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A glomerulus is the capillary tuft within a renal corpuscle. Fluid crosses its specialized filtration interface into the surrounding capsular space. The arrangement filters water and small solutes while strongly restricting most cells and large proteins; it does not produce final urine without later tubular processing.

Glucagon is a peptide hormone made by alpha cells in the pancreatic islets. Its mature polypeptide has 29 amino acid residues. Alpha cells store glucagon in secretory granules and release it by exocytosis. Glucagon is a message about fuel availability; glucose is the sugar whose supply it helps regulate. A falling blood glucose concentration favors glucagon release, whereas a rise usually suppresses it. Amino acids can also stimulate release, so a meal containing protein need not produce the same hormone pattern as a meal dominated by carbohydrates. ^[ref]

The principal blood glucose-raising target is the liver. In a hepatocyte, the glucagon receptor is a G protein-coupled receptor: activation of a stimulatory G protein drives adenylyl cyclase, increasing the second messenger cAMP and activating protein kinase A. Downstream phosphorylation, including the phosphorylase kinase relay to glycogen phosphorylase, favors glycogenolysis. Glucagon also supports gluconeogenesis. These are different supplies: glycogenolysis draws on a finite glycogen reserve, whereas gluconeogenesis makes glucose from other substrates and depends on their availability. Longer fasting depletes the glycogen reserve; a stronger hormone signal cannot release material that is no longer there. ^{[ref] [ref]}

Do not copy the liver pathway into every target of insulin. Skeletal muscle uses its own glycogen reserve locally; its glucose uptake and the liver's glucose output are different flows. Nor is direct glucagon-stimulated lipolysis in human adipose tissue as firmly established as its action in the liver. Glucagon action depends on the tissue, other hormones, and available substrates. ^[ref]

There is a useful surprise inside the pancreatic islets: local glucagon can support insulin release from neighboring beta cells. The islet perifusion experiment distinguishes that interaction from glucagon's action on the liver. Thus the two hormones can oppose each other's effects on blood glucose while cooperating locally. Delta cells supply another local brake through somatostatin. ^{[ref] [ref]}

A nuclear receptor responding to glucocorticoid ligands, with regulated localization and recruitment of transcriptional partners.

Ligand-dependent regulation changes nuclear activity and interaction with DNA-associated factors. Because the receptor directly connects a small-molecule hormone to gene regulation, its response can include delayed changes requiring transcription and protein synthesis. ^[ref]

A glucose transporter is a membrane transport protein that moves glucose across a cell membrane. The category includes facilitated transporters such as GLUT1 and GLUT4, and coupled carriers such as SGLT1. A facilitated transporter allows movement down a glucose chemical-potential difference; a sodium-coupled carrier can use the sodium electrochemical gradient to accumulate glucose. ^[ref]

For GLUT4, changing surface abundance is itself a regulatory mechanism. Insulin-responsive trafficking can deliver pre-existing carriers to the surface, increasing transport without first synthesizing more transporter. The amount of protein in the whole cell and the amount available at its outer boundary therefore need not change together. ^[ref]

GLUT4 is an insulin-responsive glucose transporter expressed especially in skeletal muscle and adipose tissue. Insulin increases its abundance at the cell surface by regulating vesicle trafficking, thereby increasing transport capacity. GLUT4 facilitates transport down the glucose concentration difference; it does not directly pump glucose using ATP. Regulation by relocation illustrates how a cell can change transport rapidly without first synthesizing a new biological population of transporter proteins. ^[ref]

Glutamic acid is an amino acid with a second carboxylic acid group in its side chain. Near physiological pH, this side chain is mostly deprotonated, giving glutamate.

Transamination connects the glutamate carbon skeleton with 2-Oxoglutarate. This allows amino groups to be transferred between carbon skeletons, making glutamate central to nitrogen metabolism as well as a constituent of proteins.

Gluten is the viscoelastic protein-rich network formed when wheat storage proteins, principally gliadins and glutenins, interact during hydration and mixing. Glutenins contribute network elasticity, while gliadins contribute extensibility and flow. Its behaviour depends on composition and intermolecular interactions rather than one unique gluten molecule. This connects protein assembly to the mechanical properties of dough. ^[ref]

Glycogen phosphorylase is an enzyme involved in glycogen breakdown. Its activity is the readout in the extract-transfer experiment under second messenger; see that experiment's primary references. A change in enzyme activity need not mean production of more enzyme protein.

A glycoprotein is a protein carrying one or more covalently attached carbohydrate groups. Attachment occurs through glycosylation; the carbohydrate component is part of the modified molecule, rather than simply dissolved beside it. Protein sequence alone therefore need not specify the complete chemical form measured in a biological preparation.

