Cell Membrane and Transport
Cell Membrane and Transport
Definition: The cell membrane is a selectively permeable phospholipid bilayer, studded with proteins, that separates a cell’s interior from its environment and controls exactly which substances move across it.
How It Works
- The membrane is built from phospholipids: each has a hydrophilic (water-attracting) phosphate head and two hydrophobic (water-repelling) fatty acid tails.
- In water, phospholipids spontaneously form a bilayer: heads facing outward toward the watery cytoplasm and fluid outside, tails facing each other in a protected interior.
- The fluid mosaic model describes this membrane as fluid, not rigid, with proteins and lipids drifting freely sideways within the layer, like logs floating on a pond.
- Small, uncharged, nonpolar molecules (O2, CO2) slip directly through the lipid bilayer by simple diffusion, no protein required.
- Larger or charged molecules (glucose, ions) cannot cross the hydrophobic interior directly. They pass through channel proteins (a pore) or carrier proteins (which change shape to shuttle the molecule across) — together called facilitated diffusion.
- Osmosis is the diffusion of water specifically, through the bilayer and through dedicated water channels called aquaporins, always moving from an area of higher water concentration to lower.
- Both diffusion and osmosis are passive transport: they move substances down their concentration gradient and require no cellular energy.
- Active transport moves substances against their gradient, low to high concentration, and requires ATP, carried out by pump proteins like the sodium-potassium pump.
- Receptor proteins sit on the membrane surface without spanning it, binding specific signal molecules (hormones) and triggering a response inside the cell without the signal itself ever entering.
- Plant cells add a rigid cell wall outside the membrane; this generates turgor pressure, the membrane pushing outward against the wall when the cell is full of water, which keeps non-woody plant tissue upright.
Illustration
Under the Hood
Direction of osmosis is set by comparing water potential (or solute concentration) across the membrane:
Hypotonic solution: lower solute concentration outside → water moves IN → cell swells (turgid in a walled cell, can burst in an animal cell)
Isotonic solution: equal solute concentration → no net water movement → cell stays the same size
Hypertonic solution: higher solute concentration outside → water moves OUT → cell shrinks (plasmolysis in plants, crenation in animal cells)- “Tonicity” always describes the solution relative to the cell, not an absolute property of the liquid.
- Active transport pumps, like the sodium-potassium pump, move 3 sodium ions out for every 2 potassium ions in, per ATP spent, a ratio that keeps the cell’s interior negatively charged relative to the outside.
- That voltage difference, the resting membrane potential, is what neurons and muscle cells later use to fire electrical signals.
Why It Matters
- Turgor pressure is what holds up a wilting houseplant after watering. It is water re-entering leaf and stem cells by osmosis, not new tissue growth.
- Kidney dialysis relies entirely on diffusion and osmosis across an artificial membrane to filter waste from blood when a patient’s own kidneys cannot.
- IV fluids must be isotonic to blood cells; a hypotonic IV solution would cause red blood cells to swell and burst.
- Salting food draws water out of bacterial and fungal cells by osmosis, which is why salt has preserved meat and fish for thousands of years.
- Root hair cells absorb soil water by osmosis because the cell’s interior is more concentrated with solutes than the surrounding soil water.
- Drug design depends on membrane transport: many medicines are engineered specifically to cross, or specifically to avoid crossing, particular cell membranes (like the blood-brain barrier).
Common Pitfalls
- Calling all membrane crossing “diffusion.” Diffusion is passive and needs no energy; active transport moves substances the opposite direction and always costs ATP.
- Confusing osmosis with diffusion in general. Osmosis refers specifically to water movement; diffusion can describe any substance.
- Assuming a plant cell in a hypotonic solution will burst like an animal cell would. The rigid cell wall resists the pressure, so the cell becomes turgid rather than rupturing.
- Thinking the membrane is a static wall. The fluid mosaic model describes proteins and lipids constantly drifting within the layer.
- Mixing up channel and carrier proteins. A channel is a passive, non-shape-changing pore; a carrier physically changes shape to move its cargo across.
Comparison
| Process | Direction | Needs ATP? | Needs a protein? |
|---|---|---|---|
| Simple diffusion | High to low concentration | No | No |
| Facilitated diffusion | High to low concentration | No | Yes (channel or carrier) |
| Osmosis | High to low water concentration | No | Sometimes (aquaporins speed it up) |
| Active transport | Low to high concentration | Yes | Yes (pump protein) |
FAQ
Why doesn’t water need a channel protein to cross the membrane at all? A small amount does slip directly through gaps between phospholipid tails, but it is slow. Aquaporins provide a dedicated, much faster route, which matters enormously in tissues like the kidney that move huge volumes of water quickly.
Can a substance cross by both passive and active transport at different times? Yes. Glucose, for example, moves into most cells by facilitated diffusion when extracellular levels are high, but the small intestine actively pumps glucose in against its gradient using a sodium-coupled transporter, because it needs to fully absorb it regardless of concentration.
Example
Put a wilted piece of celery in a glass of water: within hours, water moves by osmosis into the stalk’s cells, driving up turgor pressure until the stalk turns crisp again, a visible demonstration of exactly the process keeping every leaf on a healthy plant standing upright.
Related Terms
Referenced by