plasma membrane

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noun Cell Biology.


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Origin of plasma membrane

First recorded in 1895–1900
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2022


What is a plasma membrane?

Plasma membrane is another term for the cell membrane—the thin layer that encloses a cell’s cytoplasm, which is the substance between the membrane and the nucleus. It allows beneficial things to pass through while blocking harmful ones.

The cell is the basic structural unit of all living organisms. It is a microscopic structure consisting of a nucleus surrounded by the cytoplasm and enclosed by a membrane—the plasma membrane. In general, a membrane is a thin barrier or covering. In the context of biology, plasma is used as another word for cytoplasm.

The membrane consists of a double layer made up of lipid molecules and large protein molecules. Lipids and proteins are the basic building blocks of living cells. They move fluidly in the plasma membrane and function like a system of gates, giving the membrane its ability to allow molecules to pass through. For this reason, the membrane is described as semipermeable (meaning it is capable of allowing things to pass through it).

Plants cells also have a rigid outer wall in addition to a plasma membrane.

The plasma membrane is also called the plasmalemma.

Why is the plasma membrane important?

To understand why the plasma membrane is important, you first have to understand what it does. In essence, the plasma membrane serves as the glue that holds the components of the cell together and protects it from outside threats. The membrane regulates what comes into the cell. Among other things, it keeps potentially harmful substances out and keeps nutrients in.

The makeup of the plasma membrane includes three crucial components: phospholipids, cholesterol, and proteins. The phospholipids attract and repel water. The layer created by these opposing forces is known as the phospholipid bilayer. It’s responsible for creating the spaces within the membrane and making it a good barrier. Proteins help with transporting cell components across the barrier. Cholesterol also helps with this regulation.

Did you know ... ?

The understanding of the plasma membrane is based on what’s called the fluid mosaic model. The fluid mosaic model is a way of describing the structure of plasma membranes that likens them to a “mosaic” of different components, consisting of a fluid double layer of molecules. The model was developed by cell biologist S. J. Singer and biochemist Garth L. Nicolson in 1972. It has evolved somewhat since then, but it is now the most accepted way to understand the structure and function of the plasma membrane.

What are real-life examples of plasma membrane?

Along with the nucleus and the cytoplasm, the plasma membrane is one of the main components of a cell. It’s more commonly called the cell membrane. 



Quiz yourself!

True or False?

The plasma membrane blocks anything from entering.

How to use plasma membrane in a sentence

Medical definitions for plasma membrane

plasma membrane

cell membrane
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

Scientific definitions for plasma membrane

plasma membrane

See cell membrane.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.