unit 1 KA4

Comprehensive vocabulary flashcards covering extracellular and intracellular signalling, hydrophilic versus hydrophobic pathways, insulin and diabetes, nerve impulses and action potentials, and retinal photoreception cascades.

Extracellular signal molecules

Molecules used by multicellular organisms to signal between cells; examples include steroid hormones, peptide hormones, and neurotransmitters.

Receptor molecules

Proteins of target cells with a binding site for a specific signal molecule. Binding changes receptor conformation, initiating a response within the cell.

Tissue-specific response

The ability of different cell types within a multicellular organism to show different cellular responses to the same signalling molecule, caused by differences in intracellular signalling molecules and pathways.

Hydrophobic signalling molecules

Lipid-soluble signalling molecules (such as the steroid hormones oestrogen and testosterone) that can diffuse directly across the hydrophobic tails of the phospholipid bilayer to bind intracellular receptors.

Transcription factors

Proteins that act as receptors for hydrophobic signalling molecules; when bound to DNA, they can either stimulate or inhibit the initiation of transcription.

Hormone response elements (HREs)

Specific sites on DNA to which a steroid hormone-receptor complex binds in the nucleus, influencing the rate of transcription of many different genes.

Hydrophilic signalling molecules

Molecules (such as peptide hormones and neurotransmitters) that are not lipid soluble, cannot cross the hydrophobic plasma membrane, and therefore bind to transmembrane receptors on the cell surface.

Signal transducers

Transmembrane receptors that convert an extracellular ligand-binding event into intracellular signals, thereby altering cellular behaviour without the signal molecule entering the cell.

G-proteins

Molecules that relay intracellular signals from activated receptors to target proteins, such as enzymes and ion channels.

Phosphorylation cascades

A series of sequential kinase activations where one kinase activates the next, allowing multiple intracellular signalling pathways to be activated and resulting in the phosphorylation of many proteins.

Insulin

A hydrophilic peptide hormone that binds to a transmembrane receptor, triggering receptor phosphorylation and an intracellular phosphorylation cascade that recruits GLUT4-containing vesicles to the plasma membrane.

GLUT4

Glucose transporter proteins stored in intracellular vesicles that fuse with the plasma membrane to allow glucose to pass into fat and muscle cells.

Diabetes mellitus

A condition caused either by a failure to produce insulin (Type 1) or by a loss of receptor function (Type 2, which is generally associated with obesity).

Resting membrane potential

The electrical state of a cell membrane where there is no net flow of ions across the membrane.

Membrane potential

The electrical condition created across a plasma membrane when there is a difference in electrical charge between its two sides.

Action potential

A wave of electrical excitation that travels along the plasma membrane of a neuron.

Neurotransmitter receptors

Ligand-gated ion channels located at a synapse that open upon neurotransmitter binding, allowing positively charged ions to enter and depolarise the plasma membrane.

Depolarisation

A change in the electrical potential across a membrane toward a less negative value on the inside.

Voltage-gated sodium channels

Ion channels in a neuron's membrane that open when depolarisation exceeds a threshold, allowing $Na^+$ ions to enter down their electrochemical gradient.

Sodium-potassium pump

A membrane protein that restores and re-establishes sodium and potassium ion concentration gradients back to resting potential levels by actively transporting ions across the membrane.

Retina

The light-sensitive area within the eye that detects light and contains two types of photoreceptor cells: rods and cones.

Rods

Retinal photoreceptor cells that function in dim light but do not allow colour perception.

Cones

Retinal photoreceptor cells responsible for colour vision that function only in bright light and are less sensitive than rods due to having fewer photoreceptor molecules.

Retinal

A light-sensitive molecule found in eukaryotes that captures light energy.

Opsin

A membrane protein in animals that combines with retinal to form photoreceptor complexes in the eye.

Rhodopsin

The retinal-opsin complex located in rod cells that changes conformation to photoexcited rhodopsin upon absorbing a photon.

Transducin

A G-protein in rod cells activated by photoexcited rhodopsin; hundreds of transducin molecules are activated by a single photoexcited rhodopsin molecule.

Phosphodiesterase (PDE)

An enzyme activated by transducin that catalyses the hydrolysis of cyclic GMP (cGMP), breaking down thousands of cGMP molecules per second.

Cyclic GMP (cGMP)

A molecule that, when broken down by PDE, leads to the closure of ion channels in rod cell membranes, halting the inward leakage of $Na^+$ and $Ca^+$ ions.

Hyperpolarisation (retinal rod cells)

An increase in membrane potential (increased internal positive charge) resulting from the closure of ion channels in rod cells, which triggers nerve impulses in retinal neurons.