Cell Biology Exam 2

Chapters 12 and 15

Action Potential

Traveling wave of electrical excitation caused by rapid, transient-self-propagating depolarization of the plasma membrane in a neuron or other excitable cell.

*Also called a nerve impulse.

Active Transport

The movement of a solute across a membrane against its electrochemical gradient; requires an input of energy, such as that provided by ATP hydrolysis.

Antiport

Type of coupled transporter that transfers two different ions or small molecules across a membrane in opposite directions, either simultaneously or in sequence.

Aquaporin

Channel that facilitates the transport of water, but not ions, across cell membranes; those found in the plasma membrane greatly increase a cell’s permeability to water.

Axon

Long, thin extension that conducts electrical signals away from a nerve cell body toward remote target cells.

Ca²⁺ Pump

An active transporter that uses energy supplied by ATP hydrolysis to actively expel Ca²⁺ from the cell cytosol.

*Also called Ca²⁺ ATPase.

Channel

A protein that forms a hydrophilic pore across a membrane, through which selected small molecules or ions can passively diffuse.

Dendrite

Short, branching structure that extends from the surface of a nerve cell and receives signals from other neurons.

Depolarization

A shift in the membrane potential, making it less negative on the inside of the cell.

Electrochemical Gradient

Driving force that determines which way an ion will move across a membrane; consists of the combined influence of the ion’s concentration gradient and the membrane potential.

Gradient-Driven Pump

A protein that uses energy stored in the electrochemical gradient of ions to actively transport a solute across a membrane.

H⁺ Pump

A protein or protein complex that uses energy supplied by ATP hydrolysis, an ion gradient, or light to actively move protons across a membrane.

*Also called H⁺ ATPase.

Ion Channel

Transmembrane protein that forms a pore across the lipid bilayer through which specific inorganic ions can diffuse down their electrochemical gradients.

K⁺ Leak Channel

Ion channel permeable to K⁺ that randomly flickers between an open and closed state; largely responsible for the resting membrane potential in animal cells.

Ligand-Gated Channel

An ion channel that is stimulated to open by the binding of a small molecule such as a neurotransmitter.

Mechanically Gated Channel

An ion channel that allows the passage of select ions across a membrane in response to a physical perturbation.

Membrane Potential

Voltage difference across a membrane due to a slight excess of positive ions on one side and of negative ions on the other.

Membrane Transport Protein

Any transmembrane protein that provides a passageway for the movement of select substances across a cell membrane.

Na⁺ Pump

Transported found in the plasma membrane of most animal cells that actively pumps Na⁺ out of the cell and K⁺ in using the energy derived from ATP hydrolysis.

*Also called Na⁺-K⁺ ATPase.

Nernst Equation

An equation that relates the concentrations of an inorganic ion on the two sides of a permeable membrane to the membrane potential at which there would be no net movement of the ion across the membrane.

Neuron

An electrically excitable cell that integrates and transmits information as part of the nervous system; a nerve cell.

Neurotransmitter

Small signaling molecule secreted by a nerve cell at a synapse to transmit information to a postsynaptic cell. Examples include acetylcholine, glutamate, GABA, and glycine.

Osmosis

Passive movement of water across a cell membrane from a region where the concentration of water is high (because the concentration of solutes is low) to a region where the concentration of water is low (and the concentration of solutes is high).

Passive Transport

The spontaneous movement of a solute down its concentration gradient across a cell membrane via a membrane transport protein, such as a channel or a transporter.

Patch-Clamp Recording

Technique used to monitor the activity of ion channels in a membrane; involves the formation of a tight seal between the tip of a glass electrode and a small region of cell membrane.

Pump

Transporter that uses a source of energy, such as ATP hydrolysis or sunlight, to actively move a solute across a membrane against its electrochemical gradient.

Resting Membrane Potential

Voltage different across the plasma membrane when a cell is not stimulated.

Selectivity Filter

Part of an ion channel that determines which ions the channel can transport; located in the region where the channel is narrowest.

Symport

A transporter that transfers two different solutes across a cell membrane in the same direction.

*Example: Glucose-Na⁺ Symport allows glucose to be “dragged” into the cell by the more frequent import of Na⁺ using this symport.

Synapse

Specialized junction where a nerve cell communicates with another cell (such as a nerve cell, muscle cell, or gland cell), usually via a neurotransmitter secreted by the nerve cell.

Transmitter-Gated Ion Channel

Transmembrane receptor protein or protein complex that opens in response to the binding of a neurotransmitter, allowing the passage of a specific inorganic ion; its activation can trigger an action potential in a postsynaptic cell.

Transporter

Membrane transport protein that moves a solute across a cell membrane by undergoing a series of conformational changes.

Turgor Pressure

Force that builds as water flows into plant and yeast cells by osmosis; drives the expansion of cells that underlies plant growth and maintains the rigidity of plant stems and leaves.

Voltage-Gated Channel

Channel protein that permits the passage of selected ions, such as Na⁺, across a membrane in response to changes in the membrane potential. Found primarily in electrically excitable cells such as nerve and muscle cells.

Voltage-Gated Na⁺ Channel

Protein in the plasma membrane of electrically excitable cells that opens in response to membrane depolarization, allowing Na⁺ to enter the cell. It is responsible for action potentials in these cells.

Autophagy

Mechanism by which a cell “eats itself,” digesting molecules and organelles that are damaged or obsolete.

Clathrin

Protein that makes up the coat of a type of transport vesicle that buds from either the Golgi apparatus (on the outward secretory pathway) or from the plasma membrane (on the inward endocytic pathway).

