Cell Communication

After a minor cut, platelets at the wound release a protein that spreads only a short distance through the tissue fluid and stimulates nearby skin cells to divide. Which type of signaling does this describe?

A marine bacterium releases a small signal molecule and makes a light-producing enzyme. Researchers grow cultures at different densities and measure both (invented data). Cell density (cells/mL) | Signal molecule in medium (nM) | Light output (relative units) 10⁶ | 2 | 1 10⁷ | 20 | 2 10⁸ | 200 | 950 10⁹ | 2,000 | 1,000 Which conclusion is best supported by the data?

Cell population A triggers a response in cell population B when the two are grown mixed together. Researchers then grow them in the same dish separated by a porous filter that lets dissolved molecules and proteins pass but keeps the cells from touching. Population B no longer responds. What is the most reasonable conclusion?

A fluorescent sugar too large and polar to cross a plasma membrane is injected into a single cell of a moss leaf. Within minutes it appears in the cytoplasm of the surrounding cells. Which structures most likely allowed this?

A drug fits snugly into the ligand-binding site of a hormone receptor but does not cause the shape change that the hormone causes. What is the most likely effect of giving the drug along with the hormone?

A mutant G protein binds to its activated receptor normally but cannot release GDP, so it never picks up GTP. Which outcome is most likely when the receptor's ligand is present?

In a recording from a muscle cell, ions begin flowing across the plasma membrane less than a millisecond after a signal molecule arrives. The response needs no ATP, no enzyme activity and no second messenger inside the cell. Which kind of receptor most likely detects this signal?

In a receptor tyrosine kinase, researchers replace every tyrosine on the receptor's intracellular tail with phenylalanine, an amino acid that cannot be phosphorylated. The ligand-binding domain is unchanged. When the ligand is added, which result is most likely?

Which property of second messengers such as Ca²⁺ and cAMP best explains why they can spread a signal through the cytosol faster than most relay proteins can?

A pathway contains three kinases, K1, K2 and K3. Researchers delete each kinase in turn, add the signal, and check which kinases become phosphorylated (invented data). Cell line | K1 phosphorylated? | K2 phosphorylated? | K3 phosphorylated? | Response? Normal | Yes | Yes | Yes | Yes K1 deleted | — | No | No | No K2 deleted | Yes | — | No | No K3 deleted | Yes | Yes | — | No What order of the kinases in the cascade do the data support?

In one pathway, the final target enzyme is switched OFF when a kinase phosphorylates it, and a phosphatase switches it back on. What would most likely happen to this enzyme's activity if the cell were treated with a drug that blocks the phosphatase?

In a resting animal cell, the cytosolic Ca²⁺ concentration is 100 nM (1.0 × 10⁻⁷ M), and the concentration in the surrounding fluid is 1.2 mM (1.2 × 10⁻³ M). Which choice gives the correct ratio and explains why Ca²⁺ works well as a second messenger?

In a model pathway, 3 hormone molecules each bind one receptor. Each receptor activates 40 G proteins, and each G protein switches on one enzyme. Each enzyme makes 250 second-messenger molecules, it takes 4 second-messenger molecules to activate one kinase, and each kinase activates 30 target proteins. How many target proteins are activated in total?

Two signals, A and B, each cause a response in the same cell type. Researchers measure each response with and without a drug that blocks protein synthesis (invented data). Signal | Response at 2 min, no drug | Response at 3 h, no drug | Response at 3 h, with drug A | Strong | Strong | Strong B | None | Strong | None Which conclusion is best supported?

Histamine causes smooth muscle cells in the airways to contract, but it causes certain stomach lining cells to secrete acid. Which explanation best accounts for the two different responses to the same signal?

Two versions of a signal molecule activate the same receptor. Version N binds and lets go like a normal ligand. Version X forms a permanent covalent bond with the receptor. Both are added to separate dishes of cells and then washed away. What would most likely happen after the washout?

A virus-infected cell in the lining of the lungs dies by apoptosis rather than bursting. Which feature of apoptosis best protects the surrounding healthy tissue?

Cultured cells are treated with a drug that completely blocks protein synthesis. When a death signal is added 10 minutes later, the cells still undergo apoptosis on the normal schedule. What best explains this result?

Normal cells and mutant cells lacking a protein that detects DNA damage are exposed to the same dose of radiation. After 24 hours, 45% of the normal cells but only 4% of the mutant cells have undergone apoptosis, and the mutant survivors carry many unrepaired DNA breaks. Which long-term risk is greatest for tissue made of the mutant cells?

A drug prevents pores from forming in the outer mitochondrial membrane, so mitochondria cannot release proteins that promote cell death. Which trigger could most likely still cause apoptosis in cells treated with this drug?