Importance Of Photosynthesis Essay Questions
2. How is light from the sun transformed into the chemical energy used by living organisms on earth?
Light from the sun is transformed into chemical energy contained in organic material through the process of photosynthesis. In photosynthesis, light, water and carbon dioxide react and highly energetic glucose molecules and molecular oxygen are produced.
3. What is the chemical equation for photosynthesis?
The chemical equation for photosynthesis is the following:
6 CO₂ + 6 H₂O + light --> C₆H₁₂O₆ + 6 O₂
Chloroplasts and Chlorophyll
4. What living organisms are responsible for photosynthesis? What cell organelle is responsible for the absorption of light in the photosynthesis process in plants and algae?
There are many organisms (including all animals) that do not use photosynthesis. There are also autotrophic organisms that do not perform photosynthesis but which do perform chemosynthesis. Plants, algae and cyanobacteria are photosynthetic organisms.
In plants and algae, light is absorbed by chlorophyll, a molecule present in cytoplasmic organelles called chloroplasts.
5. Are there chloroplasts in cyanobacteria?
In cyanobacteria, there are no chloroplasts and chlorophyll layers are dispersed in the cytosol.
6. Which chemical element is at the center of the chlorophyll molecule?
The chemical element at the center of the chlorophyll molecule is magnesium. One atom of magnesium is present at the center of a combination of eight nitrogen-containing carbon rings.
7. How do chloroplasts multiply?
Like mitochondria, chloroplasts have their own DNA, RNA and ribosomes and self-replicate through binary division.
8. What evidence is there to support the hypothesis that chloroplasts and mitochondria were primitive prokaryotes that developed a relationship of mutualism with primitive anaerobic eukaryotic cells?
This hypothesis is known as the endosymbiotic hypothesis, and discusses the evolutionary origin of mitochondria and chloroplasts.
Mutualism is explained as the following in this context: mitochondria and chloroplasts can offer energy and nutrients to the cell in exchange for protection. This hypothesis is based on the fact that those organelles have their own DNA, RNA and protein synthesis machinery and divide themselves through binary division like bacteria.
9. What are the main structures of chloroplasts?
Chloroplasts are made up of two membrane layers, the outer and the inner membranes. Inside the organelle, the basic unit is called the granum, and is a coin-shaped structure that, when combined with other grana, forms structures called thylakoids. Thylakoids fill the chloroplast and an intergrana membrane permeates the interior of the organelle.
10. In which chloroplast structure are chlorophyll molecules found?
Chlorophyll molecules are distributed in an organized manner in order to enhance the exposure of thylakoid surfaces to light.
The Stages of Photosynthesis
11. What do ATP and ADP mean? What are the roles of these molecules in the energy metabolism of a cell?
ATP is the abbreviation of adenosine triphosphate, a molecule made of one adenosine molecule bound to three inorganic phosphate ions. ADP is an abbreviation of adenosine diphosphate, which is two molecules of phosphate bound to one molecule of adenosine. ATP stores energy for the cell. When ATP hydrolyzes and becomes ADP, energy is released and then consumed by several metabolic reactions.
- The Process of Photosynthesis - Image Diversity: ATP molecule
12. What is ADP phosphorylation? What are photophosphorylation and oxidative phosphorylation?
ADP phosphorylation is the addition of one inorganic phosphate molecule to the adenosine diphosphate molecule, thus creating ATP (adenosine triphosphate) and incorporating energy. The phosphorylation is oxidative when the energy incorporated comes from the breaking down of organic molecules with oxygen as reagent, like in aerobic cellular respiration. The reaction is called photophosphorylation when the energy source is light, like in photosynthesis.
The energy incorporated into ATP is disposable (released) to other cellular reactions when ATP hydrolyzes and ADP is formed again.
13. What are the stages of photosynthesis?
Photosynthesis is divided into the photochemical stage, or light reactions, and the chemical stage.
