Category: Cells

How Surface Area to Volume Ratio Limits Cell Size

 

How Surface Area to Volume Ratio Limits Cell Size

  1. A cell is a metabolic compartment where a multitude of chemical reactions occur.
  2. The number of reactions increase as the volume of metabolic volume within a cell increases. (The larger the volume the larger the number of reactions)
  3. 3.All raw materials necessary for metabolism can enter the cell only through its cell membrane.
  4. The greater the surface area the larger the amount of raw materials that can enter at only one time.
  5. Each unit of volume requires a specific amount of surface area to supply its metabolism with raw materials. The amount of surface area available to each unit of volume varies with the size of a cell.
  6. As a cell grows its SA/V decreases.
  7. At some point in its growth its SA/V becomes so small that its surface area is too small to supply its raw materials to its volume. At this point the cell cannot get larger.

 

 

Food Chemistry

 

Food Chemistry

Introduction:

All living things are made up of one or more cells, and the cells in turn contain many kinds of molecules.  In this lab we will be looking at several different macromolecules (large molecules): proteins, carbohydrates, and lipids (specifically fats).  Various chemicals will be used in this laboratory to test for the presence of these molecules.  Most often, you will be looking for a particular color change.  If the change is observed, the test is said to be positive because it indicates that a particular molecule is present.  If the color change is not observed, the test is said to be negative because it indicates that a particular molecule is not present.

You will be using these tests to determine which of the macromolecules are present in various samples of food.

In all of the procedures, you will need to include a distilled water sample as a control.  Usually, a control goes through all the steps of the experiment but lacks one essential factor (the experimental variable).  This missing factor allows you to observe the difference between a positive result and a negative result.  If the control sample tests positive, you know your test is invalid.  Some tests may also contain other controls to make sure certain additives are not contaminated with the substance for which you are testing.

Proteins:

Protein molecules are long chains of amino acids joined by peptide bonds.

Biuret reagent, which is a blue color, contains a strong solution of sodium or potassium hydroxide (NaOH or KOH) and a very small amount of very dilute copper sulfate (CuSO4) solution.  The reagent changes color in the presence of proteins or peptides because the amino group (H2N-) of the protein or peptide chemically combines with the copper ions in biuret reagent.

Carbohydrates:

Carbohydrates include sugars and molecules that are chains of sugars.  Glucose is a simple sugar, also known as a monosaccharide.  Sucrose, or table sugar is a disaccharide, two sugar units bonded together.  Starch is a polysaccharide, a long chain of glucose units.

Benedict’s reagent reacts with many sugars (both mono- and disaccharides) after being heated in a boiling water bath.  Increasing concentrations of sugar give a continuum of colored products ranging from green through yellow and orange to orange red.

Iodine solution reacts with starch to produce colors ranging from a brownish to blue black.

Lipids:

Lipids are hydrophobic molecules which are insoluble in water and soluble in solvents such as alcohol and ether.  Lipids include fats, oils, and cholesterol.

Lipids do not evaporate from brown paper, instead leaving an oily spot.  Lipids also do not mix with water, forming a separate layer, usually on top of the water.  However, some molecules mix with both water and lipids, and can be used to mix the two.  These molecules are known as emulsifiers.  The liver produces bile salts which act as emulsifiers in the digestive tract.  Soaps and detergents also act as emulsifiers.

Summary of tests:

 

Biuret Reagent
Benedict’s Reagent
Iodine Solution
Brown Paper
Reacts with proteins or peptides, turns purple (protein) or pink (peptides) Reacts with sugars, turns green through yellow to orange (green, less sugar, orange, more sugar) Reacts with starch, turns dark brown to black Lipids leave oily spot

Procedure:

Take some time to plan with your lab partner what tests you will do, and in what order before beginning the procedures.

There are available in the lab a variety of different types of common food.  Choose at least 3 foods and test each for the presence of protein, carbohydrate (both starch and simple sugars), and fats.  Be sure to plan your experiments before starting.

Form a hypothesis for each sample you have chosen to test.

Samples will need to be suspended in water for most tests.  Using a mortar and pestle if necessary, break each sample to be tested into small pieces and suspend the pieces in a small amount of distilled water.

Also available are samples of each of the types of molecules for which you will be testing.  Use these samples to try out the tests so that you will know what a positive result looks like.

