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Cell Membrane Structure & Transport
All cells need to exchange substances like food,
water, and nutrients with their environment
Cell membranes accomplish this exchange
through cell transport!
Why is the membrane’s structure described as
a “fluid mosaic”?
• Membranes are fluid and
have the consistency of
vegetable oil
• The lipids & proteins are
always in motion
• Proteins form patterns or
mosaics
• Cell Membrane is composed of
– Phospholipid bilayer: forms the basic unit of the
cell membrane
– Proteins: help transport large molecules through
the membrane
– Carbohydrates: help cells send and receive
chemical messages
Phosphate heads
interact with water:
Hydrophilic
Lipid tails repel
water:
Hydrophobic
2 Main Types of Cell Transport
• Passive Transport: NO energy
required
• Active Transport: NEEDS energy
Passive Transport: Movement of
materials across the membrane that
does not require energy
3
Types:
Diffusion
Osmosis
Facilitated Diffusion
Diffusion is the
movement of molecules
from an area of high
concentration to an area
of lower concentration
until they are evenly
spread out
Hi to the
Low YO!
Concentration Gradient and
Permeability affect whether
diffusion occurs across a membrane
Concentration Gradient: the difference
between the amount of a particular substance
in one area and the amount in an adjacent area
Permeability is the ability of a substance to
pass through the membrane
• Impermeable: Cannot
pass
• Selectively permeable
or Semi-permeable and
only allow certain
molecules to pass
Osmosis is the diffusion of water molecules through
a selectively permeable membrane
Water will continue to diffuse/move until there is an
equal concentration of molecules inside and outside
the cell.
17
Hypertonic Solutions
• Solute concentration outside of the cell is
higher than the solute concentration inside
the cell
• Water moves OUT of the cell
19
Hypotonic Solutions
• Solute concentration outside of the cell is
lower than the solute concentration inside
the cell
• Water moves INTO the cell
20
Isotonic Solution
• Solute concentration in the solution is equal
to the solute concentration in the cell
• Water moves equally in and out of the cell
21
Which way will the water move?
On the left side,
more of the space
is occupied by
22
solutes than by
water
60% H2O
90% H2O
Which way will the
water move?
80% H2O
100% H2O
What type of solution is
this cell in?
80% H2O
100% H2O
Which way will
the water move?
90% H2O
90% H2O
90% H2O
90% H2O
What type of
solution is this
cell in?
Which way will the
water move?
90% H2O
75% H2O
What type of
solution is this cell
in?
90% H2O
75% H2O
Some things like glucose and proteins are too
large to pass though the membrane by
diffusion or osmosis
Carrier proteins
transport (diffuse)
large substances
across the
membrane from
high to low
concentration
• Active transport
requires energy
• moves substances
against the
concentration
gradient
• Low concentration
to high
concentration!!
Like facilitated diffusion, ACTIVE
TRANSPORT uses proteins to transport a
substance across the membrane.
Active transport, however, requires energy!
Types of Active Transport
Bulk Transport moves LARGE
MATERIALS in and out of the cell
Endocytosis: Moving large substances
into the cell by infolding of the cell
membrane
• Pinocytosis:
engulfs liquids
• Phagocytosis:
engulfs solids
Amoebas take in food and
nutrients through phagoctosis…...
Phagocytosis of
Streptococcus
pyogenes. White
blood cells engulf
many types of
pathogenic
bacteria
Exocytosis: Large substances are
shipped out of the cell by
outfolding of the cell membrane
Think “exit”