Tutorial: Photosynthesis
Equation for Photosynthesis | ATP | ATP to ADP | Light and Dark Reaction | Simple Photosynthesis Summary | Plant Cell | Chloroplast | Chloroplast Reactions | Photosynthesis and Light | Chlorophyll | Thylakoid Magnified | The Light Reaction | Light Reaction Animated | The Dark Reaction | Dark Reaction Animated | Photosynthesis Animated | Plant Cell Summary | Quiz

Copyright © Steve Kuensting, 2004, All Rights Reserved.
This web tutorial may not be distributed by any means
without the expressed permission of the author!


All life forms require a constant source of energy for survival. Cells are constantly in a state of activity - reproduction, growth, maintenance & repair - and a constant source of energy is needed to sustain the activity. Food provides this energy. Food contains chemicals which store energy, and cells break down food chemicals for their energy.


Autotrophs vs Heterotrophs

Basically, there are two types of organisms on this planet. AUTOTROPHS are organisms that can make food molecules from light or other energy sources. HETEROTROPHS are organisms that cannot make their own food and so must eat other organisms to stay alive. The autotrophs include the plants - grass, trees, shrubs, weeds, - and alga - the green slime found in rivers and ponds. The heterotrophs include all of the animals - snakes, fish, crickets, worms, jellyfish - and many microscopic animals.

Autotrophs vs Heterotrophs

Photosynthesis vs. the Food Chain

Ultimately, the source of all food on this planet is autotrophs. Plants use light energy to produce food chemicals, and the light energy is thus stored in the bonds of the food molecules. Plants and animals use this stored bond energy as fuel for their cells. If the sun stopped radiating light, first the autotrophic plants would die, followed by all of the heterotrophic animals. Those animals that fed on plants would die first, followed by the predators that ate the plant-eating animals. Eventually, all life on the planet would cease.

Food chain

Importance of the Sun

Thus, anything you ever did was made possible by the trapped solar energy of the food chemicals you ate. The process that makes these food chemicals using solar energy is PHOTOSYNTHESIS.
Defined, photosynthesis is: - chemical reactions that convert the radiant energy of sunlight to chemical energy of sugars.

Sun importance

Equation for Photosynthesis

Photosynthesis is a very complex process. It involves many different chemical reactions that occur in a pathway or series, that is, one chemical reaction produces what the next chemical reaction needs. A chemical equation is written below which summarizes the reactants and products of the photosynthesis pathway. The equation shows that carbon dioxide and water are used with light energy to produce glucose sugar and oxygen gas.

Photosynthesis equation

History of Photosynthesis Research

One of the first records of research is attributed to a Dutch scientist named Jan Baptist van Helmont, dating back to 1648. In trying to determine where plants gained their mass, he weighed and planted a willow sprig in a known amount of potted soil and grew it for 5 years. At the end of the experiment, he re-weighed the soil and weighed the tree. The willow shoot started at 2.3 kg and the soil originally weighed 90.8 kg. After 5 years, the plant reached 76.8 kg while the soil only lost 56 GRAMS. Van Helmont concluded that the willow tree had gained 74 kg from the water that had nourished it for the duration of the 5 year experiment.

VanHelmontVanHelmont Experiment 1VanHelmont Experiment 2

History of Photosynthesis Research

In 1771, an English Scientist named Joseph Priestley discovered that plants and animals alter the air around them in complementary ways. He burned a candle in a covered jar until it burned out. If a plant was in the jar, after several days the candle would relight. He repeated the same experiment with a mouse - a mouse with a plant in the jar would live while a mouse in a jar alone would die. Priestley concluded that the plants were restoring the air that had been "injured" by the candle or mouse.


History of Photosynthesis Research

In 1779, a Dutch physician named Jan Ingenhousz discovered that submerged willow sprigs produced oxygen gas only in the presence of sunlight. By 1796, after further research, Ingenhousz was able to write the first equation for photosynthesis, which was:

Light Energy

Light is not directly usable in photosynthesis. It cannot be directly used to combine carbon dioxide and water to form sugar and oxygen gas. It is first converted to a more usable form. The light is absorbed in photosynthesis and temporarily stored in a versatile molecule called adenosine triphosphate or ATP. ATP can then be used to combine carbon dioxide and water to form sugar and oxygen.



