Here is an essay on ‘Photosynthesis’ for class 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Photosynthesis’ especially written for school and college students.
Essay on Photosynthesis
Essay Contents:
- Essay on Introduction to Photosynthesis in Crop Plants
- Essay on Light Used by Crop Plants during Photosynthesis
- Essay on the Photosynthetic Apparatus of Crop Plants
- Essay on Leaf as a Photosynthetic Organ of Crop Plants
- Essay on Photosynthate Utilization by Crop Plants
Essay # 1. Introduction to Photosynthesis in Crop Plants:
Agriculture is basically a system of exploiting solar energy through photosynthesis. The primary source of energy for humans, photosynthesis has supplied the energy for food, feed, and the fossil fuels that power electrical generating plants and many machines. A study of crop physiology soon leads to the discovery that the yield of crop plants ultimately depends on the size and efficiency of this photosynthetic system.
Crop management practices proceed from this assumption. Because photosynthesis is the cornerstone of crop production, it is important to be aware of the energy available to drive photosynthesis and to consider how the anatomical features and biochemical processes in the plant interact to capture and store radiant energy.
Visible light, the source of energy used by the plant for photosynthesis, is part of the radiant energy spectrum. Radiant energy has unique characteristics that can be explained by using two related theories, the electromagnetic wave theory and the quantum theory. The electromagnetic wave theory states that light travels through space as a wave. The number of waves passing a given point in a certain interval of time is a frequency.
v = c/λ
Where v = frequency (wavelengths/sec), c = speed of light (3 × 1010cm/sec), and λ = wavelength. If we divide the speed of light by the frequency, we obtain the wavelength.
The quantum theory states that light travels in a stream of particles called photons. The energy present in one photon is called a quantum. Because the energy present in one photon is proportional to the frequency, the quantum can be expressed in terms of wavelength and the energy per photon is inversely proportional to the wavelength.
E = hc = hc/λ or E = hcv̅
Where E = photon energy (quantum), h = Planck’s constant (6.624 × 10-27 erg. sec or 1.58 × 10-34 cal. sec), c = speed of light (3 × 1010 cm/sec), and λ = wavelength and v̅ is wave number (1/λ). Light reaction of photosynthesis is a direct result of photon absorption by pigment molecules such as chlorophyll.
Not all photons have the proper energy level to excite leaf pigments- above 760 nm they do not have enough energy; below 390 nm, they (if absorbed by cutin) have too much, causing ionization and pigment degradation. Only photons with wavelengths between 390 and 760 nm (corresponding to visible light) have the proper energy level for photosynthesis.
Because pigment excitation is a direct result of interaction between a photon and the pigment, a measure of light used in photosynthesis is often based on photon flux density rather than on energy. Photon flux density is the number of photons striking a given surface area per unit of time. Since wavelengths between 400 and 700 nm are most efficiently used in photosynthesis, light measurement for photosynthesis is usually based on photon flux density within those wavelengths.
These measurements are called photosynthetically active radiation (PAR) or photosynthetic photon flux density (PPFD). The term Einstein (E) is defined as one mole of photons, so PAR is often listed in terms of µE. m-2. sec-1, or under the international system of units as simply µmol . m-2. s-1.
Essay # Photosynthesis and Solar Radiation:
The radiant energy available for photosynthesis on earth comes from the sun. Every energy source used by humans, directly or indirectly, results from solar radiation, with the exception of atomic energy and possibly geothermal energy. For crop growth and development, the sun is the only source of energy.
The sun is a blackbody radiator, and according to Wein’s law, the maximum wavelength is inversely proportional to the heat of the body and-
Max λ = 2.88 × 106/K
Where 2.88 × 106 is Wein’s displacement constant and K is the temperature.
For example, the temperature of the sun is believed to be 5750 K, so-
Max of sun = (2.88 × 106)/5750 = 500 nm (green)
Thus, the solar radiation spectrum has a peak at λ of 500 nm. Plants have apparently adapted to solar radiation because the visible light of λ between 400 and 700 nm corresponds to 44 to 50% of the total solar radiation entering the earth’s atmosphere.
The solar constant is 2.00 cal . cm-2 . min-1 (1395 W . m-2). It is the amount of energy received on a flat surface that is perpendicular to the sun’s rays and immediately outside the earth’s atmosphere.
