Photosynthesis, the process by which green Plants and certain other organisms transform Light energy into chemical energy. During photosynthesis in green plants, light energy is captured and used to convert water, carbon dioxide, and Minerals into Oxygen and energy-rich organic compounds.
It would be impossible to overestimate the importance of photosynthesis in the maintenance of life on Earth. If photosynthesis ceased, there would soon be little food or other organic matter on Earth. Most organisms would disappear, and in time Earth’s Atmosphere would become nearly devoid of gaseous oxygen. The only organisms able to exist under such conditions would be the chemosynthetic bacteria, which can utilize the chemical energy of certain inorganic compounds and thus are not dependent on the conversion of light energy.
In plants, algae and cyanobacteria, long-term energy storage in the form of sugars is produced by a subsequent sequence of light-independent reactions called the Calvin cycle; some bacteria use different mechanisms, such as the reverse Krebs cycle, to achieve the same end. In the Calvin cycle, atmospheric carbon dioxide is incorporated into already existing organic carbon compounds, such as ribulose bisphosphate (RuBP). Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then reduced and removed to form further Carbohydrates, such as glucose.
The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen or hydrogen sulfide, rather than water, as sources of electrons. Cyanobacteria appeared later; the excess oxygen they produced contributed directly to the oxygenation of the Earth, which rendered the evolution of complex life possible. Today, the Average rate of energy capture by photosynthesis globally is approximately 130 terawatts, which is about three times the current power consumption of human civilization. Photosynthetic organisms also convert around 100–115 thousand million tonnes of carbon into Biomass/”>Biomass per year.
Steps of photosynthesis
Carbon dioxide in the atmosphere enters the plant leaf through stomata, i.e., minute epidermal pores in the leaves and stem of plants which facilitate the transfer of various gases and water vapor.
Water enters the leaves, primarily through the roots. These roots are especially designed to draw the ground water and transport it to the leaves through the stem.
As sunlight falls on the leaf surface, the chlorophyll, i.e., the green pigment present in the plant leaf, traps the energy in it. Interestingly, the green color of the leaf is also attributed to presence of chlorophyll.
Then hydrogen and oxygen are produced by converting water using the energy derived from the Sun. Hydrogen is combined with carbon dioxide in order to make food for the plant, while oxygen is released through the stomata. Similarly, even algae and bacteria use carbon dioxide and hydrogen to prepare food, while oxygen is let out as a waste product.
The electrons from the chlorophyll Molecules and protons from the water molecules facilitate chemical reactions in the cell. These reactions produce ATP (adenosine triphosphate), which provides energy for cellular reactions, and NADP (nicotinamide adenine dinucleotide diphosphate), essential in plant Metabolism.
The entire process can be explained by a single chemical formula.
6CO2 +12H2O + Light → C6H12O6 + 6O2+ 6H2O
While we take in oxygen and give out carbon dioxide to produce energy, plants take in carbon dioxide and give out oxygen to produce energy.
Light-dependent reactions
In the light-dependent reactions, one molecule of the pigment chlorophyll absorbs one photon and loses one electron. This electron is passed to a modified form of chlorophyll called pheophytin, which passes the electron to a quinone molecule, starting the flow of electrons down an electron transport chain that leads to the ultimate reduction of NADP to NADPH. In addition, this creates a proton gradient (energy gradient) across the chloroplast membrane, which is used by ATP synthase in the synthesis of ATP. The chlorophyll molecule ultimately regains the electron it lost when a water molecule is split in a process called photolysis, which releases a dioxygen (O2) molecule as a waste product.
The overall equation for the light-dependent reactions under the conditions of non-cyclic electron flow in green plants is:
Not all wavelengths of light can support photosynthesis. The photosynthetic action spectrum depends on the type of accessory pigments present. For example, in green plants, the action spectrum resembles the absorption spectrum for chlorophylls and carotenoids with absorption peaks in violet-blue and red light. In red algae, the action spectrum is blue-green light, which allows these algae to use the blue end of the spectrum to grow in the deeper waters that filter out the longer wavelengths (red light) used by above ground green plants. The non-absorbed part of the light spectrum is what gives photosynthetic organisms their color (e.g., green plants, red algae, purple bacteria) and is the least effective for photosynthesis in the respective organisms.
Z scheme
In plants, light-dependent reactions occur in the thylakoid membranes of the chloroplasts where they drive the synthesis of ATP and NADPH. The light-dependent reactions are of two forms: cyclic and non-cyclic.
