Photosynthesis

This page aims to provide an overview of photosynthesis, how it works and why it’s important.

What is photosynthesis?

One of the key goals of the RoboPlant project is to communicate the crucial importance of photosynthesis to our lives, and indeed all life. But when Elsbeth and I were asked at the very first meeting to explain what photosynthesis is, I’m rather ashamed to say we were a little bit stumped. We began to wonder if Dr. Osborne was having second thoughts about hiring us as ‘biology experts’. As it happens, despite the basic organic chemistry of photosynthesis being almost second nature to us, to put it all into simple words is quite challenging. So let’s start from the basics.

First of all, what does ‘photosynthesis’ actually mean? The word is derived from Greek: ‘photo’ means ‘light’, and ‘synthesis’ means ‘making’ or ‘building’, so the word literally translates into ‘building with light’. This is precisely what plants (and a number of other different groups of organisms) do – they build things using light energy. But what do they build, and how do they do it?

Image credit: newscientist.com

Image credit: newscientist.com

Life involves the transfer of energy

Before we begin to understand photosynthesis, we need to understand that all life requires energy.

To make anything – to do anything – requires energy. However, energy can never be created or destroyed. It can only be transferred from one form to another, but there are ways that energy can be stored. One way energy can be stored is in chemicals, such as sugars which are made by plants.

You are using energy all the time in a process called respiration, which involves breaking down sugars to release the energy stored inside. This energy then enables you to walk, move your fingers, think, and everything else your body does. These actions are fuelled by a molecule called ATP, which delivers energy and is generated from sugars in respiration. ATP is regarded as the universal energy currency, as it is used by every life form on the planet. However, it cannot be stored, which is why we rely on sugars. So when we think of respiration in terms of energy transfer, stored chemical energy (sugars) is transferred to kinetic (movement) energy. The process usually requires oxygen, and releases carbon dioxide and water as waste products. This goes for plants as well as animals and everything else, as everything needs to expend energy to grow. Respiration can be summed up in the following chemical equation:

Sugars (C6H12O6) + Oxygen (O2) –> Carbon Dioxide (CO2) + Water (H2O) + Energy (ATP)

This is why you must breathe in oxygen to survive, and breathe out carbon dioxide (although respiration is not to be confused with breathing!). Respiration can also work in different ways, and does not require oxygen in some cases, but it is the form of respiration described above that is most important to us. Respiration take place in structures called mitochondria, which are found in most of our cells. Like photosynthesis, respiration is a very complex process and involving many different chemicals and stages of energy conversion, but we will not go into it in detail here.

Mitochondria are the cells' microscopic power plants, producing ATP which powers your muscles.

Mitochondria are the cells’ microscopic power plants, producing ATP which powers your muscles.

Photosynthesis works in more or less the opposite direction to respiration, as it is the process that builds sugars which store energy. In this case, energy in the form of sunlight is transferred into stored chemical energy (sugars). The process requires carbon dioxide and water, and releases oxygen as a waste product. It can be summarised by the following equation:

Solar energy + Carbon Dioxide (CO2) + Water (H2O) –> Sugars (C6H12O6) + Oxygen (O2)

Considering that the sugars we use in respiration come from plants (even if they may have gone through other things before reaching you), it should now have become clear that the energy that you use to move your fingers originally came from the sun. That may be a little hard to swallow at first, but it makes perfect sense when you think about it! That is the basic principle of it. If you haven’t had enough by now, and if you don’t mind a bit of chemistry, read on to find out more about how photosynthesis works.

Sugars from plants provide the energy for us to move, via ATP produced in respiration

Sugars from plants provide the energy for us to move, via ATP produced in respiration

How does photosynthesis work?

So now we know that photosynthesis is the process by which plants use sunlight, carbon dioxide and water to build sugars which store solar energy. However, this is not a simple procedure. Photosynthesis is essentially a series of chemical reactions which can be broadly separated into two stages, which we call the light reactions (also called the light-dependent stage, as the reactions require light energy) and the dark reactions (also called the light-independent stage, as the reactions do not require light energy). They take place inside chloroplasts – structures within plant cells which are in some ways similar to the earlier-mentioned mitochondria. The chloroplast is divided into different regions, where different parts of the process take place.

Light reactions

The light reactions are a step-by-step process, and take place in membrane structures called thylakoids, within the chloroplast.

chloroplast map 1

A simple diagram of a chloroplast. The light reactions take place in the thylakoids. Image credit: nature.com

The first step is called photolysis, and is the splitting of water (H2O) into hydrogen ions (positively charged hydrogen) and oxygen using solar energy, with the help of special enzymes (molecules which speed-up reactions). The oxygen is released as a waste product, but hydrogen is needed later on. The splitting of a water molecule also releases two electrons, which go into a molecule of chlorophyll, a green pigment.

