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Photosynthesis: Introduction

Photosynthesis. Look at the word. Photo (light) + synthesis (production of chemical compounds from simpler elements). Plants, algae, and some bacteria use photosynthesis to capture light energy in the form of photons to produce a chemical compound (glucose, C6H12O6) from simpler elements (carbon dioxide, CO2 and water, H2O). In the process, an essential gas is also formed, one that changed the planet forever once ancient bacteria and plants began to photosynthesize. That gas is oxygen (O2).

Photosynthesis is a biochemical process in which energy derived from light is converted into the energy of chemical bonds. In plants, there are many biochemical steps involved in the process of photosynthesis involving both what are called oxidation and reduction reactions.

We do not need to discuss oxidation and reduction reactions at this time other than to make you aware that such reactions do occur and are very common in biochemistry and chemistry in general. You will learn more about these reactions as you continue studying chemistry later in your education. For now, we can greatly simplify the process of photosynthesis and still have you grasp the importance of the reaction and its impact on life on Earth.

A general overview describing the reactants and products of photosynthesis is as follows:


Looking at photosynthesis at this level, we see a rather straight-forward one-way path. Essentially, photosynthesis is a process that occurs on certain membranes inside a plant organelle called a chloroplast. The actual membrane inside the chloroplast where photosynthesis occurs is called a thylakoid membrane. The thylakoid membranes form stacks of tightly pack structures in chloroplasts called grana.

Chloroplasts are not found in animal cells. However, in plant cells, they are typically easily seen by light microscopy even in unstained preparations referred to as wet mounts. In the wet mount micrograph (photo taken through a microscope) shown below, you can easily see the individual chloroplasts as small green/greenish-yellow particles or discs in both the seaweed Elodea and the moss Plagiomnium.


As you will see in the lab for Investigation 3, one advantage of wet mount microscopy is that it is fast and easy to prepare. All that is needed is a fairly thin specimen (a leaf is typically thin enough), a glass microscope slide, coverslip, and a drop of water. The other advantage is that you can see living cells using wet mount microscopy, as in the case of the seaweed Euglena shown here. When you prepare Elodea wet mount in the lab, you may be able to see the movement inside the cells.

The simplest and chemically correct way to express the photosynthetic reaction is shown below:

One can “read” this chemical equation, “Six molecules of the gas carbon dioxide plus six molecules of water are converted to six molecules of oxygen gas and one molecule of the sugar glucose in the presence of light and chlorophyll”. This is easily the most important single reaction on Earth! Let’s see why.


Why is Grass Green?

Have you ever wondered or perhaps stopped to ask yourself on a walk through the woods or across a golf course, “Why is everything in nature so green?”. Perhaps someday you will be lucky enough to have a job that requires you to go into outer space. Looking back at the Earth from space, you might ask yourself a similar question, “Why does the Earth look so green?”. The answer to both questions is the same – chlorophyll.

Chlorophyll is a molecule that absorbs most wavelengths of light but reflects waves in the 500 nm – 650 nm range. As you learned in the Light CELL, if an object absorbs a certain wavelength of visible light, that light is not reflected and therefore does not enter our pupils and we can not perceive it. On the other hand, if a wavelength is not absorbed by an object, but is reflected, the color of that wavelength enters our eye and we perceive that color. Since chlorophyll absorbs almost all visible wavelengths except in the green range, we see green. When we see a green forest or peer out the space station at the Earth below, we see chlorophyll! Lots of chlorophyll.


Perhaps not surprisingly, the most abundant of all proteins on Earth are those involved in the photosynthetic reaction. In particular, the most abundant protein on the planet is likely the chloroplast enzyme ribulose bisphosphate, which functions, along with chlorophyll and many other molecules, in photosynthesis.

There is more than one form of chlorophyll in plants. Below is a model of chlorophyll a. Chlorophyll b is also present and is a slightly larger molecule. However, we need not go into the differences between these two pigments (pigment = molecule that reflects light in the visible range) in this CELL.


