In the sequence that unfolds after molecules of a hormone...

In the sequence that unfolds after molecules of a hormone such as ABA bind to receptors at the surface of a target plant cell: a. first messenger molecules in the cytoplasm are mobilized, then G proteins carry the signal to second messengers such as protein kinases, which alter the activity of cell proteins such as $\mathrm{IP}_{3}$ b. binding activates G proteins, which in turn activate second messengers such as IP $_{3} ;$ subsequent steps are thought to involve activation of genes that encode protein kinases. c. binding activates phospholipase $C$, which in turn activates G proteins, which then activate molecules of IP $_{3}$, a step that leads to the synthesis of protein kinases. d. binding stimulates G proteins to activate protein kinases, which then bind calcium channels in ER; the flux of calcium ions activates second messenger molecules that alter the activity of cell proteins or enter the cell nucleus and alter the expression of target genes. e. binding activates G proteins, which in turn activate phospholipase $C ;$ this substance then stimulates the synthesis of second messenger molecules, the second messengers bind calcium channels in the cell's ER, and finally protein kinases alter the activity of proteins by phosphorylating them.

In the sequence that unfolds after molecules of a hormone such
as ABA bind to receptors at the surface of a target plant cell:
a. first messenger molecules in the cytoplasm are mobilized, then G proteins carry the signal to second messengers such as protein kinases, which alter the activity of cell proteins such as $\mathrm{IP}_{3}$
b. binding activates G proteins, which in turn activate second messengers such as IP $_{3} ;$ subsequent steps are thought to involve activation of genes that encode protein kinases.
c. binding activates phospholipase $C$, which in turn activates G proteins, which then activate molecules of IP $_{3}$, a step that leads to the synthesis of protein kinases.
d. binding stimulates G proteins to activate protein kinases, which then bind calcium channels in ER; the flux of calcium ions activates second messenger molecules that alter the activity of cell proteins or enter the cell nucleus and alter the expression of target genes.
e. binding activates G proteins, which in turn activate phospholipase $C ;$ this substance then stimulates the synthesis of second messenger molecules, the second messengers bind calcium channels in the cell's ER, and finally protein kinases alter the activity of proteins by phosphorylating them.

Key Concepts

Receptor–Ligand Binding

This concept involves the initial interaction between an extracellular signaling molecule and its specific cell surface receptor. Binding typically induces a conformational change in the receptor, which triggers the downstream signaling cascade within the cell.

G Protein–Coupled Signaling

G proteins, which are activated by conformational changes in receptors, act as molecular switches to relay signals from the cell surface to intracellular effectors. Their activation is a critical step in many signal transduction pathways, enabling the propagation and amplification of the signal.

Second Messenger Systems

Second messengers are small intracellular molecules, such as inositol trisphosphate (IP3), that transmit and amplify the signal initiated by receptor activation. They facilitate a rapid and broad cellular response by modulating various downstream targets.

Phospholipase C Activation

Phospholipase C (PLC) is an enzyme that, when activated, plays a key role in signal transduction by cleaving membrane phospholipids to generate important second messengers like IP3 and diacylglycerol (DAG). This process links receptor activation to the mobilization of further intracellular signals.

Protein Kinase Activation

Protein kinases are enzymes that phosphorylate specific target proteins, thereby modifying their activity. This post-translational modification is a common mechanism through which cells regulate processes such as metabolism, cell division, and gene expression in response to extracellular signals.

Calcium Signaling

Calcium ions function as versatile second messengers in many cellular processes. Changes in intracellular calcium concentrations, often mediated by channels activated during the signaling cascade, play a pivotal role in regulating enzyme activity, cell contraction, secretion, and gene transcription.

Regulation of Gene Expression

Signal transduction pathways can culminate in the activation or repression of specific genes. This regulation of gene expression ensures that the appropriate proteins are synthesized in response to a signal, thereby facilitating long-term cellular adaptations and responses.

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