Cells

Lesson 3.1

Introduction to Cells

3.1.3 Cell membrane

Whether a cell is prokaryotic or eukaryotic, a cell possesses a cell membrane. Acting as a physical barrier, the cell membrane separates the cell from its environment. Acting as a diffusion barrier, the cell membrane selects what materials enter or leave the cell. Not all molecules that come in contact with the cell membrane will be able to enter.

This membrane is composed of a double layer of phospholipids, proteins, and other molecules. Phospholipids are macromolecules that are composed of phosphate groups, glycerol, and fatty acids. Chemically, the phosphate group and glycerol are soluble in water (hydrophilic) while the fatty acid end is not soluble in water (hydrophobic). When arranged to create a double layer of phospholipids, the hydrophilic end of one molecule will face the inside of the cell and the hydrophilic end of the other molecule will face the outside of the cell. The hydrophobic ends of the molecules will face one another. Thus, a membrane separating the outside of the cell from the inside of the cell can form.

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FIGURE 3.1.2 Diagram of the structure of phospholipids and their specific arrangement to create a plasma membrane.

One red dot attached to two twisted blue strands represents one phospholipid molecule. The red dot represents the phosphate and glycerol end of the molecule. The two twisted blue strands represent the fatty acid end of the molecule.

The cell membrane is selectively permeable, which means it acts as a gatekeeper. Materials that are needed by the cell will be allowed into the cell. Those materials not needed by the cell will not enter the cell. This selection is made possible, in part, by the variety of proteins that are embedded into the cell membrane to help with transportation of materials across the membrane.

Figure 3.1.3 below illustrates the fluid mosaic model of cell membranes. This model states that cell membranes are not static, but flexible. In addition, such membranes are not homogeneous. They contain a variety of proteins, serving a variety of functions including: transporting materials across the membrane, chemically identifying the cell (such as proteins associated with human blood types) and enabling physical connections between individual cells to create tissues. Other materials may also be present including cholesterol, glycolipids, and glycoproteins. While found in both prokaryotic and eukaryotic cells, the types of proteins and their arrangement within the cell membrane differs between these cell types.

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Figure 3.1.3 A diagram illustrating the fluid mosaic model of cell membranes. Note the number of different materials found in the membrane.

Activity VIEW VIDEO: Fluid mosaic model of the cell membrane

 As you view the video using the link above:

1. Take notes to supplement your work

2. Write down any questions you may have

3. When the video is finished, discuss as directed by your instructor.

Activity CELL MEMBRANE COLORING SHEET 

1. Create a color coded key for each of the structures in the cell membrane diagram below. For example:        red = carbohydrate attached to protein

 For the phospholipids:

 color the heads of the phospholipid molecules using one color

 color the tails of the phospholipid molecules using a different color

2. Using the key you have created, color the entire diagram.

3. Briefly describe the functions of the following membrane structures:

• Phospholipids (both the head and tail portion of this molecule)

 Integral protein forming a channel (also known as a channel protein)

• Peripheral proteins
• Carbohydrate attached to a protein

4. List any questions you have and find the answers (internet, notes, peer, instructor).

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Optional activity: Basics of a light microscope

1.Review the basics of working with a microscope (includes: background information, parts of a microscope, calculating total magnification, using a microscope, creating a wet mount, basic staining, troubleshooting and a self quiz and microscope labeling) found at:

How to use the microscope

2. For this second part of this activity, a standard light microscope, slides, cover slips, colored thread, scissors, and newspaper are needed to complete this activity. Be sure to follow all directions provided by your instructor for this activity, including lab safety information.

Microscope activity

If microscopes are not available, a virtual form of this lab can be found at:

Virtual Microscope Lab