Paper Chromatography

In the lab for Investigation 1, you will extract chlorophyll and other pigments from spinach leaves. In doing so, you will perform a procedure called chromatography. There are various types of chromatography. You will use paper chromatography in the lab.

If you put a small drop of water or other liquid on a piece of absorbent paper (paper towel, coffee filter, construction paper, etc.), you will observe that the small wet spot quickly spreads and grows larger. As the liquid spreads through the paper, you can imagine that any molecules that are soluble in the liquid may spread through the paper with it. That is essentially the principle behind how paper chromatography works.


Look at the video below of a mixture of red, blue, and yellow ink.  As the liquid is absorbed and creeps up the white filter paper, against gravity, you can easily see that the three colors are eventually completely separated from one another.

What causes the separation of molecules in paper chromatography? The answer is not so simple because there are a couple of factors involved. First, in order to be swept up by the rising solvent, the sample molecules must be soluble in the solvent to some degree. Second, each molecule in a mixture will have some affinity for the surface of the filter paper. In this instance, the solvent is simply water. Thus, if the pigment molecules in the ink were entirely insoluble in water, the original line of ink drawn across the bottom of the filter paper would simply not move at all. The water would rise up the paper and leave all of the pigment molecules behind. For example, if we used a waxy crayon instead of ink to draw the line on the bottom, little or no movement or separation of pigment would likely take place because crayon colors are not very soluble in water.

The distance a pigment moves in paper chromatography can be quantified quite simply by measuring two distances. The first distance is the distance from the origin (the spot where the sample was initially applied on the paper) to the point that it ended up at the end of the process. The second important distance is from the origin to where the front of the solvent was at the end of the procedure (below). 

By dividing the distance the spot moved by the distance the solvent moved, we can obtain the retention factor (Rf). It is called a retention factor because it is a measure of how tightly the sample is retained by the surface (paper in our case). The more affinity ta sample has for the paper, the less it will move during chromatography. The Rf of any sample will vary depending on the solvent used, the sample, of course, and other factors, such as the precise type of paper or other material on which the chromatography is performed. The    of the illustration above (assuming the squares on the graph paper are 1 cm, would be (notice that the retention factor Rf has no units):


Therefore, molecular movement and separation in chromatography depends on the differential solubility of the sample in the solvent used as well as differential affinity for the filter paper surface. Judging by the results of the chromatography video shown above, we can guess that the yellow pigment molecules have the least affinity for the filter paper and/or are the most soluble in the solvent. Conversely, the red pigment likely has a greater affinity for the filter paper and/or is less soluble in the solvent. Even though we can not actually measure the migration distances of the three pigments and the solvent front in the video above, we may reasonably deduce the following: 

Paper chromatography and other forms of chromatography are extremely important procedures in chemistry and biochemistry. Rvalues gives scientists a quantitative way of reporting and comparing chromatography results. To chemists, Rvalues can tell quite a bit about the properties of the molecules being tested and are extraordinarily useful for then isolating and purifying them. You will perform a paper chromatography experiment in the Investigation 1 lab. 


The Products of Photosynthesis

There are two products of the photosynthesis reaction for us to consider, the simple sugar glucose, and the diatomic (two-atoms) gas molecule oxygen (O2). These are obviously two of the most important molecules for life on our planet. Let’s start with glucose.



Glucose is important for both plants and animals. In living cells glucose is processed very rapidly to release the energy it contains in its chemical bonds. In addition, plants use glucose to build two closely related carbohydrate polymers – cellulose and starch. Cellulose is the polymer that plants use to build their cell walls. That makes cellulose easily the most abundant natural polysaccharide on Earth! Cellulose is the major component of wood. When wood is burned, the glucose subunits of cellulose are consumed in the combustion reaction, which, in turn, releases the chemical bond energy stored in the glucose molecule.


