Photosynthesis in Plants

Photosynthesis is the process used by photoautotrophs such as plants and algae, and also certain bacteria such as cyanobacteria and purple bacteria. This process is one of the most important in nature for several reasons. The process of photosynthesis uses the power of the sun to make chemical energy in the form of glucose, which they use as their own energy.

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But the effects of this process aid other organisms across the planet, giving them the oxygen that they need to breathe and also making sure that the gases in the air are balanced, ( the composition of air is approximately 21% Oxygen, 78% Nitrogen, and 1% other gases) which helps keep earth at a safe temperature to sustain life. Photosynthesis was first researched in the 17th century by a Belgian scientist named Jan-Baptista v. Helmont. After studying the weeping willow plant, Helmont concluded that plants do not acquire all of their total product from the soil, but that they gain it from water. At the end of the 17th century, an English scientist named Joseph Priestley used his famous candle experiment and discovered that mint sprigs could restore air that was ‘wasted’ after a candle was burned in a closed container, his final conclusion was that plants produce fresh air.

Lastly, a Dutch scientist named Jan Ingenhousz discovered that plants required light on their green parts to restore air, and he thought that plants created oxygen from carbon dioxide (CO2 ). The chemical equation for photosynthesis is 6CO2 + 6H2O ‡† C6H12O6 + 6O2 (this equation is balanced). The formula is often depicted with the word sunlight above the arrow and the word chlorophyll below the arrow. Sunlight and Chlorophyll are essential to the process of photosynthesis, but neither of them has a chemical formula that can be used in the equation. One thing to note about the formula is that water is produced during the chemical or dark reactions of photosynthesis, however, water cannot appear as a product because to mathematically balance the equation. Photosynthesis is a reversible equation because its reverse, C6H12O6 + 6O2 ‡† 6CO2 + 6H2O + 38 ATP (balanced equation), is the equation for cellular respiration. Cellular respiration is a process in humans that is directly related to photosynthesis. It is a process that humans undertake to metabolize glucose into usable energy. The products of photosynthesis are the reactants in the equation for cellular respiration. Both photosynthesis and cellular respiration are interlinked meaning without one, the other would cease to exist. But because of photosynthesis, many species are able to exist, including humans.

The organisms that undergo photosynthesis are called photoautotrophs. Photo meaning light, auto meaning oneself, and troph meaning nourished by, a rough translation of the word would be an organism that feeds itself with light. These organisms are usually plants, algae, and a few bacteria. In plants and algae, the process of photosynthesis takes place in an organelle called the chloroplast. Bacteria also undergo photosynthesis but not exactly the same way. For example, cyanobacteria do not undergo photosynthesis in chloroplasts, because they lack chloroplasts. Layers of chlorophyll are dissolved into the cytosol of cyanobacteria. Chloroplasts are green, round organelles, that are found in plant cells. At one point chloroplasts were independent organisms that were engulfed by a eukaryote. Eukaryotes evolved to have chloroplasts, but chloroplasts still have their own DNA. This DNA is what allows chloroplasts to multiply by the process of binary fission. The chloroplast replicates its DNA and components and the divides in two. This organelle gets its green color from a pigment that it contains, called chlorophyll. Chlorophyll helps with the absorption of light during photosynthesis. A molecule of chlorophyll has a hydrophobic tail that embeds the molecule in the thylakoid membrane, the head of the chlorophyll is constructed from a ring called porphyrin which has an atom of magnesium at its center.

The head of the chlorophyll molecule is the location of light absorption. Chloroplasts are double-membraned organelles containing an outer membrane that is semi-permeable and an inner membrane that is much less permeable and also contains transport proteins. Another part of the chloroplasts are the thylakoids, which are folded membranes that contain pigments and enzymes. Several thylakoids stacked on top of each other are called grana, which is the location where light-dependant reactions occur. Light-independent reactions occur in the stroma, fluid area between the thylakoid and inner membrane. The stroma also contains starches and enzymes. Image of a chloroplast The process of photosynthesis starts by pigments absorbing light energy. It is possible to divide photosynthesis into two stages; light-dependant or photochemical reactions and light-independent or the Calvin cycle. Light-independent reactions are also known as dark reactions. While these reactions do not need light to occur, they can still happen in the presence of light. Light from the sun is classified as electromagnetic energy, and it behaves like both a light wave and also particles, its moves in units called photons. This electromagnetic energy forms the electromagnetic spectrum, with every wave having a particular wavelength.