The second step in the light-dependent stage is called photophosphorylation, and begins when chlorophyll absorbs solar energy, and releases electrons. The electrons are said to be ‘excited’ as they are carrying the energy. They then pass along a chain of electron carrier molecules, along which their energy is used to power a proton pump, which causes a flow of hydrogen ions (H+) across a membrane. The energy of this flow is harnessed like a hydroelectric dam, and is used to bind phosphate to a molecule called ADP, creating the energy carrier molecule ATP (the energy is contained in the chemical bond).

Now for the last leg of the electrons’ journey through the light reactions stage. The electrons, which have lost energy, are re-charged as they pass through another chlorophyll molecule which absorbs solar energy and excites them again. They then pass along a second chain of electron carriers, at the end of which their energy is used to bind a hydrogen ion to a molecule called NADP, creating another energy carrier molecule called NADPH (again, the energy is contained in the chemical bond).

The light reactions of photosynthesis are best visualised in terms of the Z-scheme, which shows electrons at different energy levels. The 'photosystems' are the different chlorophyll molecules involved in the two electron excitation events.

The light reactions of photosynthesis are best visualised in terms of the Z-scheme, which shows electrons at different energy levels. The ‘photosystems’ refer to the areas containing chlorophyll molecules involved in electron excitation at different stages of the process. Image credit: hyperphysics.phy-astr.gsu.edu

But we are only half-way through photosynthesis now! Let’s do a sit-rep. We have tracked how solar energy is used to excite electrons, the energy of which is used to generate two energy carrier molecules, called ATP and NADPH. These aren’t sugars – they are not stable enough to be stored. So how do we get sugars from them? Read about the dark reactions stage to find out.

Dark reactions

The dark reactions are a cyclic process, often referred to as the Calvin cycle. They take place in an area of the chloroplast called the stroma, and don’t necessarily have to take place in the dark (although no light energy is required). It is a somewhat simpler process than the light reactions (we don’t need to worry about electron excitations or proton pumps). This is also the stage where carbon dioxide – the final basic component – enters the scene.

chloroplast map 2

A simple diagram of a chloroplast. the dark reactions take place in the stroma. Image credit: nature.com

Carbon dioxide enters the cycle and is combined with a five-carbon sugar called RuBP. The reaction is catalysed by the enzyme rubisco, which is the most abundant enzyme on Earth. The resulting six-carbon sugar is unstable, and breaks in half to form two three-carbon sugars. Then, energy from the energy carrier molecules ATP and NADPH (produced in the light reactions) are used to convert these sugars into another kind of sugar called triose phosphate, by donating phosphates and hydrogen ions. ATP and NADPH turn back into ADP and NADP, and the energy is passed on to the stable triose phosphates. Most triose phosphates are used to regenerate RuBP so that the cycle can go round again, but some are further converted into the final products of photosynthesis, namely sugars such as fructose, sucrose and glucose. They can also be used to make other complex molecules, such as amino acids which form the building blocks of proteins.

The dark reactions of photosynthesis are best visualised in terms of the Calvin Cycle, which can be shown at different levels of complexity. Here, triose phosphate is referred to as 3GP. Image credit: hartnell.edu

The dark reactions of photosynthesis are best visualised in terms of the Calvin Cycle, which can be shown at different levels of complexity. Here, triose phosphate is referred to as 3GP. Image credit: hartnell.edu

The long story

That was the short version of the story. The reality is a far more complicated chemical process – one which we currently don’t have the technology to replicate. Like respiration, photosynthesis can also occur in many different ways, and does not always use the same molecules or follow the same steps mentioned above. However, the light and dark reactions described are carried out in most land plants. Photosynthesis is also believed to have existed in many ancient forms before it started splitting water. Once ancient microbes evolved to do this several billion years ago, the oxygen they released into the atmosphere completely transformed the planet from one which was almost completely devoid of oxygen, to one that was filled with it. These oxygen-rich conditions permitted the evolution of new life forms with complex body plans, which became animals. As for the microbes, their photosynthetic machinery formed the basis of land plants, which today supply us with food. So photosynthesis is a story worth knowing, because it’s the story of how we came to be on this planet, and the story of why we’re still here.

The first animal life would not have appeared without oxygen from photosynthesis

The first animal life would not have appeared without oxygen from photosynthesis. Image credit: pbs.org

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