Like tractor-trailers on a highway, small sacs called vesicles transport substances within cells. This image tracks the motion of vesicles in a living cell. The short red and yellow marks offer information on vesicle movement. The lines spanning the image show overall traffic trends. Typically, the sacs flow from the lower right (blue) to the upper left (red) corner of the picture. Such maps help researchers follow different kinds of cellular processes as they unfold. Image credit: Alexey Sharonov and Robin Hochstrasser, University of Pennsylvania.

Cellular Transport: the movement of ions and molecules across membranes.

Cells must take up nutrients from the environment and export waste products. To overcome this dilemma, species have evolved a multitude of "transport proteins" that provide a passageway for the movement across "membranes." (Brooker, 97) "Channel proteins" span the width of the cell membrane and contain a fluid-filled pathway through which ions pass. (Kandel, 88) "Transporters" provide the principal “pathway” for the uptake of "organic" "molecules," such as “sugars,” “amino acids,” and “nucleotides.” (Brooker, 98)


Active Transport: the movement of a solute across a membrane against its “gradient,” meaning from a region of low “concentration” to higher concentration. Active transport is energetically unfavorable and requires the input of energy. ‘Primary' active transport involves the functioning of pumps that directly use energy to transport a solute against a gradient. (Brooker, 99) Movement of substances across biological membranes into cells or organelles other than by passive diffusion or passive transport, often occurring against concentration or "electrochemical" gradients. It involves ("transporters") and requires energy. (Lawrence)

Diffusion: the movement of materials across cell membranes and “epithelial” layers against an “electrochemical" "gradient,” requiring the expenditure of metabolic energy. (MeSH) The net movement of molecules (or "ions") from an area of high concentration to an area of low concentration. (Norman, 41) Fat-soluble molecules dissolve in the “phospholipid” part of the membrane and diffuse very readily. Molecules such as water, "oxygen" and "carbon dioxide" diffuse readily because of their very small size. Ions, which are charged particles, and larger molecules such as "glucose," cannot pass through the phospholipid layer.  Instead they go through special “channel proteins.” Since all of the molecules and ions involved are moving anyway, the cell does not require the use of energy for this process. It can therefore be described as a form of “passive” transport. (Indge, 79) Due to incessant random movement ("Brownian motion") and vibrations of molecules, diffusion is very rapid over short distances, which enables biological reactions to occur with tiny quantities of reactants in the extremely confined volumes of most cells. (Venter, 46) Factors affecting diffusion include: travel distance (shorter is faster); “gradient” difference (greater is faster); molecule size (smaller is faster); and temperature (hotter is faster). (Also affecting diffusion are) electrochemical gradients. Opposites attract so if it is positive-to-negative, the solute crosses. If it is negative-to-positive, the solute crosses. If it is positive-to-positive, the solute does not necessarily cross. (Norman Lectures, 6/3/09)

Brownian Motion: random movement due to the impact of molecules suspended in liquid or gas. (Norman, 6/3/09) The phenomenon of 'heat energy' causes atoms and molecules to vibrate and move. (Brooker, 32) Brownian movements cause the observable motion of solutions seen under a light microscope. The more massive components of the solutions are ‘literally blasting the little guys’ into other areas. (Norman, 54) Einstein proved what now we call Brownian motion was due to the molecules in water vibrating due to heat at a very rapid rate, and that's what drives all the energy in biology. (VenterNPR, 25Oct13)

Facilitated Diffusion: transport of molecules or ions across a membrane 'down' their concentration gradient by a carrier system without the expenditure of energy. (Lawrence) Involves the aid of "transport proteins." Transport proteins facilitate the movement of various nutrients and water across the membrane. (Brooker, 93)

Osmosis: diffusion of water across a "semi-permeable" membrane. In plants, referred to as ‘turgor pressure.‘ (Norman, 6/3/09) The movement of water across membranes to balance "solute" concentrations. (Brooker 93) A special case of diffusion involving the movement of water molecules. The net movement of water through a (semipermeable) membrane from a solution of higher 'water potential' to a solution of lower water potential. For example, in a solution with water molecules and larger solute molecules, the water molecules are able to pass freely through the (semipermeable) membrane, while the solute molecules are too large and cannot pass through. If the solution on one side has a higher concentration of water molecules, the water molecules will diffuse from the higher concentration to the lower concentration. (Indge, 193-194) There is a great tendency of water to move into the cell by osmosis. (Brooker 94)

