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Cell Transport
High School Biology Class
I. Cell (Plasma) Membrane
The cell membrane is composed of lipids (because
they can separate substances.)
Cell Membrane (cont’d)
The membrane is made of many phospholipids
which have two parts: hydrophilic (water loving)
heads and hydrophobic (water fearing) tails.
Cell Membrane (cont’d)
Notice also that the membrane is made of two
layers of lipids called a lipid bilayer.
Cell Membrane (cont’d)
Also found in the cell membrane are
certain proteins, which play an important
part in the cell by:
1) Coordinating cell-to-cell behaviors.
2) Adjusting the cells metabolism rates.
3) Initiating growth & division times.
Cell Membrane (cont’d)
A. Marker Proteins
Helps cells recognize
similar cells.
This is helpful for
when our defensive
cells encounter
bacteria with unknown
marker proteins, they
can then destroy them.
B. Receptor Proteins
 Helps cells receive materials/signals outside of the cell.
 When cells need stored energy from the pancreas, they
release signal molecules, which find and bind with the
pancreas cells to let them know to release some energy.
C. Channel/Carrier Proteins
Helps substances
move across the cell
membrane.
If substances are too
large to get through
the lipid bilayer
channel proteins
(doughnut shaped) act
as passage ways.
II. How Substances Move
 There are 2 types of transport across cell
membranes:
1. Passive – transport that does not need
any energy.
2. Active – transport that requires energy.
A. Passive Transport
Chapter 4 Section 1
Passive transport (no energy required)
can occur in 4 different ways:
i) Simple Diffusion
ii) Osmosis
iii) Diffusion through Ion Channels
iv) Facilitated Diffusion (Carrier Proteins)
Passive Transport (cont’d)
During passive transport substances
always move from high concentration
to low concentration (this is known as a
concentration gradient).
After passive transport when the
concentration of molecules is the same
we say equilibrium exists.
i) Diffusion
Diffusion is the movement of molecules
down a concentration gradient, from high
to low concentrations.
Examples: food coloring in water, perfume
spreading throughout a room, etc.
ii) Osmosis
Osmosis is the diffusion of water across a
semipermeable (cell) membrane.
It is the same process as diffusion except it
specifically means water movement.
a) Solutions
A solution is a mixture of solutes (things
being dissolved) and a solvent (things
doing the dissolving).
Water is typically the solvent. (i.e. sugar
cube in water, the sugar is the solute)
b) Types of Solutions
1. Hypertonic: solution with a higher concentration
of dissolved particles than its surroundings.
2. Hypotonic: solution with a lower concentration
of dissolved particles than its surroundings.
3. Isotonic: solution with an equal concentration of
dissolved particles to its surroundings.
c) Water Balance in a Cell
1. Cells placed in a hypertonic environment (salt
water) will plasmolysis. – SHRINK!
c) Water Balance in a Cell
2. Cells placed in a hypotonic environment (fresh
water) will cytolysis. – BURST!
c) Water Balance in a Cell
3. Cells placed in a isotonic environment will be
balanced.
Water Movement Problems
iii) Diffusion through Ion
Channels
An ion channel is a doughnut shaped
channel protein that is hydrophilic (polar)
inside.
Ions can pass through the channels
without contacting the hydrophobic
(nonpolar) part of the cell membrane.
Ion Channel (B)
iv) Facilitated Diffusion
Facilitated diffusion is a passive form of
transport caused by carrier proteins specific for
the molecules they transport.
Remember movement is always down the
concentration gradient. (i.e. high to low)
Also facilitated diffusion needs no energy.
a) Steps in Facilitated Diffusion
1. A molecule outside the cell binds to the
carrier protein.
2. The carrier protein transports the molecule
across the cell membrane.
3. The molecules are released inside the cell.

B) Active Transport
Chapter 4 Section 2
 There are many types of active transport,
but the 2 we will learn are:
i) The Sodium-Potassium (Na-K) Pump
ii) Movement by Vesicles
Active Transport (cont’d)
Active transport is the movement of
molecules up (against) the concentration
gradient, from low to high.
Remember this time the cell must use its
own energy (ATP) to move substances.
i) Sodium Potassium Pump
It takes 3 sodium ions out for every 2 potassium
ions put in the cell.
There is more sodium inside the cell and more
potassium outside the cell so the cell is working
against the concentration gradient.
Energy for the pump comes from ATP made by
the mitochondria.
ii) Movement by Vesicles
 There are 2 types of vesicles:
1. Endocytosis – into the cell.
2. Exocytosis – out of the cell.
1) Endocytosis
Process by which a cell engulfs a
substance that is much too large to enter
the cell by passing through the
membrane.
2 types of Endocytosis:
a) pinocytosis - liquids
b) phagocytosis - solids
a) Pinocytosis
Process where the
liquids are taken into
the cell.
b) Phagocytosis
Process where a vacuole is formed around solid
particles before they are taken into the cell.
2) Exocytosis
Process by which large molecules are passed out
of the cell without going through the membrane.
Any Questions?
“Knowledge is not simply another
commodity. On the contrary. Knowledge is
never used up. It increases by diffusion
and grows by dispersion.”
--Daniel J. Boorstin