AP Biology 2.7-2.8: Osmosis and Facilitated Diffusion

Selective permeability

The property of a membrane that allows some substances to cross while restricting others. It enables a cell to maintain different internal and external compositions.

Passive transport

Movement of substances across a membrane without cellular energy input, generally from higher concentration to lower concentration along a concentration gradient.

Concentration gradient

A spatial difference in the concentration of a substance. Each substance has its own independent concentration gradient and moves according to that gradient.

Why does passive transport not require ATP?

Random molecular motion causes particles to move down existing concentration or electrochemical gradients. The gradient represents stored potential energy that dissipates as equilibrium is approached.

What is dynamic equilibrium in diffusion?

Dynamic equilibrium occurs when a substance is evenly distributed and there is no net movement between regions, even though individual particles continue moving randomly.

Simple diffusion

The net movement of particles directly through a membrane or medium from higher concentration to lower concentration without ATP or transport proteins.

What happens to the net diffusion rate as a concentration gradient becomes smaller?

The net diffusion rate decreases because the driving force for movement becomes smaller. At equilibrium, there is no net movement.

How do temperature and molecular mass affect diffusion rate?

Increasing temperature generally increases diffusion by increasing average kinetic energy. At the same temperature and in the same medium, lighter particles generally diffuse faster than heavier particles.

How does solvent or cytoplasm viscosity affect diffusion?

A denser or more viscous medium impedes particle motion and decreases the diffusion rate. This is one reason dehydration can impair intracellular transport.

How do membrane surface area and thickness affect diffusion?

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

How does travel distance limit cell size?

As cell size increases, nutrients and wastes must travel farther, slowing diffusion. This favors small or flattened cells unless other transport mechanisms are present.

Why can small nonpolar molecules cross a phospholipid bilayer readily?

The bilayer has a hydrophobic interior, so small nonpolar and lipid-soluble molecules dissolve in and diffuse through it. Examples include $O_2$, $CO_2$, and fat-soluble vitamins.

How does molecular solubility affect diffusion through a membrane?

Nonpolar or lipid-soluble molecules diffuse through the hydrophobic membrane interior more readily than polar molecules, which generally require transport proteins.

Why do ions generally require membrane proteins to cross a phospholipid bilayer?

Although ions are small, their charges and hydration shells are unfavorable in the bilayer's nonpolar interior. Channels or carriers provide a more favorable pathway.

Facilitated diffusion

Passive movement of a polar molecule or ion down its concentration or electrochemical gradient through a membrane protein. It requires no ATP but is limited by the number and properties of transport proteins.

How do facilitated-diffusion proteins overcome the membrane's hydrophobic barrier?

They create a hydrophilic pathway or temporarily bind and shield the transported substance, allowing it to cross without entering the nonpolar lipid core.

Channel protein

A transmembrane protein containing a hydrophilic passageway through which specific ions or polar substances can diffuse down a gradient.

What distinguishes an ion channel from a nonspecific pore?

Ion channels are selective: their structure and charge distribution allow only a particular ion or limited group of ions to pass.

Gated channel

A channel protein that switches between open and closed states in response to a signal such as ligand binding or another regulatory mechanism. Ions diffuse only when the channel is open.

Carrier protein

A transmembrane protein that binds a specific solute and changes conformation to move it across the membrane. In facilitated diffusion, the net movement is down the solute's gradient.

Why does facilitated transport through carrier proteins show saturation?

Cells have a finite number of carrier proteins. When all binding sites are occupied, the transport rate reaches a maximum, so increasing solute concentration further does not increase the rate.

How do channel proteins and carrier proteins differ in transport rate?

Channels generally transport substances much faster because many particles can pass through an open pore. Carriers must bind solute and undergo conformational changes for each transport cycle.

Aquaporin

A membrane channel protein that allows water molecules to cross rapidly and selectively, facilitating osmosis.

Osmosis

The net movement of water through a selectively permeable membrane from higher free-water concentration to lower free-water concentration, often described as movement toward the side with higher effective solute concentration.

Why is osmosis considered a special case of diffusion?

Water molecules undergo random motion and move down their own concentration gradient. The membrane permits water to cross while restricting one or more solutes.

If a membrane allows water but not solute to pass, toward which side does water move?

