How a bacterium survives

1. Hold the boundary

   The cell membrane controls molecular passage; the cell wall primarily resists mechanical and osmotic stress. Small nonpolar oxygen crosses the lipid bilayer more readily than glucose or a charged sodium ion.

   Representative answer and reasoning

   Choose oxygen. Glucose is large and polar, while sodium is charged, so both usually depend on membrane proteins.

2. Move matter

   Diffusion describes net movement from higher concentration to lower concentration. Channels and carriers give selected substances paths through the membrane.

   Representative answer and reasoning

   Oxygen can cross directly; an ion can use a channel; a polar nutrient can use a selective carrier.

3. Survive salt shock

   In osmosis, water moves both ways while the net movement follows relative water availability. A saltier exterior produces net water loss; a dilute exterior produces net water entry and the wall helps resist expansion.

   Representative answer and reasoning

   After external salt rises, predict net water movement out of the bacterium.

4. Pay for movement

   Down-gradient transport can be passive. Accumulating a scarce nutrient against its gradient requires an energy-coupled process, though the transporter need not use ATP directly.

   Representative answer and reasoning

   Choose an energy-coupled transporter when the cell must keep accumulating a nutrient already concentrated inside.

5. Build a response

   DNA stores an instruction, RNA carries an expressed message, and a ribosome reads the message while assembling a protein. This useful model leaves out more complex regulation.

   Representative answer and reasoning

   Order the pathway DNA → RNA → ribosome → protein.

6. Face an antibiotic

   Simplified antibiotic targets include cell-wall assembly, membrane integrity, and ribosome function. A ribosome-targeting mechanism disrupts normal production of a new transport protein. This is not treatment guidance.

   Representative answer and reasoning

   Connect wall disruption to structural support, membrane disruption to controlled gradients, and ribosome disruption to protein production.

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