Biology Key Stage 4   - PowerPoint Presentation

1. Biology Key Stage 4 

2. Tutorial 2.4Enzymes and pH Core Practical In this tutorial we will look at:How amylase breaks down starch with reference to the lock and key modelThe aims and variables of the amylase experiment Interpreting pictorial representations of results and predicting alternative outcomes

3. Starter taskWhat can you remember about enzymes from your lessons? Use the key words and diagrams below to help you create a mind-map of everything you can remember! SubstrateCatalystActive siteOptimumChemical reactionDenatureCan you recall what all these key terms mean? Discuss which ones you’d be confident explaining and which ones you need reminding of!

4. Enzymes Enzymes are biological catalysts: they speed up a reaction without being used up or altered in the process. An enzyme works on a substrate molecule, breaking one large molecule into smaller pieces.There are three types of enzymes in the digestive system; their sites of production and function are listed below. EnzymeSite(s) of actionSubstrate(s)Product(s)AmylaseMouth and Small intestineStarchGlucoseProteaseStomach and Small intestineProteinAmino acidsLipaseSmall intestineLipids (Fats)Glycerol and Fatty acids

5. Lock and Key Model The lock and key model describes how enzymes break down substrates into products. The lock is the enzyme, while the key is the substrate. The part of an enzyme where a substrate will bind is called the active site; the shape of this active site is complementary to the shape of the substrate. The substrate therefore fits exactly into the active site like a key fitting into the lock on your front door.

6. Lock and Key Model – Substrate binding The process occurs in the same way in every location. We will use the binding of substrate to amylase as an example:Amylase is produced by the Salivary Glands in the mouth. A person eats a piece of bread (a food with high starch content) so the enzyme and substrate are now both free in the mouth.The substrate (starch) collides with the active site of the enzyme (amylase) and binds here – this is called the enzyme-substrate complex.The amylase acts on the starch breaking it into individual glucose molecules.The glucose molecules no longer have the same shape as the starch did, so they do not fit the enzyme active site anymore – these are released and continue along the digestive system.The enzyme is unchanged and can repeat its function with further starchy foods that are eaten.

7. Lock and Key Model – Enzyme specificityWe need all three types of enzyme in the body to ensure all food groups can be broken down. The active site of the amylase enzyme only matches the shape of starch, and therefore cannot break down proteins or lipids. Therefore, we need separate enzymes (each with their own specific active sites) to break down each type of food we ingest.Enzymes also have optimal temperature and pH conditions for their function:Enzymes present in the digestive system will function optimally at 37ºC as this is body temperature. Amylase enzymes function in the mouth (pH 6.7 – 7) but not in the Stomach (pH 2). When enzyme conditions are unfavourable, the shape of an enzyme changes and we call this denaturation.Optimal conditions

8. Handbook Activity  1: Lock and Key ModelUse the space in your handbook to draw a diagram that represents ‘The Lock and Key Model’ Once amylase reaches the stomach, the pH is unfavourable, so amylase denatures and no longer digests starch. We can observe the process of denaturation in the laboratory by monitoring how starch is broken down by amylase using Iodine as an indicator.Page 37 in your handbook

9. Core Practical: Amylase and pH Starch solution and amylase are both separately heated to 35ºC in a water bath.Single Iodine drops are added to depressions on a spotting tile.One drop of the starch solution is added to the first depression on the tile. This is a negative control (there is no enzyme so no starch can be broken down). The Iodine will react with the starch and turn blue-black in colour.Starch from the water bath is added to a test tube containing a buffer solution of pH 5. The mixture is stirred with a glass rod.

10. Core Practical: Amylase and pH 5. Amylase is added to the starch and pH 5 buffer solution, and the solution is mixed continually with the glass rod.6. The stopwatch is started and after 10 seconds the glass rod is used to remove one drop from the test tube. This is added to the second depression on the tile.7. This process is repeated every 10 seconds until the added drop of mixture does change the colour of the Iodine in the depression – it will stay brown if there is no starch present.8. The time it takes for the Iodine to turn blue-black is recorded in a table.9. The whole method is repeated with buffer solutions of pH 6, 7 and 8.

