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1.4. Membrane transport
Miltiadis-Spyridon Kitsos Platon IB Diploma
Edited from http://education.uoit.ca/lordec/ID_LORDEC/diffusion_osmosis/garib_diffusion_osmosis.html
The official IB Diploma Biology guideEssential idea: Membranes control the composition of cells by active and passive transport
https://ibpublishing.ibo.org/server2/rest/app/tsm.xql?doc=d_4_biolo_gui_1402_1_e&part=3&chapter=1
Phospholipid bilayers and transport of moleculesPhospholipid bilayers are selectively permeable
Selectively permeable: Certain molecules may pass through the membrane and others may not
+ + + + + + + + + + + + + + + + + + + +
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Modes of transport
Polarity
DiffusionParticles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
Does not require energy
Overall direction of movement (given that all molecules are moving all the time in all directions)
Diffusion is the passive net movement of molecules from areas of high concentration to areas of low concentration (that is down the concentration gradient).
Semi-permeable membrane
Red molecules will move towards the right, that is down the concentration gradient, through the semi-permeable membrane.
Purple molecules are bigger. Deduce their possible diffusion pattern.
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Simple Diffusion
http://commons.wikimedia.org/wiki/File:Scheme_simple_diffusion_in_cell_membrane-en.svg
DiffusionParticles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
Stephen Taylor http://sciencevideos.wordpress.com
DiffusionParticles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
Slide from
Stephen Taylor http://sciencevideos.wordpress.com
DiffusionParticles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
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Stephen Taylor http://sciencevideos.wordpress.com
DiffusionParticles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
Slide from
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DiffusionParticles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
DiffusionParticles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
Slide edited from
Facilitated DiffusionParticles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
Facilitated Diffusion is the passive net movement of molecules from areas of high concentration to areas of low concentration (that is down the concentration gradient) through carrier proteins of the membrane.
Semi-permeable membrane
Mode of transfer for polar molecules which may not pass through the phospholipid bilayer. Protein specificity, only a specific molecule may pass through the channel.
Integral channel proteins.
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Facilitated DiffusionStructure and function of sodium–potassium pumps for active transport and potassium channels for facilitated diffusion in axons.
A potassium channel is an integral protein consisting of 4 protein subunits having a narrow pore (0.3 nm)
Potassium ions, when dissolved, become attached within a cluster of water molecules.
As soon the ion with the water molecules enters the pore, bonds with the water molecules are broken, and a new series of bonds is formed between the ion and the amino acids.
These bonds are again broken, and new bonds are formed with water molecules.
Structure of a subunit.
http://www.mun.ca/biology/desmid/brian/BIOL2060/BIOL2060-13/13_08.jpg
Facilitated DiffusionStructure and function of sodium–potassium pumps for active transport and potassium channels for facilitated diffusion in axons.
http://hyperphysics.phy-astr.gsu.edu/hbase/biology/imgbio/actpot4.gif
http://physiologyonline.physiology.org/content/nips/13/4/177/F1.large.jpg
Potassium channels are voltage gated and triggered when the external side membrane becomes more positive than the inside.
Facilitated Diffusion
Media
http://highered.mheducation.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html
http://highered.mheducation.com/sites/0072943696/student_view0/chapter8/animation__voltage-gated_channels_and_the_action_potential__quiz_1_.html
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
Simple & facilitated diffusion are passive. • No energy input is required
There is a net movement of molecules from one side of the membrane to the other. • The motion of molecules is random (Brownian motion)
• But there is an overall general movement in one direction.
This net movement is down the concentration gradient. • From areas of high concentration to low concentration.
Movement is across a selectively or partially permeable membraneDependent on size or properties, some molecules can cross and not others.
Simple DiffusionOccurs when the molecule’s properties allow them
pass across the membrane.
Facilitated DiffusionSome molecules cannot cross easily, for example if they are polar the phospholipids of the bilayer will
repel them.
The rate of diffusion is affected by: • magnitude of concentration gradient
• SA:Vol ratio (more membranes, more transport per unit volume)
• Length of diffusion pathway (longer journey gives slower diffusion).
Channel proteins are integral membrane proteins that pass through the membrane.