A longer wave means the photons of that wave will have less energy. The visible part of the electromagnetic spectrum is known as the visible spectrum. This spectrum is made up of many different wavelengths when together form white light, but when separated appear in different color. The colors white light divide into are the colors of the rainbow, red, orange, yellow, green, blue, indigo, and violet. Image of the visible light spectrum Plants undergo photosynthesis by having their pigment absorb certain wavelengths of light. The most effective wavelengths for photosynthesis are red and blue. Green light is not effective for photosynthesis because plants reflect the color green, meaning it is not being used in photosynthesis. The first process that occurs in photosynthesis is the photolysis of water or the separation of hydrogen and oxygen. However, in bacteria that are sulfur photosynthetic hydrogen and sulfur are split from molecules of Hydrogen Sulfide (H‚‚S). Instead of oxygen being released in to the atmosphere from this split, sulfur is released. Oxygen and Sulfur have similarities as they have the same number of valence electrons. Molecules of water are exposed to light energy and that forces the water to release protons, which are hydrogen atoms. The process of water photolysis could be considered the basis of photosynthesis because the products are used in the dark reactions and the Calvin cycle, so without water hydrolysis, the second phase of photosynthesis would not be possible.

Oxygen is also released back into the atmosphere, along with parts of Adenosine Triphosphate (known as ATP). ATP is considered the energy currency of life. Energy is stored between the bonds of the phosphate groups of the molecule. The broken down form of ATP is called Adenosine Diphosphate or ADP because it has two phosphates instead of three. When the phosphate is removed a large amount of energy is released that is used by the cell to do work. This conversion is a cycle because until energy is needed ATP will stay as ATP, but also when energy is not needed ADP will convert itself back to ATP. When energy is not immediately needed the additional phosphate group is reattached using sunlight, this process is called ADP phosphorylation, the enzyme that synthesizes this reaction is called ATP synthase. In the mitochondria of a cell, ADP returns to ATP by a similar process called oxidative phosphorylation, where ATP synthesis is driven by electron transfer driven by oxygen. Image of ADP phosphorylation The products that result from this stage in photosynthesis are oxygen, hydrogen, and free electrons. The products from this stage that are used in the dark reactions are ATP and NADPH. Oxygen is released in the atmosphere, the hydrogen ions are used in glucose synthesis in the dark reactions, lastly, the electrons replace electrons that were lost in photophosphorylation by molecules of chlorophyll. In photosynthesis, there are two photosystems, Photosystem I and Photosystem II.

Each has a special molecule of chlorophyll at its center. In Photosystem I this is called P700 and in Photosystem II this is called P680. The light-dependent reactions of photosynthesis take place in Photosystem II after electrons transfer energy from one pigment to another, which increases the energy level until the electron reaches the center of the chlorophyll molecule. Molecules of P680 absorb the energy and eject electrons that are accepted in the thylakoid membrane, and the electron that was ejected is replaced with one from water. The remaining oxygen is released. This process is repeated and it created ATP through phosphorylation. Eventually, the electron that was taken into the thylakoid membrane travels to the electron transport chain it arrives at Photosystem I and is accepted by P700. It goes along another electron transport chain and joins the electron acceptor NADP+ (Nicotinamide adenine dinucleotide phosphate), which becomes NADPH. Image of the light-dependent reactions The dark reactions use two of the products produced in the photochemical phase of photosynthesis. ATP is used as an energy source to fuel the dark reactions, while Nicotinamide adenine dinucleotide phosphate (NADPH) is used to reduce CO2 and deliver hydrogen atoms to the molecules that are the precursors of glucose. If it were not for these byproducts of the photochemical reactions, photosynthesis would not be able to fully occur. These products are used in a process of dark reactions called the Calvin cycle.

The Calvin cycle can be broken into three parts, carbon fixation, reduction, and regeneration. Carbon fixation starts with carbon dioxide that diffused through the stomata of the leaves of a plant to the stroma of the plant. At this point, fixation can go two separate ways depending on the plant. The two types of fixation are C-3 or C-4. Peanut plants, beets, lawn grasses, and most trees are plants that use C-3 fixation. One enzyme of photosynthesis called rubisco wastes carbon that has been fixed and used oxygen instead of carbon dioxide. This event is called photorespiration and C-3 plants have no way to avoid this process. These C-3 plants also fix carbon dioxide directly from the air, and the main types of plants that do this are cool-seasonal plants Some examples of plants that undergo C-4 fixation are cacti, corn, and sugarcane or in hot and humid locations. C-4 fixation is able to bypass photorespiration. The 4 carbon acid is transported and broken down to release carbon dioxide. This will then be attached to ribulose bisphosphate (RuBP) by rubisco. RuBP produces a six-carbon compound that divides itself in two and is known as 3-phosphoglycerate (3-PGA). There is another form of carbon fixation called Crassulacean Acid Metabolism (CAM). Plants that undergo CAM are normally succulents, and plants like pineapples. In the CAM process carbon is only taken up at night and then the steps of C-4 fixation take place. Comparison of C-3 plants against C-4 plants