The Photochemical Stage of Photosynthesis
14. What are the processes that occur during the photochemical stage of photosynthesis?
The photolysis of water, the release of molecular oxygen, and the photophosphorylation of ADP, and the resulting of ATP and NADPH are the processes that occur during the photochemical stage of photosynthesis.
15. How is the light energy absorbed by chlorophyll transferred to ATP molecules during photophosphorylation? How is the resulting ATP used?
Light excites chlorophyll and energizes electrons that jump off the molecule. The energy released when these electrons escape is used in the phosphorylation of ADP, forming ATP. The enzyme that catalyzes the reaction is ATP synthase.
The resulting ATP is then consumed during the next chemical stage of photosynthesis to transfer energy to carbon dioxide for the formation of glucose.
16. Is it correct to consider the breaking down of water through the action of light the basis of photosynthesis?
Besides ADP photophosphorylation, light energy is also responsible for the breaking down of water molecules during photosynthesis through a process known as water photolysis. During this reaction, water molecules are exposed to light energy and release protons (hydrogen ions), highly energetic electrons and molecular oxygen (O₂). Later, the hydrogen atoms bind to carbon dioxide molecules to form glucose. Since water is the hydrogen donor for photosynthesis, it is correct to say that water photolysis is the basis of the process.
17. What chemical substances are produced by water photolysis? What is the purpose of each of those substances?
Free electrons, hydrogen ions and molecular oxygen are released during water photolysis.
The electrons replace those electrons lost by chlorophyll molecules during photophosphorylation. The hydrogen ions are incorporated into hydrogen acceptor molecules (NADP) and later will be used in the synthesis of glucose during the chemical stage. Molecular oxygen is released into the atmosphere.
18. In sulfur photosynthetic bacteria, what molecule donates hydrogen for photosynthesis?
In sulfur photosynthetic bacteria, the substance that donates hydrogen is hydrogen sulfide (H₂S) and not water. Therefore, there is no release of molecular oxygen and instead molecular sulfur (S₂) is produced. (Oxygen and sulfur have the same number of valence electrons.)
19. Why people say that that during photosynthesis carbon dioxide is enriched by hydrogen atoms from water to form glucose?
During photosynthesis, carbon dioxide is energetically enriched by hydrogen obtained from water. Water broken down by photolysis is the hydrogen donor of the reaction. Glucose is made of carbon and oxygen atoms obtained from carbon dioxide as well as hydrogen atoms obtained from water.
20. What is the complete chemical equation for photosynthesis?
The complete chemical equation of photosynthesis is the following:
6 CO₂ + 12 H₂O + light --> C₆H₁₂O₆ + 6 H₂O + 6 O₂
21. What is an example of a lab experiment that shows the variation in the efficiency of photosynthesis as a function of the different frequencies of light energy to which the reaction is exposed? Do you think that the green light frequency will be favorable to the reaction?
The experiment: Plants of same species and ages are each placed under (respecting their photoperiods) light sources emitting only one of the colors of the light spectrum (violet, indigo, blue, green, yellow and red). The experiment is carried out with each of the colors and, after days, each plant's development is compared. The plants whose development was normal performed satisfactory photosynthesis while those with abnormal development underused the light.
Chlorophyll is green because it reflects the green light frequency, meaning that it does not “use” the green range of the electromagnetic spectrum. Therefore, green light does not favor photosynthesis (strangely, green is the range of the light spectrum that plants “dislike”).
22. What are the divisions of white light according to the electromagnetic spectrum? Which are the two most efficient colors for photosynthesis?
The color divisions of the electromagnetic spectrum in decreasing order of frequency are: red, orange, yellow, green, blue, indigo and violet. When mixed together, these colors generate white.
It has been confirmed via experiments that the most useful colors for photosynthesis are blue and red.
23. What are NADP and NADPH?
NADP is the abbreviation for nicotinamide adenine dinucleotide phosphate cation, a hydrogen acceptor. NADPH is produced when NADP binds to one hydrogen atom. It is the form that transports hydrogen.
