Cell Membrane and Transport
Maintaining homeostasis and
providing nutrients to cells
Cell Membrane Structure
The cell membrane is composed of
lipids and proteins.
 The lipids are arranged in a bilayer.



The bilayer is a “barrier” that is
impermeable to most molecules.
The proteins are embedded in the
bilayer.

Specific molecules can be helped across
the membrane by these proteins.
Cell membranes contain special
lipids known as phospholipids

Phospholipids: Composed of

two fatty acid chains attached to a
glycerol molecule and phosphate group.
Phosphate Group
Glycerol
Fatty Acid Chains
Phospholipids, continued:

Have a hydrophilic “head” that
“loves” water (both are polar)


Hydrophilic heads form outside of
layer so they can touch water inside
and outside cell
Have a hydrophobic “tail”
(hydrocarbon chain) that “fears”
water (is non-polar)

Hydrophobic tails face interior of
bilayer so they can avoid water
Phospholipid Bilayer
Role of the Cell Membrane

The cell membrane is described as
“selectively permeable”

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How does this feature relate to the
job/function of the cell membrane?
Cell membrane acts as a “guard”
Allows nutrients into cell
 Allows for removal of wastes and release
of substances made by the cell that are
needed by other cells.

Factors that affect Passive
Transport:

Whether a molecule can move
through the membrane depends on:
1) the size of the molecule
2) the type of molecule (polar or
nonpolar, charged, etc.)

Molecules move by one of the following
methods: diffusion, facilitated diffusion,
or osmosis.
All 3 types are passive transport

Passive Transport: movement of
substances without any energy input by
a cell.
1.
Diffusion: small, nonpolar molecules (O2
and CO2) move straight through the
membrane.
Facilitated diffusion: small, polar molecules
(glucose, amino acids), or ions move through
protein channels embedded in the
membrane.
Osmosis: water molecules move through
the membrane.
2.
3.

Methods of Transport
Direction of transport
The net movement (direction) of the
molecules depends on the relative
concentrations inside/outside a cell.
 In passive transport, the net movement
is always “down the concentration
gradient”.


Molecules move from an area of higher
concentration to an area of lower
concentration.
Net Movement
Think of dye or sugar molecules in water. (Water particles NOT
shown.) Dye or sugar molecules will DIFFUSE through the water.
Example: Transport of Oxygen
HIGH
Concentration gradient for O2
LOW
Example: Transport of Glucose
HIGH
Net movement
LOW
Example: Osmosis

If the membrane is
permeable to both
water and solutes,
both will diffuse to
reach equilibrium.

Often, the membrane
is NOT permeable to
the solute(s). In this
case osmosis occurs;
water diffuses (high to
low) to balance the
concentration on both
sides. (egg & potato
labs)
Osmosis, continued:

Situation #1: a cell is surrounded by a
solution that has a high concentration of
solutes.
Inside of Cell
Outside of Cell
Due to the concentration gradient for
water, what happens to the cell?
High solutes = low water concentration
 The net movement of water is from
inside the cell (high WATER conc.) to
outside the cell (low WATER conc).
 Cell loses mass.

Egg in hypertonic solution
Water moved out of
the egg to try to dilute
the solutes outside of
the egg and make the
external environment
have a higher water
concentration to match
the inside.
Chicken egg in syrup

Situation #2: a cell is surrounded by a
solution that has a low concentration of
solutes.
Low solutes = high water concentration
 The net movement of water is from outside
the cell (high WATER conc.) to inside the
cell (low WATER conc).
 Cell gains mass.

Egg in hypotonic solution
Chicken egg in tap water
(or vinegar)
Water moved into the
egg to reduce the
water concentration
compared to solutes
outside of the egg and
make the external
environment have a
lower water
concentration to match
the inside.

Situation #3: a cell is surrounded by a
solution that has the same concentration
of solutes.
Equal concentrations = equilibrium
 NO net movement of water



Equal number of molecules move in and out
Cells mass doesn’t change.
Change in Mass of Potato Cores in Different Solutions
Change in Mass (g)
+
-
No change
in mass
Facilitated Diffusion:
 Integral
Proteins can “facilitate”
or assist in transporting a
substance by:
as a channel or tunnel
 2. acting as a carrier

1. acting
Diffusion through a carrier protein
Active Transport
Cells move molecules from an area of
LOW concentration to an area of HIGH
concentration
 Molecules move AGAINST the
concentration gradient
 Requires the cell to use energy in the
form of ATP

Animation
Example: Sodium-Potassium
Pump






Three Na+ ions in the cytoplasm bind to carrier protein
Shape of protein is changed, allowing the three Na+
out of cell
Two K+ ions outside of cell bind to protein
Shape of the protein is changed
The two K+ are allowed into the cytoplasm
Similar to facilitated diffusion:
Animation 1


Different from facilitated diffusion:


Uses a carrier protein,
Animation 2
requires energy.
Overall: Na+ (sodium) becomes concentrated on
the outside of a cell.

Important in the proper functioning of neurons and the
kidneys.
Other types of active transport
Not all active transport moves molecules
from a low concentration to a high
concentration
 Active transport used in two other
situations:

Moving very large molecules through
membrane
 Moving large quantities of smaller
molecules through membrane

Other examples of active transport:

Endocytosis: Process in which cells
ingest fluids, macromolecules, and
large particles that are outside the cell
Animation 1
Animation 2
Other examples of active transport:

Exocytosis: how cells release large
molecules (proteins) or get rid of
large amounts of wastes