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Photosynthesis

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Photosyn.jpg

Photosynthesis is the chemical synthesis of complex organic materials, especially carbohydrates, from carbon dioxide and water using sunlight as the source of energy and with the aid of associated pigments. Photosynthesis seems only able to occur when there is energy from starlight such as our Sun; the proper atmosphere, amount of water vapor and organisms equipped with biological function to harvest it on Earth.[1]

For most organisms on Earth, photosynthesis is the beginning of the food chain. Photosynthesis enables plants to produce food for themselves, which is then ultimately passed on to animals and microbial organisms when they are eaten. An important byproduct of photosynthesis is oxygen, which is a basic requirement of life for all higher lifeforms. Because plants make their own food, they are called producers (autotrophs), as opposed to animals which are consumers (heterotrophs). Photosynthesis is not only used in green plants, but also algae, and some forms of bacteria.

The chemical equation for the overall process of photosynthesis can be seen below. The reaction combines Carbon and Water - in simple terms, it is the process where carbons are hydrated (carbo-hydrate) with an approximate Carbon to H20 ration of 1:1. As an example, note that glucose has the chemical formula C6H1206, which is 6 x (C+H20).

Photosynthesis equation.png

Contents

Absorption and Distribution of Reactants

Water

The water and other dissolved nutrients that plants require to perform photosynthetic are absorbed from the soil by the roots. The continuous uptake of water into the roots, even in extremely dry climates, occurs due to the phenomenon known as osmosis. Distribution of water and nutrients throughout the plant then occurs by way of vascular tubes (xylem). Water is both a reactant and a product of photosynthesis.

Carbon Dioxide

Carbon dioxide (CO2) and oxygen are exchanged through pores in the leaves called stomata. These pores, which are located on the underside of the leaf, will typically remain open during the day while photosynthesis is occurring, and then close at night to reduce the loss of water.

Light

Plant cells with visible chloroplasts.

Photosynthesis takes place in the leaves of the plants within an organelle called a chloroplast. There are approximately 40-50 chloroplasts in each cell. The chloroplasts are filled with tiny compartments known as thylakoids, which are surrounded by a fluid filled area known as the stroma. In the thylakoids, pigments (such as chlorophyll) capture the light energy used to power photosynthesis. There may be millions of pigment trapping molecules within one leaf. [2]

Biochemical Reactions

Light Reactions

The leaf - primary site of photosynthesis in plants.

Light energy is the substance that drives the chemical reactions known as photosynthesis. The first phase of photosynthesis are "Light Dependent Reactions", which result in the formation of ATP and NADPH. These energy carrying molecules are used in the next phase of photosynthesis (dark reaction) to produce carbohydrates.

The first step in the light reactions is the absorption of photons of light by pigments within the chloroplast such as Chlorophyll. The energy that is trapped by chlorophyll excites electrons that then act as a reducing agent. The chlorophyll then passes its electron through a series of electron carriers called the electron transport chain. Each carrier in turn participating in an oxidation-reduction reaction (redox).

This series of reactions (electron transport chain) is very similar to that found in the mitochondria where ATP is produced from the breakdown or carbohydrates. Next the energy is the passed to the Photosystem II and the energized electrons are then passed along to the Photosystem I. Through the Photosystem I ATP is produced. The last step and the product of the light reaction is the energy is then stored in the ATP and the NADPH2 .

These are all components of a light reaction in photosynthesis. In the electron transport there are two things that are required which are Photosystem 1 and 2. These two are different ways that the electron transport is used. The ATP produced in phosphorylation comes from chemiosmosis.

The light is necessary for the production of oxygen. In the original model of photosynthesis they did not have the right amount of water on each side of the equation. In the revised equation they fixed the amount of water molecules going into the equation and also the number of water molecules coming out of the equation.

Calvin Benson Cycle

Dark Reactions

The dark reactions known as the Calvin Benson Cycle are light-independent. This is the metabolic pathway in photosynthesis that incorporates carbon dioxide to make carbohydrates (carbon fixation). This reaction takes place within the chloroplast stroma. Although this process is sometimes called the dark reaction, it only takes place only in the light.

