Question

___________ binds to receptors inside cells. a. Estrogen b. Epinephrine c. Epidermal growth factor d. all of the above bind to such receptors. e. none of the above binds to such receptors.

Short answer

The correct answer is (a) Estrogen.

Step-by-step solution

- Understand the substances

Firstly, it is important to know what these substances are. Estrogen is a hormone, Epinephrine, also known as adrenaline, is both a hormone and neurotransmitter, and Epidermal growth factor (EGF) is a protein.

- Understand the function of each substance

Estrogen, being a hormone, can pass through the cell membrane and bind to receptors inside the cell. Epinephrine, another hormone, however, binds to receptors located on the outside of the cell. Finally, EGF, being a large protein, cannot pass through the cell membrane and therefore also interacts with receptors on the outside of the cell.

- Choose the correct answer

Based on the functions and interactions of these substances with cells, it can be determined that among the substances provided, only Estrogen binds to receptors inside cells. Hence, the correct answer is (a) Estrogen.

Key concepts

Estrogen

Estrogen is a vital hormone that plays multifaceted roles in the human body, particularly in regulating the reproductive system in females.
Unlike many other hormones, estrogen is a steroid hormone. This allows it to pass through cell membranes with ease and reach receptors located inside the cell—these are known as intracellular receptors.
Once inside the cell, estrogen binds to these receptors and forms a complex that can move into the cell's nucleus. Here, it influences the expression of specific genes, effectively turning them on or off. This process is what drives many developmental and functional changes in reproductive tissues.

Cell Membranes

Cell membranes act as protective barriers for the cell. Composed primarily of a lipid bilayer, the membrane is semi-permeable.
This means it allows some substances to pass through while blocking others. Establishing this selective permeability is important for cell function and homeostasis.
Lipid-soluble hormones like estrogen can easily slip through this bilayer, whereas water-soluble molecules and larger molecules like proteins often cannot. Instead, they need receptors on the cell surface to relay their messages into the cell.

Intracellular Signaling

Intracellular signaling refers to the communication processes that occur within a cell once a receptor is activated by its ligand, such as a hormone.
Estrogen binding to its receptor initiates a cascade of signals inside the cell, leading to changes in gene expression and ultimately cell function.
This subgroup of signals is part of a broader signaling network, which is crucial for coordinating complex biological processes and ensuring that cells respond appropriately to environmental changes.

Epinephrine

Epinephrine, widely known as adrenaline, functions as both a hormone and a neurotransmitter.
Unlike estrogen, it does not enter the cell. Instead, it binds to receptors on the cell surface, triggering a response from outside the cell.
This binding activates intracellular signaling pathways that lead to rapid responses such as increased heart rate and metabolic shifts vital in fight-or-flight reactions.

Epidermal Growth Factor

Epidermal Growth Factor (EGF) is a powerful protein that stimulates cell growth and differentiation.
Because EGF is a large molecule, it cannot penetrate the cell membrane directly. It binds to specific receptors on the cell surface.
Once bound, these receptors undergo a conformational change and transmit signals into the cell's interior, promoting processes like cellular replication and repair.