1. Most organisms are active in a limited temperature range

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1 1. Most organisms are active in a limited temperature range Identify the role of enzymes in metabolism, describe their chemical composition and use a simple model to describe their specificity on substrates Enzyme action Enzymes - protein molecules acting as biological catalysts, increase the rate of the reactions that occur in living organisms Intracellular enzymes are used within the cells that produce them (e.g. enzymes in cellular respiration and photosynthesis) Extracellular enzymes act outside the cells that produce them (e.g. digestive enzymes) While enzymes participate in reactions as biological catalysts they are not used up and are available for reuse (the activation energy required to start the reaction is lower when an enzyme is present) Enzyme structure Enzymes are made up of proteins and the basic building block of proteins is the amino acid. Two amino acids bonded together form a dipeptide. When a number of dipeptides join together a polypeptide chain is formed. Polypeptides form proteins. These chains fold in a specific way forming active sites Many enzymes require the presence of other factors as well as the protein part before they act. These non-protein parts are called cofactors and include metallic ions like iron, calcium, copper and zinc. If the cofactor is an organic molecule like a vitamin it is called a coenzyme Figure (a) From a pool of amino acids form a dipeptide when joined by a peptide bond. (c) A polypeptide is formed when many peptide bonds are formed. (d) Polypeptides fold into specific shapes to act as enzymes. Each has its own specific active site that combines with its substrate 1 Enzymes and their substrates The compound acted on by an enzyme is called a substrate The compounds obtained as a result of the enzymes action on the substrate are called the products Enzymes are highly specific in their action each enzyme acts upon a particular substrate The shape of an enzyme at a region, its active site fits with part of the substrate molecule this is called the lock & key model In some cases the active site of the enzyme varies slightly from that of the substrate and the two fit only after contact when the substrate induces a complementary shape at the enzyme s active site this is called the induced fit theory of enzyme action Poisons like cyanide and arsenic work by blocking the active sites of enzymes and stopping their action

2 Figure Each enzyme only reacts with one kind of substrate. The active site of each enzyme is able to bind to part of the specific substrate. Two models of enzyme action are shown: (a) lock and key model; (b) induced-fit model The essential role of enzymes in metabolism There are hundreds of chemical reactions taking place in human body cells every second, most of which would never take place at the temperature and the ph of living things unless they are catalysed by enzymes Example respiration: glucose is oxidised and the energy stored in its bonds is released as ATP. Without enzymes this reaction has a very high activation energy reached only at very high temperatures. If the reaction takes place at high temperatures there are two main disadvantages: - all the energy is released spontaneously and is lost to the cell as it cannot be trapped - high temperatures can damage living molecules with enzymes present the activation energy is reduced and the reactions can take place at moderate temperatures Enzymes catalyse steps in metabolic pathways and work in teams to produce an end product needed by the organism. Every enzyme plays an essential role in the process. If one enzyme is missing or defective then the entire pathway is affected 2 Identify the ph as a way of describing the acidity of a substance The ph of a solution is the measure of the concentration of hydrogen ions per litre of the solution The ph of a neutral solution, like water is 7.0. A ph below 7.0 indicates an acid the lower the ph the more acidic the solution. A ph higher than 7.0 indicates a basic solution Most biological fluids have a ph between 6 and 8 (e.g. blood ph is maintained at about 7.4). However there are a few extremes like gastric juice which has a ph of about 2

3 PRACTICAL Identify data sources, plan, choose equipment or resources and perform a first-hand investigation to test the effect of: - increased temperature - change in ph - change in substrate concentrations on the activity of a named enzyme Renin is an enzyme found in the stomach of calves and in junket tablets it causes the proteins in milk to set into semi-solid curds Aim: To demonstrate the effect of temperature, ph and substrate concentration on the ability of renin to solidify milk (ii) Compare this time with the time normal milk takes to set at the same temperature Part 1 Temperature (i) Measure and mark 2.5cm on 4 large test-tubes containing 0.01g of junket powder (ii) Warm or cool 4 samples of milk in a beaker to 5 o C, 40 o C, 90 o C & room temperature (iii) Add the milk to the mark on the test-tubes containing 0.01g of junket powder (iv) Add one test-tube to a water bath at 5 o C, another at 40 o C and another at 90 o C and leave the fourth as a control at room temperature (v) Time how long it takes for the milk to set Results: Sample All milk sample Diluted milk sample Time to set 1 min 48 sec Over 5 min Results Temperature ( o C) Time to set Control (room temp) Over 5 min 5 o C Over 5 min 40 o C 1 min 48 sec 90 o C Over 5 min 3 Part 2 ph variation (i) To three preheated test-tubes containing 2.5cm of milk and 0.01g of junket powder in a 40 o C water bath. To one add 2mL of HCl (acid), 2mL of NaOH (base) to another, and the third as a control with no ph alteration (ii) Time how long it takes for each to set Results: ph Acidic Alkaline Control Time to set Over 5 min Over 5 min 1 min 48 sec Part 3 Substrate concentration (i) Dilute a milk sample with water, add 0.01g of junket powder, immerse in a 40 o C water bath and time how long it takes to set

