Structure of the plasma membrane • Figure 3.2
The plasma membrane's phospholipid bilayer separates the inside from the outside. The
proteins embedded within the membrane help transport material across the membrane,
distinguish the cell's identity, and participate in intercellular communication.
Integral (transmembrane) proteins
Functions of the Plasma Membrane
1. Acts as a barrier separating inside and outside of the cell.
2. Controls the flow of substances into and out of the cell.
3. Helps identify the cell to other cells (e.g., immune cells).
4. Participates in intercellular signaling.
The Plasma Membrane Is
the Cell's Gatekeeper
The plasma membrane is a flexible barrier made of two
layers of phospholipids that is referred to as a lipid bi-
layer (Figure 3.2; see also Chapter 2). There are other
lipids within the bilayer, mostly cholesterol and some gly-
colipids (lipids with sugar attachments).
Associated with the lipid bilayer are membrane pro-
teins. Some proteins are firmly embedded in the bilayer
and either extend into the bilayer or span the entire bi-
layer. These proteins are called integral proteins. Other
proteins are not firmly embedded but are rather loosely
associated with the bilayer. These proteins are called pe-
ripheral proteins. Many membrane proteins (integral or
peripheral) have sugar attachments and are called gly-
coproteins. Membrane proteins have various functions,
including transporting substances across the membrane,
binding substances to either side of the membrane, cata-
lyzing chemical reactions (enzymes), and enabling the cell
to be recognized by immune cells.
Think of the plasma membrane as a sandwich with two
layers of raisin bread and a filling held together by tooth-
picks. The plasma membrane has four basic functions:
It acts as a barrier that separates the inside of the cell
from its surroundings.
Its transport proteins (for water-soluble material)
the bilayer itself (for lipid-soluble material)
control the flow of material
into and out of the cell via
Certain external glycoproteins
on the plasma membrane help
identify the cell to immune
Its receptor proteins and some enzymes participate in
Let’s look inside the cell.
of a membrane that al-
lows some substances
to pass through but
52 CHAPTER 3
Cells and Tissues