Pmel17 illustrates why the distinction is functional. It is a glycoprotein made as a membrane protein, then processed and organized inside developing premelanosomes. Experiments followed its molecular processing and its localization to internal vesicles and striations.^[ref] Calling it a glycoprotein identifies a chemical class; explaining its function still requires knowing its processing, position and interactions with other components.

Glycosylation attaches carbohydrates to other molecules, including proteins and lipids.^[ref]

The Golgi apparatus modifies and sorts cargo arriving from the endoplasmic reticulum for delivery to other cellular destinations. The eponym refers to Camillo Golgi.

Its usual stacked organization is polarized: cargo enters on the cis side, encounters medial processing regions, and leaves through the trans side and trans-Golgi network. Golgi cisternae enclose a Golgi lumen separated from the surrounding cytosol. Processing changes cargo; sorting selects its destination.^[ref]

This division of labor explains why reaching the Golgi apparatus does not imply being secreted: cargo can take different onward routes, while resident enzymes continue working there. The conventional route discussed here is part of secretory protein routing; many cytosol proteins never enter it.

The Golgi lumen is the enclosed interior of a Golgi cisterna, distinct from the surrounding cytosol.

The membrane-sidedness rule in secretory protein routing still applies within the Golgi apparatus. For a local worked example, stipulate a membrane protein with a carbohydrate chain exposed to Golgi lumen and a tail exposed to cytosol. An enzyme confined to Golgi lumen can contact that chain, but cannot directly contact the cytosolic tail without a boundary-crossing step. A change in which Golgi cisterna contains the protein need not turn the protein over. Thus chemical accessibility and cargo location are different questions: moving cargo onward preserves its membrane sides under the budding and fusion operations described in secretory protein routing.

A G protein is a protein switch whose activity depends on the bound guanine nucleotide. The three-subunit G proteins coupled to G protein-coupled receptors relay activation to other proteins; they are distinct from the receptors themselves. See the Alberts reference under G protein-coupled receptor.

An adaptor protein linking activated receptors to downstream signalling, including a pathway required for normal mouse hypoblast allocation.

GFP is a fluorescent protein used to report cellular location or gene expression. Fluorophore maturation and protein turnover introduce delays and filtering, so measured intensity does not in general reproduce an instantaneous transcription rate.

A class of introns with a conserved catalytic RNA core and a two-step RNA splicing mechanism initiated by external guanosine. Some can self-splice under suitable conditions, while others require protein assistance; the Tetrahymena example demonstrates that RNA can provide catalytic activity.

A biological growth factor is a signaling ligand that modulates cell growth, cell proliferation, cell differentiation or cell survival. Many are proteins recognized by cell-surface receptors. The resulting response depends on which receptors and intracellular pathways the target cell contains.

For example, epidermal growth factor engages the epidermal growth factor receptor, while FGF8 acts through FGF receptors. These interactions can alter gene expression through signal transduction. The ligand and its receptor are separate molecular components, so their distribution jointly determines where a signal can act.^[ref]

Growth hormone is a protein hormone synthesized and released by the anterior pituitary. It regulates body growth and metabolism, acting on tissues directly and partly through insulin-like growth factor 1 produced by the liver and other tissues. Its release is controlled by signals from the hypothalamus, including inhibition by somatostatin.^[ref]

This makes “growth” a coordinated physiological response rather than merely a change in one chemical concentration. Growth hormone can initiate signaling in one tissue, while a second circulating signal influences other tissues. Negative feedback connects the resulting signal levels back to their control pathway.^[ref]

A revealing molecular experiment determined the structure of human growth hormone bound to the extracellular portions of the growth hormone receptor. The 2.8-angstrom X-ray crystallography structure contained one hormone molecule and two receptor portions. Two different surfaces of the hormone contacted the receptor partners. This provides a concrete example of a signaling molecule organizing a multi-part complex, rather than simply occupying one isolated pocket. The structure supported a model for receptor association; establishing the full sequence of activation in a living biological cell requires additional functional experiments.^[ref]

G. R. Stark coauthored, with G. E. Davies, a 1970 experiment using protein cross-linking to investigate the organization of oligomeric proteins. The method connected a chemical intervention to a measurable series of protein subunit masses.^[ref]

After reaction with dimethyl suberimidate, detergent gel electrophoresis separated individual chains and covalently linked combinations. The experiments included assemblies with identical subunits and assemblies containing different kinds of subunits; the latter showed unequal chemical reactivities.^[ref]

The unequal-reactivity observation gives an important interpretive control. Failure to observe a linked pair need not mean that its members were far apart in the native protein: reactive groups may have been inaccessible or poorly reactive. Chemical proximity measurements must therefore be interpreted together with the chemistry used to record them.

A factor accelerating release of bound guanine nucleotide from a target protein, permitting reloading from the free pool. It does not phosphorylate bound GDP into GTP.