In the non-cyclic reaction, the photons are captured in the light-harvesting antenna complexes of photosystem II by chlorophyll and other accessory pigments (see diagram at right). The absorption of a photon by the antenna complex frees an electron by a process called photoinduced charge separation. The antenna system is at the core of the chlorophyll molecule of the photosystem II reaction center. That freed electron is transferred to the primary electron-acceptor molecule, pheophytin. As the electrons are shuttled through an electron transport chain (the so-called Z-scheme shown in the diagram), it initially functions to generate a chemiosmotic potential by pumping proton cations (H+) across the membrane and into the thylakoid space. An ATP synthase enzyme uses that chemiosmotic potential to make ATP during photophosphorylation, whereas NADPH is a product of the terminal redox reaction in the Z-scheme. The electron enters a chlorophyll molecule in Photosystem I. There it is further excited by the light absorbed by that photosystem. The electron is then passed along a chain of electron acceptors to which it transfers some of its energy. The energy delivered to the electron acceptors is used to move hydrogen ions across the thylakoid membrane into the lumen. The electron is eventually used to reduce the co-enzyme NADP with a H+ to NADPH (which has functions in the light-independent reaction); at that point, the path of that electron ends.
The cyclic reaction is similar to that of the non-cyclic, but differs in that it generates only ATP, and no reduced NADP (NADPH) is created. The cyclic reaction takes place only at photosystem I. Once the electron is displaced from the photosystem, the electron is passed down the electron acceptor molecules and returns to photosystem I, from where it was emitted, hence the name cyclic reaction.
Light-independent reactions (dark reactions)
Calvin cycle
In the light-independent (or “dark”) reactions, the enzyme RuBisCO captures CO2 from the atmosphere and, in a process called the Calvin-Benson cycle, it uses the newly formed NADPH and releases three-carbon sugars, which are later combined to form sucrose and starch. The overall equation for the light-independent reactions in green plants is:
3 CO2 + 9 ATP + 6 NADPH + 6 H+ → C3H6O3-phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O
Carbon fixation produces the intermediate three-carbon sugar product, which is then converted to the final carbohydrate products. The simple carbon sugars produced by photosynthesis are then used in the forming of other organic compounds, such as the building material cellulose, the precursors for lipid and amino acid biosynthesis, or as a fuel in cellular Respiration. The latter occurs not only in plants but also in animals when the energy from plants is passed through a food chain.
The fixation or reduction of carbon dioxide is a process in which carbon dioxide combines with a five-carbon sugar, ribulose 1,5-bisphosphate, to yield two molecules of a three-carbon compound, glycerate 3-phosphate, also known as 3-phosphoglycerate. Glycerate 3-phosphate, in the presence of ATP and NADPH produced during the light-dependent stages, is reduced to glyceraldehyde 3-phosphate. This product is also referred to as 3-phosphoglyceraldehyde (PGAL) or, more generically, as triose phosphate. Most (5 out of 6 molecules) of the glyceraldehyde 3-phosphate produced is used to regenerate ribulose 1,5-bisphosphate so the process can continue. The triose phosphates not thus “recycled” often condense to form hexose phosphates, which ultimately yield sucrose, starch and cellulose. The sugars produced during carbon metabolism yield carbon skeletons that can be used for other metabolic reactions like the production of amino acids and lipids.
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Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to create oxygen and energy in the form of sugar. This process is essential for life on Earth, as it provides the oxygen that we breathe and the food that we eat.
Photosynthesis takes place in two stages: the light-dependent reactions and the Calvin cycle. The light-dependent reactions take place in the thylakoid membrane of chloroplasts, and they use light energy to split water molecules into hydrogen ions, oxygen, and electrons. The electrons are then used to create ATP (adenosine triphosphate), a molecule that stores energy. The hydrogen ions are used to create NADPH (nicotinamide adenine dinucleotide phosphate), a molecule that also stores energy.
The Calvin cycle takes place in the stroma of chloroplasts, and it uses the ATP and NADPH from the light-dependent reactions to fix carbon dioxide into organic molecules. The Calvin cycle is a complex process, but it can be summarized as follows:
Carbon dioxide is combined with ribulose bisphosphate (RuBP) to form 3-phosphoglycerate (3-PGA).
3-PGA is phosphorylated to form 1,3-bisphosphoglycerate (1,3-BPG).
1,3-BPG is reduced to glyceraldehyde 3-phosphate (G3P) using NADPH.
G3P is used to regenerate RuBP.
Some of the G3P is used to make glucose.
The Calvin cycle is a very efficient process, and it can fix about 10% of the carbon dioxide that plants absorb. However, it is also a very sensitive process, and it can be disrupted by a number of factors, including environmental stresses such as drought, heat, and pollution.
