Question 1 (1998)- Cells transport substances across their membranes. Choose THREE of the
following four types of cellular transport.
Osmosis
Active transport
Facilitated diffusion
Endocytosis/exocytosis
For each of the three transport types you choose,
a) describe the transport process and explain how the organization of cell membranes
functions in the movement of specific molecules across the membrane; and
b) explain the significance of each type of transport to a specific cell (you may use
different cell types as examples.)
Osmosis (Max 4)
a) DESCRIPTION OF
PROCESS
•movement of water from:
high to low water
concentration
a) STRUCTURE/FUNCTION
•high
to low water potential
•Aquaporins
•high
to low osmotic potential
•phospholipid
bilayer
allowspassage of small/
uncharged/polar molecules
allow passage
b) EXAMPLE/specific cell
type
• Any legitimate link of water
passage to cell function
Animal cells:
• can
cause lysis
• can
cause shriveling
to high solute
concentration
• in
•low
Plant cells:
•low
to high osmotic pressure
• diffusion
of water
· passive and/or not require
cellular energy
kidney (nephron) cells:
allows for osmoregulation
· provides water for
photosynthesis
· can create turgor
· can cause plasmolysis
· in guard cells: allows control
of stomatal opening
· in root hairs: allows water
uptake
Active Transport (Max 4)
a) DESCRIPTION OF
PROCESS
· movement of
molecules/ions against
their gradient
· cellular energy required
i.e. ATP; NADH; NADPH
a) STRUCTURE/FUNCTION
· proteins act as carriers
and/or pumps
· protein conformation
changes
b) EXAMPLE/specific cell
type
· Any legitimate link of
passage to cell function
Animal cell examples:
· Na+/K+ pumps:
establish polarity
assist in repolarization
· assist in glucose transport
· Ca++ pumps allow uptake
for muscle contraction
· allow uptake for
neurotransmitter release
Plant cell examples:
· K+ pumps
regulate turgor
establish polarity
· H+ pumps permit sucrose
loading in phloem
· Any specific cell example:
generate H+ gradient
Facilitated Diffusion (Max 4)
a) DESCRIPTION OF
PROCESS
· mediated movement of
molecules/ solutes/ ions/
substances along their
gradient
· passive transport/ does not
require cellular energy
a) STRUCTURE/FUNCTION
· proteins act as carriers of
large and/or charged
molecules
· ion channels transport ions
· protein conformation
changes
b) EXAMPLE/specific cell
type
· Any legitimate link of
passage to cell function
Animal cell examples:
· Cl- ion channels assist
cardiac repolarization
· Ca++ ion channels
assist neurotransmitter
release
assist muscle fiber
contraction
Bacterial cells/ mitochondria/
chloroplasts
Any specific cell examples:
· porins create permeability
to small molecules
· Na+; K+; (ion) channels help
establish polarity
· carrier proteins assist amino
acid transport assist sugar
transport
ENDOCYTOSIS/EXOCYTOSIS (Max 4)
a) DESCRIPTION OF
a) STRUCTURE/FUNCTION
PROCESS
Endocytosis (including
· Membrane fluidity/
phagocytosis and pinocytosis) solubility/fusion allows
vesicles to join membrane
· transport of substance into
cell via vesicle formation
· clathrin and non-clathrin
and/or membrane folding
coated pits allow specificity
e.g.: engulf/ invaginate/
· external/internal
envelope/ pinch in
monolayers ("sidedness")
· transport of substance into allow fusion
cell via vesicle formation may
· cytoskeleton is involved
be receptor-mediated
EXOCYTOSIS
· transport of substance out
of cell via vesicle fusion
· Both require energy
b) EXAMPLE/specific cell
type
· Any legitimate link of
passage to cell function
Examples:
· protists: for nutrition
· mast cells: for secretion of
histamines
· liver cells: for
LDL/cholesterol uptake
· Golgi & ER form vesicles for · export of cell products
secretion
· uptake of specific
substances
· growth [addition of cell
membrane]
Scoring:
· Bulleted items earn points
· Each mode of transport earns a maximum of 4 points. There is an internal maximum of one
point per category (a1, a2,b).
· One additional point may be earned from any one category for discussion of a second
bulleted item or an elegant elaboration.