UConn BIOL 1107 Exam 3

Which option correctly matches the three main anatomical regions of the vertebrate digestive tract to their primary physiological functions?

Foregut = Mechanical & chemical breakdown; Midgut = Nutrient absorption; Hindgut = Reabsorption of water

In the human gastrointestinal tract, where does the enzymatic (chemical) digestion of carbohydrates, lipids, and proteins first begin?

Carbohydrates begin in the mouth; Lipids begin in the mouth; Proteins begin in the stomach

A researcher isolates three human digestive enzymes: Pepsin, Salivary Amylase, and Trypsin. Which ranking correctly orders these enzymes from lowest optimal pH (most acidic) to highest optimal pH (most basic)?

Pepsin (pH ~2) < Salivary Amylase (pH ~7) < Trypsin (pH ~8.5)

Which of the following lists contains ONLY accessory organs of the human digestive system?

Liver, Gallbladder, Pancreas

Why do dietary fat molecules form large hydrophobic globules in the lumen of the digestive tract, and how do bile salts facilitate their breakdown?

Fats are nonpolar and cluster in water; bile salts emulsify these globules into smaller droplets so lipases can access individual fat molecules.

Which hormone is secreted by duodenal cells in response to acidic chyme entering from the stomach, and what is its target organ and response?

Secretin; stimulates the pancreas to release bicarbonate ions (HCO-3_ to neutralize stomach acid

How is the secretion of hydrochloric acid (HCI) regulated in the human stomach during meal ingestion?

Food in the stomach triggers gastrin release, which increases HCI secretion; high acidity later inhibits gastrin release via a negative feedback loop

In which specific region of the human small intestine does the majority of nutrient absorption occur, and what structural adaptations maximize this process?

Jejunum and Ileum; villi and microvilli (brush border)

How does glucose cross the apical membrane of an intestinal epithelial cell from the lumen (where glucose concentration is low) into the cell cytosol (where glucose concentration is high)?

Secondary active transport via the SGLT1 cotransporter, driven by an inward Na+ gradient

Once glucose is concentrated inside an intestinal epithelial cell, how does it exit across the basolateral membrane into the blood, and how is the vital sodium gradient maintained?

Glucose exits via facilitated diffusion through GLUT2; the Na+ gradient is maintained by the basolateral Na+/K+ ATPase pump.

Herceptin is an antibody used or treat certain breast cancers by inhibiting signaling through HER2, a receptor tyrosine kinase (RTK).

Which of the following mechanisms could reduce signaling through an RTK?

I. Preventing receptor dimerization

II. Preventing phosphorylation of the receptor’s intracellular domain

III.Increasing the removal of receptors from the plasma membrane

I II and III

Herceptin is an antibody that binds to the extracellular portion of HER2, a receptor tyrosine kinase frequently overexpressed in breast cancer cells. 

Which of the following are plausible mechanisms by which Herceptin could inhibit HER2 signaling?

I. Preventing HER2 from interacting with other receptors to form active dimers

II. Directly blocking the ATP=bing site of HER2’s intracellular kinase domain.

III. Promoting the removal of HER2 receptors from the cell surface.

I and III only

Herceptin inhibits activation of the HER2 receptor tyrosine kinase. However, some HER2-positive cancer cells have a mutation that causes a downstream signaling protein to remain permanently active/ What outcome is most likely?

Herceptin will be ineffective because downstream signaling remains active independently of HER2

What is the true of mechanical versus chemical digestion?

Mechanical digestion is the physical breakdown of larger food particles while chemical digestion is the breakdown of bonds within nutrients

Where does digestion of carbohydrates take place?

Mouth and small intestine

Where does absorption begin in the digestive tract?

Small Intestine

Where is bile produced?

Liver

Which of the following correctly matches the organ and its classification?

Gallbladder – accessory organ

What is the function of the pancreas?

Release of proteases

A signaling molecule binds to a cell surface receptor, triggering the action of multiple intracellular proteins. What best describes the action of the intracellular proteins?

Transduction

A cell releases ligands into the extracellular fluid. The ligands bind to its own membrane receptors, as well as the receptors of nearby cells. What type of cell communication is going on here?

Autocrine & paracrine signaling only

A ligand is analyzed and found to be the steroid hormone testosterone. How would you expect testosterone to interact with a cellular receptor?

Intracellularly, because testosterone is fat soluble and will cross the
phospholipid membrane

Suppose a cell surface receptor cleaved a phosphate from an intracellular protein. What kind of enzymatic activity does this demonstrate, and what might this indicate about the signaling pathway?