Biological Polymers: Cellulose

A polymer is simply a term used for extremely large molecules composed of many repeating units, often producing long chains or branching molecules. We have already seen examples of other types of polymers in proteins and nucleic acids. The figure below compares polymers formed by nucleic acids, proteins, and the carbohydrate glucose.

As this illustration indicates, biological polymers, formed by polymerization of small subunits, result in enormous molecules with extremely important functions. Nucleotides polymerize to form DNA and the various forms of RNA. Amino acids polymerize to form polypeptides of structural proteins and enzymes. We have discussed these two major biological polymers at great lengths in LabLearner. Here we see that the simple carbohydrate glucose molecule can also form very large and complex polymers. Animals polymerize glucose for energy storage into glycogen for later energy demands. Plants polymerize glucose into starches and cellulose, which is used to build cell walls for its trillions of cells. These cells then are incorporated into plant tissues and structures.

Like many other molecules we have discussed in LabLearner, the glucose molecule contains energy in the form of the chemical bonds that hold its atoms together. The energy to form the chemical bonds in the glucose molecule is derived directly from the Sun in the form of energetic photon particles, captured by photosynthesis, that have bathed the Earth since its formation some 4.5 billion years ago.

As the illustration below highlights, the chemical bond energy captured by photosynthesis and stored in large cellulose polymers can later be released through combustion (burning) reactions. The released energy from cellulose in wood has been used by ancient human ancestors for some 1.7 to 2 million years ago for cooking, heating, light, defense, and so on.

Also, as seen in the illustration above, much of the cellulose chemical energy made by plants many, many millions of years ago and subsequently deeply buried along with other fossils of past life, are mined today in the form of coal and oil. Burning (combustion) of these ancient cellulose polymers is used by modern power plants to produce electricity – breaking the energy-rich chemical bonds formed by photosynthesis so long ago.

Interestingly, the combustion of coal, wood, and oil is much the reverse of the photosynthesis reaction (below). Photosynthesis takes carbon dioxide and water and uses light (photons) energy to produce oxygen gas and glucose. The combustion of glucose-containing products like coal, wood, and oil produces, on the other hand, not only heat energy that can be converted into useful electricity, it also forms water and carbon dioxide.

It is the carbon dioxide (CO2) gas product of the combustion of coal and petroleum (like oil and gasoline) derived from ancient plant materials that can contribute to the greenhouse gases that can subsequently impact planet-level temperature changes. We discussed greenhouse gases like carbon dioxide in terms of climate change and global warming in detail in the Atmosphere CELL. A quick overview of the issue is shown in the illustration below.


In addition to the useful carbohydrate glucose, photosynthesis also produces oxygen gas. While it is difficult to see oxygen gas produced by photosynthesis in terrestrial plants, if you ever have the opportunity to closely view living and growing aquatic plants (in a fish aquarium, for example), often you can easily see oxygen-filled bubbles on leaves and stems, as shown to the right. Thus, photosynthesis not only adds oxygen gas to the atmosphere directly from terrestrial plants like trees, grasses, and shrubs, but aquatic plants add a tremendous amount of dissolved oxygen gas to oceans, lakes, and rivers. How much of the Earth’s oxygen is produced by aquatic plants? While it varies at different times of the year, scientists estimate that from 50-80% of the Earth’s oxygen is produced by the ocean alone! It is this dissolved oxygen that aquatic animals harvest through structures like gills to survive. However, a substantial amount of this O2 enters into the atmosphere as well. 

While the Sun’s energy has rained down on our planet since its very formation, it wasn’t until about 2.33 billion years ago that oxygen began to appear in the atmosphere. So for nearly the first half of its existence, the Earth’s atmosphere was entirely devoid of oxygen gas. 

Some oxygen gas would likely have existed in the oceans but would have been quickly removed through chemical reactions (oxidation reactions) or very primitive aquatic life forms. However, once terrestrial plants (land plants) evolved, everything changed! On the timeline shown below, you can see that some primitive forms of photosynthesis took place back nearly 4 billion years ago. However, beginning about 3.3 billion years ago, oxygen gas began to accumulate in the atmosphere. This period is sometimes called the Great Oxygen Event. Much later, only about 500 million years ago, plants appeared on land.