4 EXPLANATION: The rate of an enzymic reaction is affected by several factors including temperature, ph and substrate concentration Temperature: At high temperatures, enzymes are permanently denatured (their structure is permanently changed and even when the temperature returns to normal, they remain inactive as their active site is damaged and the protein molecule is unwound) Enzymes that are inactivated in low temperatures become active when the temperature returns to normal Most human enzymes have an optimal temperature of about 37 o C (normal body temp) Heat-tolerant organisms like bacteria living in hot springs have enzymes with high optimal temperatures ph: Each enzyme has an optimal ph at which it acts best A change in ph can change the shape of an enzyme s active site affecting its ability to combine with the substrate. Because the enzyme is less able to combine with its substrate it is unable to act and metabolic reaction declines Enzymes becomes less efficient if the variable value is greater or less than optimal Carbonic anhydrase found in human blood has optimal ph of 7.4 (bloods normal ph) Pepsin found in human stomachs has an optimal ph of 2 (acidic gastric juice) Trypsin found in the human small intestine has an optimal ph of 8.0 (basic) 4 Substrate concentration: The addition of more substrate to an enzyme solution will initially increase the rate of reaction if not all of the active sites of the enzyme present are occupied but then plateau The solution contains a set amount of enzyme and if no more is added the rate of reaction declines as all of the active sites of the enzyme molecules become occupied Enzyme concentration: Only a very small number of enzyme molecules are usually involved in a reaction and these produce a given amount of product per unit time. If the amount of enzyme is increased, the amount of product made per unit time increases but amount of product will still be limited by the amount of substrate Enzyme molecules are not used up in a reaction and are available for reuse Inhibition: Other molecules may compete with the normal substrate for active sites of enzymes and this compound may combine with the active site interfering with normal substrate-enzyme reactions and inhibiting formation of the normal product Enzyme inhibition can cause death Cyanide acts by inhibiting cytochrome c oxidase, an enzyme important in aerobic respiration (provides ATP needed for life) and for this reason is extremely poisonous

5 Explain why the maintenance of a constant internal environment is important for optimal metabolic efficiency The external environment can vary greatly. In spite of this living cells can exist in a relatively unchanging stable environment In healthy people, whether they are eating or fasting their blood glucose is kept within mmol/l and regardless of weather conditions their core body temperature is kept around 37 o C Multicellular organisms have mechanisms that enable an enzyme to operate at its optimal capacity by providing an environment that has relatively constant temperature, ph and substrate concentration. Since the activity of enzymes influences the outcomes of metabolism, constant internal environmental conditions equate to optimal enzyme activity and metabolic efficiency The internal environment of living cells in the human body is a liquid consisting of tissue fluid (liquid that surrounds and bathes the membranes of nearly all cells), plasma (the liquid part of the blood in which blood cells are suspended) and other fluids Describe homeostasis as the process by which organisms maintain a relatively stable internal environment Homeostasis is the condition of a relatively stable internal environment, maintained within narrow limits When deviation occurs mechanisms act to restore values to the normal state Factors like infection, trauma, exposure to toxic substances or extreme conditions such as immersion in icy water, auto-immune diseases and inherited disorders may lead to a failure of homeostasis and is potentially life threatening 5 Figure Summary of major variables that are subject to homeostasis in mammals Variable Normal Range Comments Temperature 36.1 o C 37.8 o C Temperature of internal cells of the body is called the core temperature Blood glucose mmol per L Blood glucose is typically maintained within narrow limits regardless of diet Water Daily intake must balance daily loss Body tissues vary in their water content. Bone contains about 20% water and b about 80% water. In prolonged dehydration, fluid moves from cells and tissue f into blood Ions (e.g. plasma Ca 2+ ) mmol per L Specific ions are required by some tissues ph of arterial blood 7.4 This ph is necessary for enzyme action and nerve cells Blood pressure arterial Diastolic (relaxed) Systolic (contracted) Urea (nitrogen containing wastes) in plasma Red blood cells (contain haemoglobin) 13.3 kpa (1000 mm Hg) Transport of blood depends on maintaining adequate blood volume and pressu 5.33 kpa (40 mm Hg) < 7 mmol per L Waste products of cellular processes must be removed by the kidneys to preve toxic effects on cells Haemoglobin values: Essential for transport of oxygen - Erythropoietin, a hormone produced by the Females 135g per L kidney, acts on red bone marrow and stimulates red blood cell production Males 150g per L Explain that homeostasis consists of two stages: - detecting changes from the stable state - counteracting changes from the stable state

6 Stage 1 - Detecting changes from the stable state In this stage a sensor of some kind detects a change in a specific variable from the desired stable level The fact that there has been an undesirable change is then transmitted to the next part of the control system Stage 2 - Counteracting changes from the stable state An effector receives the message that there has been an undesirable change that must be counteracted and the variable is restored to its desired level (this is a negative feedback mechanism) In positive feed back mechanisms this stage varies. While in negative feedback mechanisms the process corrects deviation positive feedback mechanisms causes the system to reinforce the deviation and change it further (positive feedback mechanisms are rare though they do exist - e.g. entry of Na + into neurons, the entry of one ion stimulates entry of more Na + ) Figure Detecting and counteracting change. A diagrammatic summary of the two interrelated stages of homeostasis. Note that their action relies on negative feedback systems. If a variable slightly overshoots the optimal as a result of effector action, the counter negative feedback system will respond to correct the overshoot. These actions occur continuously in the body so that optimal levels of variables are continually fine-tuned 6 Body systems contribute to homeostasis Various mechanisms monitor conditions inside the body and, when change is detected, body systems react to restore the balance. In humans, cells form tissues and systems that play an essential role in homeostasis. With the exception of the reproductive system, all body systems play a part in homeostasis Figure Summary of the contribution of some body systems to homeostasis

7 The hormonal and nervous systems are the major systems responsible for the control and coordination of homeostasis 7