Biology: AP Biology: Signal Transduction and Cell Communication Worksheet

Question 1

Describe the three main stages of a signal transduction pathway: reception, transduction, and response. Use a hormone binding to a target cell as your example.

Accepted Answer

Reception occurs when the hormone binds to a specific receptor protein on or in the target cell. Transduction occurs when the receptor changes shape and activates a series of relay molecules inside the cell. The response is the final cellular change, such as altered gene expression, enzyme activation, secretion, or movement.

Question 2

A signaling molecule is polar and cannot pass through the phospholipid bilayer. Predict whether its receptor is more likely to be located on the plasma membrane or inside the cytoplasm. Explain your reasoning.

Accepted Answer

The receptor is more likely to be located on the plasma membrane because polar molecules cannot easily cross the hydrophobic interior of the phospholipid bilayer. The ligand can bind outside the cell and trigger an internal response through the receptor.

Question 3

Compare G protein-coupled receptors and receptor tyrosine kinases. Include one similarity and one key difference in your answer.

Accepted Answer

Both G protein-coupled receptors and receptor tyrosine kinases are membrane receptors that change shape after ligand binding and start intracellular signaling pathways. A key difference is that G protein-coupled receptors activate G proteins, while receptor tyrosine kinases usually dimerize and phosphorylate tyrosine residues to activate multiple relay proteins.

Question 4

A scientist adds a ligand to a cell culture and observes that a receptor tyrosine kinase forms a dimer. Explain why dimerization is important for this type of receptor.

Accepted Answer

Dimerization brings two receptor tyrosine kinase molecules close together so they can phosphorylate each other on tyrosine residues. These phosphorylated sites then serve as docking sites for relay proteins, allowing the signal to be transmitted inside the cell.

Question 5

In a phosphorylation cascade, one activated protein kinase can activate many target proteins. Explain how this can amplify a signal.

Accepted Answer

A phosphorylation cascade amplifies a signal because each activated kinase can phosphorylate multiple molecules in the next step of the pathway. As the signal moves through the cascade, a small number of ligand-bound receptors can produce a large number of activated proteins and a strong cellular response.

Question 6

Cyclic AMP, calcium ions, and IP3 are examples of second messengers. Explain what a second messenger is and why second messengers are useful in cell signaling.

Accepted Answer

A second messenger is a small intracellular molecule or ion that helps relay a signal from an activated receptor to targets inside the cell. Second messengers are useful because they can spread quickly through the cytoplasm, activate many proteins, and amplify the original signal.

Question 7

A mutation prevents a G protein from hydrolyzing GTP to GDP. Predict how this mutation could affect the signaling pathway.

Accepted Answer

If the G protein cannot hydrolyze GTP to GDP, it may remain active for too long. This could cause continuous signaling even after the ligand is gone, leading to an overactive cellular response.

Question 8

A pathway activates a transcription factor that enters the nucleus. Explain how this can change the phenotype of a cell.

Accepted Answer

An activated transcription factor can bind to DNA and increase or decrease transcription of specific genes. This changes which proteins the cell makes, and those protein changes can alter the cell's phenotype, such as its metabolism, shape, secretions, or ability to divide.

Question 9

Insulin signaling causes many body cells to increase glucose uptake. Explain why only target cells respond strongly to insulin, even though insulin travels through the bloodstream.

Accepted Answer

Only target cells respond strongly to insulin because they have the specific insulin receptor and the intracellular signaling proteins needed to carry out the response. Cells without the correct receptor or pathway components will not detect insulin in the same way.

Question 10

Two cell types receive the same signaling molecule, but one cell type divides and the other cell type changes its gene expression without dividing. Explain how the same signal can cause different responses.

Accepted Answer

The same signal can cause different responses because different cell types may have different receptors, relay proteins, second messengers, transcription factors, or target genes. The final response depends on the cell's internal signaling network, not only on the signal molecule.

Question 11

A drug blocks a protein phosphatase in a signaling pathway. Predict how this drug could affect proteins that are normally turned off by dephosphorylation.

Accepted Answer

If a protein phosphatase is blocked, proteins that are normally turned off by dephosphorylation may stay phosphorylated longer. This could prolong or intensify the signaling response, depending on whether phosphorylation activates those proteins.

Question 12

Analyze this pathway: A ligand binds a receptor, the receptor activates adenylyl cyclase, adenylyl cyclase increases cAMP, cAMP activates protein kinase A, and protein kinase A phosphorylates enzymes that break down glycogen. Identify the second messenger and describe the final cellular response.

Accepted Answer

The second messenger is cAMP because it carries the signal from adenylyl cyclase to protein kinase A inside the cell. The final cellular response is increased glycogen breakdown, which releases glucose or glucose-related molecules that can be used for energy.

Biology: AP Biology: Signal Transduction and Cell Communication

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