Pre AP Biology -CELLS 4.2 Response to Stimuli - MCQ Exam Style Questions -New Syllabus 2025-2026
Pre AP Biology -CELLS 4.2 Response to Stimuli – MCQ Exam Style Questions – New Syllabus 2025-2026
Pre AP Biology -CELLS 4.2 Response to Stimuli – MCQ Exam Style Questions – Pre AP Biology – per latest Pre AP Biology Syllabus.
▶️ Answer/Explanation
The correct option is d.
Tardigrades are famous for a survival strategy called anhydrobiosis.
When facing extreme dehydration, they lose over $95\%$ of their body water.
They retract their legs and head to form a dormant, shriveled state called a tun.
In this state, their metabolism slows to less than $0.01\%$ of normal levels.
This allows them to survive for years in extreme environments.
Once re-exposed to water, they rehydrate and return to an active state within hours.
▶️ Answer/Explanation
The correct option is c. pyruvate oxidation and the citric acid cycle.
Glycolysis occurs in the cytosol and does not release any $CO_2$.
During pyruvate oxidation, each pyruvate ($3C$) is converted to acetyl-CoA ($2C$), releasing $1$ molecule of $CO_2$.
The citric acid cycle (Krebs cycle) further breaks down the acetyl group, releasing $2$ more molecules of $CO_2$ per turn.
Oxidative phosphorylation uses oxygen as a final electron acceptor to form $H_2O$ but does not produce $CO_2$.
Therefore, all respiratory $CO_2$ is generated within the mitochondrial matrix during these two specific stages.
▶️ Answer/Explanation
A. Answer: (B) — The diagram shows a G protein-coupled receptor (GPCR) pathway, typical for peptide hormones which are hydrophilic and bind to cell surface receptors (Protein B), unlike steroid hormones which cross the membrane.
A. Answer: (A) — When the G protein binds \(GTP\), it undergoes a conformational change to become active. As shown in the diagram, this active G protein moves to stimulate the enzyme adenylyl cyclase.
A. Answer: (C) — Cyclic AMP (\(cAMP\)) is synthesized intracellularly by adenylyl cyclase. It acts as a secondary messenger, diffusing through the cytoplasm to activate Protein Kinase A, thereby relaying and amplifying the signal.
A. Answer: (D) — If Molecule X is permanently bound, the receptor remains constantly active. This leads to continuous activation of G proteins and adenylyl cyclase, causing sustained high levels of \(cAMP\) and constitutive activity of Protein Kinase A.
▶️ Answer/Explanation
The correct option is A.
When photons of light strike the antenna pigments of Photosystem II ($PSII$), the energy is funneled to the reaction center ($P680$).
This energy boosts electrons to a higher energy level, causing them to be captured by a primary electron acceptor.
These excited electrons are then passed into the electron transport chain ($ETC$) to create a proton gradient.
The “hole” left in $PSII$ is filled by electrons obtained from the photolysis of $H_2O$.
Options B and C are incorrect as $ATP$ and glucose are products of later stages or the Calvin cycle.
Option D is incorrect because $CO_2$ is consumed in photosynthesis, not produced via dehydration synthesis.
▶️ Answer/Explanation
Phospholipids are amphipathic, meaning they have a hydrophilic (polar) head and hydrophobic (nonpolar) tails.
In a nonpolar environment like oil, the hydrophobic tails will seek to maximize contact with the solvent.
Conversely, the hydrophilic heads are sequestered away from the nonpolar oil to minimize unfavorable interactions.
Model B correctly shows an “inverted” bilayer where tails face the outside and heads are tucked inside.
This orientation allows the nonpolar tails to interact with the oil via van der Waals forces.
The central core of the structure becomes a sequestered polar environment for the heads.
Therefore, the correct description and model match is Option B.