Their properties allow molecules to pass through (e.g. polar molecules or ions).
Activation of these channels might be voltage-gated (e.g. in neurons) or binding-activated.
Compare and distinguish Simple and Facilitated DiffusionParticles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
Active Transport
Require energy in the form of ATP
Active transport is the active movement of molecules from areas of low concentration to areas of high concentration (that is against the concentration gradient).
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
Why do this?
To move molecules against the concentration gradient or to create a large concentration gradient across a membrane.
Protein pumps
These are integral, passing through the membrane. They are specific – only working with the target molecule.
What happens?
1. Target molecules bind to the pump. 2. ATP also binds to the pump. ATP is broken,
releasing energy and causing a conformational (shape) change in the protein pump.
3. This conformational change pushes the molecules across the membrane.
4. The molecule unbinds, and the pump reverts back to the original shape.
Examples • Sodium-potassium pump is used to re-polarise neurons after an action potential, ready to fire again.
• Proton pumps in mitochondria generate a high concentration gradient of H+ ions, ready for chemiosmosis through ATP synthase, used for generating ATP.
Active TransportParticles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
Active TransportParticles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
Active TransportStructure and function of sodium–potassium pumps for active transport and potassium channels for facilitated diffusion in axons.
Sodium potassium pumps follow a repeating cycle of events. Each cycle requires a molecule of Adenosine Triphosphate (ATP)
1. Pump opens to the interior and 3 Na+ bind to the three binding sites
1. Binding of Na+ activates the phosphorylation of the pump protein
http://study.com/cimages/multimages/16/nak_pump.jpg
Active TransportStructure and function of sodium–potassium pumps for active transport and potassium channels for facilitated diffusion in axons.
Sodium potassium pumps follow a repeating cycle of events. Each cycle requires a molecule of Adenosine Triphosphate (ATP)
3. Phosphorylation induces a conformational change in the structure of the protein. Protein closes in the interior and opens to the exterior releasing the three Na+ ions
4. K+ from the cell’s exterior binds to the two specific sites and this triggers the release of the phosphate group from the protein.
http://study.com/cimages/multimages/16/nak_pump.jpg
Active TransportStructure and function of sodium–potassium pumps for active transport and potassium channels for facilitated diffusion in axons.
Sodium potassium pumps follow a repeating cycle of events. Each cycle requires a molecule of Adenosine Triphosphate (ATP)
5. Loss of the phosphate group restores the protein to initial state.
6. K+ ion are released to the interior of the cell, the protein pump is again susceptible to receive Na+ ions and the cycle starts over again.
http://study.com/cimages/multimages/16/nak_pump.jpg
The Na+-K+ pumpStructure and function of sodium–potassium pumps for active transport and potassium channels for facilitated diffusion in axons.
The sodium-potassium pump is responsible for building concentration gradients of Na+ and K+ across membranes. These gradients are used for the transition of nerve impulses across the nerve cells.
Na+-K+ is also essential for
Absorption of minerals in the small intestine cells. Regulation of Na+ in the kidney. regulating cardiac contractions.
https://upload.wikimedia.org/wikipedia/commons/8/83/Blausen_0818_Sodium-PotassiumPump.png
Osmosis
Osmosis is the passive net movement of WATER molecules from areas of high concentration (low solute concentration) to areas of low concentration (high solute concentration) through a partially permeable membrane
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html
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OsmosisParticles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
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Osmosis in cellsAnimal cells exposed to hypertonic or hypotonic solutions change
If a red blood cell is placed in distilled water, water molecules enter the cell by osmosis. This is because the cytoplasm is hypertonic (higher solute concentration) in relation to environment which is hypotonic (low solute concentration). This increases the osmotic pressure within the cell.
The volume of the cell may grow up to a certain size and then the cell bursts, since the plasma membrane is quite weak.
