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Passive transport review

AP.BIO:
ENE‑2 (EU)
,
ENE‑2.E (LO)
,
ENE‑2.E.1 (EK)
,
ENE‑2.E.2 (EK)

Key terms

TermMeaning
Passive transportType of transport that does not require energy to occur
Concentration gradientA region of space over which the concentration of a substance changes
PermeabilityThe quality of a membrane that allows substances to pass through it
EquilibriumThe state at which a substance is equally distributed throughout a space

Types of passive transport

Diffusion

During diffusion, substances move from an area of high concentration to an area of low concentration, until the concentration becomes equal throughout a space.
Image showing the process of diffusion across the plasma membrane. Initially, the concentration of molecules is higher on the outside. There is net movement of molecules from the outside to the inside of the cell until the concentrations are equal on both sides.
Image credit: OpenStax Biology, modified from original work by Mariana Ruiz Villareal.
This is also true for some substances moving into and out of cells. Because the cell membrane is semipermeable, only small, uncharged substances like carbon dioxide and oxygen can easily diffuse across it. Charged ions or large molecules require different kinds of transport.

Facilitated diffusion

Although gases can diffuse easily between the phospholipids of the cell membrane, many polar or charged substances (like chloride) need help from membrane proteins. Membrane proteins can be either channel proteins or carrier proteins.
Even though a concentration gradient may exist for these substances, their charge or polarity prevents them from crossing the hydrophobic center of the cell membrane. Substances transported through facilitated diffusion still move with the concentration gradient, but the transport proteins protect them from the hydrophobic region as they pass through.
Left side: Image of a channel protein, which forms a tunnel allowing a specific molecule to cross the membrane (down its concentration gradient).
Right side: Diagram showing how a carrier protein can bind a target molecule on one side of the membrane, undergo a shape change, and release the target molecule on the other side of the membrane.
Image modified from "Scheme facilitated diffusion in cell membrane," by Mariana Ruiz Villareal (public domain).

Common mistakes and misconceptions

  • Not everything enters the cell through passive transport. Only the smallest molecules like water, carbon dioxide, and oxygen can freely diffuse across cell membranes. Larger molecules or charged molecules often require an input of energy to be transported into the cell.
  • Even when equilibrium is reached, particles do not stop moving across the cell membrane. Although it may seem as if the concentrations are not changing, nearly equal numbers of particles cross the membrane in both directions. This means that there is no net change in the concentration of the substances.

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