Phosphatase activity, which will likely turn off the signaling pathway within the cell

How does a Receptor Tyrosine Kinase (RTK) work?

Phosphorylates a tyrosine amino acid residue on an intracellular protein

What signaling pathways are known to have RTK mechanisms?

Insulin & Growth Factor

How do ligand gated ion channels work?

A signaling molecule binds to the extracellular domain of a transmembrane channel, opening it and allowing ions to flow through

A cell secretes a chemical signaling molecule (hormone) into the bloodstream, which carries it across the body to act on target cells in a distant organ. What type of cell communication is occurring?

Endocrine signaling

Which type of signaling molecule can freely diffuse directly across the nonpolar hydrophobic core of the plasma membrane to bind to an intracellular receptor?

Small hydrophobic steroid hormones (e.g., testosterone)

A ligand binds to a cell-surface receptor, initiating a relay cascade involving multiple intracellular proteins that amplify and convert the external message inside the cell. What term best describes this relay stage?

Response

An intracellular enzyme transfers a phosphate group from ATP onto a target protein, inducing a conformational change that usually activates the protein. Which class of enzyme catalyzes this reaction?

Kinase

Which structural feature is a hallmark characteristic shared by all G-protein-coupled receptors (GPCRs)?

Seven transmembrane α-helices passing through the phospholipid bilayer

When an extracellular ligand binds to a GPCR, what immediate event switches the associated heterotrimeric G-protein alpha (Gα) subunit into its active state?

It releases GDP and binds a molecule of GTP.

What initial structural event is required for Receptor Tyrosine Kinases (RTKs) to activate upon extracellular ligand binding?

Ligand-induced receptor dimerization and transphosphorylation

Which statement accurately compares the molecular switch mechanisms of the Ras protein (in RTK signaling) and the Gα subunit (in GPCR signaling)?

Both function as GTP-binding molecular switches that are OFF when bound to GDP and ON when bound to GTP

How does a ligand-gated ion channel alter cellular activity upon neurotransmitter binding?

It undergoes a conformational change that opens a channel pore, allowing specific ions to rapidly flow across the membrane down their electrochemical gradient

Why might two different cell types exposed to the exact same extracellular signal molecule and possessing the same cell-surface receptor produce completely different physiological responses?

The two cell types express different downstream intracellular transduction proteins and target pathways

Which structural feature is a defining hallmark common to all G-Protein Coupled Receptors (GPCRs)?

A single polypeptide chain that weaves through the plasma membrane seven times (7 transmembrane α-helices)

How does an extracellular ligand binding to a GPCR activate the associated heterotrimeric G-protein?

The receptor changes shape, causing the G$\alpha$ subunit to drop GDP and bind a molecule of GTP.

What are the components of the heterotrimeric G-protein associated with a GPCR, and which subunit acts as the GTP-binding molecular switch?

α, β, and γ subunits; the α subunit acts as the GTP-binding molecular switch

What is the initial activation step when an extracellular signaling molecule (such as EGF or insulin) binds to a Receptor Tyrosine Kinase (RTK)?

Two RTK single-pass transmembrane monomers come together in the membrane to form a dimer

Following ligand-induced dimerization of Receptor Tyrosine Kinases (RTKs), what process occurs between the two receptor subunits?

They undergo transphosphorylation, where each subunit uses ATP to phosphorylate tyrosine residues on its partner subunit

What is the immediate physiological role of the phosphorylated tyrosine residues created on the intracellular domain of an activated RTK?

They act as docking sites (scaffolds) that recruit specific cytoplasmic signaling proteins

How does the activation mechanism of the Ras protein in RTK signaling differ from that of the G$\alpha$ subunit in GPCR signaling?

The Gα subunit is directly stimulated by a shape change in its receptor, whereas Ras is indirectly stimulated via cytosolic adapter/scaffold proteins recruited to the RTK.

Which of the following enzyme-coupled receptors possesses NO intrinsic kinase activity of its own, but instead recruits cytoplasmic kinases upon ligand binding?

Cytokine Receptors (e.g., JAK/STAT Pathway Receptors)

What is the correct sequential order of protein activations in the downstream Ras/MAPK cascade following RTK activation?

