Computer Chips Communicating With Light

As computer chips get faster and faster moving information around them is getting more difficult, as sending information very fast electrically uses lots of energy. One solution is to use light to communicate, you can transmit far more information with much lower energy losses - this is actually how information is moved long distances, between mobile phone masts, across the atlantic etc.
There is however a major problem, computer chips are made of silicon, but you can't make a laser out of silicon, instead of getting light out you just get heat. There are other semi-conductors which work well such as Gallium Arsenide, ore Indium Phosphide, but you can't grow them on a silicon wafer as the distance between the atoms is different, so it would be like tring to fit 2 lego bricks on top of each other, one 20% bigger than the other.
John Bowers in the university of California Santa Barbra may have the solution. Make some Indium phospide and some silicon increadibly smooth and clean, oxidise them a bit, then just push them together and they stick. This way you can use the right material for making the laser on a standard silicon chip. He ahs put 26 lasers on a chop but is hoping to scale it up to thousands, leading to faster computers.

23rd Sep 2006

A Strange Supernova is worrying cosmologists

A supernova is basically an exploding star, the explosion can be cause for a variety of reasons. A very important type is 1a, this is caused where an old star ember called a white dwarf is slowlyy aquiring matter from another star (probably a red giant), eventually it gets so heavy that it can't support itself and it collapses. In this collapse carbon and oxygen start to fuse together creating heavier elements ( almost all the heavy elements in the universe were made in supernovae), this releases an immense amount of energy and blows the star apart creating an explosion 5 billion times brighter than the sun.
  Because the point at which the white dwarf collapses should always be about the same, the explosion should always have about the same amount of fuel so it should be the same brightness.

  This makes typoe 1a supernovae increadibly useful as a way of judging distance because the further away something is the dimmer it appears, so if you know the brightness you can work out the distance. Cosmologists have used this to work out how far objects are, and with the red-shift which tells them how fast they are moving they have deduced all sorts of things about dark energy and the shape of the universe.

  Unfortunately Andrew Howell has found what appears to be a type 1a supernova that is 2.2times brighter than the model says it should be possible to be. Maybe the star was spinning fast which allowed more matter to build up before it collapsed. This could mean that astonomer's have observed lots of supernovae that are actually at lot further away than was calculated and cosmolgists are going to have to look at their data, and possibly their conclusions very carefully.

23rd Sep 2006

Science Update - the Sharpest and the Fastest.

Chelsea Wald and Bob Hirshon from AAAS, the science society

Bob - This week on Science Update, we'll talk about some ants that have set the world record for fastest-moving body part. But first, Chelsea reports on another world record - for sharpest object.

Chelsea - The tip of the world's sharpest needle is a single atom of tungsten. I asked physicist Robert Wolkow of the University of Alberta what it would be like to hold it in my hand.

Robert - It would be just like a sewing needle or a pin and you would see that it was very sharp but you wouldn't be able to see the end of it-it's so small it's invisible. And if you put it under the most powerful microscope in the world, you would only then just barely see the tip of it.

Chelsea - Wolkow says they make these needles by exposing a normal needle made of tungsten to nitrogen gas and electricity. The gas and electricity interact with the end of the needle to pluck off atoms until there's only one left.

Robert - It's kind of like sculpting. In a sculpture the final shape is already in the block of stone and the sculpture knows what to take away. Well, we're not sculptors, but we take away the atoms form the edge, leaving a tiny, tiny needle.

Chelsea - Since it's so sharp, Wolkow says the needle could prove to be the best probe ever made for use in powerful electron microscopes.

Bob - Thanks, Chelsea. A type of insect called the Trap - jaw ant has jaws that now hold the record for the fastest - moving self-propelled body part in the animal kingdom. But it's what they do with those super-speed jaws that's really interesting, according to University of Illinois entomologist Andy Suarez.

Andy - These ants will use their mandibles not only to capture food, but to propel themselves off the ground, to escape threat or predators.

Bob - Using a high-speed imaging system, Suarez found that the ants cock their jaws open against the ground and then snap them shut at close to 100 miles per hour. That's enough force for some pretty impressive flips and serious hangtime.

Andy - In the field when they start jumping around, it's just a fraction of a second-they just pop up in the air and they're on the ground again. And when you can slow this down and dissect the movement, the kinematics, of what's going on, it's really quite beautiful-it's very acrobatic.

Bob - Suarez says the ants probably first evolved the fast jaws to capture prey, but then repurposed the skill to escape becoming prey.

