AP Biology Unit 4: Cell Communication and Cell Cycle practice questions

Question 1

A cytokine targets only nearby cells. Insulin travels through the bloodstream to cells across the body. What structural difference in signaling explains this?

Accepted Answer

Cytokines act locally; insulin is a bloodstream hormone reaching distant cells. One is a local regulator, the other long-distance.

Answer

Cytokines are proteins; insulin is a lipid that cannot enter the bloodstream

Answer

Insulin is produced by neurons, so it travels along nerve pathways instead of blood

Answer

Local regulators require direct cell-to-cell contact, while hormones use gap junctions

Question 2

An antigen-presenting cell physically contacts a helper T-cell and transfers surface proteins to it. Which mode of cell communication is this?

Accepted Answer

Direct cell-to-cell contact — cells touch, so surface proteins pass the signal. No chemical messenger travels through extracellular fluid.

Answer

Paracrine signaling, because the two cells are in the same tissue

Answer

Hormonal signaling, because immune cells release cytokines into the bloodstream

Answer

Autocrine signaling, because the macrophage is signaling its own surface receptors

Question 3

Epinephrine is a charged molecule that cannot cross the plasma membrane. How does it trigger changes inside a liver cell?

Accepted Answer

Epinephrine binds a G protein-coupled receptor on the outer cell surface. Its intracellular domain then changes shape, starting transduction without epinephrine ever entering.

Answer

Epinephrine enters through a protein channel and binds directly to DNA

Answer

Epinephrine is converted to a lipid so it can diffuse through the membrane

Answer

Epinephrine triggers endocytosis so the whole receptor complex enters the cell

Question 4

One epinephrine molecule docks with one G protein-coupled receptor. Downstream, thousands of glucose molecules are released from glycogen. What accounts for this enormous amplification?

Accepted Answer

Each step of the cascade multiplies the signal — one ligand frees thousands of glucose. One G protein makes adenylyl cyclase produce many cAMP; each cAMP activates kinases that phosphorylate many enzymes.

Answer

Epinephrine is stored in vesicles and released repeatedly, increasing signal strength

Answer

The receptor duplicates itself after binding, creating more receptor-ligand complexes

Answer

Glucose molecules are pre-activated and only need a single enzyme to be released all at once

Question 5

Your friend says cAMP IS the signal from the hormone. You say cAMP is something different. What is cAMP, and why does the distinction matter?

Accepted Answer

cAMP is a second messenger — an intracellular relay made after receptor activation. It amplifies and transmits the signal inside the cell, while the hormone is the first messenger.

Answer

cAMP is the receptor protein that binds the hormone at the cell surface

Answer

cAMP is a modified hormone that enters the nucleus to activate transcription directly

Answer

cAMP is an enzyme that breaks down the hormone once it has bound to its receptor

Question 6

Testosterone crosses the plasma membrane and binds a receptor inside the cell. Epinephrine cannot cross and binds a surface receptor. What property of each molecule explains this difference?

Accepted Answer

Testosterone is lipid-soluble and crosses the membrane; epinephrine, water-soluble, binds a surface receptor. Solubility sets receptor location.

Answer

Testosterone is larger, so it must enter by endocytosis rather than direct diffusion

Answer

Epinephrine uses a nuclear pore to enter the cell but is blocked before reaching the nucleus

Answer

Testosterone receptors are absent from the cell surface because the molecule is too small to be detected there

Question 7

A mutation locks a Ras G protein in its GTP-bound (active) state permanently. What is the most likely consequence for the cell?

Accepted Answer

The pathway stays active even without ligand, driving uncontrolled cell division. That unchecked proliferation is a hallmark of cancer.

Answer

The cell enters G0 and permanently stops dividing because the signal is too strong

Answer

The receptor is degraded faster, so the overall signal is weaker than normal

Answer

cAMP levels drop because the G protein no longer activates adenylyl cyclase

Question 8

A drug blocks adenylyl cyclase in liver cells. Epinephrine binds its receptor normally. Trace what fails downstream.

Accepted Answer

Without adenylyl cyclase, ATP isn't converted to cAMP and no second messenger forms. Protein kinase stays inactive, phosphorylase isn't activated, and no glucose is released.

Answer

Epinephrine accumulates outside the cell because the receptor cannot be recycled

Answer

The G protein is permanently inactivated because it cannot release its alpha subunit

Answer

The cell nucleus stops transcription because the drug directly inhibits RNA polymerase

AP Biology — Unit 4: Cell Communication and Cell Cycle

What AP Biology Unit 4 covers

AP Biology Unit 4 practice questions

Key terms in Unit 4

Drill all of Unit 4 with spaced repetition