Cells- Part 2:
The Cell’s Plasma
Membrane
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What is the Plasma
Membrane?
The Plasma Membrane
• Forms the outer structure
of the cell (like your skin
in a way)
• Acts as a barrier between
the inside and the outside
of the cell.
• The structure (lipid
bilayer) allows the
membrane to be fluid.
The
Cell
We’ll look at each component
of the plasma membrane. As
we discuss it – label it on
your diagram and add any
notes you will find useful
The Phospholipid Molecule
Wikipedia
• One single
component of the
plasma membrane is
this molecule: The
phospholipid. You
noted how the
phospholipids
orientated themselves
in the video.
What’s a Phospholipid?
• It’s a pair of fatty acid chains and
a phosphate group attached to a
glycerol backbone.
–Polar (water-soluble) heads face
out and the nonpolar fatty acids
hang inside.
Outside
the cell
Inside
the
cell
Fatty Acid Tails
• Flexibility
• The fatty acid tails are
flexible, causing the lipid
bilayer to be fluid. This
makes the cells flexible.
At body temperature,
membranes are a liquid
with a consistency that is
similar to cooking oil.
Membrane Structure
• Plasma Membranes consist of a phospholipid bilayer (and
proteins) in a specific arrangement.
• The surfaces of cell membranes are hydrophilic (water-loving);
the interiors are hydrophobic (water fearing).
Hydrophilic molecules tend to interact with water and with
each other. Hydrophobic molecules avoid interaction with water
and tend to interact with other hydrophobic molecules.
Outside the Cell
Inside the Cell
WATERY
All cells live in a
watery
environment
Proteins
• Proteins are also embedded in
the Membrane
• Proteins are scattered
throughout the membrane.
• They may be attached to inner
surface, embedded in the
bilayer, or attached to the outer
surface.
• Hydrophilic (polar) regions of
the protein project from the
inner or outer
surface. Hydrophobic
(nonpolar) regions are
embedded within the membrane.
Proteins
Transport Proteins
A few substances move
freely across the cell
membrane by passive
diffusion. Most small
molecules or ions require
the assistance of specific
protein carriers to
transport them across the
membrane.
Note that we will discuss
Transport to a greater
extent in section 4 of this
Cell unit
Cholesterol
• In animals, cholesterol is a
major membrane lipid. It may be
equal in amount to
phospholipids.
• It is similar to phospholipids in
that it one end is hydrophilic
(water loving -polar), the other
end is hydrophobic (water
hating non-polar). Note that the
polar head of the cholesterol is
orientated the same way as the
polar head of the phospholipids.
• Cholesterol makes the
membrane less permeable to
most biological molecules.
Cholesterol
Cholesterol helps maintain the fluidity of cell membranes and by adding firmness
to the plasma membrane also prevents it from becoming overly fluid so Cholesterol
generally helps the plasma membrane maintain it’s structure
Carbohydrate Chains
• Lipids and proteins within the
membrane may have a
carbohydrate chain attached.
These carbohydrates often
function as cell identification
markers, allowing cells to
identify other cells. This is
particularly important in the
immune system where cells
patrolling the body's tissues
identify and destroy foreign
invaders such as bacteria or
viruses.
Cytoskeleton
• The cytoskeleton is a network of protein
elements that extend through the cytoplasm
in eukaryotic cells.
• It provides for the distinctive shape of cells
such as red blood cells, muscle cells, and
nerve cells (neurons). It produces
movement of cells and is associated with
movement of materials within cells.
The Green structure is
the cytoskeleton of the
cell
The Fluid Mosaic Model
• The phospholipids create what is called
the Fluid Mosaic.
• A fluid can be described as a substance
that can change positions without
separating.
• A mosaic in art is a picture made up of
many small pieces like little tiles or glass.
Watch this short video for a
good explanation of why the
phospholipid bilayer is called a
fluid mosaic
For further study
• Here’s a good link to help you further
understand the cellular membrane
• The Next lesson in this unit on the cell is
• The Cell – Part 3 Organelles