Chelsea - Thanks, Bob. That's all for this week. Next week we'll discuss a new estimate of how many types of dinosaurs are still waiting to be dug up. Until then, I'm Chelsea Wald.

Bob - And I'm Bob Hirshon, for AAAS, The Science Society. Back to you, Naked Scientists…

September 2006

The Science of Catalysts And Catalytic Converters

Dr Emma Schofield, Johnson Matthey Technology Centre

 

First of all, everyone uses the word 'catalytic converter' but what is a catalyst and why is it important? -

A catalyst is a substance that makes a chemical reaction happen more easily. You can get some really stroppy reactions in which you want to rearrange the atoms in them to make something useful, but it's just not playing. The starting material just isn't interested into becoming the product that you want. IF you put the right catalyst into the reaction, you can make this reaction happen either more quickly or using a lot less energy. Often you have to go into high temperatures and pressures to get the reaction to work. -

How do they do that? -

Imagine that you've got yourself a beach cottage and the beach is about a mile away from your cottage. Between you and the beach there is this massive great mountain. You have several options for reaching the beach. One of them is that you put lots of energy into it, so you put lots of energy to walk up the mountain and down the other side. Not great. The second option is to walk all the way around the mountain but it takes a heck of a long time. But if someone's gone and dug a tunnel from one side to the other then you can get to the beach pretty quickly and with a lot less energy. This is exactly what a catalyst does. So the catalyst does for a chemical reaction what the tunnel does for you: it takes an alternative pathway that allows the reaction to happen a lot more easily. -

And just like a tunnel, it's not used up in the reaction. It's available forever if you like. -

Yeah and that's why you only need a very small amount of catalyst when you have a chemical reaction. Because although the catalyst is changed during the reaction, it's regenerated at the end of it. So each little atom or each little molecule of catalyst that's in there can go on a react with hundreds and thousands of millions of reactant molecules. -

Sounds fantastic, but how do we find these things? Why isn't there a catalyst for everything? Why isn't there a catalyst for my homework? How do we discover the chemicals that do these clever jobs? -

There are quite a lot of metals that are used as catalysts because there are two different types of catalysts: homogeneous catalysts and heterogeneous catalysts. In homogeneous catalysts, the reactants and the products are in the same phase. So if there are gases reacting, the catalyst will also be a gas. In heterogeneous reaction, the reagents are in a different phase from the catalyst. An example of homogeneous would be making plastic bags - high density polyethylene. The ethene and the catalyst are all going on in the same phase, in solution. That would be a homogeneous catalyst. In a heterogeneous catalyst, that would be carbon monoxide turning into carbon dioxide. That would happen on platinum metal. -

They've got expensive tastes these things! -

Platinum is very expensive but you often find that some of the most expensive metals turn out to be the best catalysts. -

Why is platinum so good? What's special about the metal? How does it do what it does? -

When people ask questions like this, a scientist's usual answer is: oh well, it's quantum. Actually it's to do with how well these gas molecules can stick to the surface. Platinum is very good at sticking molecules onto the surface of it. That's very important because that's where the reaction actually happens. The other thing that platinum is good for is, well, when you have a molecule, the atoms in the molecule are stuck together with chemical bonds, which are electrons. Platinum is very good at rearranging these electrons and allowing the molecules to turn into something else. Again, forming this alternative pathway by which a chemical reaction can happen. -

So if you could zoom in to the surface of the platinum, what would it look like to make it so sticky and that things like it? -

We always imagine it as lots of little balls stuck next to each other. One of the aims of being a catalyst chemist, which is what I am, is to try and make as much surface as possible. So we have our tiny little pieces of platinum which are stuck onto a ceramic support. We want as much platinum on the surface and as little platinum in the middle of these balls as possible. The platinum is part of the periodic table which has lots of d-orbitals, and it's these magic d-orbitals that makes it so good at catalysis and making things stick to it. So it very easily forms bonds with lots of different types of molecule. -

And it brings them together in just the right way that they want to get married or do whatever you want them to do. -

And provides them with a route which requires so little energy that it can happen essentially spontaneously or with very little energy on the metal's surface. -

Ok so turning now to what comes out of your exhaust pipe, how does a catalytic converter on a car actually work? What are they doing? -

The catalyst on a catalytic converter is essentially a can which is next to the engine. What it does is it purifies the exhaust gases. If it was ideal, we'd just get carbon dioxide and water out when fuel was burnt. -