Be sure to include a blank control (distilled water) with each test so you know what a negative result looks like.  You may also include a positive control, a sample which you know contains the substance for which you are testing.

The procedures for testing for each type of molecule are given below.

Proteins and Peptides

Proteins:

  1. Use a separate test tube for each sample to be tested, as well as one for a control.
  2. Label each test tube.
  3. Place about 1 mL of each sample (and control) in its test tube.
  4. Add 5 drops of copper sulfate solution to each tube.
  5. Add 10 drops of potassium hydroxide solution to each tube and mix.
  6. Record the tube contents and final color in a data table.
  7. Conclusions: which tubes contained protein?

Carbohydrates: Sugars and starch

Starch

  1. Use a separate test tube for each sample to be tested, as well as one (or two) for a control.
  2. Label each test tube.
  3. Place about 1 mL of each sample (and control) in its test tube.
  4. Add 5 drops of iodine solution to each tube and mix.
  5. Record the tube contents and final color in a data table.
  6. Conclusions: which tubes contained starch?

Sugar

  1. Use a separate test tube for each sample to be tested, as well as one (or two) for a control.
  2. Label each test tube.
  3. Place about 1 mL of each sample (and control) in its test tube.
  4. Add about 2 mL of Benedict’s reagent to each tube and mix.
  5. Heat the tubes in a boiling water bath for 5-10 minutes.
  6. Record the tube contents and final color in a data table.
  7. Conclusions: which tubes contained sugar?

Lipids

  1. Place a small sample of the material to be tested on a square of brown paper.
  2. Place a small drop of water on the square of brown paper.
  3. Compare the drop of water to the sample.
  4. Wait at least 5 minutes.  Evaluate which substance impregnates the paper and which is subject to evaporation.  Record your results.
  5. Conclusions: which sample contained lipids?

Conclusion Questions:

  1. Why do experimental procedures include control samples?
  2. How would you test an unknown solution for each of the following:
    1. Sugars
    2. Fat
    3. Starch
    4. Protein
  3. Assume that you have tested an unknown sample with both biuret solution and Benedict’s solution and that both tests result in a blue color.  What have you learned?
  4. What purpose is served when a test is done using water instead of a sample substance?
  5. Compare your results.

Lab report:

Lab reports must include the following:

  1. A Title to the lab.  A Purpose: What was studied in this lab, and why did we study it?
  2. Procedure: a brief description of each type of test, what constitutes a positive test and what constitutes a negative test.
  3. All data tables.
  4. For each food sample, state your hypothesis and your conclusions.  Did your results confirm or refute your hypothesis?
  5. Answers to questions.
  6. A brief analysis of what worked in this lab and what didn’t work, and why.

 

Food Testing

 

Chemical Tests for Nutrients in Food

INTRODUCTION:

Cells are made up of small molecules like water; ions such as sodium and magnesium, and large organic molecules. There are four important types of large organic molecule in living organisms — proteins, carbohydrates (sugars & starches), lipids (fats), and nucleic acids. Proteins, carbohydrates, and fats serve as nutrients in the food that we eat.

In this experiment you will evaluate the nutrient content of unidentified food samples. You will use chemical reagents to test the unknown for specific nutrients. By comparing the color change a reagent produces in the unknown with the change it produces in the known nutrient, you can estimate the amount of that nutrient. Use small samples.

MATERIALS:

400-ml beaker
Hot plate
8 test tubes
Test tube rack
4 medicine droppers
Glass stirring rod
Tongs
Several unknown food substances
Glucose
Cornstarch
Non-fat dry milk
Lard
Distilled water
Benedict’s solution
Iodine-potassium iodide solution
10% aqueous sodium hydroxide solution
0.5% Copper sulfate solution
Sudan III solution

PROCEDURE:

Monosaccharide (simple sugar) test

1. Fill a 400-ml beaker to about 300 ml with water and heat on the hot plate.

Be sure to label all test tubes.

2. Place pea-sized portions of glucose and the unknown substance you are testing in separate test tubes. Add about 2.5 ml of distilled water and 10 drops of Benedict’s solution to each test tube. Mix with a stirring rod, or holding the tube between the thumb and index finger of one hand, thump it with the middle finger of the other hand to mix.

