LAB TWO CARBS

Lab 2: Carbohydrate Lab

Name and Date

  • Name: ___
  • Date: ___

Learning Objectives

  1. Describe the basic and chemical structure of a monosaccharide.
  2. Differentiate between monosaccharides, disaccharides, and polysaccharides.
  3. Identify the storage form of carbohydrates in animals and plants.
  4. Describe the major functions of carbohydrates.
  5. Know the purpose and principle of Benedict’s Test.
  6. Know how to apply the scientific method, including formulating a hypothesis, designing an experiment, collecting data, and concluding.

Pre-Lab Activity

Questions to Answer:

  1. Name the most important sugars (monosaccharides) found in nucleic acids.

    • Answer: Ribose and deoxyribose.

  2. What are the sugars (monosaccharides) that have 6 carbons called? Name the most important sugars that have six carbons.

    • Answer: Hexoses; the important hexoses are glucose, fructose, galactose, and mannose.

  3. Name the major storage polysaccharide of:
    a. Plants: Starch.
    b. Animals: Glycogen.

  4. Why do plants make cellulose?

    • Answer: For structural purposes.

  5. Why can’t we digest cellulose?

    • Answer: Humans lack the enzymes necessary to break down cellulose.

  6. What macromolecule(s) does Benedict’s reagent identify?

    • Answer: Benedict’s reagent identifies reducing sugars, primarily monosaccharides and some disaccharides (excluding sucrose).

Lab Safety

  • Full Personal Protective Equipment (PPE) required for this lab:
    • Lab coat
    • Long pants
    • Closed-toe shoes
    • Gloves
    • Safety goggles

Introduction to Carbohydrates

  • Carbohydrates are also known as saccharides, derived from the Greek word sakcharon, meaning sugar.
  • Composition: Carbohydrates consist of Carbon (C), Hydrogen (H), and Oxygen (O), typically with a general chemical formula of (CH₂O)ₙ, where n indicates the number of carbohydrates present.

Monosaccharides

  • Simplest form of carbohydrates.
  • Types of Monosaccharides:
    • Pentoses (5 carbon atoms): Important in biochemical pathways.
    • Significant pentoses: Ribose and Deoxyribose (critical for nucleic acids).
    • Hexoses (6 carbon atoms): Important structural roles and preferred energy source for cells.
    • Key hexoses: Glucose, Fructose, Galactose, and Mannose.

Disaccharides

  • Formed when two monosaccharides link together.
  • Important disaccharides include:
    • Sucrose (glucose + fructose)
    • Lactose (glucose + galactose)
    • Maltose (glucose + glucose).

Polysaccharides

  • Long chains of monosaccharides.
  • Major polysaccharides include:
    • Starch: Primary storage polysaccharide in plants, composed of glucose.
    • Glycogen: Major storage polysaccharide in animals, also composed of glucose but with structural differences from starch.
    • Cellulose: A structural polysaccharide formed by long chains of glucose. It is not an energy storage form but provides rigidity to plant cell walls. Animals produce enzymes to digest starch and glycogen but lack those to digest cellulose, which is referred to as dietary fiber.

Background of the Experiment

  • Benedict’s Reagent: A biochemical test for establishing the presence of monosaccharides and most disaccharides (except sucrose).
    • Composition: Copper and citrate in a buffer with basic pH.
    • Reaction: In presence of reducing sugars (like glucose), heated copper (II) ions are converted to copper (I) ions, yielding a red precipitate (Cu₂O).
  • Color changes in Benedict's Test:
    • Blue: No sugar
    • Green: Trace amounts of sugar
    • Yellow: Increased amount of sugar
    • Orange: High concentration of sugar
    • Red/Rusty Brown: Very high concentration of sugar
  • Note: Benedict's reagent is not specific for any one sugar, thus quantitative determinations of carbohydrate concentration are not reliable; it is best for making relative comparisons.

