Date post: | 13-Dec-2015 |
Category: |
Documents |
Upload: | peter-douglas |
View: | 219 times |
Download: | 0 times |
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.
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.
• 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.
• 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
Highest water potential
0kPaPure water No solute
Lower water potential-50kPa
Dilute solutionSmall amount
of solute dissolved
Very low water potential -500kPa
Concentrated solution
Large amount of solute dissolved
Decre
asin
g w
ate
r pote
ntia
l
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)
Starting Mass (g) Finishing mass (g) Change in mass (g)%age change in
mass
0.1
0.2
0.3
0.4
0.5
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.