The solar radiation level decreases as it passes through the earth’s atmosphere due to absorption and scattering. The solar radiation at the earth’s surface, when that surface is perpendicular to the sun’s rays, is reduced from 2.0 to between 1.4 and 1.7 cal . cm-2 . min-1 on a clear day.
The axis, the earth spins around is tilted in relation to the sun. Therefore annual cycles and diurnal (daily) cycles of solar radiation are governed primarily by latitude.
Because of this latitude effect, the following factors influence the amount of solar radiation received in one day:
1. Angle of the sun’s rays directed on that spot. When solar radiation comes in at smaller and smaller angles from perpendicular to the earth’s surface, the light spreads out over a larger ground area, reducing the light level per unit of ground area.
2. Day length.
3. The amount of atmosphere the radiation passed through as a function of the angle of the sun’s rays. If the sun is 90° overhead, the number of atmospheres light must pass through equals one; at 60° it is equal to two atmospheres, and at 30° it is equal to five atmospheres.
4. The number of particles (e.g., dust or condensed water particles such as fog or clouds) in the atmosphere. In many tropical regions much less light hits the earth’s surface in the cloudy monsoon season than in the cloudless dry season.
5. Other minor factors, such as fluctuations of solar output, distance of the earth from the sun, and the earth’s reflecting ability.
Of the solar radiation absorbed during the daytime by a crop surface, 75 to 85% is used to evaporate water; 5 to 10% goes into sensible heat storage in the soil; 5 to 10% goes into sensible heat exchange with the atmosphere by convection processes; and 1 to 5% goes into photosynthesis.
Since the maximum solar radiation level occurs in June and July for the northern hemisphere, the untutored observer might expect agriculturalists to always have their crops ready to make their peak growth at that time (e.g., to have sorghum at the grain-filling stage).
However, the opportunity to utilize this radiation peak is limited by seasonal temperature boundaries and the fact that most crops must develop from small seeds or other small organs before the economic yield (the harvested portion of the dry matter) can be produced. The challenge to crop physiologists and plant breeders is to develop crops and crop management practices that will place the crop in the appropriate growth cycle to take maximum advantage of this radiation peak.
Essay # 3. The Photosynthetic Apparatus:
i. Light Reaction:
Electron microscopy has made it possible to look more closely at the chloroplast, which is the photosynthetic apparatus of the plant.
The chloroplast, a lens-shaped organelle 1 to 10 µm across, displays two key areas:
(1) The lamellae (membranes), consisting of stroma lamellae (a double lamella) and grana lamellae (stacked lamella), both of which are concentrated areas of photosynthetic pigments.
(2) The stroma, a less dense, fluid area where the reduction of carbon dioxide (dark reaction) occurs. The transformation of light energy to chemical energy (photophosphorylation) occurs in lamellae and consists of the oxidation of water and production of chemical potential, or reduced nicotinamide adenine dinucleotide phosphate (NADPH) and the phosphorylation of adenosine diphosphate (ADP) to adenosine triphosphate (ATP).
The NADPH is one of the most powerful reductants (acceptors of electrons and suppliers of hydrogen ions) known in biological systems. ATP is synonymous with available energy in the biological system; when a phosphate group is released from ATP, energy is also released.
The released phosphate, attaching to some molecule (phosphorylation) by an energy input, raises the energy of the molecule and allows it to undergo even more chemical reactions. Both NADPH and ATP are needed to convert carbon dioxide (CO2) to organic molecules.
The electron transport system is fairly well understood. There are two reaction centers where energy from absorbed photons is used to drive the system. These reaction centers have many pigment molecules. When a pigment like chlorophyll or a carotenoid absorbs a photon, the energy lifts an electron (e–) from lower (ground) energy state to a higher (excited) state.
While in this excited state the pigment molecule can donate and accept electrons from other molecules. Photosystem II catalyzes the removal of electrons from water molecules, and these electrons are accepted by a substance labeled Q. Photosystem I, using more energy from absorbed photons, catalyzes the removal of electrons from Q. This sets up the energy needed for photophosphorylation (ATP formation) and the reduction of NADP+. The chloroplast lamellae are specialized membranes containing the pigments, proteins, and lipid materials that facilitate electron transport.
The pigments in the chloroplast lamellae consist largely of two kinds of chlorophyll (a and b) and two kinds of yellow to orange pigments classified as carotenoids (carotenes and xanthopylls). The structures of some of these pigments are shown in Figure 1.11. Experiments indicate that the porphyrin ring of chlorophyll is associated with the protein component of membranes, and the phytol tails as well as the hydrophobic carotenoids are probably associated with the lipid interior of the lamella.
