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Photosynthesis is the process by which plants use sunlight to produce energy and add oxygen to the atmosphere. The oxygen atoms from this process then combine with hydrogen to form H2O (water).


 What is photosynthesis?

Photosynthesis is the process by which plants use sunlight to produce energy and add oxygen to the atmosphere. The oxygen atoms from this process then combine with hydrogen to form H2O (water).

How does photosynthesis occur?

The molecules in chlorophyll, which give a plant its green color, absorb light of various wavelengths and are excited. This excitation gives energy for them to perform their function as an antenna that collects electrons that were previously used in chemical reactions but not needed anymore.


An electron is then passed down a chain of carriers that ends up with the ultimate electron acceptor, which produces a molecule called NADPH. As the electrons travel down this chain, the carbon dioxide is fixed through a series of chemical reactions to form glucose. This process is called carbon fixation.


Sunlight energy is used to convert this fixed carbon into chemical forms such as glucose and later used to build new products such as starches, proteins and lipids (fats).

Plants produce oxygen during this process through a series of events:

 1) First, water acts in its usual role as an electron donor and removes two electrons from NADP+.

2) Next, the electrons move to the oxygen molecule and produce a superoxide intermediate.

3) The superoxide is then broken down into hydrogen peroxide by an enzyme called superoxide dismutase.

4) Finally, the hydrogen peroxide is broken down into water and oxygen by an enzyme called peroxidase.

In summary, photosynthesis occurs when light energy from the sun is absorbed by green plants to start chemical reactions that produce glucose from carbon dioxide. The glucose is stored and used later as a source of energy, while the oxygen produced is released into the atmosphere.

What happens to the water molecules formed during photosynthesis?

Photosynthesis produces not only glucose, but also water. It takes six molecules of water to form one molecule of glucose, which is why photosynthesis is a process that consumes water. Also, notice that 6CO2 + 6H2O → C6H12O6 + 6O2 and that a net gain of oxygen occurs after glucose has been made from carbon dioxide.


Does photosynthesis take place in the dark of night?

No. Photosynthesis occurs during the day when plants are actively using it. However, even at night green plants will use photosynthesis to some extent (you can do it too by putting a plant in a dark room and looking to see if it is moving). The plastids that are used for photosynthesis also use water that was formed during the nuclear reactions of photosynthesis, so it is possible for them to still be active in the absence of light even if no light reaches them.


The efficiency of photosynthesis as measured by oxygen produced for ATP synthesis seems to have changed little over about 100 million years since the evolution of land plants.


Davies and Bowman (2005) have described the evolution of photosynthesis by land plants, describing in some detail how this process developed from that in aquatic plants.

The following is an extract from their paper:


Thus, the role of light energy in the evolution of photosynthesis was essential for early land plants to be able to compete successfully with their aquatic ancestors. Nevertheless, even though there is evidence for a large change between aquatic and terrestrial plants in terms of efficiency, there does not appear to have been a major change between the evolution of water- and land-based photosynthetic systems rather than in the magnitude of rates of evolution.


The first land plants evolved from the earliest green algae over 500 million years ago, and later evolved a faster hydrogenation rate. The evolution of land plants caused a number of changes to occur which have led to the ability to manufacture ribulose-1,5-bisphosphate (RuBP) (an electron donor) faster than water can be lost through chemical reactions.

This increased efficiency means that more carbon dioxide is fixed, leading to more growth, which in turn leads to more mechanical and chemical energy being available for photosynthesis. This feedback loop leads to a stable initial level of photosynthesis taking place: the biochemical efficiency limits change very little over time.


These fundamental features of photosynthesis have changed very little in the course of plant evolution, and have remained virtually unchanged since the origin of land plants. However, there have been minor changes in some aspects of photosynthesis since then, mainly involving the response to changing environmental conditions. Thus, in order to try to understand more fully how these mechanisms work, it is useful to consider some of the major changes that have taken place during this time. These include adaptations to different environments and varying levels of light intensity.

Light Intensity

Traditionally, light intensity has been measured by color temperature (in units of Kelvin), which is a measure of how much heat energy a light bulb gives off. The lower the temperature, the more energy can be given off, so a light bulb with a high color temperature (very high heat output) is able to give off more heat energy. The greater the intensity of light and the higher its color temperature, the greater the photosynthesis efficiency.