11. Handbook Activity 2 Try these questions independently, then discuss with your tutor and classmates. Write the Independent Variable (1)Write the Dependent Variable (1)Write the investigative question for this experiment (1)Identify three variables which should be controlled in this experiment and explain why each one should be controlled (3)Why are the starch and amylase solutions incubated at 35°C in a water bath prior to the experiment start? (1)Why might some scientists find different results even when they follow this method? (1)One student says that taking a drop every ten seconds is too tricky and suggests the group try every 30 seconds instead. Is this a good idea? How might this affect their data? (1)Discussion question: How would this experiment need to be adapted to find the optimum temperature for amylase?Pages 37 & 38 in your handbook

12. Interpreting Data When we interpret data from this experiment, we might get a table showing the number of seconds it took to break down starch or we could get an image of a spotting tile. In an exam the questions are in black and white, so you are likely to get a key to read the data.Both these tiles allow us to work out how long it took for the amylase to break down starch.We know that drops were taken every 10 seconds so we can count how many ticks there are and times this number by 10.We have 13 ticks so 13 x 10 = 130 seconds – it took 2 minutes and 10 seconds for the amylase to break down the starch into glucose.Here:A tick would mean starch is present.A cross would mean starch has been broken down into glucose.

13. Handbook Activity 3- Interpreting dataThe spotting tile below shows the results of the experiment above when completed using pH 6 buffer. Drops were taken from the mixture and added to the tile every 10 seconds.How long did it take the amylase to break down the starch?How do you know this? Page 38 in your handbook

14. We can also be asked to complete ticks and crosses when given dataFor example: It took 50 seconds for the amylase to digest the starch, complete the spotting tile with ticks and crosses to show this.Interpreting data: Exam-style question

15. This core practical can be modified in exams to discuss how temperature might impact amylase function. We know that the optimal temperature range for human digestive enzymes is between 37 and 42°C and you could be expected to write a hypothesis or complete a spotting tile prediction for how temperatures would impact amylase activity.In this case, amylase and starch were incubated together over a range of temperatures to determine the optimum conditions for starch digestion. This spotting tile shows the results:Temperature and amylase function

16. We would be expected to complete this tile with ticks and crosses to show how quickly starch digestion took place. We know that:20°C is too low to be optimal so it has taken until the final depression to change colour .30°C is higher but still not optimal – one tick and two crosses in this line will show that the enzyme is working faster but not optimally. 40°C is closest to optimal so crosses from the second depression is most suitable.50°C is too high and amylase will have denatured.Temperature and amylase function

17. Complete the question independently then discuss with your tutor. Protease is used in the body to digest protein and yield amino acids.An experiment was carried out to identify the optimal pH for this enzyme. Prior to mixing enzyme with substrate, both solutions were incubated at 35°C for 10 minutes.Complete the tile to show what you would expect the results to show.Explain why you would expect to see these results (2)A second student repeats this method and obtains a cross in the first depression of her pH 2 row. Her teacher says this data is not reliable. Why not? (2)Why were the solutions incubated at 35°C prior to mixing? Give 2 reasons (2)Handbook Activity 4 Pages 38 & 39 in your handbook

18. Complete the match up of the key terms and ideas from today’s session. Handbook Activity 5 Page 39 in your handbook

19. Knowledge Check 2This module aimed to help you:Identify variables in an experiment, as well as define accuracy, precision, reliability, reproducibility, and repeatability. Plan and evaluate experiments using random sampling or transects appropriately. Know the method of osmosis core practical, know how to calculate percentage change, and plot your results on a graph. Understand the aims and variables of the amylase experiment and explain, using the lock and key model, how amylase breaks down starch. Page 40 in your handbook

20. Knowledge Check 2 To complete the Knowledge Check go to this link:[TUTOR TO ADD LINK]

21. Knowledge Check 2 Define precision: How close the results are to the mean (or to each other)How close to 100% accurate the instrument is every time How close the results are to the mode How close the results are to the mean (or to each other)

22. Knowledge Check 2 2. A 3.5cm long carrot cylinder is cut. This is placed in pure water for 6 hours before being removed and measured again. The cylinder is now 3.8cm long. Select the correct percentage change to two decimal places8.75% 8.57% 8.32% 9.57% 3. What is the definition of a dependent variable? The factor being measured in the investigationA factor changed by the scientist in the investigationA factor being changed during the course of the investigationA factor being disproved during the investigation 

23. ReflectionPage 41 in your handbook

24. Title/subheadingOver to you!<<Insert learning activities here>>Page X in your handbook

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