Carotenoids serve as auxiliary pigments in light absorption. Some are inactive; others absorb light and transfer excited electrons to chlorophyll as well as from one photosystem to another, a phenomenon known as Emerson enhancement. In addition, they appear to have the capacity to slow the rate of chlorophyll photodestruction.
Through adaptation the spectrum of light absorbed by chlorophyll and other leaf pigments corresponds to the visible light range of the human eye. The light absorption by the leaf is quite different from light absorption by chlorophyll in ether. Figure 1.12 shows that quantum efficiency (moles of CO2 reduced per mole of photons) in field beans ranges from 8 to 12% in monochromatic (single wavelength) light from 400 to 700 nm.
The red range is most efficient, blue next, and green the least. There is not as much variation in either the action spectrum or the adsorption spectrum of leaves from 400 to 700 nm as one might expect when looking at the variation in light absorption by individual leaf pigments.
Acceptors and donors of e– other than pigments are associated with the lamella proteins. One of these compound types is cytochromes, which are proteins that have porphyrin rings similar to chlorophyll. However, the central metallic element is iron (Fe), not magnesium that gives up or accepts the e–. In other compounds copper (Cu) donates or accepts e–.
e– + Cu++ = Cu+
e– + Fe+++ = Fe++
ii. Carbon Dioxide Fixation:
Agriculture is based on the yield or weight of crop products. Since the harvested weight is usually calibrated at specific moisture content, yield is equated to dry matter production by the plant, which is the balance between CO2 uptake (photosynthesis) and CO2 evolution (respiration).
During growth daily respiration for most crop species under cropping environments is 25 to 30% of total photosynthesis, so the plant increases in dry weight. When respiration is more than photosynthesis, the plant loses dry weight, as can be shown by putting a plant in the dark, preventing photosynthesis.
Light reactions transform light energy to the short-term chemical energy of NADPH and ATP. These compounds are then used to reduce CO2 to stable organic compounds from which dry weight results.
Essay # 4. Leaf as a Photosynthetic Organ:
The leaf serves as the major photosynthetic organ of higher plants. Leaf evolution has provided a structure that will withstand environmental rigors and yet provide both effective light absorption and rapid CO2 uptake for photosynthesis.
Most crop leaves have:
(1) A large, flat external surface.
(2) Upper and lower protective surfaces.
(3) Many stomata per unit area.
(4) Extensive internal surface and interconnecting air spaces.
(5) An abundance of chloroplasts in each cell.
(6) A close relationship between the vascular and photosynthetic cells.
A leaf ideal for gaseous exchange and light interception would be only one cell thick, but the rigors of the natural environment demand several layers of cells and a protective surface for survival.
The large, flat external surface of the leaf allows for maximum light interception per unit of volume and minimizes the distance CO2 must travel from leaf surface to chloroplast, a distance of around 0.1 mm for the leaves of most crops.
The epidermis serves as a barrier to gaseous exchange primarily because epidermal cells are covered by a waxy layer called the cuticle. Both the cuticle and the epidermis are nearly transparent and readily allow visible light to enter the leaf.
The cuticle prevents gaseous exchange between leaf and atmosphere, which is important in preventing excess water loss. Most of the gaseous exchange in leaves occurs through stomata. There are many stomata on the leaf surface (12 to 281 stomata. mm-2), which allows for maximum CO2 diffusion into the leaf when stomata are open (Table 1.1).
Guard cells surrounding the stomatal opening control opening and closing. Stomatal closing is important in preventing water loss when water is limiting, but at the same time it limits CO2 uptake for photosynthesis. Most crop species are grown under full solar radiation and have stomata on both sides of the leaf. Most shade species have stomata only on the abaxial (lower) epidermis.
Inside the leaf are many mesophyll cells and intercellular spaces. Dicot and grass types have different leaf anatomies, but there is little indication that any one structure is more efficient in intercepting light or in CO2 diffusion. However, anatomical differences among C3, C4, and CAM species do affect photosynthetic efficiency.
The many mesophyll cells in leaves increase the total internal surface area (6 to 10 times the exterior area) to allow CO2 to come into more contact with cell walls. The intercellular spaces allow for rapid CO2 diffusion from stomata to cell surfaces. The pathway of CO2 into the leaf is from stomata to the cell walls, where it dissolves in water and then diffuses to the chloroplast due to a gradient established by CO2 fixation.