Active Ras > MAP Kinase Kinase Kinase > MAP Kinase Kinase > MAP Kinase > Cellular Response

If a muscle cell is injected with a non-hydrolyzable GTP analog (a form of GTP that cannot be cleaved into GDP) and then treated with epinephrine (which acts via a GPCR), what will happen to the signaling pathway?

The Gα subunit will pick up the GTP analog and remain permanently ON, leading to continuous downstream activation and depletion of glycogen.

Scorpion alpha toxin, a component of scorpion venom, dramatically prolongs the change in membrane potential during the firing of a nerve impulse. Which one of the following hypotheses best explains how alpha=toxin exerts its effects?

It slows the inactivation of the voltage-gated Na+ channel

Which structural region of a neuron acts as the site where incoming signals are integrated and an action potential is typically initiated if threshold is reached?

Axon hillock

In a resting neuron maintaining a resting membrane potential of approximately -70 mV, what is the relative distribution of sodium (Na+) and potassium (K+) ions across the cell membrane?

Na+ is higher outside the cell; K+ is higher inside the cell

How does the Na+/K+ ATPase pump contribute to maintaining the resting membrane potential, and how much cellular energy does it consume?

Pumps 3 Na+ out and 2 K+ in using ATP; consumes ~30% of a neuron's total ATP

A resting neuron receives a stimulus that causes its membrane potential to shift from -70 mV to -60 mV. Will an action potential fire?

No, because the membrane potential did not reach the threshold voltage of -55 mV

During the depolarization (rising phase) of an action potential, which ion channels open, and in which direction do the ions move?

Voltage-gated Na+ channels open; Na+ rushes into the cell

What ionic events account for the repolarization (falling phase) and subsequent hyperpolarization (undershoot) of the membrane potential?

Na+ channels inactivate; slow-closing voltage-gated K+ channels open, allowing K+ efflux out of the cell

Why are action potentials unable to move backward ("upstream") toward the soma as they travel along an axon?

The preceding segment is in a refractory period due to inactivated voltage-gated Na+ channels and hyperpolarization

What role does the myelin sheath play in electrical signal propagation down a myelinated axon?

It acts as an electrical insulator, allowing the action potential to "jump" quickly between Nodes of Ranvier

When an action potential arrives at the axon terminal of a presynaptic neuron, what immediate event triggers the exocytosis of neurotransmitters into the synaptic cleft?

Opening of voltage-gated Ca2+ channels and subsequent influx of Ca2+ ions

If a postsynaptic neuron receives a neurotransmitter that opens ligand-gated K+ channels, what effect will this have on the postsynaptic cell?

K+ rushes out of the cell, hyperpolarizing the cell and making it less likely to fire an action potential

What is resting membrane potential?

-70mV

A cell is depolarized from -70mV to -60mV. Will an action potential fire?

No, the membrane potential did not reach threshold of -55mV

Why are action potentials unable to move “backwards”?

Voltage gated Na+ channels are inactivated and cannot open, leading to hyperpolarization

If all calcium was cleared from the body, what effect would this have on cell signal propagation?

Neurotransmitters would not be released into the synaptic cleft

What are the four essential elements of cell signaling?

cell communication are a signaling cell, a signaling molecule, a receptor protein, and a responding cell.

What are the steps that occur when a signaling molecule binds to a receptor on a responding cell?

receptor activation, signal transduction, response, and termination

If a hormone is released into the bloodstream so that it comes into contact with many cells, what determines which cells in the body respond to the hormone?

Only cells that have receptors for the hormone respond to the signal, so signaling can be specific for particular cells. These receptors are not present on every cell; different cell types express a different assortment of receptor types on their surface. Thus, only a subset of cells responds to given signaling molecule, even if that signaling molecule comes into contact with many cells.

What is one way in which endocrine, paracrine, autocrine and contact-dependent signaling are similar to one another?

The steps involved are all the same: receptor activation, signal transduction, response, and termination.

What is one way in which endocrine, paracrine, autocrine and contact-dependent signaling are different to one another?

The distance between the signaling and responding cell.

How are receptors that bind polar and nonpolar signaling molecules similar, and how are they differnet?

both act as specific molecular switches that change shape (undergo a conformational change) upon binding their target ligand to initiate a cellular response

How can cells respond to external signaling molecules, even when those signaling molecules cannot enter the cell?

signal transduction, driven by cell-surface receptors

What are three possible initial steps following the binding of a signaling molecule to a receptor?

G-protein-coupled receptors bind and activate G proteins, receptor kinases are activated and phosphorylate other proteins, and ion channels open or close

Is the term “G protein” just a shorter name for a G-protein coupled receptor?