ATP is a molecule which consists of three smaller types of molecules: 1) adenine, 2) ribose, 3) phosphates. One ATP molecule consists of one adenine, one ribose, and three phosphates bonded together. It is pictured below. The wavy lines between the phosphates represent high energy bonds.


Phosphate Bonds

Most of the chemical energy of ATP is stored in the bond between the 2nd and 3rd phosphates. When the energy of ATP is used to make glucose, the third phosphate is broken off and attached to another molecule, thus transferring the energy from the ATP to the other molecule. This transfer of the phosphate to another molecule is called PHOSPHORYLATION. Since the ATP has lost one phosphate, it is now called adenosine diphosphate, or ADP. It is drawn below.



Photosynthesis must constantly make more ATP by restoring ADP molecules. It does this by bonding phosphate molecules to the 2nd phosphate of ADP, thus converting it back to ATP, by USING LIGHT ENERGY. The ATP would then be ready to be used again to make sugar. ATP is involved in other chemical reactions, such as respiration, which will be covered in a later program.

ATP-ADP cycle

Equation for Photosynthesis

Photosynthesis converts moving radiant energy into stored chemical bond energy. The chemical equation is written below which summarizes the chemical reactions of photosynthesis. Photosynthesis is actually much more complex than what is represented by the equation below.

Chemical equation

Light and Dark Reaction

The complex process of photosynthesis actually consists of TWO interrelated complex reactions -- the LIGHT and DARK reactions.
In the light reactions, light energy is converted into the chemical energy of ATP, while water is broken down for its hydrogen atoms. The ATP and hydrogen atoms are fed to the dark reaction which uses them to produce glucose sugar from carbon dioxide. Ultimately the light energy ends up in the glucose, but it does not get their easily.

Light reaction

Dark Reaction Dependence on Light Reaction

The dark reaction depends on the light reaction for energy, just as this computer depends on electricity. The dark reaction need not occur in the dark, it simply cannot occur without the ATP and H atoms provided by the light reaction. The light reaction cannot occur without light, however. Only in the presence of light does it produce ATP and H atoms. Thus, the light reactions act as a power plant to feed the dark reactions the necessary fuel to produce sugar. The dark reactions serve as a sugar factory, using carbon dioxide and H atoms as raw materials to make sugar.

Dark reaction

Simple Photosynthesis Summary

The light reaction trapped the solar energy and transferred it to the dark reaction via ATP so that the dark reaction had the energy to make glucose. Essentially, the solar energy is being packaged into the chemical energy of glucose. The process is summarized below.

Simple photosynthesis

Plant Cell

Photosynthesis takes place only in green plant parts such as leaves and some stems, not in roots. In leaf cells, small green organelles called chloroplasts can be seen with the aid of a microscope. These chloroplasts are what color the leaf green and they are the site of photosynthesis. The only part of a plant cell that can participate in photosynthesis is the chloroplast.

Plant cell


Below is a chloroplast magnified to see its internal parts. Within a chloroplast there are small disks called thylakoids. The thylakoids are located in stacks, like stacks of pennies, which are called grana. The fluid outside of the thylakoids is called the stroma.


Chloroplast Reactions

Photosynthesis takes place inside of the chloroplast. To be more specific, the light reactions take place on the surface of the thylakoid membranes, and the dark reaction takes place in the stroma fluid. Note: the light reaction does not take place inside of the thylakoid, only on its surface.

Chloroplast reactions


There are special pigments found in a chloroplast, which function in photosynthesis, that are found nowhere else in a plant. (A pigment is a molecule that has a color.) The important pigment involved in photosynthesis is CHLOROPHYLL. Chlorophyll is found only on the surfaces of the thylakoid membranes, and is not found in the stroma. There is participates in the light reactions of photosynthesis.


Photosynthesis and Light

Sunlight is made of a mixture of colors and therefore it appears white when it "streams" through a window. There are basically 6 colors found in sunlight, which are listed below. Chlorophyll can only absorb some of these colors, such as red, orange, blue, and violet. It cannot absorb green and yellow. A plant leaf appears green because it is reflecting the color green predominantly to your eye. If chlorophyll absorbed all colors it would appear black, because black is the absence of all colors.


Light Absorption

Since chlorophyll absorbs red, orange, blue, and violet light, it can store these types of light energy in the chemical energy of glucose sugar through photosynthesis.