The reduction phase of the Calvin cycle is next. It involves the ATP and NADPH that were produced in the light-dependent stages of photosynthesis. Every molecule of 3-PGA that was produced in carbon fixation is given a phosphate group from ATP, which then creates a bisphosphoglycerate. This molecule then gains two electrons from a molecule of NADPH, and lose a phosphate group, which converts them to glyceraldehyde 3-phosphate (G3P).Two molecules of G3P are used to create one molecule of glucose. The byproducts of this reduction reaction are ADP and NADP+, which are able to be reused in the light-dependent reactions. The final stage of the dark reactions is regeneration. During this phase, G3P is used to synthesize carbohydrates and to create RuBP. It takes three molecules of carbon dioxide to produce six molecules of G3P. Five molecules of G3P are regenerated as RuBP and one is left for the synthesis of glucose. The glucose that is synthesized is stored as starch in plants. Image of the Calvin Cycle One thing that allows photosynthesis to occur is the green leaves of plants. Green leaves are the main location of photosynthesis. Leaves have features that promote photosynthesis. The surface area of leaves is the largest green area and it catches the most sunlight, and they are thin which lets light and gases travel without issue. The leaf can be further broken down into several separate regions, the upper epidermis, palisade mesophyll, spongy mesophyll, and the lower epidermis.

The upper epidermis is the top layer of the leaf and often has a wax-like layer on top of it that helps retain water. This layer is thin and lacks a sizable population of chloroplasts, so it is easy for light to pass through to the next layer. The palisade mesophyll is the layer that contains the population of chloroplasts. These chloroplasts are very tightly packed and absorb much of the sunlight and a majority percentage of the photosynthetic process occurs here. The spongy mesophyll the next layer and it is a critical part of the gas exchange between the atmosphere and the leaf itself. This layer contains the special pores called stomata, which allow gas exchange. Carbon dioxide is taken up her and oxygen is released. The bottom layer of the leaf is the lower epidermis. The layer has guard cells that cover the stomata, which control when carbon dioxide is taken in and water is allowed to evaporate out. Image of the layers of green leaves Photosynthesis is a process that needs particular environmental conditions to be fully effective, or to occur at all. There are three main factors that can limit the process of photosynthesis which are light intensity, carbon dioxide concentration, and temperature. Without enough light, plants are not able to photosynthesize quickly, but increasing the light will increase the rate of photosynthesis, but another limiting factor will make the rate steady. Plants cannot photosynthesize properly without enough carbon dioxide in the air, even with sufficient light and the right temperature.

In the case of temperature, if it is too cold the rate of photosynthesis will decrease because of a lack of enzyme activity. Conversely, if the temperature is too high (37 „? ) photosynthesis will not occur. This is because enzymes control photosynthesis, and enzymes are proteins, at high temperatures, proteins begin to denature. Limiting factors are part of the reason that the leaves on some trees change color during the autumn season. As the days get shorter and nights get longer there is less time for photosynthesis, as winter approaches the temperature decreases which also slows the rate of photosynthesis in plants. Some plants move nutrients that were made in the leaves of plants into nutrient stores elsewhere in the plant, often the branches, roots, or trunk. Less chlorophyll is produced so the green color fades away. Some trees lose their leaves to save water and the harsh cold temperatures would kill the leaves regardless. Other trees have needles which are strong enough to survive winter temperatures. Aerobic respiration is considered the anti-process to photosynthesis because its fuel is glucose. This process goes hand-in-hand with light intensity, although the relationship is not direct.

The rate aerobic respiration does not depend on the intensity of light energy, photosynthesis does. Aerobic respiration depends fully on the energy needs of the plant. If the rate of respiration is higher than the rate of photosynthesis, then it means a plant is using more glucose than it is consuming. Conversely, if the rate of respiration is lower than that of photosynthesis, there is a positive balance of glucose. If the rate of both respiration and photosynthesis are equal all the carbon dioxide that is released is consumed by photosynthesis and all of the oxygen produced in photosynthesis is used in respiration, and there is not a positive balance of glucose or any depletion in stored carbohydrates. Whatever the light energy level is when the rate of photosynthesis and aerobic respiration are equal is called the compensation point. This is also the point where a plant will stop growing. Breakdown of Aerobic Respiration Photosynthesis is an important process, its effects are felt all across the planet from plants creating their own food to providing oxygen for humans and animals to breath, to helping keep the earth’s temperature a comfortable one to sustain life on the planet. This process was first discovered in the 17th century by European scientists. Plants, algae and a few types of bacteria provide energy for not only themselves but an entire planet. Without these photoautotrophs, life on earth would cease to exist.

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