However, when red blood cells are placed in a hypertonic solution they shrink. This makes the cells appear wrinkled.
http://1.bp.blogspot.com/-pveNOT3ckRg/UG8ySTId5fI/AAAAAAAAADw/gWCtYBemsmc/s1600/Tonicidad+de+disoluciones.png
http://1.bp.blogspot.com/-pveNOT3ckRg/UG8ySTId5fI/AAAAAAAAADw/gWCtYBemsmc/s1600/Tonicidad+de+disoluciones.png
Common medical procedures in which an isotonic saline solution is useful:• fluids introduction to a
patient’s blood system via an intravenous drip, e.g for rehydration
• used to rinse wounds, skin abrasions etc.
• keep areas of damaged skin moist before applying skin grafts
• eye drops/wash• frozen and used pack donor
organs for transportation
http://www.defenseimagery.mil/imageRetrieve.action?guid=8c9d5fade029a4f5a68fe667d1ae802ba9f30dd5&t=2
Tissues or organs to be used in medical procedures must be bathed in a solution with the same osmolarity as the cytoplasm to prevent osmosis.
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Osmosis in cellsPlant cells exposed to hypertonic or hypotonic solutions undergo plasmolysis
As you remember, the plant cell wall protects the cells from changing shape and prevents bursting. If a plant cell is placed in a hypotonic solution, water enters the cell by osmosis. This increases the osmotic pressure within the cell and thus its turgidity.
When a plant cell is placed in a hypertonic solution, then the plasma membrane detaches from the cell wall and the cell undergoes plasmolysis.
http://1.bp.blogspot.com/-pveNOT3ckRg/UG8ySTId5fI/AAAAAAAAADw/gWCtYBemsmc/s1600/Tonicidad+de+disoluciones.png
http://www.one-school.net/Malaysia/UniversityandCollege/SPM/revisioncard/biology/movementacrossmembrane/images/hypertonicplantcell.png
https://youtu.be/4JyT__Dea8Q
Osmosis in cellsEstimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions. (Practical 2)Osmotic concentration, or osmolarity, is the measure of solute concentration, defined as the number of osmoles (Osm) of solute per litre (L) of solution.
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EndocytosisThe fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis. Vesicles move materials within cells.
Endocytosis is the active transfer of large molecules within the cell, via an engulfment of the plasma membrane.
When endocytosis? • to transfer larger molecules needed by
the cell (fetus absorbs mother antibodies) • Food digestion by certain protozoa • White blood cells kill bacteria or viruses
https://upload.wikimedia.org/wikipedia/commons/thumb/1/1a/Endocytosis_types.svg/672px-Endocytosis_types.svg.png
https://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120068/bio02.swf::Endocytosis%20and%20Exocytosis
https://youtu.be/JnlULOjUhSQ
EndocytosisThe fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis. Vesicles move materials within cells.
Describe endocytosis.
endocytosis occurs when a membrane encloses a target particle;
fluidity of membrane permits movement of membrane;
membrane sinks inwardly/forms pit/invaginates to enclose particle;
membrane seals back on itself / edges fuse;one membrane layer / two phospholipid layers enclose particle makingvesicle;
inner phospholipid layer of (original) membrane becomes outerphospholipid layer of vesicle membrane;outer phospholipid layer of (original) membrane becomes innerphospholipid layer of vesicle membrane;
vesicle breaks away from membrane/moves into cytoplasm;changes in membrane shape require energy;
specific example of endocytosis (e.g. pinocytosis, phagocytosis);
From the IB Question bankhttp://www.mun.ca/biology/desmid/brian/BIOL2060/BIOL2060-12/12_13.jpg
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EndocytosisThe fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis. Vesicles move materials within cells.
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EndocytosisThe fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis. Vesicles move materials within cells.
http://www.sumanasinc.com/webcontent/animations/content/vesiclebudding.html
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ExocytosisThe fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis. Vesicles move materials within cells.
Exocytosis is the active transfer of large molecules outside the cell, via vesicles fusing with the plasma membrane.
When endocytosis? • To release materials from the cell (e.g.,
release of neurotransmitters. • Production of secretory enzymes • Waster products removed (e.g., Para-
mecium’s contractile vacuole)
https://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120068/bio02.swf::Endocytosis%20and%20Exocytosishttps://youtu.be/qXThS39gLso
Annotate this diagram to explain vesicle transport & exocytosis.
Animated tutorial: http://bcs.whfreeman.com/thelifewire8e/content/cat_040/0504003.html
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