Photosynthesis is a process that occurs in plants, few bacteria, and few protists. The definition of photosynthesis is the trapping of radiant energy and converting it to chemical energy.

The overall equation for photosynthesis is:

The reactants are carbon dioxide (CO2) and water (H20). The products are glucose (C6H12O6) and oxygen (O2). The equation yields with light and chlorophyll. The overall reaction is endothermic, or a reaction in which more energy is required than released. The energy from photosynthesis is stored in the bonds of the glucose molecules formed.

In plants, photosynthesis occurs mainly in the leaves. In the leaves, the cells that form the inner layer of tissue are called mesophyll. They are highly concentrated with chloroplasts. Photosynthesis occurs in the chloroplasts of the mesophyll. The outer protective cell layer of the leaf is the epidermis. Stomata (singular: stoma) are in the epidermis, which are openings in the leaf that allow for gas exchanges. Air spaces allow for the rapid diffusion of gases inside the leaf. A vein transports food and water to the leaf.


Leaf Diagram



Plant Cell Diagram


Chloroplasts are sac-like structures or organelles that are found in only plant cells. They contain the chlorophyll needed for photosynthesis. They are very similar to mitochondria (sac-like organelles found in both plant and animal cells that are needed for cellular respiration) because they both can reproduce on their own and have a little bit of their own DNA. Scientists believe that chloroplasts and mitochondria were once bacteria. When a cell consumed them, the cell wasn't able to digest them. They remained in the cell, and when the cell reproduced, chloroplasts and mitochondria appeared in the offspring and remain in cells now. Also, the chloroplast and mitochondrion both have a similar structure. Both have two membranes: an outer membrane and an inner membrane. The chloroplast has a thick liquid enclosed by the inner membrane called the stroma. Enzymes float in the stroma to make chemical reactions in photosynthesis occur. The stroma is similar to the matrix of the mitochondria which is a gel-like liquid enclosed by the inner membrane. The mitochondria has a space in between the inner and outer membrane called the inner membrane space. This a chloroplast doesn't have. Inside the chloroplast, there are disk-shaped sacs suspended in the stroma called thylakoid discs. Each have a membrane surrounding an interior space called the thylakoid space. A stack of thylakoid discs is called a grana (plural: granum). The thylakoid discs are like the folds in the inner membrane, or cristae, in the mitochondria.


Chloroplast Diagram


Enzymes are organic catalysts. Catalysts are chemicals which help speed up chemical reactions without being used up in the reaction. They allow reactions in organisms to occur at body temperature. The only difference between catalysts and enzymes is that enzymes are only made by living things and are composed of carbon. Catalysts aren't. Almost all enzyme names end with -ase. Also, almost all enzymes are proteins which have different blob-like shapes that fit only certain chemicals. The space where the chemical fits in the enzyme is called the active space. The chemicals that enter the enzyme are its substrates.

The chloroplast contains pigments needed for photosynthesis. Pigments are chemicals that can absorb or reflect certain wavelengths of light. They are chlorophyll, carotenoids, and xanthophylls. Chlorophyll is the main pigment and reflects green light, giving it a green color. There are two different types of chlorophyll: chlorophyll a and chlorophyll b. The length of day and a change in temperature causes chlorophyll to break down. Carotenoids and xanthophylls are accessory/helper pigments. Carotenoids reflect orange light, giving it a orange color, and it is usually found in carrots. Xanthophylls reflect red and yellow light, giving it a red or yellow color. The pigments are located on photosystems that are on the thylakoid membrane.

On the thylakoid membrane, there are proteins that have clusters of chlorophyll and other pigments which are called photosystems. A photosystem is a group of proteins that hold pigments in a certain way. They are located on the thylakoid membrane. Pigments that are on the outside of the photosystem are called antenna pigments. This is because they act like antennas since they attract light rays and send it to other antenna pigments. The pigment in the center is and has to be chlorophyll a. This chlorophyll a is in the reaction center, which is where the electron transfer occurs. Sometimes the chlorophyll a is called the reaction-center chlorophyll. There is also a primary electron acceptor, which receives the electron transferred from the reaction-center chlorophyll, in the reaction center. Light comes in and is absorbed by an antenna pigment. One of the electrons gains energy and is “excited”. This state is very unstable. Right after it enters the “excited state” it returns to its low-energy state or “ground state”. The energy is then transferred to another antenna pigment and the same thing happens. This happens until it hits the reaction-center chlorophyll. The electron that is “excited” in the reaction-center chlorophyll “jumps” to the primary electron acceptor. The primary electron acceptor will send this electron somewhere later in the light reactions. It is important to remember that pigments on photosystems don’t send electrons. They send energy.


Photosystem Diagram


The structures in the chloroplast organize the complex series of chemical reactions that make up the overall process of photosynthesis. Some of the steps take place in and on the thylakoid discs, and others take place in the stroma.


Photosynthesis is performed in two different steps: the light reactions and the Calvin cycle. The light reactions occur in and on the thylakoid discs and require light to occur. There are three products of the light reactions: O2, ATP, and NADPH.

