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Plasma Membrane of a Cell: Definition, Function & Structure

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Derrick Arrington

Derrick has taught biology and chemistry at both the high school and college level. He has a master's degree in science education.

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Brenda Grewe

Brenda has 25 years of experience teaching college level introductory biology and genetics. She earned her PhD in Genetics from Indiana University.

The plasma membrane of a cell creates a boundary between a cell and its environment and regulates the molecules that enter and exit the cell. Explore the definition, function, and structure of the plasma membrane. Updated: 09/16/2021

What is a Plasma Membrane?

Imagine a water balloon. Prick it with a needle and the water would burst out everywhere; no more water balloon. Now, imagine a new water balloon in a bathtub full of water. What if the balloon itself was porous and could choose when water could flow in and out of the balloon? That's sort of like what a plasma membrane is to a cell.

The plasma membrane is the boundary between the cell and its environment. It regulates what enters and exits the cell. Cells must maintain an appropriate amount of molecules to function inside them. They must also have a way to keep things out or to allow things to enter. This is the job of the plasma membrane. The plasma membrane is like the guard at a gated community. The guard must inspect those who enter and those who leave to make sure that only the people and things needed in the community are there.

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  • 0:01 What Is a Plasma Membrane?
  • 0:55 Plasma Membrane Structure
  • 2:08 Function of the Plasma…
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Plasma Membrane Structure

The plasma membrane is composed of a phospholipid bilayer, which is two layers of phospholipids back-to-back. Phospholipids are lipids with a phosphate group attached to them. The phospholipids have one head and two tails. The head is polar and hydrophilic, or water-loving. The tails are nonpolar and hydrophobic, or water-fearing.

Since water is the key component of living organisms, both inside and outside of the cell, this causes the phospholipids to line up in two layers with the heads pointing outward and the tails hidden in the middle. It basically looks like a phospholipid sandwich with the heads forming the bread and the tails forming the meat in the middle.

Located all throughout the surface of the plasma membrane are cholesterol molecules. These molecules help stabilize the phospholipids and keep them in position. The plasma membrane is also dotted with proteins. Some of the proteins are peripheral proteins that only go halfway through the membrane, while others are integral proteins that go entirely through the membrane. These proteins serve as channels to allow the molecules to enter and leave the cell.

Function of the Plasma Membrane of a Cell

You are comfortable in your house largely because the thermostat maintains the temperature within a certain range regardless of what is happening outside. Similarly, all living cells must maintain a balance regardless of external or internal conditions. This includes regulating what can enter and exit the cell. The plasma membrane serves as a gateway to allow or block the entry or exit of materials.

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Additional Activities

Review of Structure and Function of the Plasma Membrane:

The lesson included three learning outcomes:

  1. Understand the role of the plasma membrane
  2. Outline the structure of the membrane
  3. Explain what the plasma membrane of a cell does

The first two activities below will help you to achieve these outcomes.

Activity 1: Plasma Membrane Crossword Puzzle

Review the lesson if necessary. All words to complete the puzzle can be found there.

Clues


Across

3. stabilizes the phospholipid bilayer

5. water-fearing

7. polar part of a phospholipid

8. membrane components that can function as channels

9. water-loving

10. moves freely across the plasma membrane

12. cannot move freely across the plasma membrane

13. protein type that goes partway through the phospholipid bilayer

Down

1. negatively charged groups that form the outside of the phospholipid sandwich

2. property of the plasma membrane that allows some substances into the cell and keeps others out

4. main structural component of the plasma membrane

6. nonpolar part of a phospholipid

11. protein type that spans the plasma membrane

Activity 2: Label the Drawing

The illustration of the plasma membrane is from your lesson. Test your memory of the structural components and their organization by identifying components A-D.

Activity 3: Challenge Questions

1. Examine the illustration from Activity 2. Which of the labeled components could regulate the entry of calcium into a cell?

2. Calcium exists as an ion (Ca{eq}^++ {/eq}) both inside and outside of cells. It cannot cross through the phospholipid bilayer, and proteins are required for it to move across the plasma membrane. In contrast, small nonpolar molecules can move directly across the phospholipid bilayer. What is the chemical basis for the difference in the movement of calcium and small nonpolar molecules across the plasma membrane?

Teacher Answer Key:


Activity 1:

Activity 2:

A = phospholipid

B = peripheral protein

C = integral protein

D = cholesterol

Activity 3:

1. Component C, an integral protein, could function as a channel that regulates the movement of calcium into or out of a cell.

2. As an ion, calcium is highly hydrophilic and the hydrophobic interior of the phospholipid bilayer acts as a barrier to calcium's movement across the bilayer. Nonpolar molecules are hydrophobic and can diffuse across the phospholipid bilayer via hydrophobic interactions with the phospholipid tails.

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