Quantitative and Qualitative Data

  • Quantitative Data: Numerical information that answers how many or how much.
  • Qualitative Data: Based on observations that cannot be precisely measured, often represented by symbols or code (e.g., Benedict’s Test results indicated by symbols such as -, +).

Activity: Benedict’s Reagent and Carbohydrates

Experiment Overview

  • Testing sugar levels in various solutions.

Materials Needed for Group of Four:

  • Hot plate
  • Beaker (400 mL)
  • Test tube holder
  • Test tubes (9)
  • Marking pencil
  • Measuring cylinder
  • Sugar solutions (5% glucose and 5% sucrose) for controls
  • 6 drink solutions
  • Benedict’s reagent
  • Distilled water

Controls and Predictions

  • Importance of controls:
    • Positive control: Positive test response.
    • Negative control: Negative test response.
  • Predictions for controls:
    1. 5% glucose: Expected positive test (color change).
    2. 5% sucrose: Expected negative test (no color change).
    3. Distilled water: Expected negative test (blue).

Hypothesis

  • Develop a hypothesis predicting results based on sugar concentration in drink solutions.

Procedure: Steps for the Experiment

  1. Set up a boiling water bath; fill a beaker half with water, bring to boil, then maintain simmer.
  2. Label test tubes from 1 to 3 and A to F.
  3. Add to tubes:
    • 2 ml of 5% glucose to tube 1
    • 2 ml of 5% sucrose to tube 2
    • 2 ml of distilled water to tube 3
  4. Record color of each tube before adding Benedict’s reagent.
  5. Add 2 ml of Benedict's solution to test tubes 1–3 and mix.
  6. Let tubes sit for 5 minutes and then place them in the boiling water bath for 5 minutes.
  7. After boiling, remove tubes and record resulting color without remixing.
  8. Record results in Table 1:
    • Record colors before and after the test, indicating solid or liquid, and type of control.

Table 1. Benedict’s Test with Controls Results

SampleColor before TestColor after TestSolid or LiquidType of ControlMatch Hypothesis
5% Glucose
5% Sucrose
Distilled Water
  1. Analyze drink solutions A-F, guessing types of sugars present.
  2. Make predictions for solutions A-F.
  3. Add specified amounts of solution A to tube A, solution B to tube B, etc.
  4. Record color of each solution before the Benedict’s test.
  5. Repeat steps with provided solutions, only boiling 3 tubes at a time.
  6. Record results in Table 2.

Table 2. Benedict's Test with Solutions

BeverageABCDEFGrams of sugar/servingType of sugarColor before TestPredicted Result (color)Actual ResultColor?Precipitate?
  1. Compare results within the class.
  2. Clean up:
    • Turn off the hot plate but leave the beaker to cool.
    • Dispose of Benedict's solutions appropriately.
    • Clean test tubes and return them to drying rack.

Reference

  • Griner, Richard. Biological Molecules: Structure and Methods of Analysis. Department of Biological Sciences, Augusta University, 2003.
  • Helms, Doris R. et al. Organic Molecules, Biology in the Laboratory, 3rd Edition, W.H. Freeman, 1997.

Post-Lab Activity: Carbohydrate Lab Assignment

Questions to Answer:

  1. What carbohydrates does Benedict’s reagent test for?
  2. What color(s) indicates a positive result?
  3. What color(s) indicates a negative result?
  4. Purpose of control tubes:
    • Control tube 1 (5% glucose):
    • Control tube 2 (5% sucrose):
  5. During the Benedict’s test, what does a positive test indicate?
  6. What is the chemical basis for a positive Benedict’s test?
  7. Record results from Table 2 for each tested drink.
  8. Identify solution(s) with the most sugar based on Benedict's reaction.
  9. Compare predicted vs. actual results for each solution.
  10. Were any results surprising? Yes/No. Explain unexpected results.
  11. Rewrite your initial hypothesis from page 4.
  12. State a conclusion based on results and hypothesis analysis.