Photosynthetic pigments are molecules that absorb light energy and use it to drive the photosynthetic process. The most important photosynthetic pigments are chlorophylls a and b, which absorb light in the blue and red parts of the spectrum. Other important photosynthetic pigments include carotenoids, which absorb light in the green and yellow parts of the spectrum.
The photosynthetic electron transport chain is a series of proteins that are embedded in the thylakoid membrane of chloroplasts. The electron transport chain uses the energy from light to pump protons across the thylakoid membrane. This creates a proton gradient, which drives the synthesis of ATP.
ATP synthase is an enzyme that uses the energy from the proton gradient to synthesize ATP. ATP is a molecule that stores energy, and it is used to power many of the cell’s activities.
NADPH is a molecule that stores energy. It is used in the Calvin cycle to reduce 1,3-BPG to G3P.
RuBisCO is an enzyme that catalyzes the first step of the Calvin cycle. It is the most abundant protein on Earth.
The Calvin cycle ENZYMES are a group of enzymes that catalyze the reactions of the Calvin cycle. These enzymes are essential for the fixation of carbon dioxide into organic molecules.
Photosynthetic carbon fixation is the process by which plants take carbon dioxide from the atmosphere and use it to build organic molecules. The Calvin cycle is the main pathway for photosynthetic carbon fixation.
Photosynthetic water splitting is the process by which plants use light energy to split water molecules into hydrogen ions, oxygen, and electrons. This process takes place in the light-dependent reactions of photosynthesis.
Photosynthetic oxygen evolution is the process by which plants release oxygen into the atmosphere as a byproduct of photosynthesis. This process takes place in the light-dependent reactions of photosynthesis.
Photosynthetic efficiency is the ratio of the amount of oxygen produced by photosynthesis to the amount of light energy absorbed by the plant. Photosynthetic efficiency is typically measured in terms of grams of oxygen produced per kilowatt-hour of light energy absorbed.
Photosynthetic productivity is the rate at which plants produce organic matter. Photosynthetic productivity is typically measured in terms of grams of organic matter produced per square meter per day.
Photosynthetic acclimation is the process by which plants adjust their photosynthetic rates in response to changes in environmental conditions. Photosynthetic acclimation is essential for plants to survive in a variety of environments.
Photosynthetic Stress is any environmental condition that disrupts the photosynthetic process. Photosynthetic stress can be caused by factors such as drought, heat, pollution, and disease.
Photosynthetic research is the study of photosynthesis. Photosynthetic research is important for understanding how plants function and for developing new ways to improve crop yields.
Photosynthetic applications are the uses of photosynthesis in human activities. Photosynthetic applications include the production of food, fuel, and pharmaceuticals.
1. What is the difference between a virus and a bacterium?
A virus is a small infectious agent that replicates only inside the living cells of other organisms. Viruses can infect all types of life forms, from animals and plants to bacteria and archaea. Viruses are not considered living because they do not have cells or cell membranes. They are simply a collection of nucleic acids (DNA or RNA) surrounded by a protein coat.
Bacteria are single-celled organisms that are prokaryotes, meaning they do not have a nucleus or other membrane-bound organelles. Bacteria are found in almost every Environment on Earth, and they play a vital role in the cycling of nutrients and the decomposition of organic matter. Some bacteria are beneficial to humans, while others can cause disease.
2. What is the difference between an atom and a molecule?
An atom is the smallest unit of matter that retains all of the chemical properties of an element. Atoms are made up of three types of subatomic particles: protons, neutrons, and electrons. Protons and neutrons are found in the nucleus of the atom, while electrons orbit the nucleus.
A molecule is a group of two or more atoms that are held together by chemical Bonds. Molecules can be made up of atoms of the same element (e.g., oxygen gas, O2) or of different Elements (e.g., water, H2O).
3. What is the difference between a gene and a chromosome?
A gene is a segment of DNA that codes for a specific protein or RNA molecule. Genes are located on Chromosomes, which are long, linear molecules of DNA that are found in the nucleus of every cell.
Chromosomes are made up of two strands of DNA that are twisted together in a double helix. The DNA molecule is made up of four types of nucleotides: adenine (A), thymine (T), guanine (G), and cytosine (C). The order of these nucleotides in a gene determines the sequence of amino acids in the protein that the gene codes for.
4. What is the difference between a mutation and a genetic disorder?
A mutation is a change in the DNA sequence of a gene. Mutations can be caused by errors in DNA replication, exposure to radiation or chemicals, or viruses. Most mutations are harmless, but some can cause genetic disorders.