There are actually two types of chlorophyll, called chlorophyll a and chlorophyll b. Each absorbs colors slightly differently than the other. Both are involved in the light reaction of photosynthesis. Both are located on the surfaces of the thylakoid membranes. Chemical formulas are written for both chlorophylls below. Notice that they are almost identical. Chlorophyll a absorbs violet light better than b, chlorophyll b absorbs blue and orange light better than a.

Chlorophylls a and b

Thylakoid Magnified

The light reactions take place on the thylakoid membranes. In the following part of the program, the thylakoid membrane is enlarged so that the light reaction can be seen. The enlargement is shown below.

Magnified thylakoid

Electron Pumping

The light reactions of photosynthesis are actually an electron pumping system located on the thylakoids of the chloroplast. As particles of light strike a chlorophyll molecule (a or b), electrons are knocked off and forced to travel to another molecule. This literally sets up an electrical circuit where electrons travel as if they were in a wire, but they are passed from one molecule to another.

Electron transport

The Light Reaction

The source of the electrons is water molecules. Water molecules are split into oxygen atoms and hydrogen ions and electrons. The leftover oxygen atoms from splitting water form waste oxygen gas, the Hydrogen ions are used to make ATP, and the electrons are passed down the "electron transport chain".

Electron source


There are actually two electron pumping stations, called photosystems, in the light reaction, which is more efficient than one.

Electron pumps

Hydrogen Ions

Hydrogen ions are pumped into the interior of the thylakoids by the passing of the electrons. The hydrogen ions are then used to produce ATP by a process called Chemiosmosis.



At the end of the electron transport chain, the electrons are picked up, along with hydrogen ions, by a special molecule called NADP which carries them to the dark reaction.

ETS and proton pumping


Remember, the purpose of the light reaction was to produce ATP and H atoms for the dark reaction. The electron pumping of the light reaction makes this possible, by pumping H ions into the thylakoid interior which are used to make ATP, and by NADP picking up the electrons and H ions to carry them to the dark reaction.
In the following animation, the light reaction is shown in operation. The first frame is shown below.

The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.
Speed = | Delay = milliseconds | Frame # =


Even though the light reaction is quite complex, the products were simply the ATP and H atoms. The H atoms were carried by the NADP molecules, and each NADP can carry 2 H atoms represented NADP-H-H. Water molecules were broken down for their electrons and Hydrogen ions which both aided in the production of ATP. The H atoms carried by the NADP will end up as part of a glucose molecule. The ATP energy produced in the light reaction is actually captured light energy which will end up STORED in a glucose molecule.

ATP and stored energy

Calvin Cycle

The dark reactions are often called "The Calvin Cycle" after the botanist Melvin Calvin who discovered the chemical reactions that occur in the dark reaction. The dark reaction is a cyclic reaction in that it ends up producing part of what it actually used -- as in recycling an aluminum can after drinking the soda.

Calvin cycle


The dark reaction occurs in the stroma of the chloroplast which is the fluid outside of the thylakoids. So while the light reaction occurs in the thylakoid surface, the dark reaction occurs in the fluid which is flowing against the thylakoid surface. The dark reaction uses ATP and H atoms from the light reaction plus carbon dioxide gas from the air.

Calvinc cycle in the stroma

Dark Reaction Details

The dark reaction starts out with carbon dioxide reacting with a 5 carbon sugar molecule called RuBP or ribulose biphosphate. This binding of carbon dioxide to another molecule is called Carbon Fixation. This reaction forms two small 3-carbon molecules called PGA or phosphoglyceric acid.



Then the 2 PGA molecules are converted to 2 new PGAL molecules by the use of ATP and H atoms from the light reaction.

PGA to G3P


The PGAL is then used to make glucose and also to recycle RuBP. The recycling of RuBP also requires some ATP. The products of the dark reaction are glucose and more RuBP, mostly RuBP.

G3P to sugar

The Dark Reaction

Six "turns" of the calvin cycle (dark reaction) are needed to produce a single molecule of glucose. This is because most of the PGAL has to be recycled to reproduce more RuBP. The faster the cycle "turns" or reacts, the more glucose that is made per minute.


The Dark Reaction Animated

In the following animation, the dark reaction is shown in operation. Below is the first frame of the animation.