Osmotic Pressure: the minimum pressure that must be exerted to prevent the passage of pure "solvent" into the "solution" when the two are separated by a semipermeable membrane. (Lawrence) The tendency for a "hypertonic" solution to take up water when separated from pure water (which has no solutes) by a semipermeable membrane. (Norton Lectures, 6/3/09) 

Passive Diffusion: unaided diffusion of small uncharged molecules. (Lawrence) When diffusion occurs through a membrane without the aid of a transport protein. The rate of passive diffusion depends on the chemistry of the solute and its concentration. (Brooker, 92) Also referred to as 'passive transport.'

Endocytosis: process by which cells take up "extracellular" material by "invagination" of the "plasma membrane" to form "vesicles" enclosing the external material. (Lawrence) The plasma folds inward to form a vesicle that brings substances into the cell. (Brooker, 102)

Receptor Mediated Endocytosis: a common form of endocytosis in which a “receptor” is specific for a given 'cargo.' When a receptor binds to that cargo, this stimulates the binding of “coat proteins” to the membrane, and initiates the formation of a vesicle. Once inside the cell, the vesicle sheds its coat. In most cases, the vesicle fuses with an internal membrane “organelle,” such as a “lysosome,” and the receptor releases its cargo. (Brooker, 102)

Exocytosis: a process in which material inside the cell... is excreted into the extracellular environment. (Brooker, 101) Process by which "proteins" and some other molecules are secreted from cells. They are packaged in membrane-bound vesicles which then fuse with the plasma membrane, releasing their contents to the outside of the cell. (Lawrence)

Filtration: movement of a substance due to ‘hydrostatic pressure’ forcing solutions across a membrane. Passive process, does not require energy. (Norton Lectures, 6/3/09) (Also, in the lab a) separation process in which a liquid is passed through a porous material (the filter) to separate out any particles or solid matter. (Lawrence)

Intercellular Transport: any subcellular or molecular event, process, or condition involved in translocation of a biological “macromolecule” from one site or compartment to another, entirely within a cell. (NCIt)

Spontaneous Movement: movements of “lipids” and “integral proteins” within a membrane that does not require energy. This can be lateral (sideways) or rotational (“lipid” spins on its axis). (Norton Lectures, 6/3/09)

Transport Protein(s): proteins that provide a passageway for movement across membranes. (Brooker, 97) Molecules that pump substances across a membrane. (Kolb, 153) Responsible for moving essential nutrients from the environment into cells. (Venter, 59) Embedded within the phospholipid bilayer. Facilitate the uptake and export of substances. (Booker, 11) Can categorize transport proteins into two classes, 'channels' and 'transporters,' based on the manner in which they move solutes across the membrane. (Booker, 97) Also referred to as 'transporter proteins' and 'membrane transport proteins.'

Channel Proteins: a protein or cluster of proteins that function as a channel. A passage created in a “membrane” by a “conformational change” in membrane proteins.  (GHR) Proteins that form an open passageway for the facilitated diffusion of ions or molecules across the membrane. When the channel is open, the movement of “solutes” can be extremely rapid, up to 100 million ions or molecules per second. (Booker, 97) Also referred to as 'channels.'

Ion Channels: proteins that line holes in the plasma membrane. They can open on demand to let ions in and out of the cell. (Batiza, 15) Special openings in the cell membrane that allow "potassium" (and other) ions to flow from the inside of the cell, where they are present in high concentrations, to the outside, where they are present in low concentrations. (Kandel, 81) Ion channels are present in every cell of the body not just neurons, and they all use essentially the same mechanism to generate the “resting membrane potential.” (Kandel, 89)

Calcium Channel: an ion channel with selective “permeability” to “calcium.” (NCIt) Voltage-dependent cell membrane glycoprotein (“receptor”) (that is) selectively permeable to calcium ions. (MeSH)

Chloride Channel: “anion” channel in the membrane through which chloride ions pass. (Lawrence) Cell membrane glycoprotein that forms a channel to selectively pass chloride ions. (MeSH)

Ligand-Gated Channel: an ion channel whose opening and closing is regulated by the “binding” of a chemical “messenger,” such as a "hormone" or a "neurotransmitter.” Can be “excitatory” or “inhibitory.” They are involved in neuron-to-neuron communication, whereas "voltage-gated channels" are involved in generating an “action potential” within a single neuron. (Kandel, 451) Also referred to as 'ligand-gated ion channel.'

