Answers to last issue's Do You Know?
1. What does a candle flame look like in space?
Ans: Fire and flames have fascinated human beings from ancient times. With the advent of science and technology we would expect that we know all about flames. Not really. Scientists at the International Space Station (ISS) in space got quite a surprise in 2013 when they saw something strange about flames on ISS.
Firstly, flames burn differently in space: they form little spheres. In an ordinary candle flame, thousands of chemical reactions take place. Hydrocarbon molecules from the wick are vaporised and cracked apart by heat. They combine with oxygen to produce light, heat, carbon dioxide and water. Some of the hydrocarbon fragments form ring-shaped molecules called polycyclic aromatic hydrocarbons and, eventually, soot. Soot particles can themselves burn or simply drift away as smoke. The familiar teardrop shape of the flame is an effect caused by gravity. Hot air rises and draws fresh cool air behind it. This is called buoyancy and is what makes the flame shoot up and flicker. 
Unlike flames on Earth, which expand greedily when they need more fuel, flame balls in space let the oxygen come to them. Oxygen and fuel combine in a narrow zone at the surface of the sphere, not throughout the flame. 
This is fine, but the ISS scientists saw something odd. They were learning how to put out fires in microgravity when they saw that even after the flames went out, small droplets of fuel (called heptane) continued burning. They seemed to be burning without flames!
Ordinary, visible fire burns at a high temperature between 1500K and 2000K. Heptane flame balls on the ISS started out in this "hot fire" regime. But as the flame balls cooled and began to go out, a different kind of burning took over. Cool flames burn at the relatively low temperature of 500K to 800K, and their chemistry is completely different. Normal flames produce soot, carbon dioxide and water. Cool flames produce carbon monoxide and formaldehyde. On the ISS, however, cool flames can burn for long minutes.
Scientists are now working on whether this can be used in car ignition to make it safer and cleaner.

2. How does water reach to the very top of tall trees?
Ans: Water reaches from root to leaves by a very interesting process called transpiration. Plants contain a vast network of conduits, or pipes, which consists of tissues called xylem and phloem tissues. While phloem carries food to different parts of the plant, xylem carries water.
Xylem tissues consist of different kinds of cells. The cells that conduct water (along with dissolved mineral nutrients) are long and narrow. While some cells have holes, all have pits in their cell walls through which water can pass. Water moves from one cell to the next when there is a pressure difference between the two. But there is a problem: these cells are dead and so they cannot be actively pumping water. So what is happening? 
What actually happens is suction within the water-conducting cells, and this arises from the evaporation of water molecules from the leaves. Each water molecule has both positive and negative electrically charged parts. As a result, water molecules tend to stick to one another. This "adhesion" is why water forms rounded droplets on a smooth surface and does not spread out into a completely flat film like substance. As one water molecule evaporates through a pore in a leaf, it exerts a small pull on water molecules nearby, reducing the pressure in the water-conducting cells of the leaf and drawing water from cells nearby. This chain of water molecules extends all the way from the leaves to the roots and even extends out from the roots into the soil. 
So the simple answer to the question is this. What propels water from the roots to the leaves is that the sun's energy does the job: heat from the sun causes the water to evaporate, setting the water chain in motion.

3. What is the use of air bags in cars?
Ans: An airbag is an inflatable safety device designed to protect the occupants of a car in case of a collision. During a crash occupants are thrown forward at great momentum and this can cause grave injury. That is the reason we wear seat belts, they act as a restraint. Air bags add another form of protection. In the event of an accident, the airbag fills up very quickly and provides a cushion for the people in the car to ensure they are protected during the crash.
There are crash sensors present in the front of the car which detect sudden decelerations and sends electrical signals to activate an initiator. They are designed not to trigger when the car goes over a pothole, a bump, or even in the case of minor collision. A thin wire provided in the initiator heats up and penetrates the propellant chamber of the car. This results in the chemical propellant inside the inflator to undergo a rapid chemical reaction. This reaction is often referred to as a pyrotechnic chain.
This reaction produces nitrogen gas that fills the air bag. This expanding gas inflates the airbag in less than one-twentieth of a second. This fast expansion opens up the plastic module cover and inflates it in front of the person seated in the car. The bag is inflated for just one-tenth of a second and deflated three-tenths of a second after impact. The inner side of the airbag is provided with a coating of cornstarch or talcum powder which is released from the bag as it is opened. All this mitigates the impact on the seated person.
All of these parts are interconnected and are powered by the battery in the car. A backup power is provided for the airbags to work even after the battery has been disconnected. Note that seat belts are still needed as the airbags generally offer protection only for front-end collisions. Rear-end collisions, secondary impacts and crashes are not covered by the airbags.
Sometimes the airbags open with such force that they can hurt people.  These days manufacturers are producing "smart" airbags to prevent this. Since it is rarely used, people are likely to forget to check the airbag system, and that is why cars come with an airbag light which indicates if it is in working condition.