Since that time, both terrestrial and aquatic plants, particularly single-cell plant microbes called phytoplankton, have produced copious amounts of oxygen gas, flooding both the Earth’s waters and atmosphere with precious, life-giving oxygen gas.



  • Fun Facts: Animal Respiration
  • More Fun Facts: The Oxygen Cycle
  • Learn the Lingo
  • Get Focused



Animal Respiration and Oxygen Consumption

Take a deep breath. Hold it for as long as you comfortably can. Now exhale. Next, look at the data table below the illustration to see the gases you just inhaled and exhaled.


Notice that inhaled air is approximately 79% nitrogen (N2), 21% oxygen (O2), and only a trace (0.04%) carbon dioxide (CO2). Nitrogen is not affected by respiration and is present as the major component of both inhaled and exhaled air. On the other hand, while exhaled air is still around 16% oxygen (about 5% of inhaled O2 is consumed by cells), carbon dioxide jumps from 0.04% to 4%, approximately a 100-fold increase!

The exchange of oxygen and carbon dioxide gas occurs in the lungs, of course. Capillaries bring in oxygen-depleted blood to the small alveoli of the lungs where CO2 from the body’s trillions of cells is exchanged for O2 captured by inhalation from the air around us (shown below).

Notice that the oxygen gas molecules pass through the thin walls of the alveoli and capillaries and enters the bloodstream where it binds to red blood cells. It is a protein molecule in red blood cells – hemoglobin – that binds O2 and transports it throughout the body.


The Oxygen Cycle

Photosynthesis. Look at the word. Photo (light) + synthesis (production of chemical compounds from simpler elements). Plants and some bacteria and algae use photosynthesis to capture light energy in the form of photons to produce a chemical compound (glucose, C6H12O6) from simpler elements (carbon dioxide, CO2, and water, H2O). In the process, an essential gas is also formed, one that changed the planet forever once ancient plants began to photosynthesize. That gas is oxygen (O2).



The following list includes Key Terms that are introduced within the Backgrounds of the CELL. These terms should be used, as appropriate, by teachers and students during everyday classroom discourse.

Note: Additional words may be bolded within the Background(s). These words are not Key Terms and are strictly emphasized for exposure at this time.

Investigation 1:
  • Photosynthesis: process carried out by bacteria, algae, and higher-order plants in which energy derived from light is converted into the energy of chemical bonds. It consists of light reactions and dark reactions.
  • Chloroplasts: cytoplasmic organelles present in plant cells
  • Chlorophyll: the green pigment found in the chloroplasts of plants
  • Carbon dioxide: a gas that is necessary for photosynthetic reactions
  • Oxygen: a gas that is a product of photosynthetic reactions and necessary for the life of animals and other living organisms
Investigation 2:
  • pH: a property of matter which describes the acidity or alkalinity of a substance. A pH of 7 indicates a neutral substance, lower than 7 indicates that the substance has acidic properties, and greater than 7 indicates that the substance has basic properties.
Investigation 3:
  • There are no Key Terms introduced in Investigation 3.
Investigation 4:
  • There are no Key Terms introduced in Investigation 4.


The Focus Questions in each Investigation are designed to help teachers and students focus on the important concepts. By the end of the CELL, students should be able to answer the following questions:


Investigation 1:
  • Which pigments or colors are present in spinach leaves?
Investigation 2:
  • How do light and photosynthesis affect carbon dioxide levels?
  • How do light and photosynthesis affect oxygen levels?
Investigation 3:
  • What is the importance of light in photosynthesis? 
  • In which parts of the plant does photosynthesis occur? 
Investigation 4:
  • In which parts of a Coleus leaf does photosynthesis occur? 
  • Which pigment is required for photosynthesis?