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It is this light independent stage which is affected by the temperature because it involves an enzyme, Ribulose Bisphosphate Carboxylase (rubisco), which catalyses the reaction between carbon dioxide and a five-carbon sugar known as Ribulose Bisphosphate (RuBp). Enzymes are catalyst made of protein. They convert substrate molecules into products by possessing an active site where the reactions occur. The optimum temperature of rubisco is 45o C. Exceeding this temperature causes the bonds that hold the polypeptides in specific shapes to be broken and thus the active site changes shape3.
The substrate (. i. e. Ribulose Bisphosphate) is unable to fit into the active site and therefore no photosynthesis occurs. The enzyme is said to be denatured.Powered by the energy of sunlight, plants perform this central task of carbon fixation. Inside plant cells, rubisco forms the bridge between life and the lifeless, creating organic carbon from the inorganic carbon dioxide in the air. Rubisco takes carbon dioxide and attaches it to Ribulose Bisphosphate, a short sugar chain with five carbon atoms.
Rubisco then clips the lengthened chain into two identical Glycerate -3-Phosphate pieces, each with three carbon atoms. Glycerate -3-phosphates are familiar molecules in the cell, and many pathways are available to use it. Most of the Glycerate -3-phosphate made by Rubisco is recycled to build more Ribulose bisphosphate, which is needed to feed the carbon-fixing cycle. But one out of every six molecules is skimmed off and used to make sucrose to feed the rest of the plant, or stored away in the form of starch for later use.
In spite of its central role, rubisco is remarkably inefficient. As enzymes go, it is very slow. Typical enzymes can process a thousand molecules per second, but rubisco fixes only about three carbon dioxide molecules per second. Plant cells compensate for this slow rate by building lots of the enzyme. Chloroplasts are filled with rubisco, which comprises half of the protein. This makes rubisco the most plentiful single enzyme on the Earth. Plants and algae build a large, complex form of rubisco composed of eight copies of a large protein chain and eight copies of a smaller chain.
Many enzymes form similar symmetrical complexes. Often, the interactions between the different chains are used to regulate the activity of the enzyme in the process known as allostery. Rubisco, however, seems to be rigid as a rock, with each of the active sites acting independently of one another. In fact, photosynthetic bacteria build a smaller rubisco composed of only two chains, which performs its catalytic task just as well. By packing many chains together into a tight complex, the protein reduces the surface that must be wetted by the surrounding water.
This allows more protein chains, and thus more active sites, to be packed into the same space. The active site of rubisco is arranged around a magnesium ion. Above it is a small sugar molecule that is similar to the product of the rubisco reaction, and a short stretch of the protein chain is shown at the bottom. In reality, the rubisco protein chains completely surround these molecules. The magnesium ion is held tightly by three amino acids, including a surprising modified form of lysine. An extra carbon dioxide molecule is attached firmly to the end of the lysine side chain.
In plant cells, this “activator” carbon dioxide, which is different from the carbon dioxide molecules that are fixed in the reaction, is attached to rubisco during the day, turning the enzyme “on,” and removed at night, turning the enzyme “off. ” The exposed side of the magnesium ion is then free to bind to both ribulose bisphosphate, holding onto two oxygen atoms and the carbon dioxide molecule that will be attached to sugar.
Blackman’s law states that ‘the rate of any process which is governed by two or more factors is limited by the factor in least supply. At low light intensities, increasing the temperature has little effect on the rate of photosynthesis, because the light intensity is a limiting factor. But at high light intensities, increasing the temperature (within limits) increases the rate of photosynthesis, by increasing the rate of light independent (temperature dependant) stages. At higher temperatures (above 45o C for RuBp) the proteins of the cell begin to denature and the rate falls.
Below 0 C, the cells freeze and are destroyed by ice crystals. This explains why graphs cover a small temperature range. When light intensity is varied and all other factors remain constant, the rate of photosynthesis increases linearly with increased light intensity over a range of low intensities. The light intensity is said to be the limiting factor. At high light intensities, increasing the intensity has little effect on the rate of photosynthesis. A factor other than light intensity is limiting the rate of reactions ( e. g. the temperature).
As with increasing the light intensity, increasing the concentration of carbon dioxide initially increases the rate of photosynthesis. Over this range of concentrations, the carbon dioxide is rate limiting. At higher concentrations, some other factor is rate limiting.
Prediction: A fundamental of chemical thermodynamics is that all reaction rates will increase as the temperature increases. In the case of rubisco, increasing the temperature to the optimum rate (45o C) will inevitably increase the rate of which a water molecule is split during photolysis and hence there will be an increase in the volume of oxygen observed.
This is because increasing the temperature increases the proportion of particles (i. e.substrates) that collide with energies greater than the activation energy (Ea). Further more enzymes change the mechanism of a reaction to one having a lower value of activation energy (Ea), by providing an active site where reactions occur more easily than elsewhere.
Therefore, the proportion of successful collisions at a given temperature increases because they can be reached without a need of substantial energy and hence an increase in rate can be followed. This is had lead me to believe that the volume of oxygen will increase for each temperature up to the optimum temperature of rubisco, which is 45o C.
At temperatures of 45o C, the enzyme is carrying out maximum catalysis as all its active sites are filled and in use. The enzyme is said to have reached its v- max. However, for reactions that are dependant on enzymes such as this one, means that if you were to go over the optimum temperature of the enzyme, although chemically you are increasing the chances of increasing the volume of oxygen, you are also increasing the chances of the breakdown of the three-dimensional structure of the enzyme. As the heat in the system increases, the vibrational energy of the entire rubisco molecule also increases.
This puts a strain on the weak interactions that hold the enzyme together. At temperatures just above optima, there may be a situation where the enzyme is in a sort of equilibrium where it temporarily loses some of its structure and then regains it to work again. At higher temperatures these bonds literally get shaken apart and the three -dimensional structure of the protein destabilises. This has lead me to believe that if the temperature is to exceed 45o C, the enzyme will begin to denature and so the process of photolysis will decrease and eventually stop as would the volume of oxygen being produced.
Part of the experiment involves investigating the volume of oxygen produced when the elodea specimen is placed in ice. A leaf of a plant is an organ adapted for photosynthesis. The leaf structure varies greatly and is related to the environment in which the plant is in, e. g. the artic11. Therefore placing a plant in a freezing environment does not mean that it will not photosynthesise. This is evident in plants which are able to carry out photosynthesis in the artic.
Subsequently Rubisco, the enzyme involved in photolysis of water can work in at freezing levels, though the rate of photosynthesis is extremely slow due to the very slow diffusion of enzyme and substrate molecules through the ice lattice. Therefore I can make a firm prediction that for this particular investigation, the volume of oxygen produced will be rather small as oppose to being zero.
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