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Questions and Answers
Cholesterol helps to prevent membranes from freezing and maintains membrane fluidity.
Cholesterol helps to prevent membranes from freezing and maintains membrane fluidity.
True
Phospholipids are found in equal numbers in both the outer and inner leaflets of the plasma membrane.
Phospholipids are found in equal numbers in both the outer and inner leaflets of the plasma membrane.
False
All cells, whether prokaryotic or eukaryotic, are surrounded by a cell wall.
All cells, whether prokaryotic or eukaryotic, are surrounded by a cell wall.
False
Phagocytosis is a type of endocytosis that involves the uptake of small particles.
Phagocytosis is a type of endocytosis that involves the uptake of small particles.
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The phospholipid bilayer is the fundamental structure of the plasma membrane.
The phospholipid bilayer is the fundamental structure of the plasma membrane.
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Cholesterol has distinct effects on membrane fluidity regardless of temperature.
Cholesterol has distinct effects on membrane fluidity regardless of temperature.
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Lipid rafts are formed by the interactions of sphingomyelin, glycolipids, and cholesterol.
Lipid rafts are formed by the interactions of sphingomyelin, glycolipids, and cholesterol.
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The glycocalyx is primarily responsible for cell-cell recognition and protection of the cell surface.
The glycocalyx is primarily responsible for cell-cell recognition and protection of the cell surface.
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Peripheral proteins are embedded within the phospholipid bilayer.
Peripheral proteins are embedded within the phospholipid bilayer.
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Phospholipids in a membrane cannot move laterally or rotate within the bilayer.
Phospholipids in a membrane cannot move laterally or rotate within the bilayer.
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Study Notes
Membrane Structure and Function
- Membranes surround all cells (prokaryotic and eukaryotic)
- Membranes are dynamic, not fixed
- Define cell boundaries
- Separate internal cell contents from the environment
- Act as selective barriers for molecule passage
Plasma Membrane Structure
- Phospholipid bilayer: The fundamental structure, with phospholipids comprising more than half the lipids, and glycolipids and cholesterol also present.
- Cholesterol: A rigid ring structure that alters membrane fluidity (less fluid at warmer temperatures; more fluid at lower temperatures).
- Proteins: Integral, peripheral, single-pass, multi-pass, lipid-anchored, and GPI-anchored, carrying out various functions.
- Glycocalyx: A carbohydrate coat on the cell surface, formed by glycolipids and glycoproteins, protecting the cell surface and acting as markers for cell-cell recognition.
- Lipid rafts: Structures formed by the interaction of sphingomyelin, glycolipids, and cholesterol, with GPI-anchored proteins preferentially located here.
Membrane Mobility
- Phospholipids can rotate (on their axes) and move laterally within the bilayer (frequent). Flip-flop is rare and requires energy or help from proteins/enzymes.
- Membrane proteins are free to diffuse laterally through the phospholipid bilayer, but their mobility can be restricted by interactions with other proteins or specific lipids, or tight junctions between epithelial cell membranes.
Transport of Molecules Across Membranes
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Passive Transport:
- Simple diffusion: Small, uncharged molecules (e.g., O2, CO2, H2O, glycerol, ethanol) cross freely.
- Facilitated diffusion: Larger, polar molecules (e.g., glucose, amino acids) use transport proteins (channels or carriers) for passage.
- Channels form open pores for ions or small molecules.
- Carriers bind to molecules, undergo conformational change, releasing them on the opposite side of the membrane.
-
Active Transport:
- Driven by ATP hydrolysis: Proteins utilize ATP energy to move molecules against their electrochemical gradients. An example is the sodium-potassium pump.
- Driven by ion gradients: The concentration gradient of one ion (e.g., Na+) can be used to power the active transport of another molecule.
Endocytosis
- Mechanism for ingesting large particles (bacteria) or taking up fluids or macromolecules in small vesicles.
- Phagocytosis (cell eating) involves engulfing large particles.
- Pinocytosis (cell drinking) involves taking up fluids or macromolecules.
- Receptor-mediated endocytosis: Selective uptake of specific macromolecules.
- Protein trafficking in endocytosis: Endocytosed molecules are transported to endosomes, where they are sorted for recycling to the plasma membrane or degradation within lysosomes.
Important Functions of the Plasma Membrane
- Structural: Provides shape to cells and organelles.
- Barrier: Controls passage of ions and other molecules into and out of the cell.
- Contact: Allows cell-cell interactions.
- Receptors: Receives signals from the environment.
- Transport: Facilitates the movement of molecules across the membrane, including active and passive transport.
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Description
Test your knowledge on the structure and function of cell membranes, including the plasma membrane and its components. This quiz covers essential concepts like phospholipid bilayers, cholesterol's role, and the function of membrane proteins. Suitable for students of cell biology and related fields.
