Chapter 4: The Structure and Function of the Plasma Membrane

Vocabulary flashcards related to the structure and function of the plasma membrane.

Plasma Membrane

Thin, fragile structure that separates cells from the world. It is only 5-10 nm wide.

J. D. Robertson

Duke University, generated early electron micrographs that portrayed plasma membrane as three-layered structure, consisting of darkly staining inner and outer layers and a lightly staining middle layer.

Compartmentalization

Membranes are continuous, unbroken sheets and the plasma membrane encloses the entire cell, while the nuclear and cytoplasmic membranes enclose diverse intracellular spaces.

Scaffold for biochemical activities

Membranes are a distinct compartment, components embedded in it are no longer floating free and can be ordered for effective interaction.

Selectively permeable barrier

The plasma membrane prevents an unrestricted exchange of molecules from one side to the other, so it controls what gets into and out of cell.

Transporting Solutes

Plasma membrane has transport machinery to move substances from one side to the other, often from a region of low solute concentration to a region of much higher solute concentration.

Responding to external stimuli (signal transduction)

The plasma membrane plays a critical role in the cell's response to external stimuli (e.g. hormones, growth factors, neurotransmitters).

Cell-cell communication

The plasma membrane of multicellular organisms mediates the interactions between a cell and its neighbors, since it is situated at the outer edge of every living cell.

Energy transduction

Membranes are intimately involved in processes by which one type of energy is converted to another type (energy transduction), done mostly by membranes of chloroplasts and mitochondria.

Lipid-protein assemblies

Membranes are lipid-protein assemblies in which the components are held together in a thin sheet by noncovalent bonds; the core of the membrane consists of a sheet of lipids arranged in a bimolecular layer.

Lipid bilayer

The lipid bilayer serves as the structural backbone of the membrane and provides the barrier preventing the random movements of water-soluble materials into and out of cell.

Amphipathic lipids

Most membrane lipids have phosphate groups and are phospholipids (except cholesterol, glycolipids).

Phosphoglycerides

Most membrane phospholipids are built on a glycerol backbone.

Sphingosine

An amino alcohol containing a long hydrocarbon chain, which is a component of sphingolipids.

Cholesterol

A sterol that may constitute up to 50% of animal cell plasma membrane lipids.

Liposomes

Fluid-filled spherical vesicles whose walls are made of a single continuous lipid bilayer organized in same way as natural membrane.

Asymmetry of Membrane Lipids

Lipids are distributed in distinctly different patterns between the two leaflets of the bilayer; some more prevalent in outer leaflet, others on inner leaflet.

Membrane Carbohydrates

Eukaryotic cell plasma membranes also contain carbohydrate.

Glycosylation

The addition of carbohydrate to protein structure.

Integral proteins

Penetrate the lipid bilayer and pass entirely through the bilayer so they are transmembrane proteins.

Peripheral proteins

Located entirely outside of bilayer on either the extracellular or cytoplasmic side, and are associated with the membrane surface by noncovalent bonds.

Lipid-anchored proteins

Found outside the bilayer on either extracellular or cytoplasmic side, but they are covalently linked to membrane lipid that is situated within the bilayer.

Fluidity

A measure of ease of flow.

Viscosity

A measure of resistance to flow.

Transition temperature

The temperature at which membrane goes from fluid state to crystalline gel.

Lipid rafts

Artificial lipid bilayers where cholesterol and sphingolipids tend to self-assemble into microdomains.

Flippases

Enzymes that move certain phospholipids from one leaflet to the other.

Cell fusion

A technique in which 2 different cell types or cells from 2 different species can be fused to produce one cell with a common cytoplasm and a single continuous membrane.

Net flux

Indicates that movement of substance into the cell (influx) and out of the cell (efflux) is not balanced, one exceeds the other.

Osmosis

Water moves readily through a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration.

Hypertonic

The compartment of higher solute concentration relative to the compartment of lower solute concentration.

Hypotonic

The compartment of lower solute concentration relative to the compartment of higher solute concentration.

Isotonic

External and internal fluids are at equal solute concentration, so no net movement of H2O into or out of cells occurs.

Aquaporins

A family of small integral proteins that allows the passive movement of water from one side of the plasma membrane to the other.

Ion channels

Openings in membranes that are permeable to specific ions.

Voltage-gated channels

Their conformational state depends on the difference in ionic charge on the 2 sides of the membrane.

Ligand-gated channels

Their conformational state depends on the binding of specific a molecule (the ligand), which is usually not the solute that passes through the channel.

Mechano-gated channels

Their conformational state depends on mechanical forces (e.g., stretch tension) that are applied to the membrane.

Facilitated Diffusion

Diffusion of a substance that can occur, not through bilayer or channel, but by binding selectively to a membrane-spanning protein, a facilitative transporter, to facilitate the diffusion process.

Insulin

Hormone produced by endocrine cells of the pancreas that plays a key role in maintaining proper blood sugar levels.

Active Transport

Depends on integral membrane proteins that selectively bind a particular solute and move it across the membrane in a process driven by changes in the protein's conformation.

Na+ -K+ pump

Found only in animals and is the primary means to maintain cell volume and establish the steep gradients needed for nerve-muscle impulses.

Secondary active transport

The potential energy stored in ionic gradients utilized by a cell to perform work, including the transport of other solutes.

Symport

The 2 transported species move in the same direction.

Antiport

The 2 transported species move in opposite directions.

Inherited disorder

All disorders affect the movement of ions across the plasma membranes of excitable cells (i.e., muscle, nerve and sensory cells), reducing the ability of these cells to develop or transmit impulses.

Cystic Fibrosis

Results from a defect in epithelial cell ion channels.

Irritability

The ability to respond to external stimulation, a property referred to as irritability.

Nerve cells (neurons)

Specialized for the collection, conduction and transmission of information, which is coded in the form of fast-moving electrical impulses.

Voltage

The voltage, or electric potential difference, between 2 points.

Resting potential

When a nerve or muscle cell is in an unexcited state, the membrane potential is referred to as resting potential, since it is subject to dramatic change.

Potassium equilibrium potential

The membrane potential that would be measured at equilibrium if the nerve cell plasma membrane were permeable only to K+ ions.

Depolarization

Membrane is less polarized.

Action potential

The threshold, a new series of events is launched called an ___ .

Propagation of Action Potentials as an Impulse

AP at one site has an effect on the adjacent site; the large depolarization that accompanies an AP creates a difference in charge along the inner and outer surfaces of the plasma membrane.

Saltatory conduction

The conduction speed along a myelinated axon; is propagation by this mechanism.

Synapses

Specialized junctions between neurons.

Presynaptic cell

Conducts impulses toward a synapse, which it must cross to get to the postsynaptic cell (neuron, muscle, or gland cell).

Postsynaptic cell

Lies on the receiving side of synapse.

Synaptic vesicles

Neurotransmitters are released here.

Excites the cell

Can trigger the opening of cation-selective channels in the postsynaptic cell plasma membrane, which leads primarily to Na+ ion influx and a less '-' (more '+') membrane potential.

Inhibits the cell

Can trigger the opening of anion-selective channels in the postsynaptic cell plasma membrane, which leads mainly to an influx of Cl- ions and a more '-' membrane potential (hyperpolarized).

AChase

An enzyme located in the synaptic cleft where it hydrolyzes acetylcholine (ACh).

Cochlear implant

Implants designed to restore partial hearing to deaf patients by direct stimulation of auditory nerves.