However, it is important to take into account two other factors that could reduce photosynthesis efficiency under some conditions:

1. Two photons of light can react with one enzyme to form an excited state (one of the electrons is transferred to a higher energy level). If this excited state is not further activated, it will revert to its original state by releasing heat energy. This will slow down the reaction and reduce efficiency.


2. The rate of photosynthesis is limited by the availability of carbon dioxide (CO2) in the atmosphere, which depends on factors such as temperature, pressure and concentration. Thus, in low concentrations or high temperatures (when CO2 becomes less soluble), photosynthesis increases more slowly than in high concentrations or lower temperatures.


It is obvious therefore that, in order to understand how the efficiency of photosynthesis has evolved and still evolves today, it is necessary to look at changes in all three factors: increasing light intensity, the ability to activate more excited states of oxygen, and more efficient fixation of carbon dioxide.


The source of energy in the photosynthetic process comes from solar radiation that can be captured by chlorophyll compounds known as porphyrin pigments. Chlorophyll can absorb energy from red/orange and blue light but cannot capture infrared or green wavelengths. The remaining wavelengths are referred to as non-absorbed or 'white' light. The importance of absorption spectra to a plant's ability to capture light energy and employ it in the process of photosynthesis, is demonstrated by the fact that photosynthesis can occur at low levels with limited availability of red and blue light, if green light is available. This means that some form of chlorophyll molecule exists that can absorb green wavelengths. The chlorophyll "a" molecule is known to be able to absorb red and blue light as well.


The absorption spectrum for chlorophyll "a" shows that the amount of energy captured will vary depending on the intensity (or strength) of incoming light. This is due to the ability of chlorophyll light-harvesting complexes to vary the amount of energy they capture when illuminated.


It should be mentioned that the absorption spectrum of chlorophyll a is only moderately sensitive to changes in light intensity in that there is a significant margin over which absorption will occur at any given intensity level. This means that even though the measured light intensity may vary over a wide range, there are likely to be only minor differences in plant performance (photosynthesis) under changing intensities.


The most important factor required for photosynthesis to occur, is not just the energy absorbed from visible light but also includes energy absorbed from non-visible wavelengths such as ultraviolet and infrared. All plants absorb energy in the visible wavelengths and red/orange/yellow light is important when these wavelengths are at a high intensity. In low light intensity, energy from infrared rays becomes of importance and green light energy becomes significant at even lower light intensities.


There are two main reactions that occur in photosynthesis and they are called the Z scheme (also known as the Calvin cycle) and the Calvin-Benson Cycle. The Z scheme occurs only in plants, algae, and cyanobacteria. The Calvin-Benson Cycle does occur in plants, algae, and cyanobacteria but can also occur in some species of fungi and protozoans as well.


The Calvin Cycle is a photosynthetic reaction and occurs in the chloroplasts of plants and algae.

This process takes place in the the thylakoid of green and cyanobacteria. This membrane contains granules called thylakoids that are made from stacks of flattened sacs (each with a central stromal lumen). Each sac is formed from a flattened membrane-bound compartment that contains chlorophyll complexes as well as proteins, lipids and carbohydrates (depending on the species). The membranes that are formed by these various compartments are stacked one on top of another to form stacked sacs, each containing a single thylakoid.

The process of photosynthesis in the Z scheme occurs when light energy is absorbed by the thylakoid. The energy causes electrons to be excited to higher energy levels that can be used as a source of energy when released as heat. The electron is first directed through an electron transport chain that includes a number of molecules. It then reaches a series of molecules that act as electron carriers (electron acceptors). These electrons are passed on to oxygen molecules, which contain free or loosely bound oxygen ions. The oxygen ions are attracted to their nearest neighbors and combine with the electrons (to form water) and pass the electrons on again to the next electron acceptor molecule. This continues until the electrons reach a molecule called plastoquinone (PQ), which shuttles the electrons to a compartment called the cytochrome system. The electrons then pass through this system before they are used to reduce NADP and produce NADPH.


