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Mcat answer ?

Answer: A. System or cell
Here's the reasoning:
Question: Carrier molecules always require a configurational change to function. For Passive transport, this configurational change also requires energy - who provides this energy?
Why A is correct:
  • Carrier proteins (transporters) used in facilitated diffusion (passive transport) undergo conformational changes to move molecules across the membrane.
  • The energy for this conformational change comes from the concentration gradient itself - meaning the system/cell provides the driving force through the electrochemical gradient stored in the system.
  • The molecule moving down its gradient releases potential energy, which drives the carrier's conformational change.
Why the other options are wrong:
  • B. Molecule itself - the molecule doesn't carry its own energy for this purpose; it simply moves down a gradient.
  • C. Carrier will use ATP - ATP is used in active transport, NOT passive transport. Facilitated diffusion does NOT require ATP.
  • D. Non-metabolic energy by the system - this is a distractor; while the energy is technically non-ATP, the phrasing "non-metabolic" is misleading. The correct framing is that the system/cell provides the energy via the concentration gradient.
Key concept: In passive/facilitated transport, the thermodynamic energy stored in the concentration gradient (a property of the system) drives both molecular movement AND the carrier's conformational change - no ATP needed.
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Qno 7 answer

Answer: D. Arginase
Here's the reasoning:
Question 7: Which of the following will NOT be channelized through RER?
Why D (Arginase) is correct:
The RER processes and transports proteins that are:
  • Secreted outside the cell (e.g., antibodies, hormones)
  • Destined for the plasma membrane
  • Targeted to lysosomes
Arginase is an enzyme involved in the urea cycle and functions inside the cytoplasm/liver cells (it's an intracellular cytosolic enzyme). It does NOT need to pass through the RER because it stays within the cytosol - it is NOT a secretory protein.
Why the others DO go through RER:
  • A. Immunoglobulins - antibodies are secretory proteins, made on RER and exported
  • B. Thyroid stimulating hormone (TSH) - peptide hormone that is secreted, processed through RER → Golgi → secretory vesicles
  • C. Interferons - secretory proteins (cytokines) released from cells, processed via RER
Key Rule: Any protein that is secreted, membrane-bound, or sent to lysosomes goes through the RER. Cytosolic/intracellular metabolic enzymes like arginase bypass the RER entirely and are made on free ribosomes.

Qno 8 answer?

Answer: A. It is involved in Glycogenolysis
Here's the reasoning:
Question 8: Which of the following statements about the endoplasmic reticulum is correct?
Why A is correct:
The Smooth ER (SER) contains the enzyme glucose-6-phosphatase, which is the final step of glycogenolysis (and gluconeogenesis). This enzyme converts glucose-6-phosphate → free glucose, which is then released into the bloodstream. This reaction occurs on the luminal surface of the SER membrane - so yes, the ER is directly involved in glycogenolysis.
Why the others are WRONG:
  • B. It consists of a collection of unconnected vesicles - FALSE. The ER is a continuous network of membranes (cisternae and tubules), not disconnected vesicles.
  • C. It is continuous with outer mitochondrial membrane - FALSE. The ER is continuous with the outer nuclear envelope/membrane, NOT the mitochondrial membrane. These are separate organelles.
  • D. It contains enzymes for the breakdown of cellular organelles - FALSE. That function belongs to lysosomes, which contain hydrolytic enzymes for autophagy/breakdown of organelles.
Key fact to remember:
ER (Smooth) → Glycogenolysis (via glucose-6-phosphatase) Lysosomes → Breakdown of organelles ER is continuous with the nuclear envelope, not mitochondria
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