Most mesophyll cells contain a large number of chloroplasts (20 to 100 per cell) where the light reaction for photosynthesis takes place. When light illuminates the leaf, the chloroplasts often congregate along the side of the cell wall, orienting themselves to intercept the most light under dim conditions or sometimes to intercept the least light under high-light conditions. Being close to cell walls also facilitates rapid CO2 diffusion from cell walls into chloroplasts.
Leaf cells are not far from vascular tissue, which allows for rapid movement of water and minerals to the photosynthetic cells and of photosynthetic products from the cells and from the leaf. Reduction in the movement of raw materials to the chloroplasts or of products from the chloroplasts can reduce the photosynthetic rate.
Essay # 5. Differences in Photosynthetic Rates among and within Species:
Figure 1.20 shows CO2 exchange rates (photosynthesis) in response to light. Response A is typical for crop species with the C, pathway. Responses B and C represents crop species with the C3 pathway.
Response D represents C3 plants that show adaptation to shade conditions; these include certain hardwood trees and house plants. Plants with response D have stomata only on lower sides of leaves (this does not account completely, however, for their low photosynthetic rates) and are inefficient at dry matter production. Few if any crop species have response D photosynthetic rates.
Many studies have shown that varieties within a crop species have different CO2 exchange rates, with 2-fold to 3-fold ranges in CO2 uptake between the lowest and highest examples. This has encouraged speculation that yields might be increased by selecting for, and developing populations with, higher CO2 uptake rates.
Since C4 species have a high CO2 uptake rate and are among the most productive crop species (e.g., maize, sorghum, and sugarcane), it would appear desirable to introduce the C4 mechanism into C3 crop species. Several attempts have been conducted with soybeans, barley, and a few other crops to determine if any C4 plants occur in C3 crop species, but without success. In the future other methods may be tried to change a C3 to a C4 species.
Essay # 6. Photosynthate Utilization by Crop Plants:
i. For Storage and Structure:
Although it is convenient to regard photosynthesis as ending with the formation of hexose sugar, many further changes may occur. The hexose may immediately interconvert from glucose to fructose, or combine to form sucrose for translocation to other cells, or polymerize to starch for temporary storage in the chloroplast.
The sucrose may go to enlarging cell walls, where it may be transformed to structural components such as cellulose. The sucrose might also be transported to other areas of the plant where there is active growth (meristems) or where it is converted to polysaccharides as storage or structural compounds.
ii. Respiration and Growth:
Hexose can also enter into the respiratory system of the cell where it is broken down to release energy or be converted to organic components used in important structural, metabolic, and storage compounds. The first step is the anaerobic respiratory process called glycolysis, in which reduced nucleotides and ATP are formed to do work in the cells by splitting a hexose phosphate sugar to pyruvic acid.
The pyruvic acid then loses a carbon through oxidation to CO2, reduces NAD+ (since the reduced form of nicotinamide adenine dinucleotide [NADH] is used to reduce O2 to H2O the production and utilization of NADH is called aerobic respiration), and forms acetyl-coA. Acetyl-coA enters the Krebs cycle by combining with a four-carbon molecule from the Krebs cycle to make a six-carbon molecule.
In the Krebs cycle, more carbon is oxidized to CO2, which is coupled to the reduction of NAD+. At the same time, the compounds and energy produced in the Krebs cycle are used to form and transport amino acids and nucleic acids for polymer synthesis (i.e., proteins, RNA, and DNA). This energy comes from NADH oxidation, which is coupled to the reduction of O2 to H2O and the phosphorylation of ADP plus inorganic phosphorus (Pi) to ATP.
Since O2 is used in this process, it is called oxidative phosphorylation. The Krebs cycle occurs in the mitochondria between membranes, and oxidative phosphorylation occurs in the inner membrane of mitochondria. This membrane is very similar to chloroplast membranes except that it does not contain the photosynthesizing pigments. Oxidative phosphorylation utilizes an electron transport system similar to photophosphorylation; the major proteins involved are cytochromes.
Photosynthesis and respiration, although very similar, are in many ways opposing reactions. Both use energy for synthesis, but respiration must catabolize organic molecules to obtain energy for its processes (i.e., for the synthesis of storage, structure, and metabolic compounds and for processes such as translocation and nutrient transport across membranes).
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