Sept. 19, 2021

Episode 5 - Photosynthesis

Welcome to Untangling Science, a podcast about science that is for everyone, with me Darragh Ennis. You probably know me as The Menace from ITVs quiz show The Chase, but my day job is as a scientist at the University of Oxford. In this podcast I want to bring the world of science to people who think it’s too complicated to understand in a way that is fun and straightforward. We have a website, and you can follow us on Twitter @untanglings . I have a blog on the website that I leave useful information, links and diagrams from each episode in, so check that out. This time we will be talking about the most important chemical reaction for life on earth, photosynthesis.


As usual we will try to answer some important questions along the way; What exactly is photosynthesis? How does it work? When did it first start? How does it impact on the planet as a whole? What would happen if photosynthesis just stopped? Once we’ve answered these questions I’ll do a little bit on some recent research in the field to finish things off. I hope that sounds good to you all, so let’s get started with the first question, what exactly is photosynthesis.


As we discussed in the last episode, a lot of what happens on our planet gets its energy from the Sun. This is especially true of the things on our planet that are alive. One of the fundamental laws of physics is that energy cannot be created, it has to come from some source. So if you have a petrol car, the energy comes from burning the fuel you put into the tank. And that energy came from the living things that over millions of years were turned into oil. And those living things at some point got their energy either by eating other living things or they used sunlight to make something they can consume by photosynthesis. So even the internal combustion engine relies on this process. But I’m not answering the question! What exactly is it. Well, in a nutshell it is a chemical reaction that plants perform to turn a carbon source, usually carbon dioxide, and water into sugar. And as this takes some energy to do they use solar power to do it. So in an extremely indirect way your car is kind of solar powered, and all of this starts with two key ingredients which for most plants are carbon dioxide from the air or water they live in, and water. First the carbon dioxide, how do they get this gas inside the plant? It’s not like plants breathe. Well in a way they do exactly that.


One of the first things we did in our botany practical classes when I was a student was to paint the bottom of a leaf with clear nail varnish. When it dried we carefully peeled it off so it was like a very thin cast or mould of the leaf surface. When we looked at this under a microscope we saw the veins of the leaf and all of the other obvious shapes but once you looked closer we saw what looked like tiny mouths. Indeed the scientific name for these openings in leaves is stomata which is from the Latin word for mouth. It’s through these little holes in their leaves that most plants on land do what in animals would be called breathing. Unlike us, the plants are after carbon dioxide in the air as it’s this that they use as raw material to make sugar. Carbon dioxide contains two elements in it, carbon and oxygen but the sugars that plants store their energy in have another element in them, hydrogen. This is why as a group they are called carbohydrates. In order to get this hydrogen plants take it from water which, as I hope most people know, is H2O. Ok, now it’s a well known story that Stephen Hawking was told that for every equation he added to his book he would cut the sales in half, but we’re going to have to risk it. So we can’t create matter from nowhere and neither can plants. One of the key rules of chemistry is that when you have a chemical reaction all of the atoms have to balance. So what goes into the reaction has to go out, even if it’s in a different form. With me so far? Good. Now the most basic form of sugar is glucose which has 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms. To get that number of carbon and hydrogen atoms we would need 6 molecules of both water and carbon dioxide. Ok, I know it’s maths so I’ll go over it again: we need 6 carbons, 6 oxygens and 12 hydrogens to make up glucose and the plant makes them from carbon dioxide and water. CO2 has one carbon and 2 oxygen, so we need 6 of those to get the 6 carbons we need. With me so far? And we’ve also got the oxygen we need, so now we need hydrogen. Well there’s two of them in each water molecule and one oxygen, H2O. So if we need 12 hydrogens then 6 water molecules will give the plant those. Now, for those of you who are paying attention we will have some leftover oxygen in all of this, 6 atoms of it. What does the plant do with the leftover oxygen? It releases it back into the atmosphere as a waste product. That’s right, to plants oxygen is a waste product that it releases into the atmosphere and animals make use of that oxygen to live and function. In fact, the basic way the most animals get their energy is just the reverse of photosynthesis. We take in sugars and oxygen and release water and carbon dioxide as our waste products. And because the energy is already stored in the sugar molecule we don’t need sunlight to help us with it. But why do plants actually need sunlight for this process? And how does that all work? We’ll get on to that now, but first a quick recap:


  • The living world depends on the sun for its energy, either directly or indirectly
  • The direct method is almost exclusively done by plants through photosynthesis
  • Plants need more than sunlight, they need carbon dioxide and water as raw ingredient
  • The end product is usually a sugar like glucose with oxygen released as a waste product



