Chemical messengers and receptors

The core of the bilayer is made of hydrophobic fatty acid tails. Small hydrophobic molecules dissolve through it; ions and large hydrophilic molecules cannot.

• Correct: A small hydrophobic (lipid-soluble) molecule:
• A charged ion:
• A large hydrophilic protein:

Transcription factors bind DNA near genes and increase or decrease their transcription, which changes which proteins the cell makes.

• Correct: A protein that binds DNA and switches genes on or off:
• An enzyme that copies DNA before division:
• A ribosome that reads messenger RNA:

In exocytosis, a vesicle fuses with the plasma membrane and empties its contents outside the cell.

• Endocytosis:
• Correct: Exocytosis:
• Osmosis:

Only target cells respond to a messenger, and a target cell is one with receptor proteins that fit it. The skin and fat cells meet the hormone but have no receptor proteins for it.

• The hormone is broken down before reaching the skin and fat tissue: The question says the hormone reaches all three tissues equally. Reaching a cell is not enough; the cell must have a matching receptor protein.
• Liver cells have more ATP than skin and fat cells: Energy supply does not decide which cells respond. Receptor proteins do.
• Correct: Only the liver cells carry receptor proteins that bind this hormone: Correct. Matching receptor proteins make the liver cells target cells.
• The hormone enters every cell equally but produces an effect only in the liver: Whether a messenger enters a cell depends on its solubility, not the tissue. The response still requires a matching receptor protein.

Autocrine signaling (auto- = self) is a messenger binding receptor proteins on the same cell that released it.

• Paracrine: Paracrine signaling acts on neighboring cells. Here the releasing cell is also the target.
• Correct: Autocrine: Correct. The cell signals itself.
• Endocrine (hormonal): The interleukin never travels in the blood to a distant target.
• Synaptic: No neuron or synapse is involved.

A small hydrophobic molecule dissolves through the phospholipid bilayer, so it can reach receptor proteins inside the cell. Steroids made from cholesterol bind intracellular receptors.

• Correct: Inside the target cell, in the cytosol or nucleus: Correct. Lipid-soluble messengers cross the membrane and bind intracellular receptors.
• In the plasma membrane, facing out: Membrane receptor proteins serve water-soluble messengers that cannot cross the bilayer.
• In the blood plasma: Proteins in the plasma carry lipid-soluble messengers, but they are transport proteins, not the receptor proteins that start the response.
• On the outer surface of the nuclear envelope, facing the cytosol: Intracellular receptors sit in the cytosol or inside the nucleus, not on the outer surface of the nuclear envelope.

The signal passes from the first messenger outside, through the receptor protein and G protein in the membrane, to the enzyme making cAMP (the second messenger) inside, and then to phosphorylated proteins that carry out the response.

• Correct order: 1. The first messenger binds a receptor protein in the membrane 2. The receptor protein switches on a G protein 3. The G protein switches on a membrane enzyme 4. The enzyme converts ATP into cAMP 5. cAMP switches on enzymes that phosphorylate target proteins 6. The phosphorylated proteins change the cell's activity

Blocking cAMP breakdown leaves the first messenger's binding unchanged but raises cAMP, strengthening and prolonging everything downstream.

• Number of receptor proteins occupied by the messenger: no change. The drug acts inside the cell, after the receptor protein; it does not change how much messenger binds.
• cAMP concentration inside the cells: up. cAMP is still made at the usual rate but broken down more slowly, so it builds up.
• Phosphorylation of the target proteins: up. More cAMP switches on more of the enzymes that phosphorylate target proteins.
• How long the response lasts after the messenger leaves: up. Breaking down cAMP is what ends the response; slowing that step lets the response continue longer.

When a messenger stays low for a long time, target cells add receptor proteins. Without the nerve's neurotransmitter, the fibers insert its receptor proteins across their whole surface and become far more sensitive.

• Correct: The fibers up-regulated receptor proteins for that neurotransmitter: Correct. Long-term absence of a messenger leads to up-regulation.
• The fibers down-regulated receptor proteins for that neurotransmitter: Down-regulation would reduce receptor proteins and weaken the response. The response became stronger.
• The neurotransmitter became lipid-soluble: Solubility is a property of the molecule and does not change. The change is in the target cells.
• The fibers became autocrine cells: Autocrine signaling means a cell responding to its own messenger. The fibers are responding to a neurotransmitter applied from outside.

Nervous signaling sends action potentials along axons and releases neurotransmitter across synapses onto specific muscle fibers within milliseconds. A hormone must travel through the blood and reaches every target cell, so it is too slow and too widespread for a precise, split-second action.

• Hormones cannot reach the arm: Blood reaches the arm, so hormones can too. The problem is speed and precision.
• Hormones only affect organs, not muscles: Many hormones act on muscle. The issue is the time a hormone takes to arrive and act.
• Neurotransmitters travel through the blood to the arm muscles much faster than hormones do: Neurotransmitters are not carried in the blood to reach muscles; they cross the synaptic cleft at each synapse.
• Correct: Nerve signals reach specific muscles in milliseconds; hormones take seconds or more: Correct. Fast, precise responses are the strength of nervous signaling.