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(6Co2 + 6H2O ––> C3H6O3 + 6O2)

What is Photosynthesis?

Synopsis of Photosynthesis
Stages of Photosynthesis in more Detail
What affects Photosynthesis?

What Photosynthesis Is

            Photosynthesis is the method by which light energy is converted to chemical energy. Autotrophs, organisms that use energy from sunlight or inorganic substances to make organic compounds, go through photosynthesis. These autotrophs include plants, algae, and some bacteria, which go through photosynthesis where they get energy (one percent) of the sun and convert it to chemical energy. Through the course of many steps, photosynthesis help heterotrophs, humans and other animals that cannot get their energy from the sun directly. Therefore, photosynthesis helps provide energy for almost all of life.

  Synopsis of Photosynthesis

            Taking place in the chloroplasts of plant cells, algae, and the cell membrane of certain bacteria, photosynthesis is summarized in 3 basic stages. In the first stage, energy is captured from sunlight by pigment molecules in the thylakoids of chloroplasts. Electrons in the pigments are excited by light and move through electron transport chains in thylakoid membranes. For energy, these electrons are replaced by electrons from water molecules, which are broken up by an enzyme, resulting in oxygen atoms from water molecules to form oxygen gas and hydrogen ions. In the second stage, light energy is changed to chemical energy because hydrogen ions build up inside thylakoids, producing a concentration gradient, making available the energy needed to make ATP and NADPH.  In stage three, with the use of CO2, carbon dioxide, the chemical energy stored in ATP and NADPH powers the development of organic compounds.
            Photosynthesis can be summarized further with its chemical equation:

            The equation means that carbon dioxide, water, and light are needed to form sugar, and oxygen. Furthermore, the numbers mean that six carbon dioxide molecules, six water molecules, and light are needed to form one three-carbon organic sugar and six molecules of oxygen. Photosynthesis make organic compounds during photosynthesis, and use them later to carry out their life processes. For instance, the sugar caused by photosynthesis can form starch, which can be stored in stems or roots. The starch may continue to break down to make ATP to power metabolism. Portions of these sugars also make up all of the proteins, nucleic acids, and other molecules of the cell.


Carbon dioxide = starts the Calvin Cycle
Water = from electron transport chain, replaces excited electrons that move from chlorophyll molecules to other nearby molecules in the thylakoid membrane
3-carbon sugar = formed at the Calvin Cycle, because of carbon dioxide
Oxygen Gas = formed at the electron transport chain when the chlorophyll molecules take electrons from hydrogen atoms, leaving H+ and O, remaining oxygen atoms from water molecules spilt up to form oxygen gas, O2

Stages of Photosynthesis in More Detail

Stage 1
Stage 2
Stage 3

  STAGE 1:

            In stage one of photosynthesis, energy in the form of light is captured from the sun, because the light captured helps make “light reactions” which help power photosynthesis. Pigments, light absorbing compounds that absorb certain wavelengths of visible light and reflects all the others, capture this light energy.

            Two very important pigments are chlorophyll, a primary pigment involved in photosynthesis and carotenoids. Chlorophyll absorbs mostly blue and red light and reflects green and yellow light; leaves look green because of the reflection of green and yellow light. There are two types of chlorophyll in plants, chlorophyll a and chlorophyll b, which both play an important role in photosynthesis. Carotenoids, the pigments that produce yellow and orange fall leaf colors, as well as the colors of fruits, vegetables, and flowers, absorb different wavelengths of light from chlorophyll, so both pigments enable plants to absorb more light energy during the process of photosynthesis.

            Pigments are involved in plant photosynthesis and located in the chloroplasts of leaf cells. Thylakoids are disk-shaped structures where groups of pigments are embedded. Plants can capture energy from sunlight in a series of steps. First, Light hits a thylakoid in a chloroplast and the energy absorbed is transferred to electrons in chlorophyll and other pigments. Then, the transfer in energy cause electrons to have a higher energy level and to become “excited. After that, the electrons leap from chlorophyll molecules to other nearby molecules in the thylakoid membrane, which causes the plant to get replacement electrons for energy from H2O, commonly known as water. An enzyme inside the thylakoid splits these water molecules. After the split, chlorophyll molecules take the electrons from the hydrogen atoms, H, leaving the hydrogen ions, H+, and O, the remaining oxygen atoms from the water molecules that are split up combine to form oxygen gas, O2.

  Roles in Stage 1:

Pigments: absorb light energy
Chlorophyll: Pigment that absorbs blue and red light and reflects green and yellow light, play a key role in photosynthesis
Carotenoids: Pigment that produce yellow and orange fall leaf colors, and colors of fruits, vegetables, and flowers
Thylakoid: where clusters of pigments are embedded in the membranes of disk-shaped structures
Chloroplasts: where pigments involved in photosynthesis are located

  STAGE 2:

            In stage two of photosynthesis, the “excited” electrons from the chlorophyll molecules that jumped to other molecules in the thylakoid membrane earlier are used to make new molecules that briefly store chemical energy. The electron does this by passing through a series of molecules down the thylakoid membrane, called the electron transport chains. The “excited” electrons follow the path of the electron transport chains and goes through a series of steps before it can make new molecules that temporarily store energy.

Each electron transport chain has a different role that help to make molecules that temporarily store energy in the cell. They do this by producing ATP and NADPH, two forms of chemical energy.

