A genetic mutation causes a cell to produce lysosomal hydrolase enzymes that lack their mannose-6-phosphate tag. What is the most likely consequence?
AThe enzymes accumulate inside lysosomes but become inactive without the tag
BThe enzymes are secreted outside the cell instead of being delivered to lysosomes
CThe enzymes digest the lysosomal membrane from the inside
DThe cell upregulates lysosome production to compensate for the reduced enzyme delivery
Mannose-6-phosphate is a molecular address label added by the Golgi apparatus that directs newly synthesized hydrolases to the lysosomal pathway. Without this tag, the enzymes are misrouted to the default secretory pathway and released outside the cell — exactly what happens in I-cell disease. The lysosomes become depleted of functional enzymes, substrates accumulate inside them, and the cell loses its ability to degrade endocytosed material and perform autophagy.
Question 2 Multiple Choice
A lysosome ruptures and spills its hydrolase enzymes into the cytoplasm. Why is the cellular damage more limited than the enzyme concentration alone would suggest?
AThe cytoplasm contains universal inhibitor proteins that neutralize all lysosomal enzymes
BLysosomal hydrolases are optimized for acidic pH (~4.5) and lose most activity in the neutral cytoplasm (~7.2)
CThe endoplasmic reticulum immediately sequesters the spilled enzymes before they can act
DThe lysosomal membrane rapidly re-forms around the spilled enzymes, containing them
Lysosomal hydrolases require pH 4.5–5.0 to function efficiently — roughly 100 times more acidic than the cytoplasm at pH 7.2. In the neutral cytoplasm, most of their enzymatic activity is lost. This pH-dependence is a deliberate safety feature: the same acidic environment that enables digestion inside the lysosome is what renders the enzymes relatively harmless if they escape. The proton pumps that maintain lysosomal acidity are thus both functional machinery and a containment strategy.
Question 3 True / False
Autophagy is an emergency response that cells activate primarily during starvation; under normal conditions, lysosomes process mainly material captured from outside the cell.
TTrue
FFalse
Answer: False
Autophagy is a constitutive, ongoing quality-control process that operates under normal conditions, not only during starvation. Cells continuously engulf and degrade damaged organelles, misfolded proteins, and surplus cellular components as routine maintenance. During starvation, autophagy intensifies to provide nutrients, but characterizing it as exclusively an emergency response misses its normal function. Failures in basal autophagy contribute to neurodegenerative diseases (e.g., Parkinson's) where protein aggregates accumulate that functional autophagy would normally clear.
Question 4 True / False
Lysosomal storage diseases — in which specific substrates accumulate in lysosomes — demonstrate that lysosomes use substrate-specific enzymes rather than a single all-purpose digestive mechanism.
TTrue
FFalse
Answer: True
Each lysosomal storage disease is characterized by accumulation of a specific class of molecule — sphingolipids in Tay-Sachs, glucocerebrosides in Gaucher disease — because the one enzyme responsible for that substrate is absent or defective. If lysosomes used non-specific enzymes, losing any single enzyme would cause broad general dysfunction rather than the buildup of one particular substrate. The disease pattern is direct evidence that each of the ~50 lysosomal hydrolases handles a distinct molecular class.
Question 5 Short Answer
Why is the acidic interior of the lysosome not only a functional requirement for digestion, but also a safety mechanism protecting the cell from self-destruction?
Think about your answer, then reveal below.
Model answer: Lysosomal hydrolases are optimized to work at pH 4.5–5.0. In the neutral cytoplasm (pH 7.2), these enzymes lose most of their activity. This means that if the lysosomal membrane ruptures, spilled enzymes cannot efficiently digest cytoplasmic components — limiting the damage. The cell uses the same acidic pH that enables digestion inside the lysosome as an automatic containment strategy if the membrane fails.
This is an elegant design principle: the property that gives lysosomes their digestive power is also what makes them safe to keep in the cell. If the hydrolases worked at neutral pH, any membrane rupture would risk catastrophic self-digestion. By coupling enzymatic activity to an acidic environment that must be actively maintained by ATP-powered proton pumps, the cell ensures that containment failure doesn't automatically mean enzymatic function outside the lysosome — the enzymes need the acidic environment to do their job.