7 Introduction

Module 4 – The Plasma Membrane


You learned in the last module that cells are the basic unit of life. We explored some of their internal structures and how those internal structures facilitate the life of the cell. One of the most important features of the cell is the external barrier of the cell, the plasma membrane. All cells have a plasma membrane. As you know from your work in the last module, the plasma membrane is made of a phospholipid bilayer with proteins, and other molecules, embedded and provides the cell will some measure of control over what is entering/ exiting the cell (selective permeability).

Why is this important? Every cell, whether it represents a unicellular organism or one cell of a multicellular organism, interacts with it’s environment. Each cell will in the least require nutrients (entering) and make waste (exiting) the cell.

In this module, we will review the composition of the plasma membrane in more detail and then discover several ways in which molecules enter/ exit the cell through the plasma membrane. In addition to phospholipids, you will explore roles of proteins embedded in the plasma membrane. Also, it is important to note that the membranes of the endomembrane system (please define) are also composed primarily of phospholipids and proteins.

Once you have a strong understanding of the general composition of the plasma membrane in general, investigate passive and active movement across the membrane. You should be able to describe the examples discussed within the text (simple diffusion, osmosis, facilitated diffusion, active transport, and bulk transport: exocytosis and endocytosis).

While you work on understanding these concepts, it is also important to consider that while all cells have a plasma membrane only some additionally have a cell wall. Consider the plant cell wall and how this structure will influence the response of the plant cell to different tonicities. Figures 3.22 and 3.23 in your readings provide an overview of animal and plant cells exposed to isotonic, hypotonic, and hypertonic environments. 1

Learning Outcomes

This module addresses the following Course Learning Outcomes listed in the Syllabus for this course:

  • Demonstrate knowledge of biological principles.
  • Demonstrate knowledge of scientific method.
  • Communicate scientific ideas through oral or written assignments.
  • Interpret scientific models such as formulas, graphs and tables.
  • Demonstrate problem solving methods in situations that are encountered outside of the classroom .

Module Objectives

Upon completion of this module, the student will be able to:

  • Review the composition of the plasma membrane.
  • Review the composition of the plant cell wall (NOTE: other organisms have cell walls that may differ in composition, but have some similar functions).
  • State which types of molecules freely pass the plasma membrane.
  • Define the Fluid-Mosaic Membrane model as it relates to the plasma membrane.
  • Describe the concepts of passive and active transport and for each of the examples covered in this module define: (simple diffusion, osmosis, facilitated diffusion, active transport).
  • Describe the general idea of tonicity – see Figures 3.22 and 3.23 in the readings.
  • Define isotonic, hypotonic, hypertonic solutions and state how they influence the cell in general.
  • Define bulk transport: endocytosis and exocytosis .


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