Cell Membrane Transport & Signal Transduction: Key Concepts and Processes

Transport

Movement of substances across the cell membrane.

Passive Transport

Movement down a concentration gradient. Key point: No ATP required.

Active Transport

Movement against a concentration gradient. Key point: Requires ATP.

Diffusion

Particles spreading from high → low concentration.

Osmosis

Diffusion of water across a selectively permeable membrane.

Simple diffusion

Small/nonpolar molecules crossing the membrane without proteins.

Facilitated diffusion

Molecules moving through protein channels or carriers, no ATP.

Solvent

A Substance that dissolves another (usually water).

Solute

Substance being dissolved (salt, sugar, ions).

Hydrophilic protein channel

Membrane protein allowing polar/charged molecules to pass.

Countercurrent exchange

Fluids flowing in opposite directions to maximize diffusion efficiency.

Hypertonic

Cell in solution with higher solute outside → cell shrinks.

Hypotonic

Lower solute outside → cell swells/lyses.

Isotonic

Equal solute concentration → no net water movement.

Osmotic potential

Effect of solute concentration on water movement (always ≤ 0).

Water potential

Formula: Ψ = Ψs + Ψp. Defined separately: Ψs = solute (osmotic) potential, Ψp = pressure potential.

Turgid

Plant cell swollen due to water entering, firm cell wall.

Turgor pressure

Pressure of water pushing against the cell wall.

Plasmolysis

Plant cell membrane pulling away from the wall in a hypertonic solution.

Aquaporin

Specialized channel protein for rapid water transport.

Gated channels

Channels that open/close in response to a signal.

Pumps / carriers

Proteins that use ATP to move substances against gradients.

Sodium-potassium pump

3 Na⁺ out, 2 K⁺ in, Uses ATP, Maintains membrane potential.

Electron transport chain

A series of membrane proteins that transfer electrons to create a proton gradient used to make ATP.

Exocytosis

Vesicle fusing with membrane, releasing contents outside.

Endocytosis

Membrane engulfing material into a vesicle.

Receptor mediated endocytosis

A selective process where molecules bind to specific receptors and are brought into the cell in vesicles.

Ligand

Molecule that binds to a receptor.

Coated vesicle

Vesicle with protein coat (often clathrin).

Bulk flow

Movement of fluids due to pressure differences (not diffusion).

Cell membrane receptors

Proteins on the cell surface that bind signaling molecules and trigger a cellular response.

Ion channel receptors

Open/close channels directly.

G-protein-coupled receptors

Use G proteins & second messengers.

Protein kinase receptors

Activate phosphorylation cascades.

First messenger

External signaling molecule (hormone, neurotransmitter).

Second messenger

Intracellular signal (e.g., cAMP, Ca²⁺).

Hydrophilic signaling molecules

Water-soluble signals that bind to surface receptors.

Hydrophobic signaling molecules

Lipid-soluble signals that cross the membrane.

Signal transduction pathways

Steps that convert a signal into a response.