4. How do viruses mutate?
Ans: We keep hearing about the SARS-CoV-2 virus mutating. In other words it is acquiring genetic changes. As a virus replicates, its genes undergo random “copying errors” and these are called genetic mutations. Over time, these genetic copying errors can, among other changes to the virus, lead to alterations in the virus’ surface proteins or "antigens". Let us see how this happens.
The template for viruses to reproduce is located in DNA (or sometimes
RNA). These give the instructions for the virus to replicate. DNA is a more stable molecule than RNA, and DNA viruses have a proofreading check as part of their reproductive process. They manage to use the host cell to verify viral DNA replication. If the virus makes a mistake in copying the DNA, the host cell can often correct the mistake. DNA viruses therefore do not change, or mutate, much. RNA, however, is an unstable molecule, and RNA viruses do not have a built-in proofreading step in their replication. Mistakes in copying RNA happen frequently, and the host cell does not correct these mistakes. RNA virus mutations are frequent and can have important consequences for their hosts.
Once a mutation occurs, if it changes the function of a resulting protein, a virus or organism is then changed. Because cells and viruses interact with the environment or surrounding cells, this change is either going to give the mutated cell or virus an advantage, allowing it to thrive more easily in its environment, or will make it disadvantaged, making it more difficult to survive. This is a process called natural selection. If the mutation confers an advantage, the mutated sequence then spreads within a population and if the mutation confers a disadvantage, the mutated sequence dies out. 
Viruses typically mutate more rapidly than human cells do. This is because human cells have mechanisms to "proofread" the genome while copying and also mechanisms to repair a sequence if an error is detected. Mutations can vary in severity from having zero consequence to majorly altering a protein and its function. Mutations can involve the substitution of one DNA base to another. Or mutations can involve the insertion of additional DNA bases or the deletion of existing DNA bases. 
Just as natural selection has shaped the evolution of humans, plants, and all living things on the planet, natural selection shapes viruses too. Viruses are not technically living since they need a host organism to reproduce, but they are also subject to evolutionary pressures.
The human immune system uses many tactics to fight viruses. The virus’s job is to evade the immune system, create more copies of itself, and spread to other hosts. Characteristics that help a virus do its job tend to be kept from generation to generation. Characteristics that make it difficult for the virus to spread to another host tend to be lost.
Take, for example, a virus with a mutation that makes it particularly deadly to its human host and kills the host within a few hours of infection. The virus needs a new, healthy host for its descendants to survive. If it kills its host before the host infects others, that mutation will disappear.
One way hosts protect themselves from a virus is to develop antibodies to it. Antibodies lock onto the outer surface proteins of a virus and prevent it from entering host cells. A virus that appears different from other viruses that have infected the host has an advantage: the host has no pre-existing immunity, in the form of antibodies, to that virus. Many viral adaptations involve changes to the virus's outer surface.
Does all this also happen with SARS-CoV-2? From what has been observed thus far regarding the genetic evolution of SARS-CoV-2, it appears that the virus is mutating relatively slowly as compared to other RNA viruses. Studies to date estimate that the novel coronavirus mutates at a rate approximately four times slower than the influenza virus, also known as the seasonal flu virus. 
Sources:
science.nasa.gov, scientificamerican.com, National Highway Traffic Safety Administration USA, cdc.gov