Water moves toward the side with higher effective solute concentration, or higher osmolarity, because that side has the lower concentration of free water.

Osmolarity

The total concentration of dissolved particles in a solution, accounting for the number of particles produced by each solute. It is commonly expressed in osmoles per liter.

How does the number of dissolved particles affect osmotic behavior?

Osmotic effects depend on the total number of solute particles rather than simply the identity or mass of the solute. For an electrolyte, dissociation increases the number of particles contributing to osmolarity.

Osmotic pressure

The pressure required to stop osmosis. For a dilute solution, it can be estimated with $\pi = iMRT$, where $i$ is the van't Hoff factor, $M$ is molarity, $R$ is the gas constant, and $T$ is absolute temperature.

What happens when osmotic movement continues until hydrostatic pressure balances osmotic pressure?

Water movement reaches a condition in which the pressure opposing further movement balances the osmotic driving force. There is then no further net water transfer.

Water potential

The tendency of water to move from one area to another. Water moves spontaneously from higher water potential to lower water potential.

What determines a solution's water potential?

Water potential is determined primarily by solute potential and pressure potential: $\Psi = \Psi_S + \Psi_P$.

How does solute concentration affect solute potential?

Adding dissolved solute lowers solute potential, making it more negative. A solution with more solute therefore tends to have a lower water potential and attract water.

What is pressure potential?

Pressure potential is the physical pressure exerted on water. Positive pressure, such as turgor pressure in a plant cell, raises water potential and can oppose additional water entry.

What is the water potential of pure water under standard conditions?

Pure water under standard conditions has a water potential of $0$ MPa. Dissolved solutes make water potential negative, while positive pressure can increase it.

How is water potential used to predict osmosis?

Water moves across a selectively permeable membrane from the side with higher water potential to the side with lower water potential until equilibrium is reached or opposing pressure balances the movement.

Tonicity

The ability of an extracellular solution to cause a cell to gain or lose water and change volume. Tonicity depends on solutes that cannot freely cross the membrane and is evaluated relative to the cell interior.

What is the difference between osmolarity and tonicity?

Osmolarity is the total concentration of solute particles. Tonicity predicts the effect on cell volume and depends especially on solutes that cannot cross the membrane.

Hypotonic solution relative to a cell

The extracellular solution has lower effective osmolarity than the cell. Water enters the cell, causing it to swell and potentially lyse.

Hypertonic solution relative to a cell

The extracellular solution has higher effective osmolarity than the cell. Water leaves the cell, causing it to shrink.

Isotonic solution relative to a cell

The extracellular solution has the same effective osmolarity as the cell. Water still crosses in both directions, but there is no net water movement or sustained change in cell volume.

Why can a solution with many dissolved particles be hypertonic even if it appears clear?

Tonicity depends on the number of dissolved particles, not visual cloudiness. A clear solution may contain more solute particles than a cloudy suspension of cells.

A red blood cell is placed in a solution with lower effective solute concentration than its cytoplasm. Predict the result.

The solution is hypotonic, so water enters the cell by osmosis. The cell swells and may lyse if the membrane cannot withstand the resulting volume increase.

A red blood cell is placed in a solution with higher effective solute concentration than its cytoplasm. Predict the result.

The solution is hypertonic, so water leaves the cell. The red blood cell shrivels, or crenates, and its concentrated cytoplasm can impair cellular function.

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

Water enters these cells, but the rigid cell wall resists unlimited expansion and prevents lysis. The resulting turgor pressure can provide structural support.

Turgor pressure

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

Plasmolysis

The separation of a plant cell's plasma membrane from its cell wall after water leaves the cell in a hypertonic environment. Loss of turgor pressure causes wilting.

How does a contractile vacuole help a freshwater protist?

It collects and expels excess water that enters by osmosis from the hypotonic environment, reducing the risk of cell lysis.

Osmoregulation

The control of water and solute balance to maintain suitable osmotic conditions in an organism or cell.

How do freshwater and saltwater fish counter osmotic water movement?

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 excrete salts through their gills and produce concentrated urine.

What role do kidneys and blood proteins play in vertebrate water balance?

Kidneys regulate water and solute excretion, while brain osmoreceptors detect blood solute concentration and promote hormonal responses that conserve or release water. Large blood proteins such as albumin contribute to osmotic pressure because they do not readily cross capillary membranes.