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Questions and Answers
Which component is primarily responsible for the fluidity of a cell membrane composed of unsaturated phospholipids?
Which component is primarily responsible for the fluidity of a cell membrane composed of unsaturated phospholipids?
- Presence of cholesterol
- The length of fatty acid tails
- Degree of saturation in fatty acids (correct)
- Protein content in the membrane
Which statement best describes the movement of substances across a phospholipid bilayer?
Which statement best describes the movement of substances across a phospholipid bilayer?
- Small nonpolar molecules can readily cross the bilayer without assistance. (correct)
- Large polar molecules can easily diffuse through the bilayer.
- Charged molecules generally pass through more readily than uncharged molecules.
- Nonpolar molecules require carrier proteins to traverse the membrane.
How does the addition of cholesterol to a phospholipid bilayer affect membrane permeability?
How does the addition of cholesterol to a phospholipid bilayer affect membrane permeability?
- It selectively decreases permeability only to polar molecules.
- It increases permeability by allowing more small molecules to pass.
- It has no effect on permeability.
- It decreases permeability by providing more rigid structure. (correct)
Which sequence correctly ranks the permeability of different types of molecules across a phospholipid bilayer, from most to least permeable?
Which sequence correctly ranks the permeability of different types of molecules across a phospholipid bilayer, from most to least permeable?
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What is the main reason for the behavioral differences of phospholipids in water?
What is the main reason for the behavioral differences of phospholipids in water?
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Study Notes
Phospholipid Structure and Behavior
- Phospholipids possess a hydrophilic (polar) head and two hydrophobic (nonpolar) tails.
- In an aqueous environment, phospholipids arrange themselves into a bilayer, with hydrophilic heads facing outwards and hydrophobic tails facing inwards.
- This arrangement forms a barrier that separates the internal and external environments of cells.
Cell Membrane Components
- The cell membrane consists of four primary components:
- Phospholipids, which form the fundamental structure.
- Proteins, embedded within or associated with the membrane, functioning in transport and signaling.
- Cholesterol, interspersed among phospholipids, enhancing membrane stability and fluidity.
- Carbohydrates, typically attached to proteins or lipids, involved in cell recognition and communication.
Substance Permeability Across Membranes
- Small nonpolar molecules (e.g., O2, CO2) can cross the phospholipid bilayer easily.
- Small polar molecules (e.g., H2O) have limited permeability but can pass through slowly.
- Large molecules (e.g., glucose) and charged ions struggle to cross the bilayer without assistance.
- Uncharged molecules typically cross more readily than charged ones.
Direction of Substance Movement
- Substances move from areas of high concentration to areas of low concentration (down their concentration gradient).
- This passive movement can be influenced by the membrane's permeability to specific substances.
Membrane Fluidity and Permeability
- Saturated phospholipids result in a more rigid membrane due to closely packed tails, leading to lower fluidity and permeability.
- Unsaturated phospholipids contain kinks that prevent tight packing, thus increasing fluidity and permeability.
Impact of Cholesterol on Membrane Properties
- The addition of cholesterol stabilizes the membrane, reducing permeability by filling spaces between phospholipids.
- Cholesterol maintains membrane fluidity at varying temperatures and prevents excessive movement of phospholipids, thereby enhancing membrane integrity.
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