MLB 111 Unit 2 Summary

MLB 111 UNIT 2
STUDY UNIT 2: CELL MEMBRANES
(5 lectures)
STUDY MATERIAL: Hillis et al., Chapter 6
Lecturer: Dr Nicky Olivier
Core Concepts:
2.1 Membrane Structure
2.2 Membrane Transport
2.3 Cell junctions

LEARNING OUTCOMES:
You have mastered this study theme when you can:
1. Explain what an amphipathic molecule is and show how lipids associate with water
depending on their amphipathic properties. (Chapter 3.4, p63-64)
• Characteristics of polar and non-polar molecules. (Has both hydrophilic and
hydrophobic parts)
• Phospholipid:
• Phosphate functional group has negative electric charge
◦ hydrophilic - attracts polar water molecules
• Two fatty acid tails
◦ hydrophobic (CH2)
◦ avoid water
◦ aggregate together (bind to selves) or with other hydrophobic substances
• Forms phospholipid bilayer spontaneously - polar and non-polar sides.
◦ Polar molecules line with polar, non-polar with non-polar
◦ "tails" - non-polar; "head" - polar
◦ line up so tails of one molecule lines with tails of other molecule thus "tails" are as
far from water as possible (see image from textbook below)
◦ This is a membrane (bilayer - 2 layers of phospholipids, forms an inside and
outside)




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,Changing components of head can change characteristics of membrane


2. Describe the “fluid mosaic model” of membrane structure. (p113)
• Lipids establish physical integrity of membrane
• Create a barrier to rapid passage of hydrophilic materials (e.g. polar molecules and
ions)
• Phospholipid bilayer serves as lipid "lake" in which a variety of proteins "float" - fluid
mosaic model
• Fluid: movement. Mosaic: mixture that makes up membrane differs
• Not all membranes are identical: some membranes need more protein, some need just
lipids, some have lots of cholesterol (membranes are 20-50% cholesterol), some have
carbohydrates
• Bilayer controls what can cross membrane




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, 3. Apply your knowledge on the properties of lipids to the properties and functions of cell
membranes by discussing membrane fluidity. (p114)
IMPORTANT SECTION: HAVE TO UNDERSTAND
• Phospholipids form bilayer
• Affecting fluidity: differ in:
◦ fatty acid chain length
◦ degree of saturation (no. double bonds: saturated has no double bonds)
◦ polar head groups (lipid composition)
• Fat is more solid - not as easy to move around in
More fluid = more movement
• Can travel to other end of membrane in one second
• Usually: movement of lipids mainly in plane of membrane
• Movement between membrane layers ("flip") very rare - because inside is so
hydrophobic so polar head of molecules do not want to go in
Affecting fluidity: *Understand why
Chain length:
• shorter chain = weaker interaction = more fluid
◦ movement is easier
◦ don't associate as closely with neighbours
• longer chain = stronger interaction = less fluid
• Van der Waals forces associate strongly thus chain length affects strength of bonds
Saturation:
• saturated = no double bonds = tight packing = less fluid
• unsaturated = double bonds (forms a kink in the chain which prevents closeness
between molecules) = weaker interaction = more fluid
Lipid composition (cholesterol):
• more cholesterol = more interaction = less fluid (more interaction between
phospholipids, binds tightly, less fluid, less movement)
• less cholesterol = less interaction = more fluid



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