|
|
 |
|
|
|
|
|
Photosynthesis
Index: 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.
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.
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. CHEMICAL EQUATION FOR PHOTOSYNTHESIS: WHAT IT REPRESENTS Carbon
dioxide = starts the Calvin Cycle Stages of Photosynthesis in More Detail 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. Pigments: absorb light energy 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+. Electron Transport
Chains: series of molecules through which excited electrons are passed
along a thylakoid membrane
Diagram:
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. Carbon dioxide fixation: transfer of carbon dioxide
to organic compounds 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   
  NADPH
 Plant Cell
 Hydrogen Return to Top Return to Top Return to Top |