I'm sure people would argue that it would be ideal if we just had water and burning hydrogen, which is what Fraser is going to be talking to us about in a minute. -

The problem being that you put fuel in at the beginning and you can destroy atoms as you go along. But what we also get out is carbon monoxide, which is a poison and binds so strongly to the blood that you can't bind oxygen anymore. There are also what's called NOx gases, which are oxides of nitrogen responsible for acid rain; and hydrocarbons which come together with NO-x to form smog. This is why in the 1970s Los Angeles got buried under this cloud of photochemical smog and what triggered all of the legislation about car pollutants. Also what comes out are particulates, which are essentially soot. This is linked with respiratory illnesses as well as cancer. So we obviously don't want these coming out of the backs of our cars. We need to put the catalytic converter between the engine and the exhaust pipe to catch these things as they go out. Inside the catalytic converter we have the monolith and the metal. The monolith is a ceramic and it's a honeycomb with a very large surface area and it's coated to give it an even greater surface area. If you spread it out it would cover about three football pitches. On the channels of this monolith you have little globules of the metal platinum, palladium, rhodium in various mixtures depending on whether you're a petrol or a diesel car, and these are so small that we call them nanoparticles. This is what we were talking about before. As these gases go past, which is a very quick reaction, it goes from the engine through the catalytic converter in less than a tenth of a second. -

So it must be very fast. -

Yes and it wouldn't happen normally unless there was a catalyst there. -

So how much of the gases does the catalytic converter scavenge or convert? Does it do the lot? -

It causes about a 90% decrease in the amount of pollution coming out, and what you mostly get out of the other end is nitrogen, water and carbon dioxide. -

So it does a good job but there was a motivation for people to stop using leaded fuel because it makes your brain rot and causes dementia but also lead's quite toxic to catalysts. -

Exactly. If you think about these little metal particles, the lead will stick onto the surface of them. The more you reduce the amount of surface there is, the less chance these pollutant gases have of sticking to the surface and making the catalyst catalyse. -

So it's better to do without lead if we can for more reason than one. -

It's better to do without lead and it's better to do without sulphur in petrol too, because sulphur is responsible, or used to be responsible when we had high sulphur petrol, for this eggy smell some people associate with catalytic converters. Now there's less sulphur in fuel, this is much less of a problem. -

Now Emma, it's impossible to miss your t-shirt and on the subject of noisy engines, I was wondering if this was what we're talking about! It says NOISE. What is NOISE and why are you here today? -

NOISE is the New Outlooks In Science and Engineering campaign. It's a group of young scientists who are there to give an alternative image for what scientists are like. Chris, when you think about the stereotype of a scientist, what is it that springs to mind? -

Glasses more powerful than the Hubble space telescope, shocking teeth, 1960s get-up and muttering unintelligibly in a way that no-one can understand. -

And the words fun and dynamic don't really feature in those descriptions. -

But that's why people listen to the Naked Scientists! -

And that's why NOISE is there. We need to change this. We're the new generation of young scientists and we have a website www.noisemakers.org.uk where there's this whole group of scientists that do lots of fun science that we want to tell people about. We have a snowboarding physicist and we have somebody who does robotics who is a scuba diver. The idea is to point out to people, especially kids who are thinking of going to university, that there's more to being a scientist than a white coat. -

September 2006

Hydrogen

A FUEL FOR THE FUTURE? - Professor Fraser Armstrong, Oxford University

Dave - With us this evening we have Fraser Armstrong from Oxford University. You work on fuel cells - can you explain what a hydrogen fuel cell is?

Fraser - Well first of all hydrogen is a energy carrier much like petrol or any sort of oil or coal. But it's a very different type of energy carrier because it's a gas, and it's not necessarily a very convenient energy carrier because hydrogen is a gas all the way down to something like 20 Kelvin. This is about minus 250 degrees Celsius. So it's not a very convenient fuel, but when combined with oxygen, hydrogen and oxygen make a bang together and give off water.

Chris - The people on the Hindenburg knew a bit about that.

Fraser - Well they did unfortunately, yes. Hydrogen is a very good and light fuel; that's why it's used in spacecraft even if it's not very useful in terms of being able to transport it efficiently.

Chris - Just as aside on the Hindenburg disaster, it was actually a bit of a myth that it was the hydrogen, although that didn't help. When the Germans built it they thought it looked nice as a silver colour because it showed up the Nazi swastika very nicely. In order to get that colour they sprayed it with aluminium, and aluminium particles burn beautifully.

