-hormone complex influences gene expression.

Mechanism of Hormone Action:
Water-soluble Hormones

Unlike lipid-soluble hormones, water-soluble hormones like protein hormones, peptide
hormones and catecholamines cannot diffuse through lipid bilayer of the plasma membrane. They bind to receptors on the target cell membrane. This causes the production of a second messenger e.g. cAMP (cyclicAdenosineMonoPhosphate) inside the cell. The cAMP then causes subsequent intracellular effects of the hormone by activating a cascade of enzymes.

Mechanism of Thyroxine Action

Thyroid hormones are derived from the amino acid tyrosine and they have iodine incorporated into their structures. There are two thyroid hormones, which have either 3 or 4 atoms of iodine per molecule; they are known respectively as T3 (tri-iodothyronine) and T4 (thyroxine). Both are synthesized within the thyroid follicles and secreted into the bloodstream when the cells are stimulated by TSH to extrude them. When the appropriate level of T3/T4 is attained in the blood, thyroid hormones switch off TSH production. This is an example of a negative feedback system.
T3, the form of thyroid hormone which contains only 3 atoms of iodine per molecule, is now considered to be the physiologically active hormone, and T4 to be a precursor of T3, which can be converted to T3 by specific enzymes within the target cells. Since T4 circulates at a concentration about a hundred-fold higher than that of T3, it can therefore be considered to be a storage form of the active hormone.

Osteoporosis

Osteoporosis literally means "porous bones. PTH increases the activity of osteoclasts. This results in resorption of calcium phosphate ions from bone into the blood. Hypersecretion of PTH results in osteoporosis.

Jellyfish help mix the world's oceans

Small sea creatures such as jellyfish may contribute to ocean mixing by pulling water along as they swim, according to a new study. The collective movement of animals could generate stirring of the same order as winds and tides.

Pulsating jellyfish stir up the oceans with as much vigor as tides and winds, scientists have found. The new study, which is published in the July 30 issue of the journal Nature, reveals a mixing mechanism first described by Charles Darwin's grandson that is actually enhanced by the ocean's viscosity, making these tiny sea creatures major players in ocean mixing.

In their field experiments, the researchers squirt fluorescent dye into the water in front of the Mastigias jellyfish and watched what happened as the animals swam through the dyed water. Rather than being left behind as the jellyfish swam by, the dyed water travelled along with the swimming creatures.

As the jellyfish swims, water gets pulled along with the animal, seen as swirls of red or green dye that was injected into the water.

Here's how the researchers think it works: As a jellyfish swims, it pushes water aside and creates a high-pressure area ahead of the animal. The region behind the jellyfish becomes a low-pressure zone. Then, the ocean water rushes in behind the animal to fill in this lower pressure gap. The result: Jellyfish drag water with them as they swim.

Jelliy fishes have huge variation in their body shapes. Moon jellyfish (the kind typically seen at aquariums) have saucer-shaped bodies and can carry a lot of water with them. But other bullet-shaped jellyfish would drag smaller volumes of water.

Global impact
The ocean churning has broader implications.
With this mechanism the animals can pull nutrient-rich fluid up to nutrient-poor areas and pull oxygen-rich fluid down to oxygen-poor regions,".
On larger scales, the biologic blender could impact the ocean circulation, which affects the Earth's climate.