So now that we’ve caught up let’s see how sunlight is involved. Before that I want you to think about how you would describe plants. I bet very high up on that list for most people is the word green, and that is not a coincidence. A huge proportion of plants are green and for a very good reason, chlorophyll. You’ve probably heard the word before and it is this wonderful green stuff in leaves that makes photosynthesis possible. But what actually is chlorophyll? To put it simply chlorophyll is a molecule that absorbs light from the red and blue parts of the spectrum while reflecting back much of the green light. This is why it looks green to us. So if you think of a rainbow running from red to violet light, chlorophyll absorbs either end and leaves the greenish light in the middle for us to see. When this energy is absorbed by the atoms of the molecules it is able to shake loose some electrons. These electrons then cause a chain reaction where more electrons are shaken loose. At each point of this reaction when an electron is freed up there is energy available for the plant to use and it uses this energy to create a chemical called ATP. Now this is the first time we’ve come across ATP but we will keep it simple, ATP is like the energy currency for cells. It’s a readily usable form of energy that pretty much all cells use to power things. In earlier episodes we described our cells as busy factories, and in that analogy ATP is like a charged battery pack. It’s ready to use and can be plugged into any of our cells machinery when they need the energy. So the plants have now converted the light energy absorbed by chlorophyll into energy that can be used in the cell. But photosynthesis doesn’t end here, there is a second phase that doesn’t need light to work, and it’s this phase that makes the sugar that is the end goal of the process. In this second phase, the ATP from the first phase is used to convert the CO2 and water the plant takes in into useful sugar and other carbohydrate molecules. This process requires the energy from ATP to split up the molecules, form them into simple carbohydrates and then assemble these into things like glucose or starch. The leftover oxygen molecules are then released from the cells where they leave the plant through the same openings that it uses to take in carbon dioxide. You might ask why the plant bothers making sugars when it already has usable energy in ATP. Well a lot of plant structures are made up of more complex carbohydrates and are not used as energy. Also it’s a much more efficient and stable method of long term storage to keep sugar or starch than ATP, especially in seeds that may not grow for months or even years. Whatever the reason, it is very good for us that plants evolved photosynthesis and are still doing it.



So what would happen if photosynthesis stopped working? Well it would not be ideal for life on earth to put it mildly. The supply of organic matter as a source of food for animals would eventually disappear completely, though that would be the least of our worries. Without plants giving off oxygen the atmosphere would pretty quickly run out of oxygen, there would be a very high greenhouse effect and almost all living things would be dead. There would be a few species that would keep going, certain bacteria that don’t need oxygen or organic matter to live, some ecosystems that live off deep sea vents and so on. But pretty much everything else needs plants doing their thing to survive. On that happy note, time for a recap


  • The green colour of most plants comes from chlorophyll
  • Chlorophyll is a pigment that allows plants to absorb energy from light
  • This light energy is used to cause a chain reaction of electrons which the plant uses to make ATP
  • ATP is the battery cells use to power all of the things that happen in the cell
  • For long term storage and to make some structural material for the plant to grow the cells use this ATP to convert carbon dioxide and water into carbohydrates
  • Oxygen is released as a waste product
  • If photosynthesis stopped then pretty soon most of life on earth would stop too


Now a lot of what I have been talking about involves land plants. They take their carbon dioxide from the air, they’re on the surface of the planet so if they can compete with other plants they have lots of sunshine and those not in deserts usually have lots of water. But land plants only represent a small proportion of the plant life on earth. A huge amount of it is in the sea. The rainforests have been called the lungs of the earth but really seaweed, and more precisely microscopic plants floating in the sea should get this name. Seaweed and related plants are from a group called the algae, and the tiny floating microscopic members of this group produce about ¾ of the oxygen that is used by us animals on the surface. At the other end of the scale algae are represented by giant kelp that can form huge underwater forests to rival most trees in height. So how does photosynthesis work for these algae? Where do they get their carbon dioxide and their sunshine from? We’ll start with light first. One of the big issues for many algae is that light doesn’t penetrate the sea very well. So most are only able to photosynthesise fairly near to the surface. But even there the type of light that does penetrate is usually in the green section of the spectrum, which if we were paying attention earlier we would know is the wrong part of the spectrum for chlorophyll. Luckily the algae have a workaround for this issue. They have special proteins that are fluorescent. Now this doesn’t mean they are coloured like highlighters or anything, fluorescent means that something takes in one type of light and gives out another. In this case they take in this green light and emit red light which is exactly what’s needed to get chlorophyll doing it’s thing. These proteins can give a lot of seaweeds a brown or reddish colour, which is why so many of them are not green in colour. So that’s how they get their light, but what about CO2? That is more of an issue. There is some CO2 dissolved in seawater and the plants extract this, but it is often this that is a limiting factor on algae growth in the sea. Some algae that float are able to take it directly from the air but for the rest they just have to take whatever is in the water around them. Added to the difficulty of getting sunlight and some key minerals this means that for many algae photosynthesis is pretty slow, but it’s done on such a vast scale that they can pump oxygen into the air for animals to survive.