Fraser - That was a bad choice.

Dave - So what does an actual fuel cell look like?

Fraser - Well a fuel cell consists of two electrodes: one on which hydrogen is oxidised to protons. Of course, as we've just heard from Emma, this needs a catalyst and the catalyst in this case is generally platinum or platinum with other precious metals. Hydrogen is oxidised to protons and at the other electrode, oxygen is reduced to oxide. The oxide and the protons combine to form water. We find that we have a large amount of energy produced from this, and it's the same amount of energy as would be produced if we deliberately burned hydrogen and oxygen and got an explosion. Now the energy is converted directly into electricity, which can be used to power devices.

Dave - I guess there's a problem if the hydrogen gets on the wrong side and the hydrogen gets on the wrong side. How do you normally solve that problem?

Fraser - Well normally the anode and the cathode as the two electrodes are called, are separated by a membrane called the proton exchange membrane. Hydrogen is directed at one of the electrodes and air is directed at the other electrode. Generally there is very little in the way of cross-over, which is the mixing of gases.

Chris - Since this show is about catalysts, I've got to ask, what is the catalyst that's doing this in your fuel cells?

Fraser - In the conventional fuel cell, which is called the proton exchange membrane fuel cell, the catalyst is platinum, as we heard from Emma. My research group is investigating the possibility of other types of catalysts for this type of technology, particularly ones that are based on enzymes that occur in microbes. These particular enzymes do not of course contain platinum at their active centre but contain other elements that are much more familiar: in particular iron and most often nickel as well.

Dave - I guess that this is a big advantage because if you powered all the cars with platinum fuel cells you'd run out of platinum quite quickly.

Fraser - Well either we'd run out or the price would go up and up. There's always a good point to having catalysts that are on the market.

Chris - So why do bacteria need to be able to do this with hydrogen? Why do we need to do that?

Fraser - Very interestingly the bacteria have used hydrogen as a fuel for over 2.5 billion years. If we go back in time to the earliest life forms, at the particular time there was no oxygen on the Earth and many microbes would use the proton as an oxidant. Of course, when one reduces a proton, we obtain hydrogen. So many bacteria have the ability to make hydrogen from protons, that is, from water. Equally, other bacteria have the ability to use hydrogen as a fuel. So there's a kind of cycling that's possible in the microbial world.

Chris - Is it possible for us to co-opt this efficiently enough to run our cars though?

Fraser - No I don't see this ever running cars because as it stands at the moment, the problem with enzymes is that they're not designed to last forever and they're not designed to withstand very high temperatures and reaction conditions. However, we can learn a considerable amount by studying the active sites of the enzymes and the molecular structure.

Chris - In other words the business end that does the catalysis.

Fraser - Yes the business end at which catalysis occurs.

Chris - And what, you'd hope to make a model of that or reproduce that more stabley?

Fraser - For the purposes of high energy orhigh power, it may be possible in the future to make catalysts which are alternatives to platinum that use the chemistry of the active sites of enzymes as we currently understand them. It may also be possible to actually use enzymes themselves for power production, which is much less demanding than the automotive industry.

Dave - So the advantage of your design with the enzymes is that you don't need to keep the oxygen separate any more.

Fraser - In principle that may be quite correct. It is possible to mix hydrogen and oxygen to get non-explosive mixtures. However, the amount of hydrogen that one requires for this is less than 4% in air in order to avoid hazardous mixtures.

September 2006

	On a recent trip to Sweden I noticed at the check in desk that my mobile battery was running low and was just about to turn off. As I went through airport security, my mobile was sent through the x-ray machine. When I next looked at my phone there was almost a full charge. I was wondering if you had any ideas as to why this might be so? Adamski via email
	There are a couple of reasons but it's unlikely that it's anything to do with the x-ray machine itself, but more to do with the phone being turned off. One explanation might involve it being cold when you went into the airport. Batteries work by a chemical reaction going on inside. When they get cold, the chemical reaction slows down, which means that they can't produce as much current. This makes the battery appear to be a lot emptier than it is. This is why you sometimes get a flat battery on a cold day. If you have a battery and you've been using it a lot, it will drain quickly and the voltage will go down. If you turn it off, that voltage will slowly build up as long as you're not drawing a current. This means that when Adamski turned his mobile phone back on, it was full.
	September 2006