In order to understand how photosynthesis takes place in both reactions and why there are two of them, it is important to understand how the reactions occur and what their products are. The Calvin Cycle and the Calvin-Benson Cycle are both distinct chemical reactions that take place in separate compartments of the plant cell. However, their products are similar and they form pairs of compounds that are important to both reactions.


The products formed in the Z reaction include:

The products formed in the Calvin-Benson reaction include:


The Z-reaction (the Calvin Cycle) takes place at the upper surface of chloroplasts, in the thylakoid membrane. It is energetically more favourable to use energy derived from light from the green portion of visible light spectrum so that reaction proceeds until PQ is consumed. This results in a proton gradient being produced which drives an ATP synthase, which synthesizes ATP degradatively via phosphorylation and hydrolysis of ADP.


Using this mechanism, photosynthesis levels off at a optimum level where photosynthesis is maximised because PQ is consumed as fast as it can be supplied by photosystem II.


This photolysis of PQ is where one molecule of PQ combines with two molecules of NADP to form NADPH. This process takes place at the cytochrome system, which then passes the electrons to plastoquinone (PQ), which is a component of the photosystem II.


If, as occurs when light intensity increases, there is not enough energy being absorbed by chlorophyll, reaction will switch from the Z-reaction to the Calvin Cycle and PQ will also be consumed. The Calvin cycle proceeds until C is consumed and then it switches back to photosynthesis in the Z reaction.


The Calvin-Benson Cycle takes place in the stroma of the endoplasmic reticulum. It is a series of chemical reactions in which CO, derived from both CO and O, is combined with and separated into molecules of 3-Phosphoglycerate (3PGA) by the addition of ATP, ADP and Pi molecules. The result is then synthesized into glucose by the continuing addition of Pi molecules to each molecule of 3PGA until 6C are linked together to form glucose.


It should be mentioned that during photosynthesis, plants break down starch from their food source through a process called photorespiration. Phospohorespiration is a process by which plants maintain the elevated levels of carbon dioxide within the plant's cells while using oxygen in the production of sugars.

The role of the Calvin-Benson Cycle in photosynthesis is to replenish those carbondioxide molecules used to produce glucose. In photorespiration, plants use CO rather than O as a reactant and product of photosynthesis, producing acetic acid instead of glucose as its major end product. Plants generally do not favor this process over normal photosynthesis, so they use the Calvin-Benson cycle to convert CO back into O for reuse in photosynthesis.


The Calvin-Benson Cycle is also used by plants in the production of amino acids, which are necessary for the synthesis of proteins. The cycle is also used to make lipids, vitamins and purines.


Plants make both glucose and amino acids using the Calvin-Benson Cycle, which supplies O for use in photosynthesis. The cycle can be activated by light or the hormone abscisic acid (ABA). When light hits chloroplasts, a process called phototropism causes chloroplast movement toward or away from light to occur. The movement is achieved by the control of wall tension and also movement through plasmodesmata (a structure composed of pores between cells) and, to a lesser extent, by motor proteins.


Protein synthesis in mitochondria provides energy for the cells to function. It is aided by the ATP synthase, which turns ADP into ATP molecules.


When a plant goes to sleep during the day, chloroplasts go into a state called "deactivation". In this state, starch granules are broken down and turned over by endomembranes to give a supply of ready glucose for every molecule of oxygen that can be used in photosynthesis. Photosynthesis is also inhibited by the hormone, abscisic acid.


All plants are equipped with a method of synthesizing nitrogen in a process known as nitrogen fixation. Nitrogen fixation occurs when organic nitrogen, such as the amino acids found in proteins and nucleotides found in DNA and RNA, are converted into ammonia and nitrate by bacteria. This process is important for growing plants since it enables them to make use of all the nutrients that they have been provided with during photosynthesis once they have been harvested.


The biochemical pathway that fixes nitrogen uses energy derived from sunlight and ATP produced by photosynthesis to convert atmospheric nitrogen (N) into ammonia (NH). Nitrate is then converted into nitrite, which can be further converted into nitrate.