REMEMBER: If you use a stirring rod, wash it after every use, so you won’t contaminate one solution with another.

3. When the water boils, use tongs to place the test tubes in the water bath. Leave the test tubes in the water bath for 10 minutes.

Do not let the water bath boil hard. Control the boiling by turning the hot plate on and off as needed.

4. Remove the test tubes with tongs and place the tubes in a test tube rack. Unplug the hot plate to cool. When the tubes cool, an orange or red precipitate will form if large amounts of glucose are present. Small amounts of glucose will form a yellow or green precipitate. Record your observations in the DATA TABLE.

Polysaccharide complex sugar) test

5. Place cornstarch in a clean test tube and some of the unknown substance in another. Use a clean dropper to add 10 drops of iodine-potassium iodide solution to each test tube. Observe the results and record in the DATA TABLE.

Protein test

6. Place non-fat dry milk in a clean test tube and some of the unknown in another. With a clean dropper slowly add an amount of sodium hydroxide solution about equal to the amount of the milk sample, and mix carefully. Then add 10 drops of copper sulfate solution one drop at a time. Mix gently between drops. Observe the results and record in the DATA TABLE.

7. Repeat step 6 with the unknown substance.

Lipid test

8. Place a small piece of lard in a clean test tube and some of the unknown in another. Use a clean dropper to add 10 drops of Sudan III solution to each test tube. Mix well, observe and record your results in the DATA TABLE.

DATA TABLE:

Mark your results in the appropriate boxes. Indicate relative amount by H for high, M for medium, L for low, or 0 for none.
Monosaccharide test Polysaccharide test
SUBSTANCE: RELATIVE
AMOUNT:		 SUBSTANCE: RELATIVE
AMOUNT:
Unknown Unknown
Glucose Corn starch


Protein test Lipid test
SUBSTANCE: RELATIVE
AMOUNT:		 SUBSTANCE: RELATIVE
AMOUNT:
Unknown Unknown
Non-fat dry milk Lard

CONCLUSIONS:

Question 1 . What is the main nutrient in the unknown?

Question 2. What are the controls in this investigation?

 

Diffusion and Osmosis

Teacher correction: Glucose and fructose are reducing sugars. Sucrose is not a reducing sugar unless first hydrolyzed; do not use untreated sucrose as an expected positive result with Benedict’s reagent.

 

  Diffusion and Osmosis

Introduction:
In this exercise you will measure diffusion of small molecules through dialysis tubing, an example of a semi permeable membrane. The movement of a solute through a semi permeable membrane is called dialysis. The size of the minute pores in the dialysis tubing determines which substance can pass through the membrane. A solution of glucose and starch will be placed inside a bag of dialysis tubing. Distilled water will be placed in a beaker, outside the dialysis bag. After 30 minutes have passed, the solution inside the dialysis tubing and the solution in the beaker will be tested for glucose and starch. The presence of reducing sugars such as glucose and fructose will be tested with Benedict’s Solution. The presence of starch will be tested with Lugol’s solution (iodine-potassium-iodide).

Procedure:

  1. Obtain a 30 -cm piece of 2.5-cm dialysis tubing that has been soaking in water. Tie off one end of the tubing to form a bag. To open the other end of the bag, rub the end between your fingers until the edges separate.
  2. Place 15 mL of the 15% glucose/ 1% starch solution in the bag. Tie off the other end of the bag, leaving sufficient space for the expansion of the bag’s contents. Record the color of the solution in Table 1.1.
  3. Test the 15% glucose / 1% starch solution in the bag for the presence of glucose. Your teacher may have you do a Benedict’s test. Record the results in Table1.1.
  4. Fill a 250 mL beaker or cup 2/3 full with distilled water. Add approximately 4 mL of Lugol’s solution to the distilled water and record the color in Table 1.1. Test the solution for glucose and record the results in Table 1.1.
  5. Immerse the bag in the beaker of solution.
  6. Allow your set up to stand for approximately 30 minutes or you see a distinct color change in the bag or the beaker. Record the final color of the solution in the bag, and of the solution in the beaker, in Table 1.1.
  7. Test the liquid in the beaker and in the bag for the presence of glucose. Record the results in Table 1.1.

 

Table 1.1

Initial Contents Initial Solution Color Final Solution Color Initial Presence of Glucose Final Presence of Glucose
Bag 15% Glucose & 1% starch
Beaker H2O + IKI

Analysis of Results:
1. Which substance(s) are entering the bag and which are leaving the bag? What experimental evidence supports your answer?

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2. Explain the results you obtained. Include the concentration differences and membrane pore size in your discussion.

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3. Quantitative data uses numbers to measure observed changes. How could this experiment be modified so that quantitative data could be collected to show that water diffused into the dialysis bag?

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4. Based on your observations, rank the following by relative size, beginning with the smallest : glucose molecules, water molecules, IKI molecules, membrane pores, starch molecules.

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5. What results would you expect if the experiment started with glucose and IKI solution inside the bag and only starch and water outside? Why?

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Osmosis:
In this experiment you will use dialysis tubing to investigate the relationship between solute concentration and the movement of water through a semi permeable membrane by the process of osmosis. When two solutions have the same concentration of solutes, they are said to be isotonic to each other. If the two solutions are separated by a semi permeable membrane, water will move between the two solutions, but there will be no net change in the amount of water in either solution. If two solutions differ in the concentration of solutes that each has, the one with more solute hypertonic to the one with the less solute. The solution that has less solute is hypotonic to the one with more solute. These words can only be used to compare solutions.

Procedure:
1. Obtain six 30-cm strips of presoaked dialysis tubing.

2. Tie a knot in one end of each piece of dialysis tubing to form six bags. Pour approximately 25 mL of each of the following solutions into separate bags:

  • Distilled water
  • 0.2 M sucrose
  • 0.4 M sucrose
  • 0.6 M sucrose
  • 0.8 M sucrose
  • 1.0 m sucrose

Remove most of the air from the bags by drawing the dialysis bag between two fingers. Tie off the other end of the bag. Leave sufficient space for the expansion of the contents in the bag.

3. Rinse each bag gently with distilled water to remove any sucrose spilled during filling.

4. Carefully blot the outside of each bag and record in Table 1.2 the initial mass of each bag.

5. Fill six 250 mL beakers 2/3 full with distilled water.

6. Immerse each bag in one of the beakers of distilled water and label the beaker to indicate the molarity of the solution in the dialysis bag. Be sure to completely submerge each bag.

7. Let them stand for 30 minutes.

8. At the end of 30 minutes remove the bags from the water. Carefully blot and determine the mass of each bag.

9. Record your group’s results in Table 1.2. Obtain data from the other lab groups in your class to complete Table 1.3: Class Data.

Table 1.2 Dialysis Bag Results: Individual Data

Contents in Dialysis Bag Initial Mass Final Mass Mass Difference % Change in Mass
a). Distilled Water  

 
b). 0.2 M  

 
c). 0.4 M  

 
d). 0.6 M  

 
e). 0.8 M  

 
f). 1.0 M  

 

To Calculate:

% change in mass = Final Mass-Initial Mass X 100
———————–

Initial Mass

Table 1.3 Dialysis Bag Results: Class Data

percent change in Mass of Dialysis Bags

 

Bag Contents Group 1 Group 2 Group 3 Group 4 Group 5 Group 6 Total Class Average
Distilled Water
0.2 M
0.4 M
0.6 m
0.8 M
1.0 M

10. Graph the results for both your individual data and class average on the following graph. For this graph you will need to determine the following:

a). the independent variable. __________________________________

b). the dependent variable. ___________________________________

Graph Title ______________________________________________

Analysis of Results:
1. Explain the relationship between the change in mass and the molarity of sucrose within the dialysis bag.

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2. Predict what would happen to the mass of each bag in this experiment if all the bags were placed in a 0.4 M sucrose solution instead of distilled water. Explain your response.

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3. Why did you calculate the per cent change in mass rather than using the change in mass?

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4. A dialysis bag is filled with distilled water and then placed in a sucrose solution. The bag’s initial mass is 20 g. and its final mass is 18 g. Calculate the percent change of mass, showing your calculations in the space below.

 

 

 

 

5. The sucrose solution in the beaker would have been ___________________ to the distilled water in the bag.

 

Related teaching resources

Review osmosis, diffusion, and tonicity before interpreting membrane experiments. An alternative model investigation is the egg osmosis lab with percentage mass change; review its separate safety and preparation requirements.