Chapter 5 Membrane Transport and Cell Signaling

Plasma membrane

The boundary that separates the living cell from its surroundings

Selective permeability

Allowing some substances to cross the plasma membrane more easily than others

Amphipathic

A molecule that contains both a hydrophobic and hydrophilic region

Fluid mosaic model

The plasma membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids

Integral proteins

Proteins that penetrate the hydrophobic interior of the lipid bilayer; most are transmembrane, which span the membrane with ends that protrude out either side

Peripheral proteins

Proteins that are not embedded in the lipid bilayer at all but are loosely bound to the surface of the membrane, often to exposed parts of integral proteins

Glycolipids

Carbohydrates covalently bonded to lipids

Glycoproteins

Carbohydrates covalently bonded to proteins

Polar

A molecule which utilizes polar covalent bonds, leaving areas with partial charges

Nonpolar

A molecule which utilizes nonpolar covalent bonds, leaving a neutral charge over the entire molecule

Hydrophobic

A molecule that is 'water-fearing' due to its nonpolar covalent bonds, which are repelled from the polarity of water

Hydrophilic

A molecule that is 'water-loving' due to its polar covalent bonds, which are attracted to the polarity of water

Transport proteins

A transmembrane protein that helps a certain substance or class of closely related substances to cross the membrane; carrier and channel proteins are types of transport proteins

Aquaporins

A type of channel protein that facilitates the passage of water molecules across the plasma membrane

Diffusion

The movement of particles of any substance from areas of high concentration to areas of low concentration

Concentration gradient

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

passive transport

The diffusion of a substance across a biological membrane

Osmosis

The diffusion of free water across a selectively permeable membrane, whether artificial or celluar

Tonicity

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

Isotonic

no net movement of water across the plasma membrane

Hypertonic

A solution that has more solute particles than the inside of the cell; water will move out of the cell, causing it to shrivel (crenate)

Hypotonic

A solution that has less solute particles than the inside of the cell; water will move into the cell, possibly causing it to burst (lyse)

Osmoregulation

The control of solute concentrations and water balance of cells

Turgid

Very firm; the healthy state of most plant cells

Flaccid

Limp, not firm; lacking vigor or effectiveness

Plasmolysis

When a plant cell is placed into a hypertonic environment, the cell will shrivel and its plasma membrane pulls away from the cell wall at multiple places

Facilitated diffusion

Diffusion across the plasma membrane with the aid of transport proteins

Gated channels

Transport proteins (usually of ions) that open or close in response to a stimulus, such as an electrical change

Ion channel

A transmembrane protein channel that allows a specific ion to diffuse across the membrane down its concentration or electrochemical gradient.

Active transport

Transport of a solute across the plasma membrane against its concentration gradient, which requires energy

sodium-potassium pump

A type of carrier protein that exchanges sodium for potassium across the plasma membrane, moving both against their concentration gradients and thus requiring energy

Membrane potential

The voltage difference across a plasma membrane. A negative voltage indicates that the inside of the cell is negatively charged relative to the outside

Electrochemical gradient

The diffusion gradient of an ion, which is affected by both the concentration difference of an ion across a membrane (a chemical force) and the ion's tendency to move relative to the membrane potential (an electrical force)

Electrogenic pump

An active transport protein that generates voltage across a membrane while pumping ions.

Proton pump

A protein that actively transports protons

Cotransport

The coupling of the downhill' diffusion of one substance to the 'uphill' transport of another against its own concentration gradient

Exocytosis

The cellular secretion of certain biological molecules by the fusion of vesicles with the plasma membrane

Endocytosis

When a cell takes in molecules and particulate matter by forming new vesicles from the plasma membrane

Reception

The target cell's detection of a signal molecule coming from outside the cell.

Signal transduction pathway

A step or series of steps that converts the signal to a form that can bring about a specific cellular response

Signal transduction

The conversion of a signal into a cellular response.

Response

In cellular communication, the change in a specific cellular activity brought about by a transduced signal from outside the cell

Ligand

A molecule that specifically binds to another molecule, often a larger one

G-protein coupled receptor (GPCR)

A signal receptor protein in the plasma membrane that responds to the binding of a signaling molecule by activating a G protein

Ligand-gated ion channel

A membrane receptor that has a region that can act as a 'gate' for ions when the receptor assumes a certain shape

G protein

A GTP-binding protein that relays signals from a plasma membrane signal receptor, known as a G protein-coupled receptor, to other signal transduction proteins inside the cell.

Second messengers

In many signaling pathways, small, nonprotein, water-soluble molecules or ions that are activated by a first messenger binding to the plasma membrane receptor

cyclic AMP (cAMP)

Cyclic adenosine monophosphate, a ring-shaped molecule made from ATP that is a common intracellular signaling molecule (second messenger) in eukaryotic ells.