7. Photosynthesis

What you need to know...

    • Chemistry of photosynthesis, as a series of enzyme-controlled reactions, in a two-stage process.

      • Light reactions:

        • The light energy from the sun is trapped by chlorophyll in the chloroplasts and is converted into chemical energy in the form of ATP.

        • Water is split to produce hydrogen and oxygen.

        • Excess oxygen diffuses from the cell.

      • Carbon fixation:

        • Hydrogen and ATP produced by the light reaction is used with carbon dioxide to produce sugar.

    • The chemical energy in sugar is available for respiration or can be converted into plant products such as starch and cellulose.

    • Limiting factors: carbon dioxide concentration, light intensity and temperature and their impact on photosynthesis and cell growth.

Source: SQA


As we've already learnt, enzymes catalyse the many chemical reactions which take place in cells. In these final two topics of this unit, we'll now learn about two enzyme controlled series of reactions which are particularly crucial to life on earth.

We'll begin with photosynthesis. You should already know that photosynthesis is the chemical reaction which takes place in plant cells in order to produce sugar using light energy from the sun. You may even know the word equation for photosynthesis:

In this topic we're going to zoom in on the steps involved in photosynthesis in more detail and consider the affect of different conditions on the rate of photosynthesis. Before we begin however, we're going to be mentioning the molecule ATP in both this topic and the next, so it makes sense to explain what this is a little first.


You don't need to know what ATP stands for, but it might help you understand how it works. ATP stands for Adenosine Triphosphate. So, it's a molecule with three (tri) phosphate groups attached. The best way to imagine ATP is like the charged battery of the cell. If a cell wants to do anything which requires energy (such as active transport) then it needs some energy in the form of ATP. The cell releases the energy by breaking ATP up into ADP (the D standing for Di instead of Tri), which has only two phosphate groups attached, and a separate inorganic phosphate molecule (Pi). To recharge the ADP to ATP, energy is required to add a phosphate molecule. These processes are summarised in the following diagram:


As you might have already guessed, photosynthesis is actually much more complicated than the word equation mentioned above. We actually divide photosynthesis into the following two distinct stages:

Light Reactions: The light reactions of photosynthesis take place in the chloroplasts of the plant cell. Here, the green pigment chlorophyll absorbs light energy from the sun. This energy is used to do two things. Some energy is used to produce ATP as discussed above. This ATP acts as an energy store and can be used in the second stage of photosynthesis. At the same time, light energy is used to split water into hydrogen and oxygen. The hydrogen also goes to the second stage of photosynthesis whereas the majority of the oxygen diffuses out of the cell, and out of the leaf, as a waste product.

Carbon Fixation: The second stage of photosynthesis also takes place in the chloroplast, but does not require chlorophyll as it does not require light energy directly. During Carbon Fixation the ATP and hydrogen from the Light Reactions is used to convert Carbon Dioxide into the sugar glucose.

These two stages of photosynthesis are summarised in the following diagram:

The sugar which is produced by photosynthesis can be used in lots of different ways by the cell. Some of it will be used for respiration, which we'll look at in more detail in the next topic. However, some if it will be joined together into long chains to form molecules of starch. This acts as an energy store for the plant, which is why storage organs such as the food store in seeds and tubers (potatoes) are packed full of starch. The plant can also join the sugar molecules together to form a different form of long-chained molecule called cellulose. This is the molecule which makes up the majority of the structure of plant cell walls which helps give the plant structure and support.

Limiting Factors

The last thing you need to understand in this topic is the impact of limiting factors on the rate of photosynthesis and cell growth. Limiting factors are a common biological concept, and so we'll briefly look at the idea in general before brining it back to photosynthesis.

For any set of chemical reactions taking place within a cell, limiting factors will be at play. A limiting factor is any factor which stops the rate of the reaction increasing further. If we imagine the following simple enzyme catalysed reaction:

If we were to carry out a series of experiments measuring the rate of this reaction, each time increasing the concentration of X, we might expect to get a graph of results like this:

If you look at the graph, you can see what happens in this sort of experiment. To begin with, as you increase the concentration of X, the rate of Z production increases. However, we eventually reach a point when the rate levels off. The sort of question you might be asked is, what is the limiting factor at various points on the graph? I've highlighted these points using numbers which indicate the experiment number.

So, what is the limiting factor in experiment number 5? This is easy. If you're asked what is the limiting factor when the line is going up, then the answer is always written on the horizontal axis of the graph. When the concentration of X was increased from experiment 5 to experiment 6, the rate of Z production increased. Therefore, the concentration of X must've been limiting the rate as the rate went up when more was added.

And, what's the limiting factor in experiment number 20? This one's more tricky. At this point the graph has levelled off. This means that when the concentration of X was increased from experiment 20 to experiment 21, the rate of Z production didn't increase. So, at this point the limiting factor has to be something other than whatever is on the horizontal axis. You can't tell for certain from this graph, but you shouldn't just guess...you can estimate from the reaction equation that the limiting factor in experiment 20 could be the concentration of Y or the concentration of enzyme A - adding more of either of these is likely to increase the rate of Z production. Or, it could even be another factor such as the temperature of the reaction mixture.

I've tried to summarise these two explanations on the diagram below:

So, that's how limiting factors work...but what have they got to do with photosynthesis?

Limiting Factors & Photosynthesis

Now that we know what Limiting Factors are, how do they apply to photosynthesis? Well, the presence or absence of various factors can limit the rate of photosynthesis. As the products of photosynthesis are used in cell growth, any limit in the rate of photosynthesis will effect the rate of growth also.

Possible Limiting Factors in Photosynthesis include:

Carbon Dioxide: As you know, carbon dioxide is a crucial molecule in photosynthesis. It is the raw material in the carbon fixation process. If the Light Reactions are proceeding at their maximum rate but there is a low concentration of carbon dioxide in and around the leaf, then the rate of photosynthesis will be limited, as will the rate of growth of the cell and the plant.

Light Intensity: Light intensity is obviously crucial for the rate of photosynthesis as it provides the energy source for the Light Reactions, which in turn transfer the energy to Carbon Fixation through Hydrogen and ATP. If all other raw materials are present in abundance but there is a low level of light intensity, then the rate of photosynthesis will be limited as will growth.

Temperature: Photosynthesis is a series of enzyme-catalysed chemical reactions, and just like any series of enzyme-catalysed reactions it can be affected by temperature. Low temperatures can result in a reduced rate of photosynthesis and growth, however high temperature can also greatly limit the rate of photosynthesis and growth. Why? Because photosynthesis is catalysed by enzymes and so high temperatures will result in denaturation as discussed in a previous topic.

So, let's bring it all together now...can you interpret the following graph now? What is the limiting factor at points A, B & C on the graph and why?

A: This is the easy one remember. The line is going up, which means the rate of photosynthesis increased when the light intensity (on the horizontal axis) was increased. Therefore, the limiting factor at point A is light intensity.

B: The graph has levelled off here, so a higher light intensity doesn't result in a higher rate. Something other than light intensity must be limiting. On this graph it also shows that the experiment was repeated at two carbon dioxide concentrations. At point B the rate increased when the carbon dioxide concentration increased, therefore carbon dioxide concentration was limiting.

C: It is more difficult to decide what is limiting at point C. It is definitely not light intensity as the graph has levelled off meaning a higher light intensity didn't result in a higher rate. It could be carbon dioxide concentration again, or it could be temperature.

Crash Course has a great video on Photosynthesis, but it does go into a fair bit more depth than you need.