Cell Transport and Homeostasis
Transport Across Membranes
Passive and active transport
Cells move substances across membranes by diffusion, osmosis, facilitated diffusion, active transport, endocytosis, and exocytosis. Direction of movement relative to concentration gradients determines whether energy is required.
Key distinction
Passive transport moves down a gradient; active transport uses energy to move against a gradient or to bulk-transport material.
Osmosis and tonicity
Osmosis is the movement of water across a selectively permeable membrane. Tonicity describes the effect of a solution on cell volume. In hypotonic conditions, water tends to enter the cell; in hypertonic conditions, water tends to leave the cell.
Which process requires ATP directly or indirectly?
Active transport uses energy to move substances against their gradients.
Correct answer: Active transport
What happens to an animal cell placed in a strongly hypertonic solution?
Water exits the cell because the surrounding solution has a higher solute concentration.
Correct answer: It loses water and shrinks.
How to solve osmosis problems
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1
Identify the solute concentration inside and outside the cell.
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2
Determine whether the external solution is hypotonic, isotonic, or hypertonic.
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3
Predict the direction of water movement.
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4
Infer the cell's volume change and physiological consequence.
Facilitated diffusion differs from simple diffusion because it:
Facilitated diffusion is passive but depends on channel or carrier proteins.
Correct answer: Uses membrane proteins to help substances move down a gradient