The passage given below is followed by a question. Choose the best answer.
Cells are ultimate multitaskers: they can switch on genes and carry out their orders, talk to each other, divide into two, and much more, all at the same time. But they could not do any of these tricks without a power source to generate movement. The inside of a cell bustles with more traffic than Delhi roads, and, like all vehicles, the cell’s moving parts need engines. Physicists and biologists have looked “under the hood” of the cell – and laid out the nuts and bolts on molecular engines.
The ability of such engines to convert chemical energy into motion is the envy of nanotechnology researchers looking for ways to power molecule-sized devices. Medical researchers also want to understand how these engines works. because these molecules are essential for cell division, scientists hope to shut down the rampant growth of cancer cells by deactivating certain motors. Improving motor-driven transport in nerve cells may also be helpful for treating diseases such as Alzheimer’s, Parkinson’s or ALS, also known as Lou Gehrig’s disease.
We wouldn’t make it far in life without motor: proteins. Our muscles wouldn’t contract. We couldn’t grow, because the growth process requires cells to duplicate their machinery and pull the copies apart. And our genes could be silent without the services of messenger RNA, which carries genetic instructions over to the cell’s protein-making factories. The movements that make these cellular activities possible occur along a complex network of threadlike fibers, or polymers, along which bundles of molecules travel like trams. The engines that power the cell’s freight are three families of protein, called myosin, kinesin and dynein. For fuel, these proteins burn molecules of ATP, which cells make when they break down the carbohydrates and fats from the foods we eat. The energy from burning ATP causes changes in the proteins’ shape that allow them to heave themselves on the polymer track. The results are impressive: In one second, the molecules can travel between 500 and 100 times their own diameter. If a car with a 5-foot-wide engine were efficient, it would travel 170 to 340 kmph.
Ronald Vale, a researcher at the Howard Hughes Medical Institute and the University of California at San Francisco, and Ronald Milligan of the Scripps Research Institute have realized a long-awaited goal by reconstructing the process by which mysom and kinesm move, almost down to the atom. The dynein motor, on the other hand, is still poorly understood. Myosin molecules, best known for their role in muscle contraction, from chains that lie between filaments of another protein called actin. Each myosin molecule has a tiny head that pokes out from the chain like oars from a canoe. Just as rowers propel their boat by stroking their oars through the water, the myosin molecules stick their heads into the actin and hoist themselves forward along the filament. While myosin moves along in short strokes, its cousin kinesin walks steadily along a different type of filament called a microtubule. Instead of using a projecting heads as a lever, kinesin walks on two “legs”. Based on these differences, researchers used to think that myosin and kinesin were virtually unrelated. But newly discovered similarities in the motors’ ATP-processing machinery now suggest that they share a common ancestor-molecule. At this point, scientists can only speculate as to what type of primitive cell-like structure this ancestor occupied as learned to burn ATP and use the energy t change share. “We will never really know, because we can’t dig up the mains of ancient proteins, but that was probably a big evolutionary leap, “ says Vale.
On a slightly larger scale, loner cells like sperm or infectious bacteria are prime movers that resolutely push their way through to other cells. As I., Mahadevan and Paul Matsudaira of the Massachusetts Institute of Technology explain, the engines in this case are springs or ratchets that are clusters of molecules, rather than single proteins like myosin and kinesin. Researchers don’t yet fully understand these engines fueling process or the details of how they move, but the result is a force to be reckoned with. For example, one such engine is a springlike stalk connecting a single-celled organism called a vorticellid down at speeds approaching 3 inches (8 centimeters) per second.
Springs like this are coiled bundles of filaments that expand or contract in response to chemical cues. A wave of positivity charged calcium ions, for example, neutralizes the negative charges that keep the filaments extended. Some sperm use springlike engines made of actin filaments to shoot out a barb that penetrates the layers that surround an egg. And certain viruses use a similar apparatus to shoot their DNA into the host’s cell. Ratchets are also useful for moving whole cells, including some other sperm and pathogens. These engines are filaments that simply grow at one end, attracting chemical building blocks from nearby. Because the other end is anchored in it place, the growing eng pushed against any barrier that gets in its way.
Both springs and ratchets are made up of small units that each move just slightly, but collectively produce a powerful movement. Ultimately, Mahadevan and Matsudaira hope to be better understand just how these particles create an effect that seems to be so much more than the sum of its parts. Might such an understanding provide an inspiration for ways to power artificial nano-sized devices in the future? “The short answer is absolutely,” says Mahadevan. Hopefully, studying these structures will not only improve our understanding of the biological world, it will also enable us to copy them, take apart their components and re-create them for other purposes.
Select the one which includes the statement(s) that are representative of an argument presented in the passage.
- Myosin, kinesin and actin are three types of protein
- Growth processes involve a routine in a cell that duplicated their machinery and pulls the copies apart.
- Myosin molecules can generate vibrations in muscles
- Ronald and Mahadevan are researchers at Massachesetts Institute of Technology
- a and b but not c and d
- b and c but not a
- b and d but not a and c
- a, b and c but not d