1. MEMBRANE TRANSPORT Topic 1.4 IB Biology Miss Werba
Membranes control the composition of cells by active and passive transport.
Topic 1.4
IB Biology
Miss Werba

2. ULTRASTRUCTURE OF CELLS
TOPIC 1 – CELL BIOLOGY
1.1
INTRODUCTION TO CELLS
1.2
ULTRASTRUCTURE OF CELLS
1.3
MEMBRANE STRUCTURE
1.4
MEMBRANE TRANSPORT
1.5
THE ORIGIN OF CELLS
1.6 CELL DIVISION
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3. THINGS TO COVER U.1 U.2 A.1 A.2 S.1 Statement Guidance NOS 3.1
Movement of particles across membranes
- inc. simple diffusion, facilitated diffusion, osmosis and active transport
U.2
Membrane fluidity allows materials to move between and within cells
- inc. endocytosis and exocytosis, vesicular transport
A.1
Structure and function of membrane pumps and channels
- eg. Na+/K+ pump, K+ channels
A.2
Matching osmolarity of the cytoplasm when using tissues or organs in medical procedures
S.1
Estimation of osmolarity in tissues by bathing sample in hypotonic and hypertonic solutions (Practical 2)
Use osmosis experiments to stress the need for accurate mass and volume measurements in scientific experiments.
NOS 3.1
Experimental design
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4. PERMEABILITY The phospholipid bilayer is selectively permeable.
U.1
The phospholipid bilayer is selectively permeable.
This means that it controls the entry and exit of molecules into and out of the cell.
Movement will depend on:
Size
Charge
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5. PERMEABILITY
U.1
Remember that the polar (charged) heads of the phospholipids will attract other polar molecules – eg. water.
The non-polar tails will prevent this movement, therefore it is sometimes necessary to use a carrier molecule or channel to help move a substance across the membrane.
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6. MEMBRANE TRANSPORT Movement can take the form of: Passive transport:
U.1
Movement can take the form of:
Passive transport:
Simple diffusion – inc. Osmosis
Facilitated diffusion
Active transport
Protein pumps
Vesicular transport – inc. endo-/exo- cytosis
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7. PASSIVE TRANSPORT Does not require energy
Movement of substances is along the concentration gradient (high  low)
2 types of passive transport:
Simple diffusion (inc. osmosis)
Facilitated diffusion
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8. DIFFUSION
U.1
The passive net movement of particles from a region of high concentration to a region of low concentration, along a concentration gradient, across a semi-permeable membrane
Concentration gradient = term given to the difference in concentration between two regions
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9. DIFFUSION
U.1
Source: i-Biology.net
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10. DIFFUSION
U.1
The rate of diffusion through a semi-permeable membrane can be affected by:
Concentration gradient (as shown on previous slide)
Surface area
Length of the diffusion path
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11. OSMOSIS The diffusion of water
The passive net movement of water molecules from a region of low solute concentration to a region of high solute concentration , along the gradient, across a semi-permeable membrane
Aquaporins also exist:
membrane proteins
designed to speed up the movement of water molecules
can be found in the nephrons within the human kidney
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12. OSMOSIS
U.1
Low solute
High solute
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13. OSMOTIC CONTROL
U.1
To prevent osmosis in the cells of tissues and organs destined for use in medical procedures, they must be bathed in a solution that is the same osmolarity as the cytoplasm.
ie. an isotonic saline solution
Examples of use:
Saline IV drip to rehydrate a patient
Used to rinse wounds
Used to keep skin moist before applying skin grafts
Eye drops
Packing donor organs for transportation
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14. PASSIVE TRANSPORT Simple diffusion: Facilitated diffusion:
Small, non-polar molecules can freely diffuse across the membrane
eg. O2, CO2, H2, H2O, glycerol
Facilitated diffusion:
Larger, polar substances cannot freely diffuse
Require the transport proteins to facilitate their movement
eg. glucose, sucrose, Cl-, Na+, K+
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15. FACILITATED DIFFUSION
Source:
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16. FACILITATED DIFFUSION
eg. Potassium channels in axons:
Channels exist along the axons of neurons to allow ion movement in/out of the cell.
