IB Biology2 Cells2.4 Cell MembranesAll
syllabus statements ©IBO 2007All images CC or public domain or link to original material.Jason de Nys http://www.flickr.com/photos/edsweeney/6346198056/

2.4.1 Draw and label a
diagram to show the structure of membranes. http://www.youtube.com/watch?v=w9VBHGNoFrY

2.4.2 Explain how the hydrophobic
and hydrophilic properties of phospholipids help to maintainthe structure of cell membranes. What happens when you put a drop of oil in water? http://www.flickr.com/photos/zorin-denu/5385963280/

The Oil droplet stays together
and makesa perfect circular shape. The oil molecules are Hydrophobic Oil Molecules are non- polar and water molecules are polar. See 3.1.5 http://www.flickr.com/photos/zorin-denu/5385963280/

Phospholipid molecules have a polar
(charged) phosphate head and long non-polar lipid tails * The head is attracted to water and the tails are not.*h not b! http://commons.wikimedia.org/wiki/File:Cell_membrane_detailed_diagram_4.svg

When put into water, an
emergent property is thatphospholipids will self-organise to keep their heads ‘wet’ and their tails ‘dry’ micelle liposome http://commons.wikimedia.org/wiki/File:Micelle_scheme-en.svg http://commons.wikimedia.org/wiki/File:Liposome_scheme-en.svg

In this 3D representation you
can see that a phospholipid bilayer is one way that the tailscan be removed from the water.Phospholipid moleculescan flow past each otherlaterally but can’t move vertically http://commons.wikimedia.org/wiki/File:Phospholipids_aqueous_solution_structures.svg

But wait! there’s more!The plasma
membrane is not just made of phospholipids http://commons.wikimedia.org/wiki/File:Cell_membrane_detailed_diagram_en.svg?uselang=en-gb

Proteins:Integral proteins are permanently embedded,
many go all the waythrough and are polytopic (poly = many, topic = surface), integralproteins penetrating just one surface are monotopic.Peripheral proteins usually have a temporary association with themembrane, they can be monotopic or attach to the surface Some human examples on the next page

Beta-secretase 1 (peripheral) has a
role in creating myelin Extracellular sheaths on nerve cells (outside)These are just threeexamples, they arefrom different celltypes. Beta-secretase 1 membraneThere are thousands In the Glycophorin (integral) carries sugarthat have been found molecules into red blood cellsso far and thousandsmore are added eachyear.Protein structure covered Cytoplasm Glycophorin A (inside)in AHL 7.5 Muscle Fatty acid binding protein (peripheral) is involved in the transport of fatty acids Muscle fatty acid binding protein http://opm.phar.umich.edu/protein.php?pdbid=2wf1 http://opm.phar.umich.edu/protein.php?pdbid=1afo http://opm.phar.umich.edu/protein.php?pdbid=1hmt

Glycoproteins:Are proteins with an oligosaccaride
(oligo = few, saccharide = sugar)chain attached.They are important for cell recognition by the immune system andas hormone receptors

Cholesterol: (It’s not all bad!)It
makes the phospholipids pack more tightly and regulates thefluidity and flexibility of the membrane. more later in 2.4.8Bad analogy: imagine a room full of people wearing fluffy jumpers (sweaters).It is crowded but they can slip past each other easily enough.Now sprinkle the crowd with people wearing Velcro™ suits…

2.4.3 List the functions of
membrane proteins List: Give a sequence of names or other brief answers with no explanation Proteins associated with membranes have many functions. Can you think of any ‘jobs’ that proteins could help cells do? Let’s look at

1 Cell Adhesion Molecules: Enable
cells to make tight connections to one another They may play a part in the immune response. http://www.flickr.com/photos/silveraquarius/1395277674/

2 Channel Proteins: Allow or
help ions and large molecules to pass through the membrane by diffusion http://www.flickr.com/photos/gilderic/2728059828/

3 Protein Pumps move ions
across the membrane to create and maintain concentration gradients. They require energy to carry out this active transport http://www.flickr.com/photos/erix/298857080/

4 Hormone Binding sites (hormone
receptors) bind to specific hormones and start signalling processes to change the behaviour of the cell e.g. insulin http://www.flickr.com/photos/18735339@N00/2926816259/

5 Cell to cell communication:
e.g. receptors for neurotransmitters at synapses Outside cell Inside membrane Cytoplasm Nicotinic acetylcholine receptor, beta2 subunit http://ifaketext.com/ http://opm.phar.umich.edu/protein.php?pdbid=2ksr

6 Enzymes on the surface
of the cell Outer surface of membrane Plasma platelet activating factor acetylhydrolase http://opm.phar.umich.edu/protein.php?pdbid=3d59

