Unit 5 - Cell Communication Q & A

Regardless of the type of ligand or receptor, what do they all have in common when it comes to ligand/receptor binding?

A: They all change the shape of the receptor.

E: When a ligand binds to a receptor, it causes a conformational change in the receptor's structure. This shape change is the first step in activating downstream signaling pathways.

All of the following are true of cell communication systems EXCEPT

A: Most signal receptors are NOT bound to the outer membrane of the nuclear envelope.

E: Most receptors are located on the plasma membrane or intracellularly, not specifically on the nuclear envelope.

Calcium's role in cell signaling is:

A: Acting as a second messenger.

E: Calcium ions amplify and relay signals within the cell, often by binding to proteins like calmodulin to trigger specific responses.

All of the following are part of the phosphorylation cascade model EXCEPT:

A: GTP DOES NOT donates a phosphate group to an inactive protein kinase.

E: Protein kinases are activated by phosphorylation, not by GTP. GTP activates G-proteins, not kinases.

A G protein is activated when...

A: GTP displaces GDP on the G protein.

E: The exchange of GDP for GTP on the G protein's alpha subunit activates it, allowing it to transmit the signal.

Phosphorylation cascades involving a series of protein kinases are useful for cellular signal transduction because...

A: They amplify the original signal many fold.

E: Each activated kinase can phosphorylate multiple downstream molecules, exponentially increasing the signal's strength.

Which of the following is a second messenger?

A: cAMP.

E: cAMP is a well-established second messenger that activates protein kinase A (PKA) and propagates cellular signals.

When a signal transduction pathway includes a cascade of kinases, how is the response shut off?

A: Phosphatases inactivate enzymes.

E: Phosphatases remove phosphate groups from proteins, deactivating them and halting the signaling cascade.

The main difference between steroid receptor activation and other hormones is:

A: The receptor is in the cytosol.

E: Steroid hormones pass through the plasma membrane and bind to intracellular receptors, unlike other hormones that interact with membrane receptors.

What do steroids and growth factors have in common?

A: They both act to turn on genes.

E: Both steroids and growth factors ultimately regulate gene transcription, though their mechanisms differ.

Hormone X produces its effect in its target cells via the cAMP pathway. Which of the following will produce the greatest effect in the cell?

A: A molecule of activated, cAMP-dependent protein kinase injected into the cytoplasm of the cell.

E: This bypasses earlier steps in the pathway, directly initiating the cellular response.

Of the following, a receptor protein in a membrane that recognizes a chemical signal is most similar to:

A: A specific catalytic site of an enzyme binding to a substrate.

E: Both receptors and enzymes exhibit specificity for their respective ligands or substrates.

What could happen to target cells in an animal that lack receptors for paracrine factors?

A: They would not be expected to multiply in response to growth factors from nearby cells.

E: Without receptors, cells cannot detect or respond to paracrine signals, such as growth factors.

Membrane receptors that attach phosphates to specific amino acids in proteins are:

A: Called tyrosine kinase receptors.

E: Tyrosine kinase receptors phosphorylate specific tyrosine residues, playing a key role in signal transduction.

Testosterone functions inside a cell by:

A: Binding with a receptor protein that enters the nucleus and activates specific genes.

E: Testosterone-receptor complexes act as transcription factors, regulating gene expression.

Which example best describes a homeostatic control mechanism?

A: The kidneys excrete salt into the urine when dietary salt levels rise.

E: This feedback mechanism helps maintain stable internal salt levels, exemplifying homeostasis.

An immune cell binding to a marker protein on a pathogen and releasing an enzyme to kill the cell:

A: C) Direct.

E: Direct signaling occurs through physical contact between cells, such as immune cells and pathogens.

The release of adrenaline causing the heart to beat faster:

A: Hormone.

E: Adrenaline is a hormone that travels through the bloodstream to affect distant target organs like the heart.

A spinal cord nerve signaling a muscle cell to contract:

A: Synaptic.

E: Synaptic signaling occurs between neurons and their target cells, such as muscle fibers.

• #1: G-protein receptor
  Explanation: This structure on the membrane binds to an extracellular ligand and activates the G-protein when the ligand binds.

• #2: AC) G protein
  Explanation: This protein is activated by the receptor when GTP replaces GDP, enabling signal transduction downstream.

• #3: Second messenger
  Explanation: The molecule (e.g., IP₃ or cAMP) amplifies the signal by relaying it further into the cell.

• #6: Calcium ion as a second messenger
  Explanation: Calcium released from the endoplasmic reticulum plays a role as a second messenger, activating proteins like calmodulin.

Two ways the same messenger can have different effects in different cell types:

The receptor may be different:

Even if the messenger is the same, different cell types may express different receptors for that messenger. For example, the receptor might have the same ligand-binding pocket to recognize the messenger, but the receptor's structure and associated intracellular domains may vary. This difference determines which intracellular signaling pathways are activated.

What proteins the receptor activates inside the cell:

The downstream signaling proteins present in a cell can vary, even if the same receptor is used. For example, one cell type might activate proteins involved in gene expression, while another cell type might activate pathways controlling metabolic activity.

The advantage of having second messengers over using all enzymes in a signal transduction pathway:

Signal Amplification: Second messengers, like cAMP or calcium, can amplify a signal significantly. A single activated receptor can lead to the production of thousands of second messenger molecules, which then activate multiple downstream targets, creating a robust cellular response.

Rapid Diffusion: Second messengers are small, soluble molecules that can quickly diffuse throughout the cytoplasm, reaching multiple targets faster than enzymes, which often require direct interactions or localization. This allows for a faster and more widespread cellular response.

In spinal nerve-to-muscle transmission, how is a chemical signal converted to an electrical signal, back to a chemical signal, and back to an electrical signal?

In spinal nerve-to-muscle transmission, a chemical signal is first converted to an electrical signal when a neurotransmitter is released from the presynaptic neuron into the synaptic cleft. This neurotransmitter binds to receptors on the postsynaptic membrane, opening gated ion channels and allowing sodium ions (Na⁺) to flow into the muscle cell, generating an action potential (electrical signal). The electrical signal is then converted back to a chemical signal as the action potential travels along the muscle cell membrane (sarcolemma) and triggers the release of calcium ions (Ca²⁺) from the sarcoplasmic reticulum. Calcium ions act as second messengers that initiate muscle contraction (a chemical response). Finally, the chemical signal leads to an electrical response within the muscle fibers as the calcium ions bind to troponin, exposing binding sites for actin and myosin, which facilitates the continuation of muscle contraction.

Explain any homeostatic mechanism, detailing the role of cell signaling in the process. Give the steps.

An example of a homeostatic mechanism is the regulation of body temperature. The hypothalamus acts as a sensor, detecting deviations from the normal temperature range. If the temperature rises, the hypothalamus signals sweat glands to secrete sweat, which evaporates to cool the body, and causes blood vessels to dilate (vasodilation), increasing heat loss through the skin. Conversely, if the temperature falls, the hypothalamus signals blood vessels to constrict (vasoconstriction) to conserve heat and muscles to shiver, generating heat through cellular respiration. This process relies on cell signaling, with the hypothalamus communicating with effectors via electrical signals (nerves) or chemical signals (hormones) to ensure a coordinated response. Once normal temperature is restored, these signals stop, completing the negative feedback loop and maintaining homeostasis.