24. Photosynthesis is the most important producer of molecular oxygen (O₂) on our planet. Which molecule do oxygen atoms released by photosynthesis come from? Which other molecule could you suspect they come from? Where do these oxygen atoms end up?
The oxygen atoms released as molecular oxygen through photosynthesis come from water.
It is easy to imagine that those oxygen atoms come from carbon dioxide. However, oxygen atoms from carbon dioxide are incorporated into glucose molecules and the water molecules released in the chemical stage of photosynthesis.
The Chemical Stage of Photosynthesis
25. Where do the photochemical and the chemical stages of photosynthesis occur?
The photochemical stage of photosynthesis occurs mainly in the thylakoids (the green part) and the chemical stage occurs in the stroma (the colorless framework) of the chloroplasts.
26. Which byproducts of the photochemical stage are essential for the chemical stage of photosynthesis?
The chemical stage of photosynthesis depends on NADPH and ATP produced through “light reactions” (the photochemical stage).
27. What are the roles of NADPH and ATP during the chemical stage of photosynthesis?
NADPH acts as a reductant of carbon dioxide, delivering highly energetic hydrogen atoms to precursor molecules during the glucose formation process. ATP is an energy source for the reactions of the chemical stage.
28. Why is the nickname “dark reactions” not entirely correct for the chemical stage of photosynthesis?
“Dark reactions” is not a correct name for the chemical stage of photosynthesis since the reactions of the chemical stage also occur in the presence of light.
29. What is the general chemical equation for photosynthesis? Why doesn't this equation clearly show the real origin of the molecular oxygen released?
The general equation for photosynthesis is:
6 CO₂ + 6 H₂O + light --> C₆H₁₂O₆ + 6 O₂
Water molecules are also produced during the chemical stage of photosynthesis as the following complete equation reveals:
6 CO₂ + 12 H₂O + light --> C₆H₁₂O₆ + 6 H₂O + 6 O₂
Water molecules are present on the reagent side as well on the product side of the equation. However, the pure mathematical simplification of stoichiometric coefficients leads to elimination of water from the product side, making it appear that 6 molecules of oxygen (O₂), that is, 12 atoms of oxygen, are made from the 6 molecules of water, that is, 6 oxygen atoms, in the reagent side. As a result, the false impression that 6 other oxygens atoms come from the carbon dioxide is created.
Limiting Factors of Photosynthesis
30. What are the three main limiting factors of photosynthesis?
The three main limiting factors of photosynthesis are light intensity, carbon dioxide concentration and temperature.
31. The rate at which photosynthesis takes place varies depending on the intensity of light energy. Does the same occur in aerobic respiration? What is the effect of these variations on glucose balance?
In a photosynthetic organism, the rate of aerobic respiration can be superior, inferior or equal to the rate of photosynthesis. The rate of respiration depends on the energy needs of the plant while the rate of photosynthesis varies depending on the availability of light energy, if all other conditions are maintained the same.
In a situation in which the respiration rate is greater than the photosynthesis rate, glucose consumption is higher than glucose production. In a situation in which the respiration rate is lower than the photosynthesis rate, glucose is accumulated (positive balance). In a situation in which the rates are equal, all molecular oxygen produced by photosynthesis is used in respiration and all carbon dioxide released through respiration is consumed by photosynthesis. As a result, there is no positive balance of glucose or depletion of carbohydrate stores.
32. What is the compensation point? What is the importance of the compensation point for plant growth?
The (light) compensation point is the light energy intensity under which the aerobic respiration rate equals the photosynthesis rate. In this situation, all glucose produced is consumed and there is no incorporation of material into the plant. As a result, the plant stops growing.
33. Why is carbon dioxide concentration a limiting factor in photosynthesis? When carbon dioxide concentration is increased indefinitely, is photosynthesis also increased indefinitely?
The availability of carbon dioxide is a limiting factor for photosynthesis because this gas is a reagent of the reaction.
