Cell Transport
The movement of molecules into and out of
a cell
How is a phospholipid constructed?
Interactions of phospholipid molecules with water
The basis of phospholipid bi-layers
The phospholipid bilayer is the
fundamental structure of the membrane
The cell membrane contains both
hydrophilic and hydrophobic regions
The Cell Membrane
• The cell membrane is composed of 2
layers of phospholipids
• Also called the phospholipid bilayer or
plasma membrane
• The Polar heads face the outside of the
membrane where water exists
• The Nonpolar tails face each other in the
inside of the bilayer creating a
hydrophobic region
Properties of Membrane
Membranes are primarily composed of
phospholipids and proteins
Membranes have the important function
of regulating the movement of
substances into or out of cells or
organelles
The Fluid-Mosaic Model of a membrane
Two types of proteins in membranes –
peripheral proteins and integral proteins
peripheral protein
integral protein
The Passage of Molecules across the Plasma Membrane
Hydrophobic molecules and non-polar molecules
(such as O2) cross the membrane with ease.
Hydrophilic molecules cannot, however, slip
through the extensive hydrophobic core of the
membrane.
The transport of hydrophilic compounds requires
the help of the integral proteins of the
membrane.
Diffusion -- a spontaneous process in which
particles move from an area of high concentration
to an area of low concentration across a
concentration gradient
When two different solutes are diffusing at the same time, they will
each follow their own concentration gradients from high to low.
The net diffusion occurs from a region of high
concentration toward a region of low
concentration.
What happens if we remove
the barrier?
High concentration
Low concentration
Diffusion – fills the space – it is spontaneous, no work is
done
High concentration
Low concentration
Diffusion Summarized
Some molecules can simply pass through
 Example: small molecules
 No energy required to pass: passive transport
 Move from high  low concentration: diffusion
 Diffusion will occur until equilibrium is reached
Osmosis – the diffusion of water across a selectivelypermeable membrane
higher solute concentration
Solute is defined as polar or
charged molecule or ion
Water is attracted to
solutes!
Osmosis – the diffusion of water across a selectivelypermeable membrane
water level rises
Osmosis Summarized
Water can simply pass through
 Special type of diffusion: osmosis
 No energy required: passive transport
Tonicity – the ability of a solution to cause a cell to loose or gain
water
Water with low solute concentration
hypotonic
Cell
with higher
solute
concentration
H2 O
Hypotonic solution – contains fewer solutes than
cell (Cell will enlarge when placed in this solution!)
Tonicity – the ability of a solution to cause a cell to loose or gain
water
Water with high solute concentration
hypertonic
Cell
(with solutes)
H2 O
Hypertonic solution – contains more solutes than
cell (Cell will shrink when placed in this solution!)
Tonicity – the ability of a solution to cause a cell to loose or gain
water
Water with equal solute concentration
isotonic
Cell
(with solutes)
H2 O
Isotonic solution – contains equal solutes as cell
(Cell will retain size when placed in this solution!)
Tonicity in animal cells
Tonicity in plant cells
Turgid
(normal)
Flaccid
Plasmolyzed
(slight water stress) (severe water stress)
Facilitated Diffusion
No ATP energy required
Molecules follow their concentration gradients
They are just “helped” across the hydrophobic core of the
membrane by a transport protein
Facilitated Diffusion
channel
gate
Net rate of
diffusion from high
concentration
toward low
concentration
Facilitated Diffusion Summarized
Some molecules are too large to pass normally
 Example: monosaccharides
 Carrier proteins assist: facilitated diffusion
 No energy required: passive transport
Facilitated Diffusion Summarized
Ions have a charge: cannot normally pass
 Examples: Na+, Ca+, Cl Can pass through ion channel
 No energy required: passive transport
Active Transport
Molecules go against their concentration
gradients
They are “helped” across the hydrophobic core of
the membrane by a transport protein, but only
when energy is expended.
Work is done by the cell!
Requires ATP (Energy)!!
Active Transport Summarized
Molecules often move from low  high concentration
 Such movement requires energy: active transport
 Carrier protein used as cell membrane “pump”
 Example: Na+ and K+ pump
Review the Differences
Bulk Transport
• Endocytosis – movement of large
molecules into the cell
– Phagocytosis – movement of foods (cell
eating)
– Pinocytosis – movement of fluids (cell
drinking)
• Exocytosis – movement of large
molecules out of the cell