AP Biology 2.5-2.6: Membrane Transport

Selective permeability

The property of a plasma membrane that allows some substances to cross while restricting others. It is essential for maintaining the cell’s internal conditions.

Concentration gradient

A difference in the concentration of a substance between two regions. It represents potential energy that can drive net movement from high concentration toward low concentration.

Passive transport

Movement of substances across a membrane down a concentration gradient without direct energy input from ATP. It includes simple diffusion, facilitated diffusion, and osmosis.

What direction does net passive transport occur relative to a concentration gradient?

Net movement is from higher concentration to lower concentration, down the concentration gradient, until equilibrium is reached.

Diffusion

The net movement of particles from a region of higher concentration to a region of lower concentration due to random molecular motion. It requires no cellular energy.

Why does diffusion not require ATP?

Random thermal motion causes particles to spread down their concentration gradients. The gradient itself stores potential energy that dissipates as the distribution becomes more uniform.

Dynamic equilibrium

A condition in which particles continue moving microscopically, but equal movement in opposite directions produces no net change in concentration.

How does the magnitude of a concentration gradient affect diffusion rate?

A larger concentration difference generally produces a faster net diffusion rate. Diffusion slows as the system approaches equilibrium.

How do temperature and molecular mass affect diffusion?

Higher temperature increases thermal energy and generally increases diffusion rate. At the same temperature, heavier particles typically diffuse more slowly than lighter particles.

How does solvent density affect diffusion?

Diffusion is slower in a denser medium because particles encounter more resistance while moving. A less dense medium generally permits faster diffusion.

How do membrane surface area and thickness affect diffusion rate?

A larger membrane surface area increases the rate of diffusion, whereas a thicker membrane increases the distance particles must cross and decreases the rate.

How does travel distance affect diffusion, and why does this limit cell size?

Diffusion becomes slower as the required distance increases. Large cells may not receive nutrients or remove wastes quickly enough from their centers, so cells tend to be small or relatively thin.

Why does increasing cytoplasmic density inhibit diffusion?

A denser cytoplasm offers greater resistance to molecular movement. For example, dehydration can make the cytoplasm more concentrated and slow intracellular transport.

How does solubility influence diffusion through a lipid bilayer?

Nonpolar or lipid-soluble substances generally diffuse through the hydrophobic bilayer more readily than polar substances.

Why can nonpolar, lipid-soluble molecules cross a plasma membrane readily?

The membrane’s interior is a hydrophobic phospholipid core, so small nonpolar molecules interact favorably with it. Oxygen, carbon dioxide, and fat-soluble vitamins are examples.

Why do ions and many polar molecules require membrane proteins to cross the plasma membrane?

Charged and polar substances interact poorly with the membrane’s hydrophobic interior. Channels or carrier proteins provide a more favorable pathway through the membrane.

Filtration

A passive process in which water and dissolved substances move through a membrane because of hydrostatic pressure.

How does pressure affect filtration in the kidneys?

Blood pressure forces water and dissolved solutes through filtration membranes into renal tubules. In this case, the filtration rate depends strongly on pressure.

Facilitated diffusion

Passive movement of a polar molecule or ion down its concentration gradient through a membrane protein. Because movement is down the gradient, ATP is not directly required.

How do channel proteins facilitate passive transport?

Channel proteins form hydrophilic pathways through the membrane, allowing specific ions or polar substances to avoid the hydrophobic lipid core.

Why are channel proteins selective?

Their pore has chemical and structural properties that favor particular ions or molecules. As a result, only a specific substance or limited subset of substances can pass efficiently.

What distinguishes gated channels from continuously open channels?

Continuously open channels permit passage whenever the appropriate substance is present. Gated channels open or close in response to a signal such as ligand binding or another regulatory mechanism.

How do carrier proteins facilitate diffusion?

A carrier binds a specific substance on one side of the membrane, changes conformation, and releases the substance on the other side. The net movement remains down the substance’s gradient.

How does carrier-protein saturation affect transport rate?

Because a membrane has a finite number of carriers, the rate reaches a maximum when all carriers are occupied. Beyond saturation, increasing the concentration gradient does not further increase the transport rate.

How do channel and carrier proteins compare in transport speed?

Channels generally transport substances much faster, potentially tens of millions of particles per second. Carrier proteins typically move about thousands to a million particles per second because each cycle requires binding and a conformational change.

What is the relationship between glucose transport and carrier saturation in the kidney?

Glucose reabsorption depends on a limited number of carrier proteins. If filtered glucose exceeds carrier capacity, some glucose remains in the filtrate and is excreted in urine.

Osmosis

The net movement of water molecules across a selectively permeable membrane from higher free-water concentration toward lower free-water concentration. If solute cannot cross, water generally moves toward the side with higher solute concentration.

Why is osmosis considered a special case of diffusion?

Both processes involve random molecular motion and net movement down a concentration gradient without ATP. Osmosis specifically describes water crossing a membrane whose permeability restricts solute movement.

Aquaporins

Hydrophilic channel proteins that allow water to cross cell membranes rapidly. They are especially important in red blood cells and kidney tubules.

How are water concentration and solute concentration related in osmosis?

They are inversely related: a solution with more solute particles has a lower proportion of free water. Thus, when solute cannot cross, water moves toward the higher-solute side.

