Chapter 4.2: Passive and Active Transport Are Used by Small Molecules to Cross Membranes

Flashcards from Chapter 4.2 of Principles of Life, 3rd Edition.

Selective permeability

A membrane characteristic that allows only some substances to pass through

Passive transport

Transport that does not require an input of energy down the concentration gradient

• Can use channel or carrier proteins as well for polar molecules

Active transport

Transport that requires an input of energy up the concentration gradient

• Applied to larger, hydrophilic molecules; this requires proteins in most cases

• Unidirectional, only moving according to specific protein function

Equilibrium

A state of solution concentration across a membrane where the distributions of solute are relatively uniform and equal, moving randomly

• Cells combat this with their membrane

Diffusion

The process of random movement of particles towards equilibrium

• Affected by surface area, temperature, and gradient severity

Simple diffusion

Diffusion without the need for a protein in a cell; this generally applies to small, non-polar molecules

Osmosis

The diffusion of water to places of higher solute concentration to disperse energy; this generally occurs in cell channels called aquaporins

• Specifically changes as a result of nonpermeable solutes that cannot diffuse across a membrane

Osmotic pressure

The pressure needed to prevent the flow of water across a membrane

Osmolarity

The concentration of all solute particles; this is proportional to osmotic pressure

• Affected by solutes that can cross the membrane that diffuse and thus attract more water

Tonicity

A measure of the relative concentration of solutes on either side of a membrane; can be higher (hyper-), lower (hypo-), or the same (iso-) to determine where water will flow

• Important in some cells to avoid bursting or shriveling

• Affected solely by solutes that cannot cross the membrane

Turgor pressure

Pressure against the cell wall that prevents water from entering, seen mostly in plant growth

Facilitated diffusion

Passive transport done with the help of a channel or carrier protein, seen with polar molecules

• Relies on hydrophilic interiors for passage

Ion channels

Protein channels created for the passage of certain ions like Na⁺

• Can aid in the transport of water alongside aquaporins

Gated channel

An ion channel that requires a stimulus, like a chemical signal, electrical difference, or ligand, to operate

Carrier protein

A protein that transports polar molecules by binding with them for passage through to the other side

• Seen with glucose and its transporters to create an energetically favorable passage

Transport saturation

Occurs when all available transporters are in use; this sets a maximum transfer rate compared to simple diffusion

Primary active transport

A type of active transport that requires the use of energy to move substances against their concentration gradient

• Seen with the sodium-potassium pump allowing sodium to be pumped out with ATP and potassium to use dephosphorylation to enter

Secondary active transport

A type of active transport that uses a concentration gradient created by primary active transport to move substances to higher concentrations with the aid of another substance

• Seen with the sodium-glucose co-transporter using the energy of sodium diffusion to move glucose inside