Coated Vesicle

Small membrane-enclosed sac that wears a distinctive layer of proteins on its cytosolic surface. It is formed by punching-off of a protein-coated region of cell membrane.

Endocytosis

Process by which cells take in materials through an invagination of the plasma membrane, which surrounds the ingested material in a membrane-enclosed vesicle.

Endomembrane System

Interconnected network of membrane-enclosed organelles in a eukaryotic cell; includes the endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, and endosomes.

Endoplasmic Reticulum (ER)

Labyrinthine membrane-enclosed compartment in the cytoplasm of eukaryotic cells where lipids and proteins are made.

Endosome

Membrane-enclosed compartment of a eukaryotic cell through which material ingested by endocytosis passes on its way to lysosomes.

Exocytosis

Process by which most molecules are secreted from a eukaryotic cell. These molecules are packaged in membrane-enclosed vesicles that fuse with the plasma membrane, releasing their contents to the outside.

Golgi Apparatus

Membrane-enclosed organelle in eukaryotic cells that modifies the proteins and lipids made in the endoplasmic reticulum and sorts them for transport to other sites.

Lysosome

Membrane-enclosed organelle that breaks down worn-out proteins and organelles and other waste materials, as well as molecules taken up by endocytosis; contains digestive enzymes that are typically most active at the acid pH found inside these organelles.

Membrane-Enclosed Organelle

Any organelle in a eukaryotic cell that is surrounded by a lipid bilayer – for example, the endoplasmic reticulum, Golgi apparatus, and lysosome.

Nuclear Envelope

Double membrane surrounding the nucleus. Consists of outer and inner membranes, perforated by nuclear pores.

Nuclear Pore

Channel through which selected large molecules move between the nucleus and the cytoplasm.

Peroxisome

Small membrane-enclosed organelle that contains enzymes that degrade lipids and destroy toxins.

Phagocytic Cell

A cell such as a macrophage or neutrophil that is specialized to take up particles and microorganisms by phagocytosis.

Phagocytosis

The process by which particulate material is engulfed (“eaten”) by a cell. Prominent in predatory cells, such as Amoeba proteus, and in cells of the vertebrate immune system, such as macrophages.

Pinocytosis

Type of endocytosis in which soluble materials are taken up from the environment and incorporated into vesicles for digestion.

*Literally, “cell drinking.”

Rab Protein

One of a family of small GTP-binding proteins present on the surfaces of transport vesicles and organelles that serves as a molecular marker to help ensure that transport vesicles fuse only with the correct membrane.

Receptor-Mediated Endocytosis

Mechanism of selective uptake of material by animal cells in which a macromolecule binds to a receptor in the plasma membrane and enters the cell in a clathrin-coated vesicle.

Rough Endoplasmic Reticulum

Region of the endoplasmic reticulum associated with ribosomes and involved in the synthesis of secreted and membrane-bound proteins.

Secretion

Production and release of a substance from a cell.

Secretory Vesicle

Membrane-enclosed organelle in which molecules destined for secretion are stored prior to release.

Signal Sequence

Amino acid sequence that directs a protein to a specific location in the cell, such as the nucleus or mitochondria.

SNARE

One of a family of membrane proteins responsible for the selective fusion of vesicles with a target membrane inside the cell.

Tethering Protein

Filamentous transmembrane protein involved in the docking of transport vesicles to target membranes.

Transport Vesicle

Membrane vesicle that carries proteins from one intracellular compartment to another – for example, from the endoplasmic reticulum to the Golgi apparatus.

Unfolded Protein Response

Molecular program triggered by the accumulation of misfolded proteins in the endoplasmic reticulum. Allows cells to expand the endoplasmic reticulum and produce more of the molecular machinery needed to restore proper protein folding and processing.

Vesicular Transport

Movement of material between organelles in the eukaryotic cell via membrane-enclosed vesicles.

Cell Charge Balancing

Outside, Na⁺ and Cl^- serve to balance the charge.

Inside the cell, K⁺ is balanced by inorganic and organic anions including nuycleic acids, proteins, and cell metabolites.

Osmolarity

The total concentration of solute particles inside a cell.

*The osmolarity of a cell is generally greater than than the solute concentration outside of the cell.

ATP-Driven Pumps

1. Na⁺ Pump (a.k.a., Na⁺-K⁺ Pump)

2. Ca²⁺ Pump

3. H⁺ Pump

Na⁺ Pump Cycle

Membrane Potential Effect on Voltage-Gated Ion Channels

Changes in membrane potential can increase/decrease the probability that channels will be open, NOT how wide the channels open.

Combined Effort of Voltage-Gated Na⁺ and K⁺ Channels

K⁺ in Animal Cells

In most animal cells, the negative value of the resting membrane potential across the plasma membrane depends mainly on the K⁺ gradient and the operation of K⁺-selective leak channels.

Piezo Channels

Stretch-activated mechanically gated channels found in locations such as the bladder or skin; when pressure stretches the plasma membrane, the channel is pulled open, allowing iiions to flow into the cell.

Percent of Eukaryotic Cell that is Membrane-Enclosed Organelle

Nearly half of the eukaryotic cell’s volume.

Ran-GTP vs. Ran-GDP

Ran-GTP is in the nucleus; it is picked up in order for proteins destined for the nucleus to be deposited. Ran-GDP is the hydrolyzed form of Ran-GTP; once it is carried out of the nucleus, it is dropped in order for new proteins to be picked up and taken inside.

*GTP → “Gets Taken for Protein (to finish being delivered). GDP → “Gets Dropped for Protein (to be picked up and brought inside)