Aquatic plants such as algae and many aquatic fungi and bacteria use symbiotic nitrogen sources such as nitrate, ammonium and nitrite for their survival. Many of these organisms do not fix nitrogen on their own but instead are associated with organisms that do the fixing. The organisms that fix nitrogen are known as rhizoids because they grow by connecting with roots or sides of stems of plants through rhizoids that extend outward from their bodies in the soil. The rhizoids are filled with a substance known as water-soluble limiting-determinant (WSLD) that contains all the bacterial genes necessary for nitrogen fixation. This symbiotic relationship is vital for plants that grow in soils since the bacteria within their rhizoids can fix nitrogen from the air and communicate with their plant host, telling it when to uptake nitrogen from the soil through its root system. The rhizoid structures of the two partner organisms are created so that there is no competition over resource allocation or territoriality.


Even though free-living bacteria are unable to fix nitrogen because they lack WSLD, they still play a vital role in supporting this process. Nitrogen fixation can occur even when the rhizoids of plants are not present due to the action of free-living bacteria. Rhizoid-less nitrogen fixation occurs by way of three different mechanisms: nitrification, anaerobic ammonium oxidation and denitrification.


Nitrification is a reaction in which ammonium is oxidised to nitrate, releasing oxygen into the environment but at the same time opening up nitrogen to fixation through autotrophy. Denitrification is another reaction that converts a portion of nitrate into nitrogen gas, but this reaction produces hydrogen as a byproduct and must therefore also be used in conjunction with oxygen for mineral nutrient uptake. Anaerobic ammonium oxidation is an alternative reaction that uses the energy produced by photosynthesis and ATP to convert ammonium into nitrite. Nitrite is then oxidised with oxygen just as in nitrification, but this time without producing hydrogen, unlike denitrification.


The addition of water to the rhizoids allows them to expand and grow. This growth results in the absorption of more nutrients from the soil for use by the bacteria within them. The increased amounts of organic nitrogen are then passed along to their plant hosts either through their roots or through the production of more nitrogen-fixing bacteria.


Plant growth and development are regulated by a number of factors, including the amount of light, water, minerals and carbon dioxide present in the environment. All plants require light for photosynthesis to occur. In many species, phototropism is used to move towards or away from light by movement of chloroplasts within their tissues. The direction that the tissues are oriented in varies according to species and geographical location; for example, some tropical tree species have their leaves at right angles to the sun while deciduous trees have their leaves arranged so they face away from it in order to reduce solar irradiation. Having their leaves facing away from the sun also reduces water loss to the air, which is usually a problem for plants growing in waterlogged soils.


Plants regulate photosynthesis to reduce transpiration rates by activating photosynthesis at varying times of day in response to light levels and temperature. For example, "Gymnosperma" species such as palms have small stem leaves with overlapping leaflets that are exposed to more light than other species such as "Nepenthes". As a result, they grow towards the sun while they are still young but then become more susceptible to wind damage during their later development. These phototropic responses allow substantial levels of growth in adverse conditions without excessive water loss.


If there is only a small amount of light available, the atmosphere of the plant itself is used to satisfy its demands for photosynthesis. This type of anaerobic photosynthesis doesn't require the use of enzymes or membrane-bound carrier systems and it occurs when water vapour trapped in the plant's vascular system diffuses into internal tissues to allow carbon dioxide to be converted into carbohydrates. Because this process does not require enzymes, it also doesn't produce waste products such as oxygen and water, but at the same time does not utilise all of the CO2 that is present in a given environment – otherwise, a quicker rate of growth would be needed in order to ensure that all CO2 can be fixed. Therefore, atmospheric anaerobic photosynthesis has a trade-off between using the carbon dioxide in the air and producing oxygen and water as waste products. This problem can be eliminated by having the plant's vascular system run through a water-filled chamber, which allows for more efficient use of the CO2 because it does not lose CO2 to the atmosphere.


By using atmospheric photosynthesis during times of low light levels, plants can also prevent an overabundance of light from overwhelming their ability to utilize that light and experience photodynergesis. This phenomenon can occur when there is a large amount of light present which prevents the green leaves from being able to photosynthesize. In order to deal with this, plants close their stomata during over-illumination periods in order to prevent excessive loss of water and nutrients.


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