Since enzymes catalyze the formation of organic molecules with carbon atoms from carbon dioxide, photosynthesis stops as soon as these enzymes become saturated, that is, when all their activation centers are bound to their substrates. In this situation, an increase in carbon dioxide concentration will not increase the photosynthesis rate.
34. Why do some trees lose their green color in the autumn?
In autumn, the days become shorter and nights become longer; as a result, there is a reduction in the photosynthesis rate. Because of this, some plants prepare themselves for the winter by making nutrient stores. In this process, nutrients from the leaves travel to storage sites: branches, the trunk and roots. With less chlorophyll produced in leaves, the typical green color of the plant fades.
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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.
Energy produced by photosynthesis carried out by plants millions of years ago is responsible for the fossil fuels (i.e., coal, oil, and gas) that power industrial society. In past ages, green plants and small organisms that fed on plants increased faster than they were consumed, and their remains were deposited in Earth’s crust by sedimentation and other geological processes. There, protected from oxidation, these organic remains were slowly converted to fossil fuels. These fuels not only provide much of the energy used in factories, homes, and transportation but also serve as the raw material for plastics and other synthetic products. Unfortunately, modern civilization is using up in a few centuries the excess of photosynthetic production accumulated over millions of years. Consequently, the carbon dioxide that has been removed from the air to make carbohydrates in photosynthesis over millions of years is being returned at an incredibly rapid rate. The carbon dioxide concentration in Earth’s atmosphere is rising the fastest it ever has in Earth’s history, and this phenomenon is expected to have major implications on Earth’s climate.
Requirements for food, materials, and energy in a world where human population is rapidly growing have created a need to increase both the amount of photosynthesis and the efficiency of converting photosynthetic output into products useful to people. One response to those needs—the so-called Green Revolution, begun in the mid-20th century—achieved enormous improvements in agricultural yield through the use of chemical fertilizers, pest and plant-disease control, plant breeding, and mechanized tilling, harvesting, and crop processing. This effort limited severe famines to a few areas of the world despite rapid population growth, but it did not eliminate widespread malnutrition. Moreover, beginning in the early 1990s, the rate at which yields of major crops increased began to decline. This was especially true for rice in Asia. Rising costs associated with sustaining high rates of agricultural production, which required ever-increasing inputs of fertilizers and pesticides and constant development of new plant varieties, also became problematic for farmers in many countries.
A secondagricultural revolution, based on plant genetic engineering, was forecast to lead to increases in plant productivity and thereby partially alleviate malnutrition. Since the 1970s, molecular biologists have possessed the means to alter a plant’s genetic material (deoxyribonucleic acid, or DNA) with the aim of achieving improvements in disease and drought resistance, product yield and quality, frost hardiness, and other desirable properties. However, such traits are inherently complex, and the process of making changes to crop plants through genetic engineering has turned out to be more complicated than anticipated. In the future such genetic engineering may result in improvements in the process of photosynthesis, but by the first decades of the 21st century, it had yet to demonstrate that it could dramatically increase crop yields.
Another intriguing area in the study of photosynthesis has been the discovery that certain animals are able to convert light energy into chemical energy. The emerald green sea slug (Elysia chlorotica), for example, acquires genes and chloroplasts from Vaucheria litorea, an alga it consumes, giving it a limited ability to produce chlorophyll. When enough chloroplasts are assimilated, the slug may forgo the ingestion of food. The pea aphid (Acyrthosiphon pisum) can harness light to manufacture the energy-rich compoundadenosine triphosphate (ATP); this ability has been linked to the aphid’s manufacture of carotenoid pigments.
Development of the idea
The study of photosynthesis began in 1771 with observations made by the English clergyman and scientist Joseph Priestley. Priestley had burned a candle in a closed container until the air within the container could no longer support combustion. He then placed a sprig of mint plant in the container and discovered that after several days the mint had produced some substance (later recognized as oxygen) that enabled the confined air to again support combustion. In 1779 the Dutch physician Jan Ingenhousz expanded upon Priestley’s work, showing that the plant had to be exposed to light if the combustible substance (i.e., oxygen) was to be restored. He also demonstrated that this process required the presence of the green tissues of the plant.
