BIOL 1000 Lecture 13: Membrane Structure and Transport

Comprehensive practice flashcards covering membrane structure, transport categories, membrane proteins, channels, gradients, and intestinal transport based on York University BIOL 1000 Lecture 13.

Fluid Mosaic Model

The model stating that biological membranes contain a diversity of proteins, lipids, and carbohydrates.

Selective Permeability

The membrane property that acts as a barrier to water-soluble (polar), charged, and large molecules, while permitting small non-polar molecules to cross.

Small Hydrophobic (Non-polar) Molecules

Molecules such as $O_2$, $CO_2$, and $N_2$ that readily diffuse through the lipid bilayer down their concentration gradient.

Passive (Simple) Diffusion

The spontaneous movement of substances from high concentration towards low concentration without transporter involvement or energy expenditure.

Thermodynamics of Passive Diffusion

The thermodynamic parameters indicating that passive diffusion occurs spontaneously: $\Delta G < 0$ and $\Delta S > 0$.

Four Types of Membrane Transport

The four main mechanisms of crossing the plasma membrane: passive (simple) diffusion, passive (facilitated) diffusion, osmosis, and active transport.

Alveolar Gas Exchange Surface

The extremely thin ($0.2\,\mu\text{m}$) membrane across which gas exchange occurs between air in alveoli and blood in capillaries.

Alveolar $P_{O_2}$

The partial pressure of oxygen in the alveolar air, measured at $13.3\,\text{kPa}$.

Alveolar $P_{CO_2}$

The partial pressure of carbon dioxide in the alveolar air, measured at $5.33\,\text{kPa}$.

Peripheral Proteins

Proteins attached to one hydrophilic surface of the membrane that do not interact with the hydrophobic core and are primarily located on the cytosolic side.

Attachment Mechanisms of Peripheral Proteins

Non-covalent attachments that can occur via an integral protein, a lipid anchor, or directly through interaction with lipid head groups.

Integral Proteins

Amphipathic proteins inserted into the membrane that possess non-polar regions interacting with the hydrophobic core and are difficult to remove.

Transmembrane Proteins

The most common class of integral proteins, extending completely across the bilayer from the extracellular space to the intracellular space.

Transmembrane Domain Structure

Segments of transmembrane proteins enriched in non-polar amino acids that are typically arranged in alpha-helices.

Amphipathic Protein

A protein containing both hydrophilic (polar and charged) and hydrophobic (non-polar) amino acid residues, enabling integration into lipid bilayers.

Transport Function (Membrane Proteins)

A primary membrane protein role involving the passage of solutes across the bilayer via channels or carrier mechanisms, sometimes using ATP.

Enzymatic Activity (Membrane Proteins)

A membrane protein function in which an active site is exposed to catalyze sequential steps of a metabolic pathway.

Signal Transduction (Membrane Proteins)

A mechanism whereby a membrane receptor binds an external signalling molecule, transmitting a message to the cell interior.

Cell-Cell Recognition

A function mediated by glycoproteins and membrane proteins that allows adjacent cells to identify and interact with one another.

Glycoprotein

A membrane protein covalently linked to carbohydrate chains, displayed on the extracellular surface of the membrane.

Glycolipid

A membrane lipid covalently attached to carbohydrate chains, oriented toward the extracellular environment.

Cholesterol (Membrane Component)

A steroid lipid located within the hydrophobic core of animal cell membranes that helps regulate bilayer fluidity.

Extracellular Matrix (ECM) Fibres

Extracellular protein fibres outside animal cells that connect to membrane proteins for support and signal coordination.

Cytoskeleton Microfilaments

Internal structural filaments situated on the cytoplasmic face of the membrane that attach to membrane proteins.

Facilitated Diffusion

Passive transport of solutes across the membrane along their concentration gradient, mediated by specific transport proteins.

Major Facilitated Diffusion Transport Proteins

The three main classes of transport proteins: channel proteins, gated channels, and carrier proteins.

Channel Proteins

Transmembrane proteins that remain continuously open to provide a facilitated pathway across the membrane for specific molecules.

Rate Determinants of Channel Diffusion

The two primary factors dictating diffusion rate through open channels: the concentration gradient and the total number of channels.

Aquaporins

Highly specific transmembrane channel proteins that permit rapid, selective diffusion of water across the membrane while blocking other molecules and ions.

Gated Channels

Membrane transport proteins that open or close in response to a specific stimulus rather than remaining permanently open.

Rate Determinants of Gated Channels

The three factors governing diffusion rate through gated channels: protein conformational state (open or closed), concentration gradient, and number of channels.

Ligand-Gated Channels

Ion channels whose open or closed conformational state is regulated by the binding of a specific chemical molecule (ligand).

Synaptic Cleft

The narrow extracellular gap separating the presynaptic membrane of an axon terminal from the postsynaptic target membrane.

Synaptic Vesicles

Membrane-bound organelles located inside the presynaptic terminal that store neurotransmitters for release upon stimulation.

Presynaptic $Ca^{2+}$ Influx

The entry of calcium ions through voltage-gated channels triggered by action potentials, which induces synaptic vesicle exocytosis.

CFTR Gated Chloride Channel

A gated transport protein in which the addition of phosphate groups and ATP binding induces an open conformational state to transport chloride ions.

Voltage-Gated Channels

Ion channels that open or close in direct response to changes in the electrical potential across the plasma membrane.

Voltage-Gated $Na^+$ Channel Activation

A conformational change that opens sodium channels when the surrounding plasma membrane undergoes depolarization.

Membrane Potential (Potential Difference)

The difference in electrical charge between the inside and outside surfaces of a cell membrane.

Resting Membrane Potential

The baseline electrical potential of an unstimulated cell, illustrated in the lecture at $-65\,\text{mV}$ inside relative to $0\,\text{mV}$ outside.

Electrochemical Gradient

The composite driving force acting on an ion, created by both its chemical concentration gradient and the electrical membrane potential.

Maximum Cation Driving Force

The state producing the strongest inward force on positive ions, combining a high extracellular concentration with a negative intracellular potential.

Depolarization

The reduction in electrical charge difference across the membrane, driven when cations enter the cell and make the interior less negative.

Carrier Proteins

Transmembrane proteins that bind specific solutes and undergo conformational changes to transport them across the membrane.

Uniport Transport

A transport process where a carrier protein transfers a single solute species across the bilayer in one direction per conformational cycle.

GLUT-1 Carrier Protein

A transmembrane carrier protein that facilitates passive glucose uptake by cycling through binding, conformational shift, and release steps.

SGLT1

A sodium-glucose symporter located on the apical membrane of intestinal epithelial cells that imports glucose against its gradient.

GLUT2

A passive glucose transporter located on the basolateral membrane of intestinal epithelial cells that releases glucose into portal blood.

GLUT5

A membrane transport protein responsible for the facilitated diffusion of fructose across intestinal epithelial cell membranes.

Sliding-Filament Model of Sarcomere Contraction

The model demonstrating that sarcomere contraction occurs as thin (actin) and thick (myosin) filaments slide past each other without changing individual lengths.