6.1: Cellular Respiration - Biology

6.1: Cellular Respiration - Biology

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Metabolism is the sum of all chemical reactions in a living organism. Catabolic reactions break down complex biomolecules, such as carbohydrates and lipids and release the energy stored within.

Think about It

  1. Is cellular respiration anabolic or catabolic? Explain.
  2. Is photosynthesis anabolic or catabolic? Explain.

Enzymes are proteins that facilitate chemical reactions in living systems by acting as catalysts in biochemical reactions. Enzymes speed the rate of the reaction by either bringing the reactants into close proximity or by binding to a single reactant and splitting it into smaller parts. Enzymes have a property known as specificity, which simply means that each enzyme catalyses a specific biochemical reaction. Enzymes are indispensable molecules of life. Enzymes are functional within a given range of temperatures and pH values for that enzyme.

Part 1: Starch Synthesis and Storage

Using energy from the sun, photosynthetic organisms convert carbon dioxide and water molecules into glucose. Plants don’t store this chemical energy as glucose. Using enzymes, plants link the glucose molecules together and store them as the polysaccharide starch. Potatoes are the primary starch storage site for the potato plant. Iodine reacts with starch to form a bluish color. You can see the starch stored in a potato cell by staining the cells with iodine.


  • Microscope slide
  • Coverslip
  • Iodine
  • Potato cells
  • Tap water


  1. Cut a thin slice or scrape a few cells from the surface of a potato.
  2. Make a wet mount of the potato cells and stain them using a drop of iodine.
  3. Observe your cells under high power.
  4. Draw a few cells and label the following structures:
    1. cell wall
    2. plastid with starch grains (stains purple)

Think about It

Where did the starch in the potato cells come from?

Part 2: Starch Digestion

When you eat starchy foods like bread or potatoes, your body must digest the complex carbohydrate into monosaccharides, such as glucose or fructose, before it can be absorbed by the body. Alpha amylase, an enzyme found in your saliva, catalyzes the first step in starch digestion.

Just for the fun of it, snag an unsalted saltine cracker. Begin chewing the cracker, but do not swallow it. Keep chewing it, and allow the amylase from your saliva to break the starch into maltose. Can you taste the difference in the cracker after chewing for a very long time? What happened?

In this lab, you will assess the effect of amylase on a carefully prepared starch solution.


  • 2 Ehrlenmeyer Flask, 250 ml
  • Glass stirring rod
  • Hot pads
  • Wax pencil
  • Amylase
  • Corn starch
  • 2 glucose test strips
  • Hot plate


  1. Label your two flasks A and B.
  2. Add 100 ml of water and 10 g of corn starch to each flask.
  3. Using the hot plate, gently heat this mixture, stirring continuously with a glass stir rod.
  4. When the mixture thickens, remove it from the heat and cool it under running water to lukewarm.
  5. Shake the Amylase solution thoroughly to mix the enzyme.
  6. Add 2 ml of Amylase to flask A. Do not add enzyme to flask B.
  7. Allow both flasks to sit for 15 minutes and note any change in the viscosity of the two starch solutions.
  8. To see if starch digestion occurred, place a small amount of solution onto each glucose test strip. Compare the color on the test strip with the known standards.
Viscosity (thickness)Glucose test (+ or –)
Beaker A:
Amylase treatment
Beaker B:
No enzyme treatment

Think about It

  1. Is there any difference in viscosity between the two beakers? If so, explain why there is a difference.
  2. Which beaker represents the control treatment in this experiment? Is this a positive or negative control? Explain.
  3. Design an experiment that would study the influence of a factor like pH or temperature on enzyme activity. Include a question, a hypothesis, and the methods you would use.

Part 3: Aerobic Respiration

Carbon dioxide is a byproduct of aerobic cellular respiration. Measuring carbon dioxide production is an indirect way of measuring whether or not cellular respiration is occurring.

Your task in this lab is to determine whether or not various sets of bean seeds are going through cellular respiration.


