Guided Inquiry Activity #14
An introduction to cells and metabolism
Model 1. The smallest unit able to sustain life is called a cell. A cell can be simply defined as a container of small and large molecules that are essential for the survival of an organism. Unicellular organisms are called prokaryotes (pronounced PRO-CARRY-OATS), and they contain all that is necessary for their survival within a single cell – bacteria are examples of prokaryotes. When cells evolved to live as a collection or group, some organisms became, by definition, multicellular; these eukaryotes (pronounced YOU-CARRY-OATS) contain many individual cells that have distinct functions – animals and plants are examples of complex organisms comprised of eukaryotic cells.
Figure 14.1. A comparison of prokaryotic and eukaryotic cells (μm is the abbreviation for micrometers or 1 billionth of a meter).
Although prokaryotes and eukaryotes are very different, several characteristics are common among all types of cells: 1) Every cell needs a barrier, called a cell membrane or a cell wall, that controls the flow of molecules into or out of the cell; 2) Inside the cell, a range of molecules including carbohydrates and 20,000 to 30,000 different proteins, play a key roles cell function; and finally, 3) each cell contains DNA[footnoteRef:0] – the genetic blueprint or hereditary information which is passed from cell to cell during cell division. Specialized compartments called organelles are found in eukaryotic cells; chloroplasts and mitochondria are examples of organelles. [0: DNA is the abbreviation for DeoxyriboNucleic Acid – the chemical name for the type of molecule that makes up genetic information. ]
|Figure 14.2. A powerful microscope image of a eukaryotic cell from an animal. The red arrow is pointing to the nucleus, while the blue arrow is pointing to the cell membrane.|
Figure 14.3. A) A cluster of prokaryotic bacteria, specifically E.coli cells.[Public domain] (B) The eukaryotic cells of a leaf [footnoteRef:1] [1: “Plagiomnium affine laminazellen” by Kristian Peters – Fabelfroh. Licensed under CC BY-SA 3.0 via Wikimedia Commons ]
1. Using the information in Model 1. Complete the Venn Diagram below by placing the following words in the appropriate area (you can use text boxes):
b. Cell membrane/cell wall
d. Organelles (e.g. mitochondria or chloroplasts)
h. An animal or plant
j. Typically multicellular
2. Using the information provided in Model 1, identify the microscope images below as either prokaryotic or eukaryotic cells. Explain your reasoning.
Image Credit: NIAID
|b) [footnoteRef:2] [2: This image from Boyer’s concepts in Biochemistry]|
3. In the image below, please place the following labels in the correct box.
a. Skin cell
Image courtesy of Dr. Gary E. Kaiser, Professor of Microbiology, Community College of Baltimore County
How did you decide which was which?
Breaking down molecules for energy
Model 3: All living things consume molecules in the form of food in order to produce energy and build molecules that make up the organism. In this activity, we will consider the metabolism of sugar by yeast – Sacchromyces cerevisae. It is this very process that produces the gas which puffs up bread dough (“the rise”) during in the process of making bread, and the ethanol that is the alcohol of beer brewing.
Wheat flour is composed of ~70-80% starch and 7-15% protein.
Figure 14.5. The overall breakdown of starch into sugar, then ethanol and CO2 by yeast.
Let’s look more closely at the breakdown of sugar by yeast. Ethanol and carbon dioxide are merely by products of yeast respiration. For yeast, the ultimate goal of consuming glucose is to generate energy to power its cellular machinery and eventually grow and multiply. How does the process of consuming glucose produce energy? The energy produced in stored in the form of high-energy bonds specifically, the bonds of ATP – adenosine triphosphate. The energy released from breaking the phosphodiester bonds can be used to drive difficult chemical reactions – in this way, the cell generates and uses energy to grow and multiply.
Figure 14.6. The structure of Adenosine Triphosphate, ATP.
When yeast consume glucose, ATP is produced. Consider Figure 14.7 and Table 14.2 below. The process of breaking down sugar to make ATP is called glycolysis – “glyco-” for sweet and “lysis” for split or break. At the end of glycolysis there are 2 molecules of ATP produced along with 2 molecules of pyruvate (pronounced PIE-RU-VATE), 2 NAD[footnoteRef:3]-H, and 2 H+. In yeast, the process of ethanol fermentation is designed to recycle the NAD+ (we will learn more about NAD+ and its role in Activity X), but the added side benefit for us is that the yeast produce ethanol (for making beer) and CO2 (for baking bread) in the process. [3: NAD is not a combination of any elemental symbols. It stands for Nicotinamide Adenine Dinucleotide. See Activity 15 for more on NAD. ]
Figure 14.7. The overall equations for glycolysis and ethanol fermentation. In red, the NAD+ molecule is nicotinamide adenine dinucleotide, while the blue NAD-H is nicotinamide adenine dinucleotide with an additional bond to another hydrogen atom.
|Table 14.2. The balance of molecules consumed and produced from yeast “eating” sugar. Areas that are grayed out indicate those molecules are both consumed and produced from the process – and therefore cancel out.|
|Glycolysis||Glucose (C6H12O6)||2 Pyruvate (C3H3O3)|
|2 NAD+||2NAD―H and 2 H+|
|2 ADP + 2 Phosphates||2 ATP and 2 H2O|
|Ethanol Fermentation||2 Pyruvate (C3H3O3)||2 Ethanol (C2H5OH) and 2 CO2|
|2 H+ and 2 NAD―H||2 NAD+|
4. According to Figure 14.5, what does the enzyme alpha amylase do?
5. Complete Table 14.3.
Table 14.3. Quantity of atoms before and after yeast act on sugar
|Quantity (#) of the atom|
(before the yeast)
|Ethanol + carbon dioxide
(after the yeast)
Compare the # of atoms of Carbon before the sugar is eaten by the yeast and after. Do the same for hydrogen and oxygen.
a) What do you notice?
b) So, if the atoms themselves didn’t change, what did change? Please explain.
6. “Proofing yeast” requires mixing yeast with sugar and warm water until a “head of foam” forms. What is causing this foam (a foam is a mass of gas bubbles in a matrix of liquid film)? And why does it “prove” the yeast are ok to use?
7. While ethanol fermentation is useful to humans in brewing beer and baking bread, why do the yeast perform ethanol fermentation?
Putting it all together:
8. Here are some images of cells. Determine if they are prokaryotic or eukaryotic and explain your reasoning:
© 2011 Jansen et al
© 2013 Ahn et al
(Salmonella species cause food poisoning)
|onion cell||Listeria monocytogenes (causes listeriosis, a food borne illness)|
9. Based on your understanding of eukaryotic and prokaryotic, predict which class the following living things belong to:
b) A chicken
c) Lactobacillus – the active ingredient in yogurt
10. Although we are not supposed to mix yeast with boiling water, we are supposed to cook eggs and poultry up to 160-165˚F. Considering, that eggs and poultry are carriers of Salmonella, what is the purpose of this instruction?
Copyright © 2016 Wiley, Inc. Page