AOL 1: Membrane Transport
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Explain how the chemical properties of a phospholipid bilayer allow it to act as a semipermeable membrane.
Structure that creates the barrier
Each phospholipid is amphipathic, with a hydrophilic head (containing a charged phosphate group) and two hydrophobic fatty acid tails.
In water, phospholipids assemble into a bilayer. The heads face the aqueous environments inside and outside the cell, and the tails face inward and form a nonpolar, hydrophobic core.
Why the core blocks some substances
To cross the membrane, a molecule must pass through the hydrophobic core.
Ions (such as Na⁺, K⁺, Cl⁻, Ca²⁺) and large polar molecules (such as glucose) are surrounded by water molecules in solution. Shedding that water shell costs a great deal of energy, and these molecules cannot form favorable interactions with the nonpolar tails. They are therefore blocked.
Why the core lets some substances through
Small nonpolar molecules (such as O₂, CO₂, and steroid hormones) dissolve easily in the hydrophobic core and diffuse across readily.
Small uncharged polar molecules (such as water and ethanol) can slip between the fluctuating lipid tails slowly.
What “semipermeable” means
The membrane allows some substances to cross freely and restricts others. This lets the cell maintain an internal environment different from its surroundings.
Cells move the restricted substances through membrane transport proteins, which gives the cell control over what enters and exits.
Define and relate the terms diffusion, tonicity, osmosis, hypertonic, hypotonic, and isotonic.
Diffusion
It is the net movement of molecules from an area of higher concentration to an area of lower concentration (down a concentration gradient), driven by random thermal motion.
It continues until the concentration is equal throughout (equilibrium), although individual molecules keep moving.
Osmosis
It is the diffusion of water across a selectively permeable membrane.
Net water movement is toward the side with the higher concentration of solutes that cannot cross the membrane (that is, the side with lower water concentration).
Tonicity
It describes how a solution affects the volume of a cell placed in it.
It depends on the concentration of non-penetrating solutes (solutes that cannot cross the membrane) in the solution relative to the inside of the cell.
Three types of solutions (always described relative to another solution, usually the cell’s interior)
Hypertonic: the solution has a higher concentration of non-penetrating solutes than the cell. Water leaves the cell, so the cell shrinks. A plant cell undergoes plasmolysis.
Hypotonic: the solution has a lower concentration of non-penetrating solutes than the cell. Water enters the cell, so the cell swells and an animal cell may burst (lyse). A plant cell becomes turgid, which is its healthy state.
Isotonic: the solute concentrations are equal. There is no net water movement, and the cell volume stays the same.
How the terms relate
Diffusion is the general process, and osmosis is diffusion of water specifically.
Osmosis is driven by differences in solute concentration, and tonicity is the label for those differences (hypertonic, hypotonic, isotonic) that predicts which way water moves and what happens to the cell.
Compare and contrast the different passive (simple diffusion, facilitated diffusion) and active (primary, secondary) transport mechanisms.
Passive transport
It requires no cellular energy input (no ATP).
Substances move down their concentration (or electrochemical) gradient, from high to low.
Simple diffusion
Molecules pass directly through the phospholipid bilayer without help from proteins.
It works for small nonpolar molecules such as O₂ and CO₂.
The rate increases in direct proportion to the concentration gradient and does not saturate.
Facilitated diffusion
Molecules cross through a membrane transport protein, but still move down their gradient.
Channel proteins form a pore (examples: ion channels and aquaporins for water).
Carrier proteins (uniporters) bind a specific molecule, change shape, and release it on the other side (example: glucose transporter GLUT).
It is specific, and the rate saturates (reaches a maximum) when all transporters are in use.
Active transport
It requires energy and moves substances against their gradient, from low to high concentration.
It always uses carrier proteins (pumps).
Primary active transport
The transporter uses ATP directly. ATP hydrolysis changes the protein’s shape and moves the substance.
Example: the Na⁺/K⁺ ATPase pumps 3 Na⁺ out and 2 K⁺ in per ATP.
Secondary active transport
The transporter uses the energy stored in an electrochemical gradient that was created by primary active transport. It does not use ATP directly.
The gradient-favored ion (often Na⁺) flows down its gradient and drives a second substance against its gradient.
Symport (cotransport): both substances move in the same direction (example: the Na⁺/glucose cotransporter).
