Questions and Answers
Which of the following are modes of mobility in a lipid bilayer? (Select all that apply)
What percentage of total membrane lipid does cholesterol make up?
45%
Cholesterol reduces phospholipid packing and increases membrane fluidity.
True
What are the two structures that amphipathic molecules can form in water?
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What is the typical protein content in biological membranes?
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Lipid bilayers allow high permeability to ions and most polar molecules.
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What effect do unsaturated double bonds in fatty acids have on membrane fluidity?
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What is the major class of membrane lipids?
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Plasmalogens are a subclass of phospholipids found in ______ systems.
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Which of the following statements about glycolipids is true?
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What major structural component distinguishes sphingomyelin from other phospholipids?
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What is a primary function of membrane proteins?
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Study Notes
Dynamics in Lipid Bilayers
- Membranes are fluid, with lipid molecules exhibiting four primary mobility modes: fast lateral diffusion, flip-flop movement, intra-chain motion, and fast axial rotation.
- Flip-flop involves lipid molecules exchanging positions from one half of the bilayer to the other.
Cholesterol
- Comprises 45% of total membrane lipids and contributes to plasma membrane structure.
- Distribution varies by tissue type and correlates with specific functions.
- Stabilizes the membrane through hydrogen bonding with fatty acid chains, reducing the endothermic phase transition of phospholipid bilayers.
- Cholesterol decreases phospholipid packing and increases overall membrane fluidity but simultaneously restricts chain motion.
Lipid Bilayer Formation
- Formed by amphipathic molecules in water, leading to spontaneous bilayer formation due to van der Waals forces among hydrophobic tails.
- Maintained by non-covalent forces, including electrostatic and hydrogen bonds, between hydrophilic parts and water.
- Serves as a preferred structure for phospholipids and glycolipids, displaying low permeability to ions and many polar molecules.
Unsaturated Fatty Acids and Cholesterol
- Unsaturated bonds disrupt orderly packing of phospholipids, enhancing membrane fluidity.
Membrane Proteins
- Perform various functions: enzymes, transporters, pumps, ion channels, receptors, energy transducers.
- Protein proportion varies, from about 18% in myelin to 75% in mitochondria, with membranes typically containing 60% protein by dry weight.
Mobility of Membrane Proteins
- Proteins in bilayers exhibit three motion modes: conformational change, rotational motion, and lateral movement, but do not flip-flop due to their large hydrophilic portions.
Restrictions on Protein Mobility
- Influenced by lipid-mediated effects, associations with other membrane proteins, and interactions with peripheral proteins such as the cytoskeleton.
General Functions of Biological Membranes
- Serve as selective permeability barriers, controlling the internal chemical environment.
- Enable communication and signal generation in response to various stimuli.
- Facilitate recognition through signaling molecules and adhesion proteins.
Membrane Composition
- Composed of approximately 40% lipid, 60% protein, 1-10% carbohydrate, and 20% water, indicating hydration in the membrane structure.
Membrane Lipids
- Comprise phospholipids, glycolipids, and cholesterol, characterized as amphipathic.
Phospholipids
- The dominant lipid type, such as phosphatidylcholine, featuring diverse polar head groups (e.g., choline, amines, sugars).
- Fatty acid chains are variable, with C16 and C18 being prevalent; unsaturated fatty acids impart a kink that reduces packing efficiency.
Non-Classical Phospholipids
- Plasmalogens, found in the nervous, immune, and cardiovascular systems, represent a subclass of phospholipids.
- Sphingomyelin, a sphingolipid found in the myelin sheath surrounding nerve cell axons, is unique as it is not based on glycerol.
Glycolipids
- Composed of carbohydrate-containing structures with hydrophobic fatty acid tails.
- Types include cerebrosides (sugar monomer head groups) and gangliosides (sugar oligomer head groups).
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Description
Test your understanding of the fluid structures of membranes and the mobility of lipid molecules within lipid bilayers. This quiz covers key concepts such as lateral diffusion, flip-flop movement, intra-chain motion, and the role of cholesterol in membrane dynamics.
