Download f211 transport across boundaries

Transport across boundaries
Learning Outcomes
• explain what is meant by passive transport
(diffusion and facilitated diffusion including
the role of membrane proteins), active
transport, endocytosis and exocytosis;
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
Cell Division, Cell Diversity and
Cellular Organisation
OCR AS Biology
Unit F211: Cells, exchange and
transport
Module 1: Cells