Movement of Materials Across Membranes

Background Information

Most cells exist in liquid, making it easy for materials to move into and out of the cell. Every cell membrane is selectively permeable, meaning that some substances but not others can cross them. Particles have a tendency to move from a region where they are more numerous to a region where they are less numerous.

A Concentration Gradient is… …a difference in the number of particles of a substance in two neighboring regions. Particles tend to move down their concentration gradients (higher to lower concentration).

Particles of a substance tend to go from areas of higher concentration to areas of lower concentration.

A. True B. False

It's not simply a case of movement in one direction. Particles move in both directions, but because one side has more particles to start with, they are more likely to move in one direction than the other. When both sides have the same number of particles, they are just as likely to move one way as the other way, so there will be no change in amounts - both sides will stay equal.

Diffusion is… …the process by which particles of a substance move from areas of higher concentration to areas of lower concentration. …the driving force behind the movement of materials across the plasma membrane.

A. A barrier separates two different substances. B. The barrier is removed, and the substances move down

their concentration gradients. C. Equilibrium is reached.

Osmosis is… …the diffusion of water molecules through a selectively permeable membrane from an area of greater concentration of water to an area of lesser concentration of water.

Based on the concept of osmosis, what would you expect to happen in this setup?

A. No movement of water will occur. B. Water will move in both directions equally. C. The net movement of water will be from side B to side A. D. The net movement of water will be from side A to side B.

Osmosis Terminology When comparing two solutions…

…solutions with higher concentrations of solute are hypertonic

(hyper means "above").

…solutions with lower concentrations of solute are hypotonic

(hypo means “below”).

…solutions in which the concentrations are equal are isotonic

(isos means “equal”).

Which of the following correctly labels this setup?

A. left: isotonic / right: isotonic B. left: isotonic / right: hypertonic C. left: hypertonic / right: hypotonic D. left: hypotonic / right: hypertonic

Imagine that the membrane shown below is impermeable to the solute molecules but permeable to water. Which way will water flow?

A. left to right B. right to left

Normally, in the process of osmosis, the net flow of water molecules into or out of the cell depends upon differences in the

A. rate of movement of molecules inside the cell

B. concentration of water molecules inside and outside the cell

C. rate of molecular motion on either side of the cell membrane

D. concentration of enzymes on either side of the cell membrane

Facilitated Diffusion Diffusion of a substance through a transport protein embedded in a membrane. In facilitated diffusion, solute particles pass through a channel in a transport protein.

Facilitated Diffusion

Outside Cell

Inside Cell

Transport Protein

Higher Concentration of Molecules

Lower Concentration of Molecules

Passive Transport Diffusion, osmosis, and facilitated diffusion are all forms of passive transport across the plasma membrane because energy is not required for them to happen. Like riding a bike down a hill, no energy is needed.

Active Transport …involves a substance being forced through a transport protein in the plasma membrane against the concentration gradient (from lower to higher concentration). Like riding a bike up a hill, an energy boost is required.

Active Transport

Outside Cell

Inside Cell

Transport Protein

Higher Concentration of Molecules

ENERGY

Lower Concentration of Molecules

Molecule to be

carried

Active Transport

Outside Cell

Inside Cell

Molecule is transported to the other side of the membrane

Which of the following types of transport across membranes was compared to riding a bike up a hill?

A. diffusion B. osmosis C. active transport D. facilitated diffusion

Like an enzyme, a transport protein recognizes a specific solute, molecule or ion. During active transport, the protein uses energy, usually moving the solute in a direction from lesser concentration to greater concentration.

Endocytosis and Exocytosis …the processes of taking material into (endocytosis) or out of (exocytosis) the cell by means of pockets of the cell membrane.

What process is shown here? A. osmosis B. exocytosis C. endocytosis D. facilitated diffusion

Osmosis Demo Procedure: The water surrounding an Elodea leaf will be replaced with salt water. Before the demo, write a paragraph describing what you expect to observe. After you have observed osmosis, write a paragraph describing what you observed and what caused it to happen. Correctly use the words equilibrium, hypertonic, hypotonic, and isotonic in your description.