Energy in Cells - Chapter 4

- Hello, my name's Lindsay Turnbull and I teach biology at the University of Oxford. In this video, I want to show you how cells have to do battle with the laws of physics in order to get the energy they need. This battle goes on in every cell, every day on our planet and it forms the key concept in chapter four of my book "Biology: The Whole Story."

(birds chirping) (frog croaking)

It's easy to think of life as truly miraculous, but there's no magic in life. It has to obey the rules. All matter, in fact, is bound by something called the laws of thermodynamics. The first law is quite easy to understand. It simply says that matter can neither be created nor destroyed so life has become brilliant at recycling. The second law though states that entropy must always increase and that's a bit harder to get your head around.

So entropy is a measure of disorder, but if disorder has always got to increase, then how is it that life can go about creating order? Because it clearly does that, cells, for example joined together smaller molecules to make larger ones. And when they have enough of those large molecules, then they can make another cell.

Well, the way they can do that is that they've got to create a lot more disorder somewhere else and that means they have to invest a lot of energy in the process of the making and the building. And by doing that, that tends to create disorder elsewhere.

So in order to get the energy they need, cells are going to have to harness chemical reactions. Now, in order to understand chemical reactions, we need to look inside atoms. And if you remember, an atom has quite a simple structure. In the centre is a nucleus that contains some positively charged protons while orbiting around, there are negatively charged electrons, and those electrons are housed in what are sometimes called shells.

So let's start by having a look at a couple of atoms that are very unreactive. The first is helium. Helium has just two electrons. And because the first outer shell that atoms can have only holds two electrons, the outer shell of helium is full. Now, let's take a look at neon. Neon has 10 electrons so it has two in the first shell and then the second shell, which in this case is the outer shell, is also full because that shell will only hold eight electrons.

And neon's got eight electrons in there. Now, helium and neon are both called noble gases. And noble gases are notoriously unreactive because electrons like being in a shell that's full and that means that they are in a low energy state. So now let's contrast that with two different atoms. So the first is hydrogen. Now, hydrogen only has a single electron and that means that that electron is on its own in a shell which does have room for two electrons.

And that means that that electron is unhappy and in a high state of energy. And this atom is oxygen, superficially a bit like neon, but it only has 8 electrons rather than 10. So that means in the outer shell, there are six electrons when there could be eight. So both hydrogen and oxygen are electron hungry. The good news for both of them is that they could get together and form a new molecule and share their electrons. And by doing that, they kind of get a full outer shell.

So both the hydrogen atoms in this molecule have a share of two electrons and the oxygen atom has a share of eight electrons. And that makes this molecule very stable indeed. And it's called water.

Now, maybe it's hard to imagine that combining hydrogen with oxygen to make water is really gonna set the world on fire. But another name for liquid hydrogen is rocket fuel. In July, 1969, three men were sitting atop a Saturn V rocket taller than Big Ben and three times heavier than the London Eye.

This was the biggest rocket that the world had ever seen, and 85% of its weight was just liquid hydrogen and liquid oxygen, the fuel that was going to push against the Earth's gravity and get that rocket off the ground and on its way to the moon. When the fuse was lit, the oxygen and the hydrogen atoms started to combine, brought together by those unhappy electrons who wanted to form water, that stable molecule. And as they were doing that, the electrons shed all of that excess energy.

And cells have worked out how to do exactly the same thing.

So we've just seen that this reaction of hydrogen and oxygen to form water is an explosive reaction releasing a large amount of energy. But most of the reactions inside the cell only need a tiny bit of energy. So the cell actually has a system of batteries which can just deliver very small amounts of energy, and it recharges those batteries using this explosive reaction. So let's go to the membrane where this reaction is taking place.

And the first thing we see are turbines embedded in the membrane. And as the turbines turn, these little batteries are recharged. Now, these are not real batteries, actually a little molecule called ATP. Now in our world, turbines are turned by the flow of particles. So we might either have a wind turbine or we might have a water driven turbine. The cell's turbines are driven by a flow of charged particles called protons. So protons flow through the turbine.

And if we look, we can see on one side of the membrane that we've got a whole pile of protons piled up there. And we have to ask ourselves, well, how did they get there? Because piling up protons together like that is creating a kind of order. And we know that in order for cells to create order, they've got to invest a lot of energy. So if we move a bit further along the membrane, we see some other structures embedded in it.

And these look like pistons and they are driving protons across the membrane and they need energy in order to do that. So if we look back, we see that the cell is shuttling hydrogen to the membrane. Those hydrogen atoms consist of a proton and this excited electron. And when that electron looks across the pistons what does it see but a waiting oxygen atom.

And so that electron leaps across, and the cell makes sure that as the energy is shed from that electron, it's used to drive the pistons and drive the protons across the membrane. And then as the protons return through the turbine, they can combine with those spent electrons and the oxygen to form water. Now, you might reasonably be asking,

but where does a cell get hydrogen from? There's no hydrogen in our atmosphere. Well, it has to get that hydrogen from inside other molecules like glucose. Glucose consists of carbon and oxygen and hydrogen. So the cell can dismember glucose and release the hydrogen and shuttle that off to the membrane. And the carbon atoms just end up combining with oxygen and being released as carbon dioxide.

And so these two processes, the dismemberment of glucose and then this process at the membrane with the turbines together form aerobic respiration. And we write that equation as glucose plus oxygen gives carbon dioxide and water and a huge amount of energy, and it's absolutely dependent on having an oxygen rich atmosphere. Without oxygen, you wouldn't get anything like the amount of energy out of that reaction as you can when oxygen is present.

So the oxygen-rich biosphere has allowed active animals to evolve, things that can run and jump and swim and fly.

You know, animals are selfish creatures. They're guzzling glucose and sucking down oxygen without a thought for where it's all coming from. But animals are lucky that there are other things on this planet that have chemical tricks that they do not possess. If you wanna make glucose and oxygen, then one of the options is to reverse that reaction that we've just seen.

You can reverse any chemical reaction, but the problem is if you get a lot of energy out one way, then when you reverse it, you're gonna have to put a lot of energy in. So the reverse reaction is carbon dioxide plus water makes glucose and oxygen. And you might recognise that. It's something called photosynthesis. And photosynthesis, the photo part in that word means the energy that you're gonna get to make that reaction happen is gonna come from the sun.

And that is another fiendishly complex operation that was invented by organisms called cyanobacteria. So they worked out how to capture the sun's energy and the energy is actually used to smash up those water molecules. Remember, where hydrogen and oxygen are both very happy. You're gonna smash them to bits, get the hydrogen out, attach it to carbon dioxide, and bingo, you've got glucose. And actually that's what the cyanobacteria want. The oxygen's just a waste product.

Photosynthesis evolved first on our planet around two to three billion years ago and that oxygen started to trickle into the atmosphere. Now, of course, you might think, but it's not cyanobacteria that do photosynthesis today. Isn't it plants? Well, it's both. Plants have their own story and we'll get to it later, but we certainly rely on the combination of plants and cyanobacteria to generate that oxygen we need.

So next time you are passing a gorgeous green thing, maybe show it a little more love than you might normally do.

Thanks for watching this episode. I hope you've enjoyed it. And if you have, please share. There's a lot more detail about how cells dismember glucose and the details of photosynthesis in chapter four of the book and the link's below if you're interested in buying it. If you wanna hang on for the next episode, that will be coming soon. And that'll be based on chapter five which is all about bacteria. (birds chirping)