Membrane Transport and Cell Signaling

Practice flashcards covering membrane structure, types of cellular transport (passive, active, and bulk), and the fundamental mechanisms of cell signaling.

Selective Permeability

A property of the plasma membrane that allows some substances to cross it more easily than others.

Phospholipids

The most abundant lipid in most membranes, characterized as amphipathic molecules containing both hydrophobic and hydrophilic regions.

Amphipathic

Molecules that possess both a hydrophilic (water-loving) region and a hydrophobic (water-fearing) region.

Fluid Mosaic Model

A model describing the membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

Cholesterol

A steroid that restrains phospholipid movement at warm temperatures ($37^{\circ}C$) and maintains fluidity at cool temperatures by preventing tight packing.

Integral Proteins

Proteins that penetrate the hydrophobic interior of the lipid bilayer; those that span the entire membrane are called transmembrane proteins.

Peripheral Proteins

Proteins that are loosely bound to the surface of the plasma membrane.

Glycolipids

Membrane carbohydrates that are covalently bonded to lipids.

Glycoproteins

Membrane carbohydrates that are covalently bonded to proteins, which is the most common form of membrane carbohydrate attachment.

Transport Proteins

Proteins that allow the passage of hydrophilic substances across the membrane; they are specific for the substances they move.

Aquaporins

Specific channel proteins that facilitate the passage of water through the plasma membrane.

Diffusion

The tendency for molecules to spread out evenly into available space, moving down their concentration gradient without energy investment.

Passive Transport

The diffusion of a substance across a biological membrane that requires no energy expenditure by the cell.

Osmosis

The diffusion of free water across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration.

Tonicity

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

Isotonic Solution

A solution where the solute concentration is the same as inside the cell, resulting in no net water movement across the plasma membrane.

Hypertonic Solution

A solution where the solute concentration is greater than that inside the cell, causing the cell to lose water.

Hypotonic Solution

A solution where the solute concentration is less than that inside the cell, causing the cell to gain water.

Osmoregulation

The control of solute concentrations and water balance, an adaptation necessary for life in hypertonic or hypotonic environments.

Turgid

A very firm state of a plant cell when it is in a hypotonic solution and the cell wall opposes further water uptake.

Plasmolysis

A lethal effect where the plasma membrane pulls away from the cell wall when a plant cell is placed in a hypertonic environment.

Facilitated Diffusion

A type of passive transport where transport proteins speed the movement of molecules or ions across the plasma membrane.

Active Transport

The process of moving solutes against their concentration gradients, requiring energy usually in the form of $ATP$.

Sodium-Potassium Pump

An electrogenic pump in animal cells that uses $ATP$ to transport three sodium atoms out of the cell and two potassium atoms into the cell.

Membrane Potential

The voltage across a membrane created by differences in the distribution of positive and negative ions.

Electrochemical Gradient

The two combined forces (a chemical force and an electrical force) that drive the diffusion of ions across a membrane.

Electrogenic Pump

A transport protein that generates voltage across a membrane, such as the sodium-potassium pump in animals or the proton pump in plants, fungi, and bacteria.

Cotransport

A mechanism where the active transport of a solute indirectly drives the transport of other solutes.

Exocytosis

A bulk transport process where transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell.

Endocytosis

A bulk transport process where the cell takes in molecules and particulate matter by forming new vesicles from the plasma membrane.

Phagocytosis

A type of endocytosis involving the cellular intake of food particles across the membrane.

Receptor-Mediated Endocytosis

A type of endocytosis where the intake of a specific substance is triggered by its binding to membrane receptors, used by human cells to take in cholesterol via low-density lipoproteins ($LDLs$).

Paracrine Signaling

A type of local signaling in animal cells where a signaling cell secretes local regulators, such as growth factors, to stimulate nearby cells.

Synaptic Signaling

A specialized type of local signaling in the animal nervous system where an electrical signal triggers the secretion of neurotransmitter molecules.

Hormones

Chemicals used by plants and animals for long-distance signaling; in animals, this is also called endocrine signaling.

Ligand

A signal molecule that binds specifically to a receptor, generally causing the receptor protein to undergo a change in shape.

G Protein-Coupled Receptors ($GPCRs$)

Plasma membrane receptors that work with the help of a G protein, which binds to the energy-rich molecule $GTP$.

Ligand-Gated Ion Channel

A membrane receptor that acts as a 'gate' for ions when it changes shape upon the binding of a specific signal molecule.

Second Messengers

Small, nonprotein, water-soluble molecules or ions, such as cyclic $AMP$ ($cAMP$) and calcium ions, that relay signals within a cell by diffusion.

Adenylyl Cyclase

An enzyme in the plasma membrane that rapidly converts $ATP$ to $cAMP$ in response to extracellular signals.