  • Flasks
  • Rubber stoppers
  • “Respiration bottle” setups
  • Phenol red
  • Germinated bean seeds
  • Germinated + boiled bean seeds
  • Dry ungerminated bean seed


  1. Fill one flask about 1/3 full of the bean seeds labeled “Germ.” These bean seeds were soaked overnight and then drained and covered in a wet paper towel for 2 days to allow the seeds to begin germinating.
  2. Fill the other flask about 1/3 full of the bean seeds labeled “Germ-Boil,” These bean seeds were soaked as for “Germ,” but were then boiled for 3–4 minutes and cooled to room temperature on the day of the lab.
  3. Your instructor will set up the control flask containing ungerminated seeds.
  4. Place rubber stoppers over each flask and allow flasks to sit for approximately one hour.
  5. After one hour, replace the rubber stopper with a second stopper containing a funnel and rubber tubing attached to a glass tube. Place the glass tube into a test tube about ½ full with water.
  6. Add several drops of phenol red solution to the test tube. Phenol red, a pH indicator, is red when pH>7 (basic) and yellow when pH<7 (acidic).
  7. Put several hundred ml of tap water into a beaker and slowly pour the water into the flask via the funnel at the top. This will force the gases in the bottle to bubble into the test tube containing the phenol red solution.
  8. Record the color of the phenol red solution as gases present in the flask bubble through the solution.
  9. Clean out the test tube, then repeat steps 5–8 for the second flask. Record your observations in the table below.
    1. Remember, the CO2 produced during cellular respiration combines with water to form carbonic acid. The carbonic acid dissociates into hydrogen and bicarbonate ions, and the hydrogen ions decrease the pH.
      CO2 + H2O ↔ H2CO3 ↔ HCO3- + H+
Bean seedsIndicator ColorResults (CO2 present or absent)

Lab Questions

  1. Which set of seeds was undergoing cellular respiration? How do you know?
  2. What might have happened to the enzymes in the germinated and boiled seed treatment?
  3. Where does cellular respiration occur inside the cells of the germinating seed?
  4. In order to obtain all the energy possible from a molecule of glucose (»36 ATPs), what substance must be available?
  5. Was the control in this experiment a positive or negative control? How could you design the opposite type of control for this experiment?

Part 4: Fermentation

During fermentation, only 2 molecules of ATP can be generated for one molecule of glucose. Pyruvate is a waste product produced during glycolysis, and unless pyruvate is metabolized, it will prevent fermentation from proceeding.

There are two ways that pyruvate can be metabolized. In yeasts and certain other microbes, pyruvate is turned into ethyl alcohol (ethanol). In animals and some bacteria, pyruvate is turned into lactic acid.

Think about It

Clearly draw the fermentation pathway in yeast.

“Wine Making” Experiment

Working in your group, you will design an experiment to test a hypothesis regarding the rate of fermentation. Some of the experimental variables you might manipulate in the fermentation process are:

  • Sugar type (Possibilities: sucrose, lactose, glucose, and dextrin)
  • Yeast quantity (use 1 gram as your control value)
  • Temperature (18˚C—room temperature, 37˚C, 55˚C, or 0˚C ice bath)

Think about It

  1. What is the question your group is asking for this experiment?
  2. Describe your hypothesis.

Experimental Design

To test your hypothesis, set up three or four fermentation tubes. Be sure to include a control in your experiment. Feel free to design your experiment using this general experimental design that you modify as necessary. Be sure to label your tubes, since you might be placing them in a water bath shared with other groups.

  1. Fill each test tube about half full with water.
  2. Add 1 gram of yeast.
  3. Add 1 gram of a sugar.
  4. Mix all the ingredients.
  5. Place a balloon over the top of each tube.
  6. Place the tubes in a test tube holder at the appropriate temperature.
  7. After 30 minutes observe tubes and measure the circumference of the balloon.

Lab Questions

Describe your experimental design:

TubeContentsVariableCircumference of Balloon (in cm)
4 (control)
  1. What was the control in your experiment? Was it a positive control or a negative control? Explain.
  2. Did the results support your hypothesis? If not, why do you think your results did not support your hypothesis? What would your new hypothesis be?
  3. What further experiment(s) would you perform to support or refine your hypothesis?

Watch the video: Notes chapter intro to cell respiration (December 2022).