ATP (Adenosine Triphosphate) has two different parts: Adenosine and Triphosphate. The Adenosine consists of a nitrogen-containing compound called adenine and a five-carbon sugar called ribose. The Triphosphate consists of 3 phosphate (PO4 or P) groups. It is the source of energy used for most cellular work. ATP is one of the most important compounds in cells and is called the energy currency of the cell. The bonds between the phosphate groups are sometimes considered "high energy" bonds, but they are really only normal covalent bonds, except they break much easier. An analogy to the bonds would be a spring, since they want to shoot off the phosphate group to break the bond. Each phosphate group is negatively charged because there are so many oxygen on the groups. Oxygen is known as an "electron hog" meaning that it hogs all the electrons that are being shared in the covalent bond between the phosphorous atom and the oxygen atom, giving it a weak negative charge. The crowding of negative charges in the triphosphate tail contributes to the potential energy in ATP. With so many negative charges, the phosphate group on the end of the tail wants to break apart. When a bond on ATP is broken, a set amount of energy is released from it. When the farthest bond, or the last phosphate group, is broken, ADP or adenosine diphosphate is formed.


The chemical equation of ATP breaking down is


 It is an exothermic reaction because all of the energy is being released and not required.

The chemical equation of ATP forming is


 It is an endothermic reaction because all of the energy is required and not being released.

NADP+ is an electron carrier chemical that is in the stroma. When it takes electrons and H+ (hydrogen ion), it turns into NADPH. NADPH then takes them to the Calvin cycle, the next step in photosynthesis.



On the thylakoid membrane, there is always a pair of photosystems together. The first one is called photosystem II, and the second photosystem I. This is because scientists discovered the second photosystem, photosystem I, before they discovered photosystem II. Basically, it is just the order in which they were discovered.

Another protein that is on the thylakoid membrane is the ATP synthase. The ATP synthase is the most complex protein ever discovered because it can move or spin as it pumps H+ out of the thylakoid disc, and it turns ADP and a phosphate group into ATP.

Here are the steps for the light reactions:

1- The first step of the light reactions is photosystem I and II absorb light energy, which causes an electron to become “excited”. Light comes in and is absorbed by one of the antenna pigments. The electron that is “excited” in the reaction-center chlorophyll “jumps” to the primary electron acceptor.

2- From the primary electron acceptor, the “excited” electrons from photosystem II are passed through a chain of proteins in the thylakoid membrane. This chain is called the electron transport chain, or ETC. As the electrons are being passed, hydrogen ions are being pumped into the thylakoid disc out of the stroma. The “excited” electron from photosystem I is passed out into the stroma. It then joins with NADP+ and 2H+ to form NADPH and H+. NADPH carries the H+ and electrons over to the Calvin cycle.

3- The missing electrons from photosystem I are replaced by the electron that passed through the electron transport chain from photosystem II. The missing electrons from photosystem II are replaced by using a process called photolysis. Photolysis is the breaking down of water due to light. H2O is broken down to 2H+ and O2. Also, two electrons are released and they replace the missing ones. The O2 is released as a waste product out of the thylakoid, out of the chloroplast, and out of the plant through the stomata. H+ build up in the thylakoid space. This creates a hypertonic solution inside the thylakoid. This also gives the hydrogen ions some potential energy.

4- The H+ that have built up in the thylakoid are then released through ATP synthase. As they are pumped out, the potential energy is released. This energy is used to turn ADP and a phosphate group into ATP. ATP then goes over to the Calvin cycle.


Light Reactions Diagram


The Calvin cycle is the second step of photosynthesis. It can also be called the dark reactions, but many scientists think that it is a misleading name because many people think that the Calvin cycle only occurs at night. This is wrong because it is called the dark reactions for it doesn’t need light to occur. Also, the Calvin cycle can occur during both day and night, using CO2, NADPH, ATP, and other compounds and enzymes to make glucose. The Calvin cycle occurs in the stroma of the chloroplast.

Here are the steps for the Calvin cycle:

1- 3 CO2 molecules enter the Calvin cycle for it needs carbon and oxygen. These CO2 molecules join RuBP, a five-carbon sugar, to form three unstable six-carbon sugars. The enzyme, rubisco, bonds them together. Rubisco it the most abundant enzyme on earth.

2- The three six-carbon sugars are then split apart and form 6 3-carbon molecules called 3-PGA.

3- Six ATP are turned into six ADP and six phosphate groups. Six NADPH are turned into six NADP+. Both the ATP and the NADPH that come from the light reactions provide energy and electrons, which are then used to convert the six 3-PGA into G3P. ATP gives each 3-PGA a phosphate group to give it energy. NADPH then takes off the phosphate group for the energy to release H+ and electrons from itself to 3-PGA, forming G3P.

4- One of the six G3P exits the cycle, which it is eventually used to make glucose and other organic compounds. The rest of the G3P remain in the cycle and are regenerated.

5- ATP provides the energy for the five G3P to turn into 3 RuBP. 3 ATP break down into 3 ADP and three phosphate groups. The ADP, the phosphate groups, and the NADP+ return to the light reactions. The RuBP are used in another turn of the cycle.


Calvin Cycle






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This site was last updated 01/17/10

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