Concept 7.3: Passive transport is diffusion of a substance across a membrane with no energy investment

Flashcards from Concept 7.3 of Pearson's Campbell Biology, Twelfth Edition.

Diffusion

The movement of particles of any substance so that they spread out evenly into the available space

• Generally directional until equilibrium is reached

Dynamic equilibrium

State of diffusion where as many molecules cross the membrane in one direction as in the other

Concentration gradient

The region along which the density of a chemical substance increases or decreases

• Substances diffuse down these, unaffected by the concentrations of other substances

Passive transport

Transport of substances that requires no cellular energy expidenture

• Potential energy represented by concentration gradient

• Rate of diffusion depends on membrane permeability

Osmosis

The diffusion of free water across a selectively permeable membrane

• Water molecules diffuse to regions of lower to higher solute concentration until equilibrium is reached

Tonicity

The ability of a surrounding solution to cause a cell to gain or lose water

• Depends on the concentration of solutes that cannot cross the membrane relative to the inside of the cell

• Water will be attracted to higher concentrations of solute, which can lead to water going in or out of each cell

Extreme hypo- or hypertonicity can cause problems for cells without walls

Isotonic

Solution where solute concnetration in a solution is the same as that inside the cell

• No net water movement is done across the membrane

• Volume remains stable in this solution

Hypertonic

Solution where the solute concentration is greater than that inside the cell

• Net water movement goes outside of the cell to the solution

• Cells may shrivel and die as they lose water

Hypotonic

Solution where the solute concentration is less than that inside the cell

• Net water movement goes to the inside of the cell from the solution

• Cells may swell and lyse (burst) as they gain water

Turgid

Term for a normal plant cell in a hypotonic environment that contains more water

• These cells swell until the inelastic wall exert a pressure on the cell, signaling a firm and healthy state

Flaccid

Term for a plant cell in an isotonic environment with less water

• No net movement of water occurs, and plant cells will become limp causing wilting

Plasmolyzed

Term for a plant cell in a hypertonic environment with low levels of water

• Water moves out of the plant cell, causing the membrane to pull away from the cell wall causing potential death

Lysed

Term for an animal cell in a hypotonic environment with too much water

Shriveled

Term for an animal cell in a hypertonic environment with too little water

Osmoregulation

The control of solute concentration and water balance

• Paramecium have a contractile vacuole to pump excess water out of the cell due to their hypotonic environment

• Bacteria and archea in hypersaline environments have mechanisms to retain water

Facilitated diffusion

Diffusion where transport proteins speed up the passive movement of molecules across the plasma membrane

• Does not require energy

• Includes transport (channel, carrier) proteins

Channel proteins

Transport proteins that provide a corridor for a specific molecule or ion to cross the membrane

Aquaporin

Channel protein that facilitates the diffusion of water

Ion channel

Channel protein that facilitates the transport of ions

Gated channels

Ion channels that open or close in response to a stimulus

• Potassium ion channels open in response to electrical stimulus in nerve cells

• Others open in response to chemical stimulus, or the binding of a specific substance to the protein

Carrier proteins

Proteins that undergo a substle shape change that moves the solute-binding site across the membrane

• Can be triggered by the binding and release of the transported molecule

• Substances are moved down the concentration gradient with no energy input required