The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.
Speed = | Delay = milliseconds | Frame # =


Even though the dark reaction is quite complex, the products were simply the glucose and more RuBP molecules. The glucose carries: 1) the stored solar energy from the ATP, 2) the H from the NADP, and 3) the atoms of carbon and oxygen of carbon dioxide. RuBP molecules are recycled because otherwise they are difficult to make. The glucose produced by the dark reaction would then be used by the plant cell who made it for food.

RuBP recycling

Photosynthesis Animated

In the next and final animation, the dark and light reaction are shown occurring simultaneously. The production of only a single glucose molecules is shown. The animation is complex because of so many interrelated processes occurring simultaneously. Try and watch only one portion at a time to see what is going on in that area only. Below are many of the symbols that are used in the animation to simplify the process. Study them before going on to view the animation using the three hyperlinks below.

Photosynthesis animation

The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.
Speed = | Delay = milliseconds | Frame # =


In the previous simulation, the original reactants were water and carbon dioxide. Using light energy they were converted to oxygen and glucose with the light energy being stored in the glucose. NADP and ATP were needed by the dark reaction to produce sugar.

Animation summary

Water and the Reactions of Photosynthesis

In the production of a single glucose molecule, more than 6 water molecules must be used. In the first equation below (shown earlier in the program), only 6 water molecules are listed as producing a single glucose. Actually, 12 water molecules are broken down in the process of producing 1 glucose molecule. It turns out that the dark reaction produces 6 water molecules as it works (shown in simulation) , so that the process overall only USES six.

Reactant and product numbers

Simultaneous Reactions

Within a chloroplast, many thylakoids are present, along with many enzymes and NADP molecules. As a result, there are many light reactions going on simultaneously all over the thylakoids with a chloroplast receiving light. Also, many dark reactions occur simultaneously to produce many glucose at the same time. Even on a single thylakoid, many light reactions can occur simultaneously. This increases the efficiency and productivity of the chloroplast.

Chloroplast overview

Plant Cell Summary

The glucose sugar that is made by the chloroplast will diffuse out into the cytoplasm of the plant cell and be used for fuel or will be sent to the plant roots to be stored in the form of starch.

Glucose purpose

Light powers Dark

So, the light reaction only exists to power the dark reaction where the sugar is actually made. The oxygen made in the light reaction is released into the air -- all the oxygen you ever breathed was from a leaf or algae somewhere. The whole process is truly complex, with many more steps than are actually listed. Enzymes are used for every chemical reaction involved in the process.


  1. What is the name of the process by which plants chemically store solar energy?

  2. What molecule is the source of electrons in the light reaction?

  3. What is the energy source that powers the light reaction?

  4. What molecule is produced as waste in the light reaction?

  5. What gaseous molecule is used by the dark reaction to build glucose?

  6. What atoms are carried to the dark reaction from the light reaction?

  7. What molecule carries trapped light energy to the dark reaction?

  8. What term broadly describes all organisms that can photosynthesize?

  9. Light energy is converted to ____________ energy in photosynthesis. (fill in the blank)

  10. What green pigment is found in chloroplasts?

  11. What colors of light does chlorophyll not absorb?

  12. What are the small disks called that are found in a chloroplast?

  13. What is the fluid called that lies outside the grana of a chloroplast?

  14. Inside of a chloroplast, on what small structures do the light reactions occur?

  15. Inside of a chloroplast, where do the dark reactions occur?

  16. What 5-carbon molecule reacts with carbon dioxide in the dark reaction?

  17. What molecule carries hydrogen atoms from the light reaction to the dark reaction?

  18. What is the name of the process of ATP production by use of hydrogen ions?

  19. How many glucose molecules are produced from 6RuBP and 6 carbon dioxide molecules completely reacting in the dark reaction?

  20. To what molecule is PGA converted by the use of ATP and hydrogen ions?

  21. What molecule is recycled in the dark reaction?

  22. To what molecules is ATP broken down by its use in the dark reaction?

  23. In what organelle does photosynthesis occur?

  24. Ultimately, what is the source of all biological energy on earth?

  25. How many different TYPES of molecules are found in an ATP molecule?

  26. Who discovered the steps of the dark reaction?

  27. How many molecules of carbon dioxide are required to make 1 glucose in photosynthesis?

  28. How many water molecules are required to make one glucose in photosynthesis? (don't count water made in dark rx)

  29. How many oxygen molecules are made in the production of 1 glucose in photosynthesis?

  30. What is another name for the dark reaction?

Diagram - Name the following photosynthesis molecules or structures.