There are three things this cycle go through. The first thing that must happen is that the reaction must fixate the carbon dioxide. Then the reduction of 3PG that was produced in the first reaction. Then this will form carbohydrates. Through this reaction it uses phosphorylation. The reaction will then need to regenerate the carbon dioxide. After this whole cycle, the final product will then be trios phosphate.(Purves, 151-156)

RuBisCO

RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) an enzyme that is used in the Calvin-Benson cycle, which is directly related to photosynthesis. This enzyme is used to fix carbon during photosynthesis. All photosynthetic organisms are largely similar. From bacteria to a plant they are almost identical in their properties. RuBisCO is a carboxylase and an oxygenase because it is both the reaction competes each other. When the RuBisCO is mixed with the oxygen it does not react with the carbon dioxide. This then causes the plant to be limited in its plant growth (Purves, 156).

C4 and CAM Plants

C4 Pathway

There are three types of photosynthetic processes. The reaction pathway used by most plants is called C3 because a 3-carbon compound (3-Phosphoglycerate) is the product of carbon fixation. C4 and CAM are alternative forms of photosynthesis utilized by a few plants.

C4

Photorespiration does not occur in C4 plants, such as corn, sugarcane, and other tropical grasses. These plants make a four carbon compound called oxaloacetate as the first product of the CO2 fixation, rather than 3PG (3-Phosphoglycerate). The C4 plants go through the Calvin-Benson Cycle, but in their reactions they do not lose carbon to photorespiration. The C4 plant has two separate ways to fixate CO2.(Purves, 158)

CAM

The CAM pathway is used by plants that have a higher humidity, which causes water loss. Some examples of plants that have this are cacti and other plants in the areas with higher temperature. This pathway uses more energy and causes stunt growth to the plant. These plants open their stomata at night to take in oxygen and then in the day time they close them. This is so that it minimizes water loss in the day time when the weather is hot [3].

History

In the beginning of the 19th century scientists knew that there were three ingredients involved in photosynthesis. These products were water, carbon dioxide and light. There are three things that scientists learned through this that water in photosynthesis comes through the roots and from the soil. They also learned that carbon dioxide is taken in and oxygen and water are the product of this reaction. They also learned that in order for this reaction to even take place the reaction must have light. In 1804 the original equation did not have water on both sides of the reaction. The matter was further studied and a century and a half past the first equation they added water to both sides of the equation [4].

Ecological Importance

Photosynthesis is considered the source of all living plants and animals. Photosynthesis may sometimes be thought to be only used in plants but on the contrary all living animals and plants use some sort of photosynthesis to keep living. Animals need oxygen to survive, and plants need the carbon dioxide that the animals produce to keep living. This whole process starts with the oxygen being released from the plants through photosynthesis.

The importance of photosynthesis is that the plant and animals would not be able to survive if photosynthesis did not occur. Animals and plants are both sources of nutrition for humans, which means that if the plants and animals did not have photosynthesis then humans would lose that source of nutrition. This would end the world as we know it because there would be no source of food or anything to nourish the human body [5].


The 2nd Law of Thermodynamics and Photosynthesis

A common claim aimed at resuscitating the notion of evolution is the response that the earth is not a closed system, because it receives energy input from the sun. The Creationist claim is that the 2nd Law of Thermodynamics (2LoT) repudiates the notion that random processes can go from a primordial soup to an increasingly higher state of order, complexity and information--it is analogous to a river flowing uphill and building a lake at the peak.

The Creationist response to this claim can be found on a number of TrueOrigin pages, such as this one and its rebuttal (to an attempted refutation of the first). In short, adding sunlight does not automatically reduce entropy. In fact, sunlight more often than not increases entropy, by aiding in decomposition. Anyone who has observed the color of their curtains fade over a course of years from sunlight will acknowledge the destructive properties of solar radiation. The fact of the matter is that unless there exists a process of conversion to harness the sunlight and turn it into mechanical energy for biological work (building cells), sunlight will only increase entropy, not decrease it.

Photosynthesis is a process by which solar energy is harnessed and used for work. It sustains nearly all of the biological life on the planet, currently. The issue is that when the evolutionist realizes that there needs to be an energy-conversion process, he will often retreat to the comfort of photosynthesis, and attribute its formation as being from "natural selection." Unfortunately, natural selection is the description of a phenomenon that 'acts on,' so to speak, already-existing replicating cells. For those cells to get there, they would require photosynthesis, or a similar process, assuming evolutionary uniformitarianism. Obviously this is circular reasoning, because natural selection cannot be the cause of photosynthetic compounds, if the compounds are required for the existence of cells, and the cells are required for natural selection to function. Moreover, the educated Creationist recognizes that natural selection, rightly considered, is not a force of nature at any rate, and cannot be appealed to as a creative agent or force of change.

Suffice it to say, Evolution remains a thermodynamic impossibility, regardless of whatever appeals to photosynthesis are made.

References

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