Chapter 4 –
Cell Transport
• Plasma Membrane Structure and
Function
• Separates the internal environment of the
cell from its surroundings.
• Regulates what materials enter and leave a
cell
• Made of 2 layers of phospholipids
(phospholipid bilayer) with proteins
embedded throughout.
• Fluid consistency (like a light oil) and a
mosaic pattern of proteins.
http://www.stolaf.edu/people/giannini/flashanimat/lipids/membrane fluidity.swf
•Because of consistency and pattern of
components, referred to as fluid-mosaic model
of membrane structure
http://home.earthlink.net/~shalpine/anim/Life/memb.htm
• Cells live in fluid environments, with
water inside and outside the cell.
• Components of plasma membrane:
–2 layers of phosphlipids
• Polar head and nonpolar tail
–Proteins
–Cholesterol
–Carbohydrates
•Phospholipid bilayer:
•Polar heads (hydrophilic) (waterloving)
•Face outside and inside of cell.
•Nonpolar tails (hydrophobic) (waterfearing)
•Extend to the interior of the plasma
membrane.
• Proteins:
• Some pass through entire
membrane, others go part-way
through
• Help large molecules pass through
cell membrane
•Cholesterol:
•Strengthens the plasma membrane.
•Carbohydrates:
•Help cells send messages and
recognize one another
The Permeability of the Plasma
Membrane
• Selectively permeable – meaning only
certain materials can cross the
membrane.
• Two mechanisms of transport:
–Active – requires energy
–Passive – does not require energy
•How do materials get across the plasma
membrane?
•Small, uncharged molecules pass through
the membrane, following their
concentration gradient •Gradual change in chemical
concentration from one area to another
•Molecules tend to move from area of
high to low concentration
•Larger macromolecules or tiny charged
molecules rely on proteins to help them to
get across.
How molecules cross the plasma
membrane
Diffusion and Osmosis
• Diffusion is the movement of molecules from
a higher to a lower concentration until
equilibrium is reached.
– Equilibrium –
• State in which all materials are evenly
concentrated
• Movement of molecules still occurs, but
there is no NET movement of
molecules
• Gases move through plasma membrane by
diffusion.
Osmosis
• Defined as the diffusion of water
across a selectively permeable
membrane due to concentration
differences.
• Molecules always move from higher
to lower concentration.
Osmosis in cells
• A solution contains a solute (solid) and a
solvent (liquid).
• Cells are normally isotonic (iso = same) to
their surroundings, and the solute
concentration is the same inside and out
of the cell.
– Cell is in equilibrium
– There is no net movement of water
across the cell membrane.
Fresh water = Isotonic solution
X = Water
Cells in isotonic solutions
http://www.linkpublishing.com/V
ideos/transport/blood_isotonic.
wmv
http://www.linkpublishing.com/
Videos/transport/elodea_norm
al.wmv
• Hypotonic (“hypo” means less than)
– Lower concentration of solute, higher
concentration of water outside than inside
cell.
– Net movement of water into the cell.
– Animal cells - may burst
– Plant cells –
• Increased turgor pressure (makes plant
cell rigid)
• Plant cells do not burst because they
have a cell wall.
Distilled water = Hypotonic solution
X = Water
Cells in a hypotonic solution
http://www.linkpublishing.com/Vide
os/transport/blood_hypo.wmv
http://www.linkpublishing.com/Videos/t
ransport/elodea_hypo.wmv
• Hypertonic (“hyper” means more than)
– Higher concentration of solute, lower
concentration of water outside than inside
cell.
– Net movement of water out of the cell.
– Animal cells - shrivel
– Plant cells - shrink
• Turgor pressure is lost as plant cells
shrink – plant will wilt or feel soft and
bendable.
Salt water = Hypertonic solution
X = Water
Cells in a hypertonic solution
http://www.linkpublishing.com/Videos/t
ransport/blood_hyper.wmv
http://www.linkpublishing.com/Video
s/transport/elodea_hyper.wmv
Transport by Carrier Proteins
• Some materials cannot enter or leave cell
due to their size and/or nature.
• Some of those molecules use the channel
proteins and carrier proteins that span the
membrane in order to enter or leave the
cell.
• Carrier proteins are specific and combine
with only a certain type of molecule.
• Facilitated transport and active transport
both require carrier proteins.
Facilitated transport
• Substances pass through a carrier protein
following their concentration gradients
(high  low concentration).
• Does not require energy.
• Brings in materials such as glucose &
amino acids; proteins are specific to
molecules taken in.
• Some molecules can be taken in faster
than others – differential permeability.
http://www.stolaf.edu/people/giannini/flashanimat/transport/channel.swf
Active transport
• Ions or molecules are moved across the
membrane against the concentration
gradient – from an area of lower to higher
concentration.
• Energy in the form of ATP is required for the
carrier protein to combine with the
transported molecule.
• Occurs in cells such as kidney cells (taking
sodium from urine), thyroid gland cells
(taking in iodine), and cells in digestive tract
(absorbing nutrients).
Active transport
http://www.brookscole.com/chemistry_d/templates/student_resources/shared_re
sources/animations/ion_pump/ionpump.html
Exocytosis and Endocytosis
• For molecules that are too large to be
transported with carrier proteins – requires
energy.
• Use vesicles – small storage units
• Exocytosis - vesicles fuse with the plasma
membrane for secretion.
• Causes cell membrane to enlarge –
process occurs during growth.
Exocytosis
http://academic.brooklyn.cuny.edu/biology/bio4fv/page/exocy.htm
• Endocytosis - cells take in substances when a
portion of the plasma membrane folds in, and
forms a vesicle around the substance.
• When vesicle fuses with lysosome, digestion
occurs.
• Endocytosis occurs as:
• Phagocytosis – large particles (food or other
cells)
• Pinocytosis – small particles or liquids
• Receptor-mediated endocytosis – form of
pinocytosis for specific particles such as
vitamins, hormones, or lipoproteins
http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter6/animations.html
Phagocytosis
•Seen in unicellular organisms like amoebas.
•White blood cells can use this process to take
in bacteria & worn out red blood cells
Pinocytosis
•Used by root cells of plants.
•Seen in blood cells, cells that line kidneys
and intestines.
Receptor-mediated endocytosis
•Seen during exchange of maternal & fetal
blood in placenta; & intake of cholesterol in
body cells (failure to do so results in high blood
pressure, blocked arteries, and heart attacks).