	I'm home schooling my son and we've been studying cell division. In one of the textbooks it - that specialised cells, such as muscle cells, don't divide. Why? Richard via email
	That's a very good question and it's the question that's been frustrating stem cell biologists and people who want to fix the human body. It's for exactly the same reason as when you have a stroke, part of your brain gets damaged forever and doesn't really recover very well. It's also the reason that when you have a heart attack, part of the muscle dies and doesn't recover. When an egg is fertilised it turns from being one cell and divides into many more cells. What these cells do is to specialise. As they divide and turn into tissues, they specialise and sub-specialise for the job that they're going to do. It's a bit like when you go into medical school: at the start you can become any kind of doctor. After you go through lots of training, you might end up as a surgeon but you don't really do much medicine anymore. The next thing you do is become, say, a specialist in vascular surgery and only work on that. Then you become so good that you only do surgery on people's aneurisms. That's the kind of specialisation that happens in our bodies as we develop. This means that the tissues in our bodies are specialised to do one kind of job and that means that they've turned off the genes that make them a general cell and they lock onto being one kind of cell. That's a process called differentiation, and it seems in some tissues to be an irreversible step. But now we're beginning to see that it might be possible to persuade cells to go back the other way with the right environment. The other way is to go back and get a stem cell that can turn into anything.
	September 2006

	Take a helium balloon and hold it on a piece of string inside a car. Watch what happens to the balloon when the car breaks - it moves backwards. So it goes the opposite way to other objects in the car. What's happening here? Graham in London
	When you do an emergency stop, the car is accelerating everything in it backwards. This means that everything relative to the car rushes forwards. The balloon is obviously floating in the air. But if it's a helium balloon, the air is heavier than the balloon itself. This means that the air wants to rush forwards more than the helium balloon is going to want to rush forward. The air will then push the helium balloon out of the way, making it move backwards.
	September 2006

	Why do cars smell like rotten eggs after a while? Is this down to catalytic converters? Andy on the A120
	This is not exactly a myth but is something that's becoming less of an issue nowadays. The reason that that happens is that there is sulphur in fuel and when sulphur burns in oxygen it forms sulphur dioxide. This only really happens in a petrol engine, which can operate under fuel-rich or fuel-lean conditions. In fuel-lean conditions there's quite a lot of oxygen so the sulphur in the fuel gets oxidised to sulphates. This is a real pain because this clogs up the catalyst and builds up on the surface of the monolith. But when you start a car, this is the point when you have fuel-rich conditions. This is going to convert the sulphate that has built up on the catalyst into H2S, which is hydrogen sulphide. That's the kind of eggy smell that you get out. But this is also why we're getting a lot more low-sulphur fuels nowadays and to be honest it's a lot less of a problem than it used to be.
	September 2006

	Why does compressed natural gas, or methane, burn cleaner than regular fuel? It's still a hydrocarbon. Why shouldn't it combust into the same pollutants as everything else? Peter via email
	There are two sides to this and that's the amount of carbon monoxide, which is what we call CO, and also the amount of soot which is much closer to pure carbon. The amount that's formed depends on the carbon to oxygen ratio in the mixture. Coal, for example, and many long-chain hydrocarbons have a much higher carbon content and it's necessary to have a much higher oxygen content in the mixture in order to avoid the formation of what we call lower oxides or soot. So if there's lots of carbon there it's much easier to get something wrong and not have enough oxygen to take it all the way to CO2.
	September 2006

	I'd like to know whether it's possible to change your hair colour without chemicals, such as with gene modification. It would save me a fortune! Are there any catalysts on the market that would catalyse a platinum blond colour?! Karen in Bamford
	The chemical that you usually dye or bleach your hair with is hydrogen peroxide. Now interestingly, that's a typical reaction that you do in school. Hydrogen peroxide decomposes into oxygen very very slowly, but if you add something like potassium iodide you can make this happen a lot more quickly. There's a fantastic experiment you can do where you put some hydrogen peroxide (also found in contact lens fluid) and add a little bit of potassium iodide and bubbles. The whole thing comes sweeping out of the top in this big fountain of bubbles. As for trying to dye your hair without a catalyst, I'd go for the catalyst way myself!
	September 2006

