4.2 Cell Membranes
Key Takeaways
The fluid-mosaic membrane is a phospholipid bilayer with proteins; animal membranes contain cholesterol, and glycoproteins and glycolipids serve as recognition tags.
Phospholipids are amphipathic, with hydrophilic heads toward water and hydrophobic tails inward; small nonpolar molecules such as O2 and CO2 cross more readily than ions and large polar molecules, while water crosses faster through aquaporins.
Simple diffusion and facilitated diffusion move substances down a gradient and do not use ATP; osmosis is the diffusion of water toward the hypertonic side.
From the cell's point of view, an isotonic solution matches the cytosol, a hypotonic solution has lower solute outside, and a hypertonic solution has higher solute outside; 0.9% saline that matches the cytosol is isotonic, and distilled water around an animal cell is hypotonic.
Active transport spends energy to move solute against a gradient; the sodium-potassium pump moves 3 Na+ out and 2 K+ in per ATP, and bulk movement uses exocytosis and endocytosis.
4.2 Cell Membranes
The plasma membrane follows the fluid-mosaic model: a phospholipid bilayer with proteins embedded in it or attached to it. Lipids form a flexible two-layered sheet, and proteins form a mosaic of different machines in that sheet. The membrane is a selective barrier. Animal membranes also contain cholesterol. Carbohydrate tags on the outer face help cells recognize one another.
A bilayer of amphipathic phospholipids
Heads toward water, tails inward
Phospholipids are amphipathic: each has a hydrophilic head that associates with water and hydrophobic fatty-acid tails that avoid water. In the bilayer, heads face the watery extracellular fluid and the watery cytosol. Tails point inward, away from both water phases, and form a hydrophobic core. That core is why the membrane can exist as a stable sheet in a wet cell.
Cholesterol and recognition tags
Proteins supply the mosaic. Integral proteins span the bilayer or sit embedded in it. Channels, carriers, and pumps are integral proteins. Peripheral proteins attach to the membrane surface or to other membrane proteins and do not cross the hydrophobic core. In animal cells, cholesterol sits among the phospholipids and modulates fluidity, limiting stiffness in the cold and excess fluidity when warm. Glycoproteins and glycolipids carry short carbohydrate chains, mostly on the extracellular face. Those chains are recognition tags. Cells use them to identify neighbors, and blood-group markers on red blood cells are carbohydrate patterns of this kind.
What can cross, and what needs help
Selective permeability
Selective permeability means the bilayer lets some substances through more readily than others. Small nonpolar molecules, and small uncharged molecules such as O2 and CO2, cross more readily than ions and large polar molecules such as glucose. Water is a special case. It crosses the bilayer itself slowly and crosses much faster through aquaporins, protein channels reserved for water.
Passive transport
Passive transport moves a substance down its concentration gradient and does not spend ATP. Simple diffusion is passive movement straight through the bilayer, the route O2 and CO2 use. Facilitated diffusion is also passive, but the substance passes through a channel or carrier protein. Glucose entering a red blood cell through a carrier, down its gradient, is facilitated diffusion. The protein provides a path. The solute still moves downhill, and that step does not hydrolyze ATP.
Osmosis and the three tonicity words
Osmosis is the diffusion of water across a selectively permeable membrane toward the side with the higher solute concentration, the hypertonic side. Tonicity is judged from the cell's point of view. An isotonic solution matches the solute concentration of the cytosol, so net water movement is about zero. A hypotonic solution has a lower solute concentration than the cytosol, so water enters the cell. A hypertonic solution has a higher solute concentration than the cytosol, so water leaves the cell.
A numeric case: 0.9% saline and distilled water
Consider a mammalian cell whose cytosol matches 0.9% saline, about 0.9 grams of NaCl in 100 milliliters of water. Bathed in that same 0.9% saline, the cell is in an isotonic solution. Cell volume stays roughly steady because water enters and leaves at similar rates. Place that animal cell in distilled water, which has essentially no dissolved solute. The bath is hypotonic. Water enters by osmosis, the cell swells, and it can lyse, or burst. A plant cell in the same distilled water also takes up water, but it becomes turgid. The cell wall presses back and prevents bursting. In a hypertonic bath the directions reverse. An animal cell shrinks and crenates. A plant cell undergoes plasmolysis: the plasma membrane pulls away from the wall as the vacuole loses water. In an isotonic bath a plant cell is flaccid, not firmly turgid, because turgor needs water pressing against the wall.
Against the gradient, and in bulk
The sodium-potassium pump
Active transport moves a solute against its gradient and requires energy. The sodium-potassium pump is the standard example. In one cycle, it hydrolyzes one ATP and moves 3 Na+ out and 2 K+ in. Both ions move uphill: sodium is already higher outside the cell, and potassium is already higher inside. The pump maintains those gradients. Nerve and muscle cells depend on them later. The membrane fact to store is the count and the direction: three sodium ions leave, two potassium ions enter, and ATP pays for the cycle.
Exocytosis and endocytosis
Bulk transport moves large amounts of material in vesicles and uses cellular energy. Exocytosis sends material out. A vesicle, often produced through the Golgi, fuses with the plasma membrane and releases its contents. Secreted proteins follow that path: made on the rough ER, processed in the Golgi, and released by exocytosis. Endocytosis brings material in by folding the membrane inward. Phagocytosis is cell eating, the uptake of large particles such as bacteria by a white blood cell. Pinocytosis is cell drinking, the uptake of droplets of extracellular fluid. Receptor-mediated endocytosis binds a specific cargo, such as LDL particles that carry cholesterol, and brings that cargo in.
| Process | Gradient | ATP at this step | What moves |
|---|---|---|---|
| Simple diffusion | Down | No | Small nonpolar molecules such as O2 and CO2 |
| Facilitated diffusion | Down | No | Polar molecules or ions through a protein |
| Osmosis | Water toward higher solute | No | Water |
| Active transport | Against | Yes | Solutes such as Na+ and K+ |
| Exocytosis | Vesicle fusion | Energy is required | Secreted proteins and other bulk cargo |
Warning
Osmosis is the diffusion of water toward the hypertonic side. It does not pump solute. Facilitated diffusion uses a protein, yet the solute still moves down its gradient and the cell does not spend ATP on that step. ATP spent to move solute uphill is active transport.
One cycle of the sodium-potassium pump uses one ATP. Which ion movement does that cycle produce?
Two sodium ions move out and three potassium ions move in, and no ATP is used
Equal numbers of sodium and potassium ions diffuse through the bilayer with no protein
Three sodium ions move out and two potassium ions move in, against their gradients
Three potassium ions move out and two sodium ions move in, both down their gradients
A mammalian cell whose cytosol matches 0.9% saline is placed in 0.9% saline. Which statement is correct?
Distilled water would be hypertonic to this cell and would crenate it by moving solute
Facilitated diffusion spends ATP to move sodium against its gradient through the sodium-potassium pump
The bath is hypotonic, and osmosis pumps sodium out until the concentrations match
The bath is isotonic to the cell, so osmosis produces no net gain or loss of water
An animal cell and a plant cell are placed in distilled water, which is hypotonic to both cytosols. What should happen?
The animal cell swells behind a cellulose wall, and the plant cell has no wall to resist the inflow
Both cells lose water, so the animal cell crenates and the plant cell undergoes plasmolysis
Both cells spend ATP on facilitated diffusion to pump the extra water back out
The animal cell may lyse, while the plant cell becomes turgid and the wall prevents bursting
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