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Cell membranes and transport
Biology · 2-page sourceTopic 1 of 2
Membrane structure, diffusion, and osmosis
Selective permeability
A cell membrane separates the cell interior from its surroundings. Its phospholipid bilayer has water-attracting heads facing watery environments and water-repelling tails oriented inward. Small nonpolar molecules, such as oxygen, can cross the bilayer relatively easily; ions and many polar molecules require transport proteins. A selectively permeable membrane allows some substances to cross more readily than others—it is not a completely sealed wall. (PDF p. 1)
Diffusion and facilitated diffusion
Diffusion is the net movement of particles from higher to lower concentration, driven by random molecular motion. At equilibrium, particles continue moving in both directions, but their movements balance, so there is no net movement.
- Simple diffusion: particles cross the lipid bilayer directly, as oxygen can.
- Facilitated diffusion: a channel or carrier protein helps particles cross, as a carrier can move glucose down its gradient.
Both are passive transport: movement down the relevant concentration or electrochemical gradient does not require direct cellular energy input. Protein involvement alone does not make a process active. (PDF p. 1)
Osmosis
Osmosis is the net movement of water across a selectively permeable membrane. In the simple case where solute cannot cross, water moves toward the side with the higher concentration of that solute. Thus, if only water can cross between a dilute sugar solution and a more concentrated sugar solution, net water movement is toward the more concentrated solution.
Aquaporins can speed water movement through the membrane; they do not pump water using ATP. Predictions about net water movement require knowing which substances the membrane permits to cross. (PDF p. 1)
Caution
Distinguish movement of individual molecules from net movement. At equilibrium, molecules still move, even though there is no net movement.
Practice · topic 1
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Source PDF · page 1
Membranes and passive transport
The membrane is selectively permeable
A cell membrane separates the cell's interior from its surroundings. Its phospholipid bilayer has water-attracting heads facing the watery environments and water-repelling tails facing inward. Small nonpolar molecules such as oxygen can cross the bilayer relatively easily. Ions and many polar molecules need membrane transport proteins. Selectively permeable means that some substances cross more readily than others; it does not mean the membrane is a completely sealed wall.
Diffusion follows a gradient
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration because of random molecular motion. At equilibrium, molecules still move, but there is no net movement in either direction. Simple diffusion crosses the lipid bilayer directly. Facilitated diffusion uses a channel or carrier protein. Both are passive transport: movement down the relevant concentration or electrochemical gradient does not require a direct input of cellular energy. A protein's involvement alone does not make transport active.
Osmosis is the movement of water
Osmosis is the net movement of water across a selectively permeable membrane. In a simple system where the solute cannot cross, water moves toward the side with the higher concentration of that solute. Suppose a membrane separates a dilute sugar solution from a more concentrated sugar solution, and only water can cross. Water moves toward the more concentrated sugar solution. Aquaporin channels can speed water movement; they do not pump water using ATP.
Source PDF · page 2
Active transport and applying the ideas
Active transport requires energy
Active transport can move a substance against its concentration or electrochemical gradient. Primary active transport couples this movement directly to an energy source such as ATP hydrolysis. For example, the sodium-potassium pump uses ATP to move three sodium ions out of a cell and two potassium ions into it per cycle. Secondary active transport uses energy stored in another substance's gradient. In a sodium-glucose cotransporter, sodium moving down its gradient can drive glucose movement against its gradient. The cotransporter does not directly hydrolyze ATP, but maintaining the sodium gradient depends on energy-consuming processes.
Compare the transport mechanisms
Simple diffusion: oxygen crosses the lipid bilayer down its concentration gradient. Facilitated diffusion: a carrier moves glucose down its gradient without a direct energy input. Osmosis: water crosses a selectively permeable membrane toward a higher concentration of nonpenetrating solute. Primary active transport: the sodium-potassium pump directly uses ATP. Secondary active transport: one substance moving down its gradient drives another substance uphill. To classify a mechanism, ask what moves, which way it moves relative to its gradient, and where any required energy comes from.
Check your understanding
A channel allows an ion to move down its electrochemical gradient. This is facilitated diffusion, even though a protein is involved. If ATP production stops, existing gradients do not disappear instantly: some passive transport may continue while gradients last, but ATP-dependent pumping cannot be sustained. A sugar solution example predicts net water movement only when the membrane's permeability is known. Always distinguish particle movement from net movement, and distinguish a protein's presence from an energy requirement.
Generated by ChatEDU from our original two-page handout: 2 topics and 4 practice questions. This public demo uses saved output; your answers stay here until you reload. Open the source PDF
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