Osmolarity

The total concentration of dissolved particles in a solution. Higher osmolarity means more solute particles and, for comparable solutions, a lower concentration of free water.

How does water move between two solutions of different osmolarity when only water can cross?

Water moves from the side with lower osmolarity and more free water to the side with higher osmolarity and less free water.

What stops osmosis?

Osmosis can continue until the water concentration gradient is eliminated or until hydrostatic pressure balances the osmotic driving force.

Tonicity

The ability of an extracellular solution to cause a cell to gain or lose water and therefore change volume. It depends on the relative effective solute concentrations across a membrane permeable to water but not to the relevant solutes.

What happens to a cell in a hypotonic extracellular solution?

The extracellular fluid has lower osmolarity than the cytoplasm, so water enters the cell. The cell swells and may lyse if the influx is excessive.

What happens to a cell in a hypertonic extracellular solution?

The extracellular fluid has higher osmolarity than the cytoplasm, so water leaves the cell. The cell shrinks and may become functionally impaired.

What happens to a cell in an isotonic extracellular solution?

The extracellular fluid and cytoplasm have equal osmolarity, so there is no net water movement and the cell’s volume remains approximately constant. Water still crosses in both directions.

How can the appearance of a red blood cell reveal whether a solution is isotonic?

A hypotonic solution causes swelling and possible lysis, whereas a hypertonic solution causes shrinking or crenation. An isotonic solution produces no net volume change.

Crenation

The shrinking of a cell, such as a red blood cell, after excessive water leaves it in a hypertonic solution.

Lysis

The rupture or bursting of a cell caused by excessive water entry, commonly when a red blood cell is placed in a strongly hypotonic solution.

Why can excessive water loss impair cell function even before the cell dies?

Water loss concentrates the remaining solutes and makes the cytoplasm denser. This can slow diffusion and disrupt cellular processes.

Osmoregulation

The control of water and solute balance within an organism or cell. It prevents harmful volume changes caused by osmotic water movement.

How do cell walls protect plant, fungal, and bacterial cells in hypotonic environments?

Water enters these cells, but the rigid cell wall limits expansion and prevents the plasma membrane from rupturing. The resulting internal pressure can support the cell.

Turgor pressure

The pressure produced when water enters a plant cell and pushes the plasma membrane and cytoplasm against the cell wall. It helps support nonwoody plants.

Plasmolysis

The separation of the plasma membrane from the cell wall as a plant cell loses water in a hypertonic environment. It causes loss of turgor pressure and contributes to wilting.

How do contractile vacuoles help freshwater protists?

They collect and expel excess water that enters by osmosis in a hypotonic environment, reducing the risk of cell lysis.

How do freshwater and saltwater fish maintain osmotic homeostasis?

Freshwater fish gain water and lose salts, so they take up salts through their gills and excrete dilute urine. Saltwater fish lose water and gain salts, so they secrete salts through their gills and produce concentrated urine.

Active transport

Energy-dependent movement of substances across a membrane, often against a concentration gradient. The energy is usually supplied by ATP hydrolysis or by a gradient established using ATP.

Why must a cell expend energy to move a substance against its concentration gradient?

Movement from low concentration to high concentration is thermodynamically unfavorable because it increases the concentration imbalance. Cellular energy, commonly from ATP, must drive the process.

Electrochemical gradient

The combined effect of a concentration gradient and an electrical gradient on an ion. Ion movement depends on both the difference in ion concentration and the difference in charge across the membrane.

How do typical intracellular and extracellular concentrations of Na$^+$ and K$^+$ differ?

Cells generally have a higher K$^+$ concentration and a lower Na$^+$ concentration than the extracellular fluid. These differences contribute to their electrochemical gradients.

How does the membrane potential affect movement of a cation into a cell with a negatively charged interior?

The electrical gradient attracts the positively charged ion inward. If the concentration gradient also favors entry, both components drive inward movement.

What electrochemical conditions generally favor sodium ion entry into a typical cell?

Sodium concentration is usually higher outside the cell, favoring diffusion inward, and the cell interior is negative relative to the exterior, electrically attracting Na$^+$. Both gradients therefore favor inward movement.

Why can an ion’s concentration gradient and electrical gradient oppose each other?

The concentration difference may favor movement in one direction while charge separation favors the opposite direction. The ion’s net movement depends on the combined electrochemical gradient.

Primary active transport

Transport that uses energy directly from ATP hydrolysis to move substances across a membrane, usually against their concentration or electrochemical gradients.

Secondary active transport

Transport that uses the energy stored in an electrochemical gradient, rather than ATP hydrolysis directly, to move another substance against its gradient. The gradient is typically established by primary active transport.

How does the sodium-potassium pump maintain ion gradients?

Using ATP, the pump moves three Na$^+$ ions out of the cell and two K$^+$ ions into the cell per cycle. This maintains high intracellular K$^+$, low intracellular Na$^+$, and contributes to the negative membrane potential.

What is cotransport?

The coupled movement of two substances through one membrane protein. The downhill movement of one substance supplies energy for the uphill movement of the other.

How do symport and antiport differ?

A symporter moves two substances in the same direction, whereas an antiporter moves them in opposite directions.

Why is the sodium-glucose cotransporter an example of secondary active transport?

It uses the downhill movement of Na$^+$ into the cell, driven by the Na$^+$ gradient, to transport glucose into the cell against glucose’s concentration gradient.