2.4.4 Define diffusion and osmosis
What is http://www.flickr.com/photos/dpup/3338948041/

Diffusion, is the motion of
all (liquid or gas) particles attemperatures above absolute zero.• The rate of this movement relates to temperature, viscosity of the fluid and the size (mass) of the particles.• Diffusion explains the net flow of molecules from a region of higher concentration to one of lower concentration.• The result of diffusion is a gradual mixing of material. In the absence of other influences, the diffusion process will eventually result in complete mixing. Diffusive equilibrium is reached when the concentrations of the diffusing substance in the two compartments becomes equal. (Wikipedia)

1. Grab your classmates or
a group Simulate! of friends. 2. Take one die each. 3. Stand together in a clump; in a room or outside. 4. Everybody rolls their die in their handIf you have 4 sided dice 5. For a 1, step forward; for a 2, then all the better. step to the right; for a 3, stepThis simulation is known as a random walk. backwards and for a 4, step left 6. If you roll 5 or 6, or if you can’tHow could you simulate move because a person or an increase of heat energy? object is in the way, roll again Do this for 3 minutes, you should have dispersed, some will have got further than others. http://www.flickr.com/photos/stevon/3145375973/

What is osmosis? http://www.flickr.com/photos/luchilu/399970490/

Osmosis may occur when there
is a partiallypermeable membrane, such as a cellmembrane.When a cell is submerged in water, the watermolecules pass through the cell membranefrom an area of low solute concentration(outside the cell) to one of high soluteconcentration (inside the cell) (Wikipedia) Aquaporin is an integral protein that, as it’s name suggests, acts as a pore in the membrane that speeds the movement of water molecules http://opm.phar.umich.edu/protein.php?pdbid=1sor

Theimportanceof osmoticcontrol http://commons.wikimedia.org/wiki/File:Osmotic_pressure_on_blood_cells_diagram.svg http://commons.wikimedia.org/wiki/File:Turgor_pressure_on_plant_cells_diagram.svg

2.4.5 Explain passive transport across
membranes in terms of simple diffusion and facilitateddiffusion. Simple Diffusion http://commons.wikimedia.org/wiki/File:Scheme_simple_diffusion_in_cell_membrane-en.svg

Facilitated Diffusion:Large and polar molecules
can’t get across the membrane viasimple diffusionTransmembrane (polytopic) proteins recognise a particularmolecule and help it to move across the membrane. Thedirection it moves is dependent on the concentration gradient. Watch the animation http://commons.wikimedia.org/wiki/File:Scheme_facilitated_diffusion_in_cell_membrane-en.svg

2.4.6 Explain the role of
protein pumps and ATP in active transport across membranes Primary active transport requires ATP. Integral protein pumps use the energy from the hydrolysis of ATP to move ions or large molecules across the cell membrane. Molecules are moved against their concentration gradient http://commons.wikimedia.org/wiki/File:Scheme_sodium-potassium_pump-en.svg

In secondary active transport, the
required energy isderived from energy stored in the form of concentrationdifferences in a second solute. http://commons.wikimedia.org/wiki/File:Scheme_secundary_active_transport-en.svg

Typically, theconcentrationgradient of thesecond solute
wascreated by primaryactive transport,and the diffusion ofthe second soluteacross themembrane drivessecondary activetransport.

2.4.7 Explain how vesicles are
used to transport materials within a cell between the roughendoplasmic reticulum, Golgi apparatus and plasma membrane. Vesicles are small spheroidal packages that bud off of the RER and the Golgi apparatus They carry proteins produced by ribosomes on the RER to the Golgi apparatus, where they are prepared for export from the cell via another vesicle

2.4.8 Describe how the fluidity
of the membrane allows it to change shape, break and reformduring endocytosis and exocytosis. Endocytosis: The taking in of external substances by an inward pouching of the plasma membrane, forming a vesicle Exocytosis: The release of substances from a cell (secretion) when a vesicle joins with the cell plasma membrane. Diagrams on following slides

Constitutivesecretionoccurscontinuously incells,depending ontheir function Regulated
secretion is in response to a trigger e.g. the release of neurotransmitters http://commons.wikimedia.org/wiki/File:Exocytosis_types.svg

“Cell eating” “Cell drinking” http://commons.wikimedia.org/wiki/File:Endocytosis_types.svg

As mentioned, phospholipids can move
freely past each other laterally but very rarely do they move vertically or flip The plasma membrane is embedded with proteins Cholesterol molecules stiffen and stabilise the plasma membrane It is because of the lateral movement and the presence ofother molecules studding it’s surface that our understanding of the plasma membrane is referred to as the fluid mosaic model

Further information: Most of the
excellent biology images in this slideshow are by graphic designer Mariana Ruiz Villarreal (LadyofHats) who has graciously released them to the public domain. Three of the best sites for IB-specific Biology information. The top link takes you to the PPT by Stephen Taylor