Non-Gated Channel: an ion channel that has no gates and is unaffected by the “voltage” across the cell membrane. (Kandel, 87) An ion channel in the membrane of nerve cells that passively conducts ions (most often potassium ions) across the cell membrane. The flow of ions through these channels is responsible for the resting membrane potential of the cell. (Kandel, 443) Also referred to as ‘resting channel’ and ‘mechanosensitive channel.’

Potassium Channel: cell membrane glycoprotein that is selectively permeable to potassium ions. At least eight major groups of potassium channels exist and they are made up of dozens of different “subunits.” (MeSH) Sits in the cell membrane and allows only potassium to pass through it, when open. Made up of four interlocking and identical protein subunits. The function is to let one, and only one, type of positively charged ion pass through from the watery soup inside the cell to the other side of the cell membrane, outside the cell. The channel acts as a filter, letting only potassium ions and no appreciable amount of any other kind of ion pass through. Potassium ions are able to pass through the hallway to the other side of the membrane at the rate of 100 million per second. Defects in the channel can cause disease. For example, "long QT syndrome" is caused by a defect in a potassium channel "encoded" by a gene on our 11th "chromosome." (Batiza, 16)

Sodium Channel: an ion channel with selective permeability to “sodium.” (NCIt) Specifically allows the passage of sodium ions. A variety of specific sodium channel subtypes are involved in serving specialized functions such as (“neurocommunication”), cardiac muscle contraction, and “kidney” function. (MeSH)

Voltage-Gated Channel: (an ion channel) that opens and closes in response to changes in the electric charge across the membrane. (Booker, 98) As an action potential moves along the “axon” toward the “axon terminal,” the gates of the sodium, and then the potassium, channels open and close in rapid succession. When the neuron is at rest, the gated channels are shut. When a “signal” reduces the cell’s resting membrane potential sufficiently, the gated channels open and ions rush into the cell. (Kandel, 86-87) These proteins span the width of the cell membrane and contain a fluid-filled pathway through which ions pass. (Kandel, 88) Also referred to as ‘voltage-gated ion channel.'

Transporters: transport proteins that bind solutes in “hydrophilic” pockets and undergo conformational change. (Norman, 6/3/09) The conformational change switches the exposure of the pocket to the other side of the membrane. (Booker, 98) (Transporters) carry only specific molecules. There is a saturation point at which all transporters are full and therefore no more solutes can cross. Other molecules may help coordinate activities of transporters. (Norman, 6/3/09) Also referred to as ‘carriers.’ 'carrier proteins,' and ‘carrier-mediated proteins.’

Antiporters: transporters that move molecules through a membrane in both directions. (Norton Lectures, 6/3/09) Bind two or more ions or molecules and transport them in opposite directions. (Brooker, 98) Transports a solute across the membrane, the transport depending on the simultaneous or sequential transport of another solute in the opposite direction. (Lawrence)

Pumps: active transport mechanism (located in the cell membrane) for ions or small molecules. (Lawrence) Transporters that directly couple their conformational changes to an energy source, such as "ATP"  "hydrolysis." A common category of pumps found in all living cells are 'ATP-driven pumps,' which have a binding site for ATP. (Brooker, 98)

Symporters: transporters that move molecules through a membrane in the same direction. (Norton Lectures, 6/3/09) Transport depends on the sequential or simultaneous transport of another solute in the same direction. (Lawrence) Bind two or more ions or molecules and transport them in the same direction. (Brooker, 98)

Uniporters: membrane proteins that transport a solute across a membrane in one direction only. (Lawrence) Single molecule transport. (Norton Lectures, 6/3/09) Bind a single molecule or ion and transport it across the membrane. (Brooker, 98)