Plants and other organisms that have the pigment chlorophyll can do something that no other living creature can – capture light energy from the sun and use it to make chemical energy through photosynthesis. In this lab, we’ll see how we can set up an experiment to measure the rate of this important process.
The Photosynthesis Reaction
Many of us appreciate the warmth of the sun when we’re on the beach or anxiously waiting for the arrival of spring.
But the sun does more than keep us warm – it also provides most of the energy that we and other animals use for fuel. How? When we eat a salad or crunch on an apple, we’re eating sugars made by a plant through photosynthesis. Photosynthesis is the process in which some organisms capture light energy from the sun and change it into chemical energy. But how can we tell if photosynthesis is occurring in a plant?Let’s start by reviewing the overall biochemical reaction. The process of photosynthesis can be summarized in the general equation:2 H2O + CO2 + light energy –> carbohydrate (CH2O) + O2 + H2OThe physical reactants in this equation are water and carbon dioxide; the products are carbohydrate (sugar), oxygen, and water.
Reaction rate, or how quickly a chemical reaction takes place, can be measured in a laboratory by looking at either how fast reactants disappear or how quickly products are formed over some period of time. So in this case, we can decide to measure how fast water or carbon dioxide disappear, or how fast carbohydrate, oxygen, or water are formed.
For simplicity, let’s eliminate water, since it’s both used and formed. And for ease, let’s count out carbon dioxide disappearance, since to do that properly, we would have to start with a closed system and measure a colorless, odorless gas. That leaves us with carbohydrate or oxygen formation.
To help us decide which of those to measure, let’s take a very quick look at leaf anatomy.The leaves of most plants have a layer called spongy mesophyll. The spaces in this layer are full of O2 and CO2; these gases cause leaves to float if we drop them in water. If we start by creating uniformly sized leaf disks by using a hole punch, then remove the gases in the leaf disks by using a vacuum, the disks will sink in a beaker of aqueous solution. We can then determine a rough estimate of how quickly oxygen is formed through photosynthesis by measuring how fast the leaf disks float to the surface of the liquid.We actually get an estimate of the rate of net oxygen formation, since the plant cells are also undergoing active cellular respiration, using oxygen to create energy.
All we need to do is add light, water, and, since we removed the existing CO2, a source of carbon dioxide. We can add these last two by immersing the leaf disks in a solution of water and baking soda.Let’s start by showing that water and carbon dioxide and light are all necessary for photosynthesis by creating a basic experimental setup of three beakers: one with leaf disks in light and water; one with leaf disks in light, water, and carbon dioxide; and one with leaf disks in water and carbon dioxide but no light. Notice the results – only the leaf disks that have all of the reactants will float.
Results and Analysis
Let’s repeat the procedure we just described, and this time, compile some data. For simplicity, we’ll just create two conditions. Condition A, leaf disks in distilled water and light, will be our control condition, and condition B, leaf disks in bicarbonate solution and light, will be our experimental condition. Let’s assume we put ten leaf disks in each beaker and keep all of the other variables constant.
Knowing that the experimental condition differs from the control condition because it contains all of the reactants needed for photosynthesis, what prediction would you make about what will happen to the leaf disks in beaker B? Let’s see.If we set a timer, we might note the following data for beaker B:
|Time in minutes||Number of disks floating|
Knowing that the median number of disks is 5 (50% of the total), we can use this value as a reference point to show that our experiment is working. Although we’re only using ten disks, it’s pretty safe to assume that something is going on if half of them are floating, and the results aren’t just due to chance. We call this value the ET50, the exposure time needed to get 50% of the sample to show an effect. You can see that the ET50 on this graph happens at 9 minutes, the time when 50% of the disks have floated to the top of the liquid.ET50 is a useful value, but it has one problem – it goes down as the rate of photosynthesis goes up. That is, if you graphed ET50 versus say, light intensity, and found a greater intensity of light meant a faster rate of photosynthesis, then you’d get a graph like this one: