IB Biology practice questions.

Which structure is present in a plant cell but NOT in a typical animal cell?

• mitochondrion
• cell wall
• nucleus
• ribosome

Answer: cell wall

A cellulose cell wall (plus chloroplasts and a large permanent vacuole) is characteristic of plant cells; the others are common to both.

The 'induced fit' model of enzyme action states that:

• The active site is a rigid shape exactly matching the substrate
• The substrate changes to fit the active site permanently
• The active site changes shape slightly as the substrate binds
• Enzymes are used up during the reaction

Answer: The active site changes shape slightly as the substrate binds

In the induced fit model the active site is flexible and moulds around the substrate as it binds, straining bonds and lowering activation energy, unlike the older rigid lock-and-key idea.

In aerobic respiration, what is the final electron acceptor of the electron transport chain?

• Carbon dioxide
• Oxygen
• NAD+
• Pyruvate

Answer: Oxygen

Oxygen accepts electrons at the end of the chain and combines with protons to form water. Without oxygen, the chain backs up and NADH cannot be reoxidised, halting oxidative phosphorylation.

The Meselson-Stahl experiment demonstrated that DNA replication is:

• Conservative
• Semi-conservative
• Dispersive
• Fully bidirectional only

Answer: Semi-conservative

Using heavy and light nitrogen isotopes, Meselson and Stahl showed each new DNA molecule contains one original and one newly synthesised strand, confirming the semi-conservative model.

Which sampling method is most appropriate for estimating the abundance of a sessile organism, such as barnacles on a rocky shore?

• Quadrat sampling
• Mark and recapture
• Collecting them with a pooter
• Following a single line transect

Answer: Quadrat sampling

Quadrats give a repeatable count of density for organisms that do not move; mark-recapture is for mobile animals and a single transect only samples one line, not overall abundance.