Transport across membranes
Learning Outcomes
• explain what is meant by passive transport
(diffusion and facilitated diffusion including
the role of membrane proteins), active
transport, endocytosis and exocytosis;
Exchange across the plasma
membrane
• The membrane provides an effective barrier
against the movement of substances, however
some exchange between the cell and the
environment is essential.
Transport across membranes
• Materials can move across cell membranes:
– Passively
• Diffusion (simple or facilitated)
• Osmosis
– Actively
• Active transport
• Bulk transport
Diffusion
• Net movement of molecules or ions from a
region of high concentration to a region of low
concentration
• Occurs along a concentration gradient
• Result = equilibrium (molecules or ions evenly
spread out within a given space or volume)
Factors affecting the rate of diffusion
• Concentration gradient
– Greater the difference in concentration the greater the
rate of diffusion
• Temperature
– At higher temperature kinetic energy particles increases
– Diffusion is faster
• Surface area
– Greater the surface area, more particles can cross
– Increases rate of diffusion
Factors affecting the rate of diffusion
• Nature of molecules or ions
– Large molecules diffuse slower
– Non-polar molecules diffuse more easily
– The respiratory gases (CO2 and O2) are small
enough to diffuse quickly through the membrane.
– Large, polar molecules (glucose and amino acids)
and ions (Na+ and Cl-) cannot diffuse through the
phospholipid bilayer
Facilitated Diffusion
• Protein molecules exist in membranes to
facilitate diffusion.
• 2 type of protein molecule
– Channel protein
• transmembrane protein that forms a tunnel through
the bilayer.
– Carrier proteins
• change shape to help molecules move into and out of
cells.
Facilitated Diffusion
Active Transport
• Energy consuming transport of molecules or
ions across a membrane against a
concentration gradient, made possible by
transferring energy from respiration.
• Energy makes the carrier proteins change
shape, transferring ions across the membrane.
Examples of active transport
• Reabsorption in kidneys
• Digestion in gut
– Helps absorb glucose from our intestines
• Load sugars into phloem
• Inorganic ion uptake in root hairs
– Magnesium ions are in short supply in the soil but
are needed for photosynthesis
Bulk transport
• This is the method of transporting large quantities of
materials into cells (endocytosis) or out of cells
(exocytosis)
– Endocytosis - Engulfing of material by cell membrane to
form a endocytic vacuole.
• 2 forms
– Phagocytosis the uptake of solid material
– Pinocytosis the uptake of liquid
– Exocytosis - Process by which materials are removed from
cells
Examples of bulk transport
• Hormones released into bloodstream from
endocrine glands
• White blood cells engulf invading
microorganisms by phagocytosis
• In plant cells materials to build the cell wall
are carried outside in vesicles.
OSMOSIS
• Special type of diffusion involving water
molecules
• Example:
– Two solutions are separated by a partially
permeable membrane. Solute molecules are too
large to pass through pores in the membrane, but
water molecules are small enough.
What would happen if the membrane were not
present?
• Net movement of solute molecules from B to
A by diffusion
• Net movement of water molecules from A to B
by diffusion
• Equilibrium – concentrations of water
molecules and solute molecules in A would
equal that in B.
What will happen if the membrane is
present?
What will happen if the membrane is
present?
• Solute molecules too large to pass through
membrane
• Water molecules pass easily from A to B
• Net movement of water from A to B until equilibrium
is reached, i.e. solution A has the same concentration
of water molecules as solution B.
• The level of liquid A will fall and the level of liquid B
will rise
• Equilibrium is brought about by the movement of
water molecules alone.
Definition of osmosis
• Water potential Ψ
– Tendency of water molecules to diffuse from one place to
another.
– Measured in kPa
– Pure water has a water potential of 0kPa
• Osmosis
– Is the net movement of water molecules from a region of
high water potential to a region of low water potential
(down a water potential gradient) across a partially
permeable membrane.
Water potential
Pure water
No solute
Dilute solution
Small amount
of solute
dissolved
Very low water
Concentrated
potential
solution
-500kPa
Large amount
of solute
dissolved
Lower water
potential
-50kPa
Decreasing water potential
Highest water
potential
0kPa
Some Important Terms
• Hypotonic
– a region of
• higher water potential.
• Lower solute concentration
• Hypertonic
– a region of
• lower water potential
• Higher solute concentration
• Isotonic
– a region where there are equal water potentials on either
side of a membrane.
Determining Water Potential in Potato
tubers
Salt Soluntion
(mol-1)
0.1
0.2
0.3
0.4
0.5
Starting Mass (g)
Finishing mass (g)
Change in mass (g)
%age change in
mass
Osmosis in Red Blood Cells
Osmosis in
Plant Cells
Important Terms
• Turgid
– the term used to describe a plant cell where the
protoplast exerts a pressure on the cell wall.
• Plasmolysed
– the term used to describe a plant cell where the
protoplast has shrunk away from the cell wall due
to loss of water by osmosis.
Osmosis in red onion cells