	I was curious about the scaremongering we're hearing about the ice caps melting and flooding everywhere. I thought the Archimedes' principle was that a body immersed in water, given that the majority of icebergs are immersed in water, when the ice melts it will only displace its own weight in water. Pom in Kent
	I understand the point you're making and it's a good one. You're half right and half not right and the reason is as follows: the north pole is ice that's already floating. If that melts, you're quite right, it's never going to overflow because when it melts it will displace an equal volume of water as itself. As it's made of water it will just turn into water and never overflow. But here's the spanner in the works: Greenland for example and Antarctica are continents and there's land under there. The ice is not under water but sitting on land, so if that melts it's going to raise sea levels drastically. In fact there's enough ice locked up in Antarctica and Greenland to give us about 7 - 70 metres of sea level rise. There's a paper that was published in this week's edition of the journal Nature in which scientists have used two satellites in space. It's called GRACE but should be called BRACE because they work in concert with each other, and they work out how much mass there is on Earth underneath them by working out how fast one is accelerating because of gravity compared with the other one. What they've done is to watch Greenland for the last two years and they've found that Greenland has lost 248 cubic kilometres, plus or minus about 60, of water every single year in the past two to four years. It's gone up 250%, so it's a scary amount. That is enough to raise sea levels every single year by about 0.5 millimetres. That's Greenland on its own in one year. So if things really do take off with global warming, we're in trouble.
	September 2006

	Is there a lost formula for using water as our main fuel? Do you guys know anything about this? Ken in Forrington
	Water itself cannot be a chemical fuel because it is the end product of a very favourable chemical reaction. However the vast amount of water on this planet, which is of course comprised of hydrogen and oxygen, can be split into hydrogen and oxygen if we apply large amounts of primary energy such as solar energy or even nuclear. From this type of reaction may come our salvation in the future when we no longer have any fossil fuels to use. Hydrogen is not a primary fuel but an energy carrier or storable fuel that can be obtained by energising water with a primary fuel, which causes it to form hydrogen and oxygen.
	September 2006

	Is the caloric content of food when it is stated on the packet the amount of energy given off when it's burned? Does it take into account the amount of energy a human can extract from it by digesting it with things like enzymes? If I were to eat two donuts at once, would I be taking in twice the number of calories and twice the fat as if I'd just eaten one? If I ate ten at once could I really process them as efficiently as if I'd just eaten one? Simon via email
	The calorie content, which is actually kilocalories, is the energy that would be obtained in an experiment were that food to be burned in an excess of oxygen.
	September 2006

	Why is it that some people are so tolerant to alcohol whereas other people just need to get a whiff to tip them over the edge? Joshua in Atlanta
	That's to do with an enzyme called alcohol dehydrogenase which contains a zinc atom. Different amounts of this enzyme would be expressed in people who have different genetic make up. This is one part of the answer. If you are a regular user you boost the enzymes in the liver to increase the numbers so you break these things down a little bit more.
	September 2006

White bread and the wonder of enzymes

Use enzymes in your mouth to break down bread in front of your very taste buds.

What you need

A slice of cheap white bread.

 

A mouth to put it in.

What to Do

1. Take half a slice of bread and chew it and chew it and chew it! Even if the bread becomes disgusting, you should still keep chewing amd remember - don't swallow or you'll spoil the experiment!
2. Pay attention to how the flavour of the bread changes.
3. Once you think you have the answer, you can swallow the bready mush or (this is probably best!) spit it out.

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What may Happen

As you chew the bread, you may have noticed that it slowly tastes sweeter. Particularly the juice that is coming out.

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What is going on?

Foods like bread, rice and potatoes are foods that are mostly made of starch. Starch is a molecule that plants make in order to store sugar. It consists of long chains of glucose (some of them branch).

 

In your saliva you have an enzyme called amylase, this will cut up the large starch molecules at random until you are left with sugars made of 1,2 or 3 glucose molecules long (glucose, maltose or dextrin).

 

 

Starch doesn't taste sweet, but glucose and maltose do, so as you chew the bread it will slowly become slightly sickly sweet.

Why do plants make starch in the first place?

All living things are made up of cells these are surrounded with membranes which will let water pass through them but not larger molecules such as salt or sugar.

If there are more salt or sugar molecules on one side of the membrane than the other, water is pulled towards the salt/sugar - this is called osmosis.  This is why slugs shrivel up if you sprinkle salt on them, all the water is sucked out of them.

Plants convert water and carbon-dioxide into glucose using the energy from the sun.  If the plant is doing particularly well it will produce huge amounts of glucose in it's cells.  This will cause water to be pulled into the cell.  The problem is that this will happen until the cell explodes.

The solution is for the plant to glue all the sugar molecules together into a starch molecule, which just being one molecule rather than thousands will suck in water much less by osmosis, reducing the problem hugely.

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