Transport of Substances Through Cell Membranes

Flashcards covering the mechanisms of substance transport across cell membranes, including diffusion subtypes, gating, osmosis, and active transport processes based on the provided lecture notes.

Diffusion

Random molecular movement of substances molecule by molecule, either through intermolecular spaces in the membrane or in combination with a carrier protein, powered by the energy of normal kinetic motion.

Active transport

Movement of ions or other substances across the membrane in combination with a carrier protein against an energy gradient, such as from a low-concentration state to a high-concentration state, requiring an additional source of energy besides kinetic energy.

Channel proteins

Transport proteins that have watery spaces all the way through the molecule and allow free movement of water, as well as selected ions or molecules.

Carrier proteins

Proteins that bind with molecules or ions that are to be transported, where conformational changes in the protein molecules then move the substances through the interstices of the protein to the other side of the membrane.

Simple diffusion

Kinetic movement of molecules or ions occurring through a membrane opening or through intermolecular spaces without interaction with carrier proteins.

Facilitated diffusion

Also called carrier-mediated diffusion, this process requires the interaction of a carrier protein which aids passage of molecules or ions by binding chemically with them and shuttling them through the membrane.

Aquaporins

Highly specialized protein pores that selectively permit rapid passage of water through the membrane; there are at least $13$ different types in various mammal cells.

Voltage gating

A mechanism where the molecular conformation of a protein channel gate responds to the electrical potential across the cell membrane to open or close.

Chemical (ligand) gating

A mechanism where protein channel gates are opened or closed by the binding of a chemical substance with the protein, causing a conformational or chemical bonding change.

$$V_{max}$$

The maximum rate at which facilitated diffusion can occur as the concentration of the diffusing substance increases, limited by the rate at which the carrier protein molecule can undergo conformational change.

Glucose transporter 4 (GLUT4)

A specific member of the membrane protein family activated by insulin that can increase the rate of facilitated glucose diffusion as much as $10-$ to $20\text{-fold}$ in insulin-sensitive tissues.

Osmosis

The process of net movement of water caused by a concentration difference of water across a selectively permeable membrane.

Osmotic pressure

The specific amount of pressure required to stop osmosis.

Primary active transport

Transport where the energy is derived directly from the breakdown of adenosine triphosphate $(ATP)$ or some other high-energy phosphate compound.

Secondary active transport

Transport where energy is derived secondarily from energy stored in the form of ionic concentration differences created originally by primary active transport.

Sodium-potassium ($Na^+-K^+$) pump

A primary active transport mechanism that pumps three $Na^+$ ions outward and two $K^+$ ions to the inside, maintaining concentration gradients and a negative electrical voltage inside cells.

$$\text{a subunit}$$

The larger globular protein of the $Na^+-K^+$ pump with a molecular weight of about $100,000$ that contains binding sites for sodium and potassium and has $ATPase$ activity.

Electrogenic pump

A pump, such as the $Na^+-K^+$ pump, that creates an electrical potential across the cell membrane by moving a net of one positive charge from the interior to the exterior for each cycle.

Co-transport

A form of secondary active transport in which the diffusion energy of sodium pulls another substance, such as glucose or amino acids, through the cell membrane in the same direction.

Counter-transport

A form of secondary active transport in which sodium ions move to the interior of the cell and cause another substance, such as calcium or hydrogen ions, to move to the exterior.