The light reactions take place in the thylakoid. There, water H 2 O is oxidized, and oxygen O 2 is released. The dark reactions then occur outside the thylakoid.
The products of this reaction are sugar molecules and various other organic molecules necessary for cell function and metabolism. Note that the dark reaction takes place in the stroma the aqueous fluid surrounding the stacks of thylakoids and in the cytoplasm.
The thylakoids, intake of water H 2 O , and release of oxygen O 2 occur on the yellow side of the cell to indicate that these are involved in the light reactions. The carbon fixation reactions, which involve the intake of carbon dioxide CO 2 , NADPH, and ATP, and the production of sugars, fatty acids, and amino acids, occur on the blue side of the cell to indicate that these are dark reactions.
An arrow shows the movement of a water molecule from the outside to the thylakoid stack on the inside of the chloroplast. Another arrow shows light energy from the sun entering the chloroplast and reaching the thylakoid stack. An arrow shows the release of an oxygen molecule O 2 from the thylakoid stack to the outside of the chloroplast.
Once the light reactions have occurred, the light-independent or "dark" reactions take place in the chloroplast stroma.
During this process, also known as carbon fixation, energy from the ATP and NADPH molecules generated by the light reactions drives a chemical pathway that uses the carbon in carbon dioxide from the atmosphere to build a three-carbon sugar called glyceraldehydephosphate G3P.
Cells then use G3P to build a wide variety of other sugars such as glucose and organic molecules. Many of these interconversions occur outside the chloroplast, following the transport of G3P from the stroma.
The products of these reactions are then transported to other parts of the cell, including the mitochondria, where they are broken down to make more energy carrier molecules to satisfy the metabolic demands of the cell. In plants, some sugar molecules are stored as sucrose or starch.
This page appears in the following eBook. Aa Aa Aa. Photosynthetic Cells. What Is Photosynthesis? Why Is it Important?
Figure 2. Figure 3: Structure of a chloroplast. Figure 4: Diagram of a chloroplast inside a cell, showing thylakoid stacks. Shown here is a chloroplast inside a cell, with the outer membrane OE and inner membrane IE labeled. What Are the Steps of Photosynthesis? Figure 5: The light and dark reactions in the chloroplast. The chloroplast is involved in both stages of photosynthesis. Photosynthetic cells contain chlorophyll and other light-sensitive pigments that capture solar energy.
In the presence of carbon dioxide, such cells are able to convert this solar energy into energy-rich organic molecules, such as glucose. These cells not only drive the global carbon cycle, but they also produce much of the oxygen present in atmosphere of the Earth. Essentially, nonphotosynthetic cells use the products of photosynthesis to do the opposite of photosynthesis: break down glucose and release carbon dioxide. Cell Biology for Seminars, Unit 1. Topic rooms within Cell Biology Close.
No topic rooms are there. Or Browse Visually. Student Voices. Creature Cast. Simply Science. Green Screen. Green Science. Bio 2. The Success Code.
Why Science Matters. The Beyond. Plant ChemCast. Postcards from the Universe. Brain Metrics. Mind Read. Eyes on Environment. Like mitochondria, chloroplasts are also thought to have originated from primitive bacterial cells through the process of endosymbiosis. Baum explained that the analysis of chloroplast genes shows that it was once a member of the group cyanobacteria , "the one group of bacteria that can accomplish oxygenic photosynthesis.
In their article, Chan and Bhattacharya make the point that the formation of secondary plastids cannot be well explained by endosymbiosis of cyanobacteria, and that the origins of this class of plastids are still a matter of debate.
Pigment molecules are associated with proteins, which allow them the flexibility to move toward light and toward one another. A large collection of to 5, pigment molecules constitutes "antennae," according to an article by Wim Vermaas , a professor at Arizona State University. These structures effectively capture light energy from the sun, in the form of photons.
Ultimately, light energy must be transferred to a pigment-protein complex that can convert it to chemical energy, in the form of electrons. In plants, for example, light energy is transferred to chlorophyll pigments. The conversion to chemical energy is accomplished when a chlorophyll pigment expels an electron, which can then move on to an appropriate recipient.
The pigments and proteins, which convert light energy to chemical energy and begin the process of electron transfer, are known as reaction centers. The reactions of plant photosynthesis are divided into those that require the presence of sunlight and those that do not.
