Final answer:
A growth factor is a signaling molecule that plays a crucial role in cell growth, division, and gene expression, often through binding to receptors like receptor tyrosine kinases (RTKs). All the choices given are correct aspects of what constitutes a growth factor.
Step-by-step explanation:
A growth factor is a substance, such as a small protein, that influences cell growth and division by binding to specific cell-surface receptors, thereby initiating a cascade of events inside the cell that lead to cellular responses like division and gene expression. These factors are involved in important processes such as wound healing and tissue repair and are crucial in cell signaling pathways. Most growth factors are linked to tyrosine kinases through their receptors, specifically receptor tyrosine kinases (RTKs).
An example of such a pathway involves the Epidermal Growth Factor (EGF), which binds to the EGF receptor (EGFR), a tyrosine kinase, leading to a series of phosphorylation events that tell the cell to grow and divide. These growth factors can also engage in paracrine signaling, influencing nearby cells. Abnormal activation of these growth factors' pathways can lead to uncontrolled cell growth and cancer.
Therefore, the correct answer to the question 'What is a growth factor?' is: All of these choices are correct—a small protein that influences cell growth and division, a signaling molecule involved in paracrine signaling, and a soluble molecule that can alter gene expression in a cell.