Now I thought we could talk a little about other things that use the sun’s energy to function. Plants aren’t the only things that can harness the sun’s energy though, some bacteria are also able to do it in a fairly similar way, but one other species uses the energy of the sun and converts it in a VERY different way. That species is of course us, humans. One of the key things we’ve realised in recent years is that we are able to pump carbon trapped by plants back into the air way quicker than the plants can take it back in again. Fossil fuels formed over millions of years by slow and steady photosynthesis of carbon that was deposited into huge reserves of oil, coal and gas have been burned through in a little over 200 years by a rapidly expanding human race. It’s become very clear that our technology driven expansion needs some technology driven solutions to this problem or very soon we will tip our world into serious trouble. One way to help with this would be to use solar energy as part of the way we drive our society, and we have all seen solar panels on people’s rooves and there are even solar farms harvesting all those lovely sunbeams. But how do they work and are there any similarities to plant photosynthesis? Well in fact they work in extremely similar ways. The cells in a solar panel are made up of two layers of silicon with each layer having slight differences. One is rich in electrons (usually by adding phosphorous) giving it a negative charge and another that is electron poor (usually by adding boron) making it positive in charge. This creates an electric field that is just ready for a flow of electrons which would create a flow of electricity. These electrons are provided by sunlight knocking electrons off the atoms in the panel which causes a chain reaction of electrons flowing through the cell. Sound familiar? It’s the same principle as the first step of photosynthesis. This flow of electrons can then be collected and sent through wires like other electricity sources.


At the end of each episode I like to bring in some research on the topic we’re discussing. For photosynthesis I will bring in two different aspects. The first is on how we humans figured out that photosynthesis was actually happening and the second is on the most important person who has ever lived but you’ve probably never heard of him. So first, how did we figure out what plants were up to anyway? The first step was taken by a Belgian scientist called Jan Baptista van Helmont in the mid 1600s. Up until that point plant growth was believed to be the plant taking in minerals from the soil and using them to grow. Van Helmont carried out experiments on willow trees in pots, showing that the weight of the soil barely changed over 5 years while the tree itself had grown by a significant amount. More than a century later, the British chemist Joseph Priestly showed that if you put a lit candle in a sealed glass container it would use up all the oxygen and go out. He then put a mint plant in the container and after a few weeks it was able to generate enough oxygen that the candle could burn again. This work was then built upon by Jan Ingenhousz who showed that this production of oxygen was only possible when the plant was exposed to light. These three experiments showed that plants are able to take in water and air and convert it to structural material for growth, but only in the presence of light what we now call photosynthesis.


The second thing I will talk about here is a researcher who it is estimated has saved over a billion lives through his research. He was the winner of many awards including a Nobel Peace Prize, the Presidential Medal of Freedom and the Congressional Gold Medal. His name is Norman Borlaug and after this podcast I hope you will look him up. So what did he do that has me raving on about him? In short he changed how we use crops but in particular cereal crops. Borlaug worked on what has become known as the Green Revolution, developing new strains of cereals that are able to be much more productive and are able to have far greater yields due to photosynthesis than those that went before. When working on wheat, strains developed by Borlaug and his team were able to double the productivity of those crops in Southern Asia, notably in Pakistan and India, greatly increasing food security in those countries and saving millions of lives. They were also able to produce disease resistant crops of wheat and rice, drastically reducing crop loss to fungi and other plant diseases. A truly amazing man who is sadly forgotten by most of the world.


Ok time for a final recap


  • Photosynthesis is vital to almost all life on earth
  • The plants in the sea are doing a lot of the heavy lifting in producing oxygen, all under pretty tough conditions
  • Plants take in CO2 and water and using sunlight they make sugars
  • Chlorophyll is the key to turning solar energy into chemical energy
  • Solar panels work in a pretty similar way
  • We should all know who Norman Borlaug is and be thankful for his work in the Green Revolution


So that’s all for this episode. Sorry there was such a big gap, but I was busy with TV stuff and then I went on holiday. I’d like to end by thanking Neal from PodKnows productions for editing the episode and all his advice on how to do a podcast and thanks also to Paul Farrer for his amazing original theme tune. The next episode which will be an interview with my good friend Dalia Gala. Dalia is a PhD student in my lab and does a lot of great work on social media promoting science, look for her under the name Dalia Science. She will be talking a bit about her work, what it’s like to be a research student at Oxford and about how we can spread our scientific message online.