Antiport (exchange): the substances move in opposite directions (example: the Na⁺/Ca²⁺ exchanger).
Key comparisons
Passive transport goes down the gradient with no energy, and active transport goes against the gradient with energy.
Simple diffusion needs no protein, while facilitated diffusion and both kinds of active transport need proteins.
Primary active transport uses ATP directly, and secondary active transport uses ATP indirectly, through a gradient.
Predict the permeability of a molecule based on its chemical properties.
Guiding principle
The bilayer’s core is hydrophobic, so molecules that are nonpolar and small cross most easily, and molecules that are charged or large and polar cross poorly.
Factors to evaluate
Polarity or charge
Nonpolar molecules dissolve in the lipid core, so they are more permeable.
Charged molecules (ions) are the least permeable, because they are strongly attracted to water and repelled by the hydrophobic core.
Size
Among molecules of similar polarity, smaller molecules cross more readily than larger ones.
Predicted permeability, from highest to lowest
Small nonpolar molecules (O₂, CO₂, N₂): highly permeable.
Hydrophobic molecules such as steroid hormones and fat-soluble vitamins: highly permeable.
Small uncharged polar molecules (H₂O, ethanol, urea): moderately permeable. Water crosses slowly, which is why cells with high water demand have aquaporins.
Large uncharged polar molecules (glucose, amino acids): poorly permeable.
Ions (Na⁺, K⁺, Cl⁻, H⁺, Ca²⁺): essentially impermeable without a transport protein.
Using this in practice
A molecule that is poorly permeable needs a channel or carrier protein to cross the membrane at a useful rate.
Predict the net movement of water based on the relative tonicity of two solutions.
Core rule
Water moves by osmosis toward the solution with the higher concentration of non-penetrating solutes (the hypertonic side). Equivalently, water moves from the hypotonic side to the hypertonic side.
Step-by-step approach
Identify which solutes cannot cross the membrane.
Compare the concentrations of those solutes on the two sides.
The side with the higher solute concentration is hypertonic, and the other side is hypotonic. If they are equal, the solutions are isotonic.
Predict that water moves toward the hypertonic side. If the solutions are isotonic, there is no net movement.
Predictions for a cell placed in a solution
Cell in a hypertonic solution: water leaves the cell, and the cell shrinks.
Cell in a hypotonic solution: water enters the cell, and the cell swells.
Cell in an isotonic solution: no net movement, and the cell volume is unchanged.
Examples
A red blood cell in pure water swells and may lyse, because the water is hypotonic to the cell.
A red blood cell in concentrated salt water shrivels, because the salt water is hypertonic.
A plant cell in a hypotonic solution becomes turgid, which is the normal healthy state.
Common mistake to avoid
Do not say solute moves in osmosis. The water moves, because the membrane blocks the solute.
Predict the rate and direction of net movement of molecules based on their concentration gradients and the absence or presence of specific membrane transport proteins.
Step-by-step approach
Decide whether the molecule can cross the bilayer on its own (small and nonpolar) or needs a transport protein (ions, large polar molecules).
Check whether the appropriate transport protein is present in the membrane.
Determine the direction of the concentration gradient (high to low).
Check whether energy (ATP or an ion gradient) is available, which would allow movement against the gradient.
Predictions by situation
Small nonpolar molecule, no protein needed
Direction: net movement down its concentration gradient.
Rate: increases in proportion to the steepness of the gradient, with no maximum.
Polar molecule or ion, no transport protein present
Direction and rate: essentially no net movement, even with a steep gradient, because it cannot cross the bilayer.
Polar molecule or ion, specific channel or carrier present (facilitated diffusion)
Direction: net movement down its gradient.
Rate: it increases with the gradient but levels off at a maximum once all transporters are occupied (saturation). It depends on the number of transporters, so more transporters give a higher maximum rate.
A different molecule that the protein does not recognize still cannot cross, because transport proteins are specific.
Active transporter present with an energy source
Direction: net movement can be against the gradient (toward higher concentration).
Rate: it depends on the number of pumps and the available ATP or ion gradient.
Without energy, the pump cannot move the molecule against its gradient.
Equilibrium
Passive movement continues until the concentrations are equal on both sides, and then net movement stops. Active transport can maintain a non-equilibrium state.
Example
Glucose, a large polar molecule, enters a cell down its gradient only if glucose transporters are present. In a cell lacking them, glucose cannot enter even if its outside concentration is high.