Potassium (K+) channels in the axon are voltage-gated.
They facilitate the movement of this ion only – the channels therefore show specificity.
The opening of K+ channels during the transmission of a nerve impulse allows K+ to leave the neuron, repolarising the cell and reestablishing the resting voltage within the neuron.
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17. ACTIVE TRANSPORT Does require energy
Movement of substances is against the concentration gradient (low  high)
Will only occur in the presence of ATP
eg. Na+/K+ Pump
vesicular transport
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18. ACTIVE TRANSPORT
U.1
They use the energy from the breakdown of ATP to translocate the molecules against the gradient
The hydrolysis of ATP to ADP causes a the protein pump to change shape, resulting in the forced movement of the substance
Protein pumps are specific for a given molecule
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19. ACTIVE TRANSPORT eg. sodium-potassium pumps:
Cycle through a series of steps
Results in sodium and potassium molecules being “exchanged” for one another
This swap is unequal – 3 Na+ ions exchanged for every 2 K+ ions
These pumps are used in the axons of neurons.
They are used to fine-tune the voltage in the neuron and reestablish resting potential.
Each cycle uses one ATP.
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20. PROTEIN PUMPS Source: http://commons.wikimedia.org/
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21. MEMBRANE TRANSPORT
U.1
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22. VESICULAR TRANSPORT
U.2
Proteins destined for secretion are directed to the endoplasmic reticulum
The protein is transferred to the Golgi apparatus via a vesicle, which forms from the budding of the membrane
The protein moves via vesicles from one side of the golgi to the other and may be modified along the way (eg. glycosylated, truncated, etc.)
The protein is transferred via a vesicle to the plasma membrane, whereby it is either immediately released or stored for a delayed release.
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23. VESICULAR TRANSPORT
U.2
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24. VESICULAR TRANSPORT Vesicular transport is possible b/c:
Membrane has some fluidity
Small amounts can be added or removed without tearing the membrane (endo- & exo- cytosis)
Membranes of all organisms are the same
Source: NogalesHighSchool/mespinoza/fig4cytosis.jpg
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25. VESICULAR TRANSPORT Exocytosis
Rough ER produces proteins intended for export
Golgi apparatus prepares substances for exocytosis
Transport vesicle formed from a section of membrane from the Golgi apparatus
This membrane then joins the cell surface membrane
Exocytosis requires energy!
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26. VESICULAR TRANSPORT Endocytosis
Transport vesicle formed from a section of the cell membrane
This process requires energy too!
Used to take up substances that are too large and/or highly polar and cant enter on their own
Requires recognition of the substance by a membrane receptor protein
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27. VESICULAR TRANSPORT Endocytosis
Two types of endocytosis :
Pinocytosis
when the substance is fluid
“cell-drinking”
Phagocytosis
when the substance is solid
“cell-eating”
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28. VESICULAR TRANSPORT Endocytosis
Source:
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29. SUMMARY of MEMBRANE TRANSPORT
ATP required
Concentration gradient
Diffusion no
down
Facilitated diffusion
Osmosis
Active transport with carrier proteins
yes
against if possible
Vesicular transport
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30. MEMBRANE TRANSPORT Experimental design
NOS 3.1
Experimental design
Accurate quantitative measurement in osmosis experiments are essential.
Practical 2: estimation of osmolarity by bathing potato tissue samples in hypotonic and hypertonic solutions.
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31. MEMBRANE TRANSPORT
Q1. What do diffusion and osmosis have in common? Q2. Describe the movement of water across membranes. [2] Q3. Distinguish between active and passive movements of materials across plasma membranes, using named examples. [5] Hint: Use a table!
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