The Plasma
Membrane
Lesson Objectives

Describe how a cell’s plasma membrane functions.

Identify the roles of proteins, carbohydrates, and
cholesterol in the plasma membrane.
Main Idea

A cell’s plasma membrane helps maintain homeostasis.
A Balancing Act

All living things must maintain a balance regardless of
internal and external conditions. * HOMEOSTASIS *

Survival is dependent upon this act.

Certain nutrients must be available for every cell to
receive.
Material Control

The plasma membrane is the boundary between the cell
and its environment.

It allows for a steady stream of nutrients to come into
the cell and regulates the amount of nutrients within
the cell.

Ex. Bouncer/security guard
Permeability

The key property of the plasma membrane is selective
permeability.

This is the process by which the membrane allows some
molecules into the cell and keeps others out.

Certain molecules, such as water, can enter at any
time. Ions however must be regulated by certain
channels and times when it can enter.
Structure of the Plasma
Membrane
Structure of the Plasma
Membrane

A phospholipid bilayer

Phospholipids have the same general structure as a
lipid, with one exception.

Glycerol backbone

2 fatty acid tails

Phosphate group attached in place of the third fatty acid
tail
Structure of the Plasma
Membrane
Structure of the Plasma
Membrane

The phosphate group greatly alters the lipid
because the phosphate group is polar.

The phospholipid bilayer forms a barrier that is
polar on the surface and nonpolar in the middle.

The polar group allows the cell membrane to
interact with watery environments.

However, substances that can dissolve in water
will not pass through the plasma membrane
because they are stopped by the nonpolar middle.

Allows the environment inside the cell to be
separate from the environment outside the cell
Structure of the Plasma
Membrane

Moving with and among the phospholipids are
cholesterol, proteins, and carbohydrates.

Receptor proteins transmit signals to the inside of the
cell.

Proteins at the inner surface anchor the membrane to
the cell’s internal support structure, giving the cell
shape.
Structure of the Plasma
Membrane



Transport proteins span the membrane
and move needed substances or waste
materials through the membrane.
Cholesterol is positioned among the
phospholipids and helps to prevent the
fatty-acid tails of the bilayer from
sticking together.
Carbohydrates and identity markers
define the cell’s characteristics and
help cells identify chemical signals.
The Fluid Mosaic Model

The model of the plasma membrane is called the fluid mosaic model.

The membrane is flexible.

The phospholipids and proteins are not held in one spot; they are free to
move around like a boat on water.

The components are in constant motion
The Fluid Mosaic Model
Where’s the Membrane?
Membranes are found
surrounding the
entire cell and the
organelles within.
 Holds the cytoplasm
away from the
internal workings of
the organelles
 Both prokaryotic and
eukaryotic cells have
plasma membranes.
