Science: Technoquest Interplantary debris/ Crocodile armour/ Volcanoes/ Hiccups

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Questions for this column may be submitted by e-mail to sci.net@campus.bt.com

Q Last week we were worried that an asteroid might hit us in 30 years' time. But how many meteors actually collide with the Earth in a typical year?

We don't know exactly, but many tonnes of material hit the Earth every year. The solar system is strewn with material left over from comets and asteroid collisions throughout history. As our planet sweeps through space, it goes through this debris which creates the streaks of light in the atmosphere that are meteorites.

In the past, larger bits of rock have collided with the Earth creating impact craters, and destroying vast tracts of the Earth's surface. But these large pieces of rocks have mostly been swept up already, so these large impacts don't happen very often - fortunately.

Q Why are crocodiles scaly?

One of the main functions of crocodile scales is protection. This function is further reinforced by some of the scales (particularly those of the back) containing a deposit of bone ("osteoderm"); the heavily ossified scales along the back of a crocodile are commonly therefore referred to as the "armour".

Q What is myelin and why is it important to nerve cells?

In vertebrates, specialised cells called Schwann cells wrap themselves round the long thin bits of nerve cells in the peripheral nervous system. The Schwann cells form a thick insulating layer rich in lipids (a sort of fat) called the myelin sheath. This insulates the axon, rather like the plastic layer round a copper wire in an electrical flex. Nerve cells with myelin sheaths are called myelinated nerves.

Curiously, there is a type of mouse which has a genetic mutation that means that it has no myelinated nerves. Without the insulation that myelin provides, nerve impulses passing along one nerve cell also affect nearby nerve cells, some of which connect to muscles. The affected mouse shivers and makes jerking movements as its muscles are stimulated. People whose myelin sheaths are damaged as a result of multiple sclerosis have similar difficulties in controlling their muscles.

Q What is the biggest volcano?

The island of Hawaii is probably the largest volcano on Earth. The distance from its base (on the floor of the Pacific Ocean) to the summit of Mauna Kea (about 13,000ft high) is some 30,000ft - higher than Everest.

Q What causes hiccups?

Underneath your lungs is a large muscle called the diaphragm. When it contracts, it pulls down on the lungs, making you breathe in. Hiccups are caused by your diaphragm suddenly contracting, making you breathe in involuntarily - generating a peculiar sound.

Q How do seed banks store seeds?

Seeds of the "orthodox" kinds are dried down to about 15 per cent moisture content, and then stored in deep-freeze at -200C. (The temperature of liquid nitrogen is -196C.) The moisture has to be removed so the water doesn't damage the cell walls as it freezes, because it expands. Some kinds of seeds can be dried further and then stored in it, but structural changes - for example in seed coats - may occur.

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