Membrane Structure and Transport
General Biology · Section 1.4 · 18 study cards
The fluid mosaic model, membrane fluidity and proteins, then diffusion, osmosis, tonicity, active transport and bulk transport.
Practice this set → Spaced repetition, card by card. No account needed.
Method
Ask two questions of any transport process
- Which way is it going relative to the concentration gradient? Down the gradient means passive; against it means active and therefore energy-requiring.
- Is a protein involved? No protein and down the gradient is simple diffusion. Protein and down the gradient is facilitated diffusion. Protein and against the gradient is active transport.
Those two questions classify every case in this unit, including osmosis through aquaporins and the sodium-potassium pump. Answer them in that order before writing anything else.
Predict membrane crossing from chemistry
The bilayer core is hydrocarbon, so it is a hydrophobic filter. Rank a molecule by charge first, then polarity, then size. Ions never cross unaided, however small. Small nonpolar molecules cross freely, however fast the question makes them sound. Water is the awkward middle case: it crosses slowly on its own and quickly through aquaporins.
Solve tonicity problems in a fixed order
- Identify which side is the cell and which the solution, since tonicity is always stated relative to the cell.
- Compare non-penetrating solute concentrations, not total solute.
- Move water towards the higher solute concentration.
- State the consequence, and say whether the cell has a wall: swelling and lysis for an animal cell, turgor for a plant cell.
Trace the energy in active transport
For primary active transport, point to the ATP. For secondary active transport, point to the ion gradient and then point to the pump that built it. A complete answer names both, because the examiner is checking that you know secondary transport is indirectly ATP-dependent rather than free.
Definitions and theorems
Worked example
A red blood cell with an internal solute concentration equivalent to 0.9 percent NaCl is placed in each of three beakers: (A) distilled water, (B) 0.9 percent NaCl, (C) 3 percent NaCl. Predict the direction of net water movement and the fate of the cell in each, then explain why a plant cell in beaker A would behave differently.
Identify the reference: tonicity is stated relative to the cell, whose internal concentration is equivalent to 0.9 percent NaCl.
Beaker A is distilled water, so it has essentially no solute and is hypotonic to the cell.
Beaker B matches the cell and is isotonic; beaker C at 3 percent has more solute than the cell and is hypertonic.
Apply the rule that water moves towards the higher solute concentration. In A water enters the cell, in B there is no net movement, and in C water leaves the cell.
Fates: in A the cell swells and lyses, because an animal cell has no wall to oppose the inward pressure. In B the volume is stable. In C the cell shrinks and crenates.
A plant cell in beaker A also takes in water, but the rigid cellulose cell wall resists expansion and builds up turgor pressure.
That turgor pressure raises the water potential inside until it equals the outside, so net water entry stops before the cell can burst. The plant cell becomes turgid, which is its healthy state.
Common mistakes
- Stating osmosis direction the wrong way round. The two correct phrasings are opposite in wording: water moves towards higher solute concentration, which is the same as towards lower water concentration. Pick one phrasing, write it out fully, and never mix the two halves.
- Assuming that anything using a protein is active transport. Facilitated diffusion uses channel and carrier proteins and is entirely passive. The only test that matters is the direction relative to the gradient, so check that before you mention ATP.
- Thinking secondary active transport needs no energy. It uses no ATP directly, but it spends an ion gradient that ATP built. Stop the sodium-potassium pump and secondary glucose uptake stops shortly after. Describe it as indirectly ATP-dependent.
- Giving tonicity without a reference point. Writing that a solution is hypertonic is meaningless on its own. Always write hypertonic to the cell or to the surrounding fluid, because the same solution can be either depending on the comparison.
- Saying cholesterol makes membranes more fluid, or less. It does both, depending on the temperature: it reduces fluidity when warm and prevents solidification when cold. A one-directional answer is marked wrong even though it is half right.
Practice it
Reading the method is not the same as being able to recall it under pressure. This set drills 18 cards one at a time and schedules each card separately, so the ones you keep missing come back sooner.