2.4 Cell Membranes IB Biology 2 Cells Jason de Nys All syllabus statements ©IBO 2007 All images CC or public domain or link to original material. 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

What happens when you put a drop of oil in water? 2.4.2 Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the 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 makes a 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/

The head is attracted to water and the tails are not. 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 that phospholipids 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 tails can be removed from the water. Phospholipid molecules can flow past each other laterally but can’t move vertically http://commons.wikimedia.org/wiki/File:Phospholipids_aqueous_solution_structures.svg

The plasma membrane is not just made of phospholipids 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 way through and are polytopic (poly = many, topic = surface), integral proteins penetrating just one surface are monotopic. Peripheral proteins usually have a temporary association with the membrane, they can be monotopic or attach to the surface Some human examples on the next page

Muscle fatty acid binding protein Beta-secretase 1 (peripheral) has a role in creating myelin sheaths on nerve cells Extracellular (outside) These are just three examples, they are from different cell types. There are thousands that have been found so far and thousands more are added each year. Protein structure covered in AHL 7.5 Beta-secretase 1 In the membrane Glycophorin (integral) carries sugar molecules into red blood cells Cytoplasm (inside) Glycophorin A 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 and as hormone receptors

Cholesterol: (It’s not all bad!) It makes the phospholipids pack more tightly and regulates the fluidity and flexibility of the membrane. more later in 2.4.8 Bad 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…

6 Proteins associated with membranes have many functions. 2.4.3 List the functions of membrane proteins List: Give a sequence of names or other brief answers with no explanation 6 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 ATP 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/

Nicotinic acetylcholine receptor, beta2 subunit 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

Plasma platelet activating factor acetylhydrolase 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

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

Diffusion, is the motion of all (liquid or gas) particles at temperatures 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)

Simulate! Grab your classmates or a group of friends. Take one die each. Stand together in a clump; in a room or outside. Everybody rolls their die in their hand For a 1, step forward; for a 2, step to the right; for a 3, step backwards and for a 4, step left If you roll 5 or 6, or if you can’t move because a person or object is in the way, roll again Do this for 3 minutes, you should have dispersed, some will have got further than others. Simulate! If you have 4 sided dice then all the better. This simulation is known as a random walk. How could you simulate an increase of heat energy? http://www.flickr.com/photos/stevon/3145375973/

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

Osmosis may occur when there is a partially permeable membrane, such as a cell membrane. When a cell is submerged in water, the water molecules pass through the cell membrane from an area of low solute concentration (outside the cell) to one of high solute concentration (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

The importance of osmotic control 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 facilitated diffusion. 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 via simple diffusion Transmembrane (polytopic) proteins recognise a particular molecule and help it to move across the membrane. The direction 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

Primary active transport requires ATP. 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 is derived from energy stored in the form of concentration differences in a second solute. http://commons.wikimedia.org/wiki/File:Scheme_secundary_active_transport-en.svg

Typically, the concentration gradient of the second solute was created by primary active transport, and the diffusion of the second solute across the membrane drives secondary active transport.

2.4.7 Explain how vesicles are used to transport materials within a cell between the rough endoplasmic 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 reform during 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

Constitutive secretion occurs continuously in cells, depending on their 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

The plasma membrane is embedded with proteins 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 of other 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