Effects of Temperature and Solvents on the Cell Membrane

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SOTM LAB: B1412/99

I. TEACHER NOTES & GUIDELINES

Title of Lab: Effects of Temperature and Solvents on the Cell Membrane

Developers of Lab: Adapted by James Kirby JD726, Jennifer Mortellaro JD449, and James Prockup JD575 from a publication by the Department of Biological Sciences at Western Michigan University, Kalamazoo, MI

Overview of Lab:
Description:
The purpose of this lab is to illustrate the effects that temperature and solvents have on the cell membrane.

Red beet tissue contains large amounts of betacyanin, a red pigment, located in the large internal membrane vacuoles. When the membrane is damaged, this pigment can cross the vacuole membrane and cell (plasma Membrane). In this experiment we will take pieces of beet root and test what types of environmental stress disrupts the membrane. This should give students an insight into the composition of the membrane.

Curriculum Considerations:
This lab is intended to be used in a Regents Biology Curriculum. This lab can be used when studying the topic of cell membrane structure and function. This lab is expected to be completed in two, 40-minute periods.

Safety Considerations:
Solvents/household liquids, which the students want to use in Part IV, must be approved by teacher before use.

Background Information

A. SCIENTIFIC VIEWPOINT:A major breakthrough in cell biology was the fluid mosaic model hypothesis of membrane design. In this model proteins are an integral part of the membrane. The fluid mosaic model proposed by Singer and Nicolson is a mixture of proteins and lipids.

The lipids are in the form of a bilayer. The hydrophobic parts of the lipid molecule face the inside while the hydrophillic portions of each lipid face either the cytoplasmic or extracellular environment. The lipids are primarily phospholipids.

Proteins in the membranes are of two types: Integral and Peripheral. Integral proteins are embedded within the bilayer. These proteins are very difficult to remove from the membrane and to study them one has to use detergents to extract them. Some of these integral proteins work as channels for the movement of ions and other molecules through the membrane. Peripheral proteins are attached to the surface of the membrane usually by a charge and are much easier to extract.

The membrane is described as fluid because the bilayer proteins and lipids can move laterally within the membrane. To maintain the fluid nature of the membrane, animal cells require Cholesterol. This steroid acts as antifreeze to keep the membrane fluid even though animal cell lipids have fatty acids that might keep the membrane solid at body temperature. Most plant cells do not require cholesterol in their membranes because of the higher polyunsaturated content of their membrane lipids.

B. MISCONCEPTIONS AND ACCEPTED SCIENTIFIC EXPLANATIONS

1.All cell membranes are not the same.
All cell membranes share the same phospholipid structure, although the lipids vary in different types of cells. The membrane proteins and associated carbohydrates vary extensively from one cell type to another.

Membranes are porous.
Membranes are selectivelyi-permeable. Only certain substances will pass through them. Permeability differs with the cell type.

Membranes are rigid.
Membranes are flexible due to the properties of the phospholipid bilayer and cholesterol.

OBJECTIVES
1. Gain an understanding of how stress affects membranes. 2. Become familiar with the use of a spectrophotometer. 3. Design and carry out an experiment using knowledge obtained from completing Part III of this lab.

EQUIPMENT AND MATERIALS
PROVIDED BY SOTM:
Spectrophotometers 20D+*Laptop Computers
Two (2) Hot...
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