Both types of reactions take place in chloroplasts : light-dependent reactions in the thylakoid and light-independent reactions in the stroma. Light-dependent reactions also called light reactions : When a photon of light hits the reaction center, a pigment molecule such as chlorophyll releases an electron. The released electron manages to escape by traveling through an electron transport chain , which generates the energy needed to produce ATP adenosine triphosphate, a source of chemical energy for cells and NADPH.
The "electron hole" in the original chlorophyll pigment is filled by taking an electron from water. As a result, oxygen is released into the atmosphere. Light-independent reactions also called dark reactions and known as the Calvin cycle : Light reactions produce ATP and NADPH, which are the rich energy sources that drive dark reactions. Three chemical reaction steps make up the Calvin cycle: carbon fixation, reduction and regeneration.
These reactions use water and catalysts. These sugars are then used to make glucose or are recycled to initiate the Calvin cycle again. Plants use much of this glucose, a carbohydrate, as an energy source to build leaves, flowers, fruits, and seeds.
They also convert glucose to cellulose, the structural material used in their cell walls. Most plants produce more glucose than they use, however, and they store it in the form of starch and other carbohydrates in roots, stems, and leaves. The plants can then draw on these reserves for extra energy or building materials. Each year, photosynthesizing organisms produce about billion metric tons of extra carbohydrates, about 30 metric tons for every person on earth.
Photosynthesis has far-reaching implications. Like plants, humans and other animals depend on glucose as an energy source, but they are unable to produce it on their own and must rely ultimately on the glucose produced by plants. Moreover, the oxygen humans and other animals breathe is the oxygen released during photosynthesis. Humans are also dependent on ancient products of photosynthesis, known as fossil fuels, for supplying most of our modern industrial energy.
These fossil fuels, including natural gas, coal, and petroleum, are composed of a complex mix of hydrocarbons, the remains of organisms that relied on photosynthesis millions of years ago. Thus, virtually all life on earth, directly or indirectly, depends on photosynthesis as a source of food, energy, and oxygen, making it one of the most important biochemical processes known.
One plant leaf is composed of tens of thousands of cells, and each cell contains 40 to 50 chloroplasts. The chloroplast, an oval-shaped structure, is divided by membranes into numerous disk-shaped compartments.
These disklike compartments, called thylakoids, are arranged vertically in the chloroplast like a stack of plates or pancakes. A stack of thylakoids is called a granum plural, grana ; the grana lie suspended in a fluid known as stroma. Embedded in the membranes of the thylakoids are hundreds of molecules of chlorophyll, a light-trapping pigment required for photosynthesis.
Additional light-trapping pigments, enzymes organic substances that speed up chemical reactions , and other molecules needed for photosynthesis are also located within the thylakoid membranes. Because a chloroplast may have dozens of thylakoids, and each thylakoid may contain thousands of photosystems, each chloroplast will contain millions of pigment molecules.
In the first stage, the light-dependent reaction, the chloroplast traps light energy and converts it into chemical energy contained in nicotinamide adenine dinucleotide phosphate NADPH and adenosine triphosphate ATP , two molecules used in the second stage of photosynthesis.
In the second stage, called the light-independent reaction formerly called the dark reaction , NADPH provides the hydrogen atoms that help form glucose, and ATP provides the energy for this and other reactions used to synthesize glucose. These two stages reflect the literal meaning of the term photosynthesis, to build with light.
AThe Light-Dependent Reaction Photosynthesis relies on flows of energy and electrons initiated by light energy. Electrons are minute particles that travel in a specific orbit around the nuclei of atoms and carry a small electrical charge. Light energy causes the electrons in chlorophyll and other light-trapping pigments to boost up and out of their orbit; the electrons instantly fall back into place, releasing resonance energy, or vibrating energy, as they go, all in millionths of a second.
Chlorophyll and the other pigments are clustered next to one another in the photosystems, and the vibrating energy passes rapidly from one chlorophyll or pigment molecule to the next, like the transfer of energy in billiard balls.
Light contains many colors, each with a defined range of wavelengths measured in nanometers, or billionths of a meter. Certain red and blue wavelengths of light are the most effective in photosynthesis because they have exactly the right amount of energy to energize, or excite, chlorophyll electrons and boost them out of their orbits to a higher energy level. Other pigments, called accessory pigments, enhance the light-absorption capacity of the leaf by capturing a broader spectrum of blue and red wavelengths, along with yellow and orange wavelengths.
0コメント