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What is the primary property of the plasma membrane that controls substance movement?
Selective permeability: it allows passage of some substances and prevents passage of others.
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What distinguishes passive transport from active transport across the plasma membrane?
Passive transport does not require cellular energy; active transport requires ATP.
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Define diffusion in the context of membrane transport.
Diffusion is passive net movement of a substance down its concentration gradient from high to low concentration.
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Which substances move across the membrane by simple diffusion?
Nonpolar, lipid-soluble plasma membrane compatible substances such as oxygen, carbon dioxide, and soluble vitamins.
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What is carrier-mediated facilitated diffusion?
Specific water-soluble molecules bind integral carrier proteins that change shape and transport them across the membrane down their concentration gradient.
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Give an example of carrier-mediated facilitated diffusion.
Glucose transporters moving glucose down its concentration gradient without using energy.
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How do channel proteins transport substances across membranes?
Channels form aqueous pathways selective for size and charge, allowing water-soluble substances to pass through.
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What is the difference between leakage channels and gated channels?
Leakage channels are always open; gated channels open in response to chemical, mechanical, or electrical signals.
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Define osmosis and its direction relative to solute concentration.
Osmosis is diffusion of a solvent (e.g., water) across a selectively permeable membrane from low solute concentration (high solvent) to high solute concentration (low solvent).
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What types of signals open gated channels and give one example for each?
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Why do some substances require carriers or channels to cross the plasma membrane?
Because they are water-soluble and incompatible with the lipid bilayer, so they need an aqueous path or specific carriers.
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What basic role does the plasma membrane play between the cytoplasm and extracellular fluid?
It acts as a barrier separating the cytoplasm from the extracellular fluid.
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What energy molecule is explicitly mentioned as required for active transport?
ATP (cellular energy).
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What is primary active transport and give an example?
Primary active transport uses ATP to move substances against their concentration gradient; example: the sodium-potassium pump exchanges intracellular Na+ for extracellular K+.
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What energy source is required for the sodium-potassium pump?
ATP is required for the sodium-potassium pump.
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What is secondary active transport?
Secondary active transport uses an ion gradient established by primary active transport to drive movement of other charged molecules down that gradient.
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How do 'symport' and 'antiport' differ in secondary active transport?
Symport cotransports molecules in the same direction; antiport cotransports molecules in the opposite direction.
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What is vesicular transport?
Vesicular transport moves substances into or out of the cell inside vesicles, which are small sacs formed at or with the plasma membrane.
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Which cellular organelle packages many proteins that are then transported by vesicles?
Many proteins produced and packaged by the Golgi apparatus are transported by vesicles.
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Define endocytosis.
Endocytosis transports substances from the extracellular fluid into the cell by forming vesicles at the plasma membrane.
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Define exocytosis.
Exocytosis is the process where intracellular vesicles fuse with the plasma membrane to release their contents into the extracellular fluid.
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What are the main categories of transport across the plasma membrane?
Transport across the plasma membrane can be either passive (simple diffusion, osmosis, facilitated diffusion) or active (requires ATP and can move substances against gradients).
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What is the primary property of the plasma membrane that controls substance movement?
Selective permeability: it allows passage of some substances and prevents passage of others.
What distinguishes passive transport from active transport across the plasma membrane?
Passive transport does not require cellular energy; active transport requires ATP.
Define diffusion in the context of membrane transport.
Diffusion is passive net movement of a substance down its concentration gradient from high to low concentration.
Which substances move across the membrane by simple diffusion?
Nonpolar, lipid-soluble plasma membrane compatible substances such as oxygen, carbon dioxide, and soluble vitamins.
What is carrier-mediated facilitated diffusion?
Specific water-soluble molecules bind integral carrier proteins that change shape and transport them across the membrane down their concentration gradient.
Give an example of carrier-mediated facilitated diffusion.
Glucose transporters moving glucose down its concentration gradient without using energy.
How do channel proteins transport substances across membranes?
Channels form aqueous pathways selective for size and charge, allowing water-soluble substances to pass through.
What is the difference between leakage channels and gated channels?
Leakage channels are always open; gated channels open in response to chemical, mechanical, or electrical signals.
Define osmosis and its direction relative to solute concentration.
Osmosis is diffusion of a solvent (e.g., water) across a selectively permeable membrane from low solute concentration (high solvent) to high solute concentration (low solvent).
What types of signals open gated channels and give one example for each?
Why do some substances require carriers or channels to cross the plasma membrane?
Because they are water-soluble and incompatible with the lipid bilayer, so they need an aqueous path or specific carriers.
What basic role does the plasma membrane play between the cytoplasm and extracellular fluid?
It acts as a barrier separating the cytoplasm from the extracellular fluid.
What energy molecule is explicitly mentioned as required for active transport?
ATP (cellular energy).
What is primary active transport and give an example?
Primary active transport uses ATP to move substances against their concentration gradient; example: the sodium-potassium pump exchanges intracellular Na+ for extracellular K+.
What energy source is required for the sodium-potassium pump?
ATP is required for the sodium-potassium pump.
What is secondary active transport?
Secondary active transport uses an ion gradient established by primary active transport to drive movement of other charged molecules down that gradient.
How do 'symport' and 'antiport' differ in secondary active transport?
Symport cotransports molecules in the same direction; antiport cotransports molecules in the opposite direction.
What is vesicular transport?
Vesicular transport moves substances into or out of the cell inside vesicles, which are small sacs formed at or with the plasma membrane.
Which cellular organelle packages many proteins that are then transported by vesicles?
Many proteins produced and packaged by the Golgi apparatus are transported by vesicles.
Define endocytosis.
Endocytosis transports substances from the extracellular fluid into the cell by forming vesicles at the plasma membrane.
Define exocytosis.
Exocytosis is the process where intracellular vesicles fuse with the plasma membrane to release their contents into the extracellular fluid.
What are the main categories of transport across the plasma membrane?
Transport across the plasma membrane can be either passive (simple diffusion, osmosis, facilitated diffusion) or active (requires ATP and can move substances against gradients).
Carrier‑mediated facilitated diffusion - Carriers (integral proteins) bind a specific molecule, change shape, and release it on the other side. - Shows specificity and can saturate when carriers are fully occupied (transport maximum). - Example: glucose transporters move glucose down its concentration gradient.
Channel‑mediated facilitated diffusion - Channels are transmembrane pores that allow passage of ions or small polar molecules via an aqueous path. - Channels are selective for size and charge. - Types: - Leak channels: always open, permit passive flow. - Gated channels: open only with a specific signal: - Ligand‑gated: chemical binds to open. - Mechanically gated: opened by physical force or pressure. - Voltage‑gated: opened by changes in membrane potential.
Endocytosis (into cell) - Vesicles form at the plasma membrane to bring substances inside. - Common forms (high‑level): phagocytosis (large particles), pinocytosis (fluid), receptor‑mediated endocytosis (specific uptake).
Exocytosis (out of cell) - Intracellular vesicles fuse with the plasma membrane and release contents to the extracellular space. - Used to secrete proteins, neurotransmitters, and membrane renewal.
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