Summarizing Passive Transport

Summary of Passive Transport

The passive forms of transport, diffusion and osmosis, move materials of small molecular weight across membranes. Substances diffuse from areas of high concentration to areas of lower concentration, and this process continues until the substance is evenly distributed in a system. In solutions containing more than one substance, each type of molecule diffuses according to its own concentration gradient, independent of the diffusion of other substances. Many factors can affect the rate of diffusion, including concentration gradient, size of the particles that are diffusing, temperature of the system, and so on.

In living systems, diffusion of substances into and out of cells is mediated by the plasma membrane. Some materials diffuse readily through the membrane, but others are hindered, and their passage is made possible by specialized proteins, such as channels and transporters. The chemistry of living things occurs in aqueous solutions, and balancing the concentrations of those solutions is an ongoing problem. In living systems, diffusion of some substances would be slow or difficult without membrane proteins that facilitate transport.

Glossary of Words

Aquaporin

channel protein that allows water through the membrane at a very high rate

Carrier protein

membrane protein that moves a substance across the plasma membrane by changing its own shape

Channel protein

membrane protein that allows a substance to pass through its hollow core across the plasma membrane

Concentration gradient

area of high concentration adjacent to an area of low concentration

Diffusion

passive process of transport of low-molecular weight material according to its concentration gradient

Facilitated transport

process by which material moves down a concentration gradient (from high to low concentration) using integral membrane proteins

Hypertonic

situation in which extracellular fluid has a higher osmolarity than the fluid inside the cell, resulting in water moving out of the cell

Hypotonic

situation in which extracellular fluid has a lower osmolarity than the fluid inside the cell, resulting in water moving into the cell

Isotonic

situation in which the extracellular fluid has the same osmolarity as the fluid inside the cell, resulting in no net movement of water into or out of the cell

Osmolarity

total amount of substances dissolved in a specific amount of solution

Osmosis

transport of water through a semipermeable membrane according to the concentration gradient of water across the membrane that results from the presence of solute that cannot pass through the membrane

Passive transport

method of transporting material through a membrane that does not require energy

Plasmolysis

detaching of the cell membrane from the cell wall and constriction of the cell membrane when a plant cell is in a hypertonic solution

Selectively permeable

characteristic of a membrane that allows some substances through but not others

Solute

substance dissolved in a liquid to form a solution

Tonicity

amount of solute in a solution

Transport protein

membrane protein that facilitates passage of a substance across a membrane by binding it