No. A G protein-coupled receptor that interacts with a G protein located inside the cell on the cytoplasmic side of the cell membrane.

What are three ways in which a signal is amplified in a G protein-coupled receptor signaling pathway?

Single receptor activates multiple G proteins, Second messenger production by enzymes, and Enzymatic cascade via protein kinases

What are three ways in which the response of a cell to a signal can be terminated?

The receptor itself could change its conformation to the inactive form; the protein interacting with the intracellular portion of the receptor could become inactivated; or any of the downstream proteins could become inactivated.

How are receptor kinase pathways similar to G protein-coupled receptor pathways?

Signaling by the receptor kinase follows the same steps as signaling by G protein-coupled receptors, including receptor activation, signal transduction, cellular response, and termination.

How are receptor kinase pathways different from G protein-coupled receptor pathways?

The responses to receptor kinase signaling tend to be long-lasting in comparison to the relatively short-term responses to G protein-coupled receptor signaling.

What are the three basic types of neurons?

Sensory neurons interneurons and motor neurons

What are the functions of these three types of neurons?

Sensory neurons respond to physical features of the environment, such as temperature, light, and touch, or to chemical signals such as molecules conveying odor and taste. Interneurons process information received by the sensory neurons and transmit it to different body regions. Motor neurons produce a suitable motor response in muscles based on information from Sensory neurons and interneurons.

How is a signal transmitted from one end of a nerve cell to another and from one neuron to another?

Nerve signals are transmitted electrically from one end of the neuron to the other. They are transmitted chemically across a synapse from one neuron to another.

Where does the digestion of proteins occur?

stomach and small intestine

Bile from the liver is secreted into which organ(s)

first to gallbladder, then to small intestine

What signals the release of bile salts?

CCK

What is the purpose of the gastrin negative feedback loop?

to prevent the over-secretion of acidic HCl, which could harm the lining of the stomach

What are the 3 regions of the small intestine?

duodenum, jejunum, ileum

What type of transporters are involved in simple diffusion?

no transporters involved in simple diffusion

An enzyme is isolated and found to operate most efficiently at a low pH. Which of the following enzymes would you expect this to be?

pepsin

What are the major parts of the hindgut?

cecum, colon, rectum

Why is dimerization necessary for RTKs?

Dimerization allows for the activation of the intracellular portion of the receptor via transphosphorylation

Suppose a toxin in the cell blocks GTP binding to the RAS protein. How would you expect the cell signaling pathway to be affected?

RAS would remain inactive and fail to activate the MAPK pathway

What is the fundamental difference between paracrine and juxtacrine signaling?

Juxtacrine signaling is contact dependent, meaning the cells needs to be touching each other for proper signaling to occur.

Cortisol, a lipid hormone derived from cholesterol, is transported from the adrenal gland of the kidney to the skeletal muscle to stimulate protein breakdown. Which of the following best describes the most likely mechanism of cortisol signal reception?

Cortisol crosses the membrane and binds to an internal receptor, triggering a change in gene expression that increases enzymatic catabolism of proteins

A membrane receptor is activated by an extracellular ligand, triggering the transphosphorylation of its associated subunits. What membrane receptor is described in this situation?

Receptor Tyrosine Kinase

Which of the following is true regarding the structure of a GCPR?

There are 7 transmembrane domains and 3 main intracellular proteins

If the kinase of an enzyme-coupled receptor is removed, which process will stop first?

transphosphorylation of the receptor subunits

A ligand gated ion channel is activated, allowing sodium to flow through with its concentration gradient. Which direction would you expect to see sodium movement?

sodium will move into the cell

Which of the following best describes the purpose of the myelin sheath?

to insulate the axon and increase the speed of the electrical signal

What is the difference between leak channels and gated ion channels?

Leaks channels are always open, while gated channels must be activated by an external factor

Say the membrane potential of a particular cell is -30mV. Which of the following best describes the movement of Ca²⁺ due to only its electrostatic gradient?

Calcium will move into the cell due to its positive charge being attracted to the negative charge inside the cell

What is the resting potential of the cell?

-70mV

Which of the following would lead to depolarization?

flow of Na+ into the cell

Researchers are experimenting by adding more sodium leak channels into the cell membrane. Which of the following is a likely result of this experiment?

Sodium will flow into the cell, increasing membrane potential

A cell's membrane potential increases from -70mV to +30mV. Which ion likely elicited this change?

sodium