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The Importance of Enzymes in Plants and Animals

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Enzymes and their importance in plants and animals (25 marks) Enzymes are biological catalysts, which accelerate the speed of chemical reactions in the body without being used up or changed in the process. Animals and plants contain enzymes which help break down fats, carbohydrates and proteins into smaller molecules the cells can use to get energy and carry out the processes that allow the plant or animal to survive. Without enzymes, most physiological processes would not take place. Hundreds of different types of enzymes are present in plant and animal cells and each is very specific in its function. Enzymes have an active site which has a complimentary base to a specific substrate, when these bind an enzyme-substrate complex is formed. There are two hypotheses for the formation of an enzyme-substrate complex; the lock and key hypothesis explains that only one substrate (they key) will fit into the active site (lock). The induced fit hypothesis is when the active site changes shape so that the enzyme moulds itself around the substrate.

Enzymes are proteins with a 3-D tertiary structure which can alter in shape if certain factors such as temperature, pH or substrate concentration change, and so the active site of the enzyme may no longer compliment the base of the substrate therefore fewer complexes can form and the reaction rate drops. Coenzymes are organic compounds, often containing a vitamin molecule as part of their structure they are not permanently bound to the enzyme but are temporarily bound for the duration of the reaction and then move away once it is completed. One example of this is NAD, which transfers hydrogen away from one molecule to become reduced NADH+. This occurs in both aerobic and anaerobic respiration in animals, whilst also in the light dependant and light independent reactions in plants. Temperature effects enzyme activity. Increasing the heat gives molecules more kinetic energy so they move around faster. This means there would be more frequent collisions between the enzymes and the substrates and the rate of reaction would increase. However if the temperature exceeds the optimum temperature the enzyme becomes denatured.

This is because there is too much energy causing the enzyme molecules to vibrate causing the bonds maintaining their tertiary structure to break. The enzyme unravels causing the shape of the active site to change so it can no longer fit with the substrate. Changes in pH also alter an enzyme’s shape. Different enzymes work best at different pH values. The optimum pH for an enzyme depends on where it normally works. For example, intestinal enzymes have an optimum pH of about 7.5 whereas enzymes in the stomach have an optimum pH of about 2. Substrate concentration also affects the rate of reaction as the greater the substrate concentration the faster the rate of reaction and all the active sites are filled. At this point the rate of reaction can only be increased if you add more enzymes in to make more active sites available. An inhibitor is a substance that slows down or stops enzyme-substrate complexes forming. Competitive inhibitors have a similar shape to the substrate which allows them to enter the active site so the substrate cannot; therefore they both compete for the active site. If you add more substrate the effect of the competitive inhibitor will be reduced.

Non-competitive inhibitors bind to the enzyme away from the active site but change the tertiary stricture of the protein and so the shape of the active sit. This means that the enzyme-substrate complex cannot form and the rate of reaction falls. Plants and animals perform some similar functions such as replicating their DNA and breaking down sugars for energy but they also perform many different functions. Consequently, animals and plant cells have some enzymes that are similar and some that are very different. In animals, enzymes are used in the formation of ATP. During the link reaction pyruvate is used to make acetate. Acetate is then picked up by co-enzyme A which forms acetyl co-enzyme A. This enzyme then enters the Krebs cycle. During the Krebs cycle coenzymes NAD and FAD remove the hydrogen and become reduced NADH and FADH. This is fundamental to the production of ATP in animals as it is the re-oxidation of NADH and FADH that produces ATP in the electron transport chain. Animals also rely on enzymes for digestion, as without them, food could not be broken down and energy couldn’t be processed to a usable state.

Some examples of enzymes secreted by animals include amylases, which digest starch, proteases, which digest protein, and lipases, which digest fat. In the digestive system, the necessary enzymes are secreted to prepare the food for absorption in the small intestine. Enzymes are also involved in the regulation of blood glucose levels in an animal through a process called glycogenesis. When high levels are detected by the hypothalamus, beta cells respond by secreting insulin. Insulin fits with receptor proteins on liver and muscle cells. When inside the cells, insulin activates an enzyme that catalyses the condensation of glucose molecules to glycogen which can then be stored in the cells cytoplasm. In plants, enzymes play a role in photosynthesis in particular the light independent stage during the Calvin cycle. Carbon dioxide combines with a 5-carbon sugar called ribulose bisphosphate (RuBP) to form a 6-carbon sugar. This process is known as carbon fixation and is catalysed by the enzyme ribulose bisphosphate carboxylase (rubisco) which is stored in the stroma in the chloroplasts.

This then splits in to 2GP then 2TP which is used to regenerate RuBP, but some is used to produce 6-carbon sugars from which complex carbohydrates, amino acids and other substances are made. Therefore without the enzyme rubisco this process could not occur and so the light independent reaction could not take place. Both plants and animals rely on the enzymes secreted by saprobiotic micro-organisms such as bacteria in the soil in the nitrogen cycle as these break down waste and decaying animals/insects to release ammonia which forms ammonium ions in the soil. Through the nitrogen cycle, these eventually become nitrate ions which are absorbed by plants and used to form proteins allowing the plant to grow and feed the consumer. Therefore, without the enzymes secreted by the saprobiotic microorganisms, plants may not grow as effectively and animals would struggle to feed. In conclusion, enzymes are essential to life. Without them, many necessary natural processes/ reactions could not take place and so plants and animals would suffer as a result.

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