One transport chain gets the energy from the excited electrons needed to make ATP. As the excited electrons pass through the protein, the transport chain uses some of their energy, which is used to pump hydrogen ions, H+ into the thylakoid. A concentration gradient is produced across the thylakoid membrane when hydrogen ions become more concentrated inside the thylakoid than outside. Hydrogen ions are more likely to diffuse back out of the thylakoid down their concentration gradient through specialized carrier proteins that operate both as an ion channel and an enzyme. Because hydrogen ions are passing through the channel portion of the protein, the protein catalyzes a reaction where a phosphate group is added to a molecule of ADP, making ATP.

Another transport chain provides the energy needed to make NADPH, an electron carrier that provides the high-energy electrons for photosynthesis. NADPH is produced when excited electrons combine with hydrogen ions and an electron acceptor called NADP+.

  Roles in Stage 2:

Electron Transport Chains: series of molecules through which excited electrons are passed along a thylakoid membrane
ATP: main source of energy in cells
NADPH: an electron carrier that provides the high-energy electrons for photosynthesis – specifically to make carbon-hydrogen bonds in the third stage of photosynthesis

                Light: Light from the sun is shining on a thylakoid on a chloroplast.
                Thylakoid: internal membranes of chloroplasts
                Path of electrons: electrons leap from chlorophyll molecules to other nearby molecules in the thylakoid membrane
                NADP+: electron acceptor that produces NADPH when excited electrons combine with hydrogen atoms and itself
                H+: Hydrogen Ions
                NADPH: electron carrier that provides the high-energy electrons for photosynthesis
                Water-Splitting Enzyme: enzyme that splits the water molecule inside the thylakoid to get replacement electrons for energy
                ATP-producing carrier protein: protein that catalyzes a reaction where a phosphate group is added to a molecule of ADP
                ADP + Phosphate Group = ATP


  STAGE 3:

            In Stage 3 of photosynthesis, carbon atoms in the atmosphere from carbon dioxide are transferred to organic compounds in a process called carbon dioxide fixation. These organic compounds help store energy. In carbon dioxide fixation, there are dark reactions or light-independent reactions that repair carbon dioxide. There are several different types of carbon dioxide fixation, but the most common way is through the Calvin cycle, a series of steps by enzyme-assisted chemical reactions that all help produce a three-carbon sugar.


           First, in the Calvin cycle, every molecule of CO2, carbon dioxide, gets added to a five-carbon compound by an enzyme, forming three six-carbon compounds. Because the three six-carbon compounds are unstable, it splits into six three-carbon compounds. Then, six three-carbon sugars are formed after phosphate groups from ATP and electrons from NADPH are added to the six three-carbon compounds. One of the three-carbon sugars is used to make organic compounds that store energy that is later used by the organism. The other three-carbon sugars are used to renew the five-carbon compounds that began the cycle. The process is repeating over and over again because of the recycled five-carbon compounds that help begin the cycle.

            Where does the energy in the Calvin cycle come from? It comes from ATP and NADPH made during stage two of photosynthesis. For the Calvin cycle to produce three-carbon sugar used to make other organic compounds, three carbon dioxide molecules must enter the Calvin cycle first.

  Roles in Stage 3:

Carbon dioxide fixation: transfer of carbon dioxide to organic compounds
Calvin cycle: series of enzyme-assisted chemical reactions that produce a three-carbon sugar
Enzyme = adds every molecule of carbon dioxide to a five-carbon compound which forms three six-carbon compounds
Three-carbon sugars = help renew the five-carbon compounds that began the cycle, therefore repeating the cycle
ATP/NADPH = gives energy for Calvin Cycle to take place

  What affects Photosynthesis

            Photosynthesis can be affected many ways, such as light intensity, carbon dioxide concentration, and temperature. Light affects photosynthesis because as light intensity increases, photosynthesis increases, until all pigments, light-absorbing compounds, are gone. When all the pigments are gone, the rate of photosynthesis decreases because pigments cannot absorb any more light. Carbon dioxide can also affect the rate of photosynthesis because if a certain concentration of carbon dioxide is present, than photosynthesis cannot go any faster. Photosynthesis can also be affected by temperature as well. Only in a certain range of temperatures is photosynthesis most efficient. If photosynthesis is faced with temperatures that are bad to it’s liking, the temperatures may inactivate certain enzymes, which only operate properly within certain temperature ranges. Enzymes play a key role in photosynthesis, because it assists the autotroph through photosynthesis.

Terms to Know

Photosynthesis = process where living organisms use energy from sunlight to make organic compounds converting light energy to chemical energy
Autotrophs = plants and certain bacteria that undergo photosynthesis
Heterotrophs = humans and other animals that get their energy by undergoing cellular respiration
Chloroplasts = organelles that use light energy to make carbohydrates from carbon dioxide and water
Cell membrane = part of a cell that surrounds and encloses the cytoplasm
Pigment = light-absorbing substances
Thylakoids = internal membranes of chloroplasts
Electron Transport Chain = series of molecules where excited electrons move through
Enzyme = protein that speeds up chemical reaction
Concentration Gradient = difference in the concentration of a substance
Proteins = chain of amino acids 
Nucleic acids = long chain of nucelotides
Chlorophyll = primary pigment involved in photosynthesis
Carotenoids = pigments that produces the fall leaf colors, and colors of fruits, vegetables, and flowers
ATP = organic molecule that acts as the main energy source of cells; adenosine triphosphate; has three phosphate groups, a base (adenine) and a sugar (ribose)
NADPH = electron carrier in photosynthesis that provides "excited" electrons
Carbon Dioxide fixation = transfer of carbon dioxide to organic compounds
Calvin Cycle = one of the many methods of carbon dioxide fixation, used to produce a three-carbon sugar with the help of enzymes





Plant Cell


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