In 1782 it was demonstrated that the combustion-supporting gas (oxygen) was formed at the expense of another gas, or “fixed air,” which had been identified the year before as carbon dioxide. Gas-exchange experiments in 1804 showed that the gain in weight of a plant grown in a carefully weighed pot resulted from the uptake of carbon, which came entirely from absorbed carbon dioxide, and water taken up by plant roots; the balance is oxygen, released back to the atmosphere. Almost half a century passed before the concept of chemical energy had developed sufficiently to permit the discovery (in 1845) that light energy from the sun is stored as chemical energy in products formed during photosynthesis.
Overall reaction of photosynthesis
In chemical terms, photosynthesis is a light-energized oxidation–reduction process. (Oxidation refers to the removal of electrons from a molecule; reduction refers to the gain of electrons by a molecule.) In plant photosynthesis, the energy of light is used to drive the oxidation of water (H2O), producing oxygen gas (O2), hydrogen ions (H+), and electrons. Most of the removed electrons and hydrogen ions ultimately are transferred to carbon dioxide (CO2), which is reduced to organic products. Other electrons and hydrogen ions are used to reduce nitrate and sulfate to amino and sulfhydryl groups in amino acids, which are the building blocks of proteins. In most green cells, carbohydrates—especially starch and the sugarsucrose—are the major direct organic products of photosynthesis. The overall reaction in which carbohydrates—represented by the general formula (CH2O)—are formed during plant photosynthesis can be indicated by the following equation:
This equation is merely a summary statement, for the process of photosynthesis actually involves numerous reactions catalyzed by enzymes (organic catalysts). These reactions occur in two stages: the “light” stage, consisting of photochemical (i.e., light-capturing) reactions; and the “dark” stage, comprising chemical reactions controlled by enzymes. During the first stage, the energy of light is absorbed and used to drive a series of electron transfers, resulting in the synthesis of ATP and the electron-donor-reduced nicotine adenine dinucleotide phosphate (NADPH). During the dark stage, the ATP and NADPH formed in the light-capturing reactions are used to reduce carbon dioxide to organic carbon compounds. This assimilation of inorganic carbon into organic compounds is called carbon fixation.
During the 20th century, comparisons between photosynthetic processes in green plants and in certain photosynthetic sulfur bacteria provided important information about the photosynthetic mechanism. Sulfur bacteria use hydrogen sulfide (H2S) as a source of hydrogen atoms and produce sulfur instead of oxygen during photosynthesis. The overall reaction is
In the 1930s Dutch biologist Cornelis van Niel recognized that the utilization of carbon dioxide to form organic compounds was similar in the two types of photosynthetic organisms. Suggesting that differences existed in the light-dependent stage and in the nature of the compounds used as a source of hydrogen atoms, he proposed that hydrogen was transferred from hydrogen sulfide (in bacteria) or water (in green plants) to an unknown acceptor (called A), which was reduced to H2A. During the dark reactions, which are similar in both bacteria and green plants, the reduced acceptor (H2A) reacted with carbon dioxide (CO2) to form carbohydrate (CH2O) and to oxidize the unknown acceptor to A. This putative reaction can be represented as:
Van Niel’s proposal was important because the popular (but incorrect) theory had been that oxygen was removed from carbon dioxide (rather than hydrogen from water, releasing oxygen) and that carbon then combined with water to form carbohydrate (rather than the hydrogen from water combining with CO2 to form CH2O).
By 1940 chemists were using heavy isotopes to follow the reactions of photosynthesis. Water marked with an isotope of oxygen (18O) was used in early experiments. Plants that photosynthesized in the presence of water containing H218O produced oxygen gas containing 18O; those that photosynthesized in the presence of normal water produced normal oxygen gas. These results provided definitive support for van Niel’s theory that the oxygen gas produced during photosynthesis is derived from water.