A genetic disorder is a condition that is caused by a mutation in a gene. Genetic disorders can range from mild to severe, and they can affect any part of the body. Some common genetic disorders include cystic fibrosis, sickle cell anemia, and Down syndrome.
5. What is the difference between a theory and a hypothesis?
A theory is a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment. A hypothesis is a tentative statement about the natural world leading to deductions that can be tested.
A theory is more than just a guess or an idea. It is a well-tested explanation that has been supported by a great deal of evidence. A hypothesis, on the other hand, is a tentative statement that is not yet supported by a lot of evidence. It is a possible explanation for something that needs to be tested further.
6. What is the difference between a fact and an opinion?
A fact is a statement that can be proven to be true. An opinion is a statement that expresses a belief or judgment that cannot be proven to be true or false.
Facts are based on evidence that can be observed and measured. Opinions are based on personal beliefs or feelings that cannot be proven or disproven.
7. What is the difference between a scientific law and a scientific theory?
A scientific law is a statement that describes a relationship between two or more variables that has been repeatedly observed and tested. A scientific theory is a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment.
A scientific law is a description of what happens, while a scientific theory is an explanation of why it happens. A scientific law is more general than a scientific theory, and it is not as open to change.
8. What is the difference between a hypothesis and a scientific theory?
A hypothesis is a tentative statement about the natural world leading to deductions that can be tested. A scientific theory is a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment.
A hypothesis is a possible explanation for something that needs to be tested further. A scientific theory is a well-tested explanation that has been supported by a great deal of evidence.
Question 1
Which of the following is not a product of photosynthesis?
(A) Oxygen (B) Glucose (C) Water (D) Carbon dioxide
Answer
(C) Water is not a product of photosynthesis. It is one of the reactants in the process.
Question 2
The process of photosynthesis takes place in which of the following organelles?
(A) Chloroplasts are the organelles in plant cells where photosynthesis takes place.
Question 3
The light-dependent reactions of photosynthesis take place in which of the following parts of the chloroplast?
(A) Thylakoid membrane (B) Stroma (C) Grana (D) Pyrenoid
Answer
(A) The light-dependent reactions take place in the thylakoid membrane of the chloroplast.
Question 4
The Calvin cycle takes place in which of the following parts of the chloroplast?
(A) Thylakoid membrane (B) Stroma (C) Grana (D) Pyrenoid
Answer
(B) The Calvin cycle takes place in the stroma of the chloroplast.
Question 5
The overall reaction for photosynthesis can be summarized as follows:
6CO2 + 6H2O + light energy C6H12O6 + 6O2
What is the role of light energy in this reaction?
(A) It provides the energy to split water molecules. (B) It provides the energy to fix carbon dioxide into organic molecules. (C) It provides the energy to drive the Calvin cycle. (D) It provides the energy to synthesize ATP.
Answer
(A) Light energy is used to split water molecules into hydrogen ions and oxygen gas.
Question 6
What is the role of chlorophyll in photosynthesis?
(A) It absorbs light energy. (B) It converts light energy into chemical energy. (C) It catalyzes the reactions of the Calvin cycle. (D) It transports electrons in the electron transport chain.
Answer
(A) Chlorophyll absorbs light energy and uses it to excite electrons. These excited electrons are then used to drive the reactions of the light-dependent reactions.
Question 7
What is the role of ATP in photosynthesis?
(A) It provides the energy to split water molecules. (B) It provides the energy to fix carbon dioxide into organic molecules. (C) It provides the energy to drive the Calvin cycle. (D) It provides the energy to synthesize NADPH.
Answer
(C) ATP provides the energy to drive the Calvin cycle.
Question 8
What is the role of NADPH in photosynthesis?
(A) It provides the energy to split water molecules. (B) It provides the energy to fix carbon dioxide into organic molecules. (C) It provides the energy to drive the Calvin cycle. (D) It provides the electrons for the Calvin cycle.
Answer
(D) NADPH provides the electrons for the Calvin cycle.
Question 9
What is the role of RuBisCO in photosynthesis?
(A) It catalyzes the fixation of carbon dioxide into organic molecules. (B) It catalyzes the reactions of the Calvin cycle. (C) It transports electrons in the electron transport chain. (D) It absorbs light energy.
Answer
(A) RuBisCO catalyzes the fixation of carbon dioxide into organic molecules.
Question 10
What is the overall purpose of photosynthesis?
(A) To produce oxygen gas. (B) To produce glucose. (C) To capture light energy. (D) To convert light energy into chemical energy.
Answer
(D) The overall purpose of photosynthesis is to convert light energy into chemical energy in the form of glucose.