Exploring Carbohydrates With Bananas Name: ________________ Partner: _________________ Most of us understand that bananas are the fruit of banana trees. Like most fruits, bananas (if ripe) are sweet. As you know from experience, many carbohydrates you come across are sweet. Carbohydrates can be simple or complex. The complexity of sugars depends on whether the sugar molecules are alone (monomer) or in chains (polymers). Monomers of carbohydrates are called monosaccharides and polymers of carbohydrates are called disaccharides, oligosaccharides or polysaccharides. Carbohydrate Type Example Units Image Monosaccharide (monomer) Glucose Fructose One Disaccharide (polymer) Sucrose Maltose Two Oligosaccharide (polymer) Glycoproteins Glycolipids 3-10 Polysaccharide (polymer) Starch Cellulose Glycogen > 10 Questions are color coded: Prelab (do before). In-lab (do during the lab) Post-lab (HW for after) Prelab Q1. Which of the carbohydrate types would you associate with bananas? Prelab Q2. Which carbohydrate type(s) would you associate with the sweetness of bananas? A fun demo…. Grab a cracker and put it in your mouth. Chew it for one minute. Don’t swallow any of it yet. In-lab Q3. Describe what you taste after one minute. Part 1 - Banana Tissue - testing for starch Obtain samples of unripe and ripe bananas by cutting a ¼” slice of banana onto your plate. Label with sharpie. In-lab Q4. Is the texture different between the green and ripe banana? If so, describe it. We will test the tissue of the banana by using iodine. Iodine stains starch but not glucose. Prelab Q5. Before we do the test, write a hypothesis (if/then/because) related to the ripeness of the bananas and starch content. Remember hypotheses are in If…(independent variable)...then…(dep var.) format. Using a sample at your workstation, place several drops of iodine onto each banana sample. In-lab Q6. Describe any differences you see between the two samples tested. Did the observations support your hypothesis? Why or why not? Part 2 - Banana Tissue - testing for simple sugars Obtain samples of unripe and ripe bananas. We will test the tissue of the banana by using Benedict’s reagent. Benedict’s reagent will create a color change going from blue to reddish to brown, in the presence of simple sugars (like glucose). Prelab Q7. Before we do the test, write a hypothesis (if/then/because) related to the ripeness of the bananas and simple sugar content. Remember hypotheses are in If…(independent variable)...then…(dep var.) format. Procedure: We will do this as a group… You will be given a piece of banana 1. Label two tubes, one for ripe, one for unripe banana. 2. Place the pieces of banana in the test tube and add enough water to fill the tube halfway full and add 20 drops of Benedict’s reagent to the tube. 3. Use a glass stir rod to mash up and mix the banana into the solution. You want this thoroughly mixed. 4. Place both test tubes upright in the beaker on the hot plate and let them heat for 10 minutes. While they are in the hot water bath, move on to part 3. After the ten minutes move on to the next step. 5. Remove the tubes from the water bath and note the color below. 6. Dump the contents of your tube in the sink and clean your tube with the brush and a drop of soap. Then place them on the mat to dry. Wash the stir rod as well. In-lab Q8. Describe any differences you see between the two samples tested. Post-lab Q9. What does the color in the tube tell us about the sugar content of each? Did the observations support your hypothesis? Why or why not? Part 3 - Banana Cells Starch is a polysaccharide made of thousands of monomers of glucose. It is primarily used as a storage molecule for sugars in cells. The organelles containing starch in banana cells (and potato cells) are amyloplasts, a type of organelle called a “plastid” that stores starch. These plastids look like little sacks inside of cells. Glucose is a product of photosynthesis. Cells build and break down polymers for many different reasons. To build a polymer, monomers are combined through a process called dehydration synthesis. In this process, portions of the monomer molecule are extracted by an enzyme and combined to form water. The resultant open bonds are combined between the two monomers. Below is an example of combining two glucose molecules together by the enzyme maltase to form a disaccharide called maltose. Dehydration Reaction Cells also take polymers and break them into monomers. This processes is called hydrolysis. Hydrolysis is also performed by an enzyme that grabs the polymer, deforms it so that water be broken into parts and added to the two resulting monomers. Below is an example of breaking the polymer maltose into two monomers of glucose. Hydrolysis Reaction Post-lab Q10. Why would the green banana fruits be high in starch? Think about starch’s main use in organisms (reread in the book if you have to)… Post-lab Q11. Make a prediction about what will happen to the proportion of starch to sugar once ripening starts to happen. Post-lab Q12. Which of the two processes (hydrolysis and dehydration synthesis) would be occurring: ○ As starch is stored in a green banana? ○ As the banana fruit ripens and becomes sweeter? Prelab Q13. Before we do the test, write a hypothesis (if/then/because) related to the ripeness of the bananas and the number of plastids counted. Remember hypotheses are in If…(independent variable)...then…(dep var.) format. Procedure 1. Obtain 3 microcentrifuge tubes and add 100uL of water. Also, add 50uL of iodine (your teacher will show you how). Cap. Shake to mix. 2. Obtain your banana sample, but don’t use too much or you won’t see individual cells. Use the inoculating loop and scrape the banana gently to fill the loop. (your teacher will show you how) 3. Place the loop into the solution in your microcentrifuge tube. Using the other end, spin the loop and check that the loop is empty. This will mix your sample. 4. Use a pipette to transfer 1 drop of solution to your slide. 5. Place a coverslip on top of the drop, starting with one side to avoid catching air bubbles under it. 6. Place the slide on the microscope, start with 4x (red) to locate cells and move to 10x (yellow) objective in position and find a field of view containing the cells. 7. You will see cells in your field of view. Use 5 cells in your field of view and count the plastids (see description on pp2) inside. Make 4 counts and average them. Log into your data chart. 8. Repeat 1-5 again for the ripe and overripe bananas. Treatment Count 1 Count 2 Count 3 Count 4 Green Ripe Overripe Post-lab Create a graph showing the mean number of plastids per treatment. Use a simple caption to describe the graph. Average Analysis Ripening of fruit (including bananas) comes from a substance produced called ethylene. Ethylene is a hormone that is made during certain developmental conditions to signal seeds to germinate, prompt leaves to change colors, and trigger flower petals to die. Ethylene causes genes to be turned on, which in turn, causes a chain reaction in the fruit. Enzymes called pectinases are made to break down cell walls and soften the fruit; enzymes called amylases convert carbohydrates into simple sugars; and enzymes called hydrolases degrade the chlorophyll content of the fruit resulting in color change. Such changes invite animals to consume the fruit and disperse the mature undigested seeds via their defecation. Post-lab Q14. Using the info above explain how the polymers of glucose disassembled into sugars. Post-lab Q15. Using the info above what causes bananas to ripen? Post-lab Q16. How do the graphs above correlate to your bananas in terms of starch → sugar content if green bananas are 1 day, ripe bananas at 7 days, and overripe at 15 days? Post-lab Q17. Why would the banana plant spend time and energy sending glucose from the leaves of the banana plant to the fruit (what is the point of that, especially if that fruit will be eaten by animals)? Post-lab Q18. If you’ve ever tried an unripe fruit, you have probably noticed a bitter taste. Often times plants produce bitter or toxic compounds in fruits, but these compounds usually break down before the fruit is ripe. Why do you think the plant does this? (think about the previous question as you answer)
