IB DP Biology- A2.2 Cell structure - IB Style Questions For SL Paper 1A -New Syllabus

Question

The electron micrograph shows part of a eukaryotic cell. What is the organelle labelled X?

(A) Mitochondrion
(B) Smooth endoplasmic reticulum
(C) Lysosome
(D) Nucleus

▶️ Answer/Explanation

Correct Answer: \( \boxed{\mathbf{B}} \)

The labelled structure is the smooth endoplasmic reticulum (SER), a network of interconnected membranes involved in lipid synthesis, detoxification, and intracellular transport. It is not a mitochondrion, lysosome, or nucleus.

Question

The table shows features of three types of cells (X, Y, and Z).

Feature	X	Y	Z
Cell wall	absent	present	present
Centrioles	present	absent	absent
Large vacuole	absent	present	present
Plastids	absent	present	absent

To which kingdom do each of the cells belong?

X	Y	Z
Animal	Fungi	Plant
Fungi	Animal	Protist
Animal	Plant	Fungi
Protist	Animal	Plant

(A) Animal – Fungi – Plant
(B) Fungi – Animal – Protist
(C) Animal – Plant – Fungi
(D) Protist – Animal – Plant

▶️ Answer/Explanation

Cell X lacks a cell wall but has centrioles, identifying it as an animal cell.
Cell Y has a cell wall, large vacuole, and plastids, identifying it as a plant cell.
Cell Z has a cell wall and large vacuole but no plastids, typical of a fungal cell.
✅ Answer: (C)

Question

The micrograph of a section through a plant stem shows at least ten different types of cells.

What explains the differences between these cells?

A. Only one gene is expressed in each cell type.
B. Different genes are expressed in each cell type.
C. Only useful genes remain in the DNA of each cell type.
D. Changes in the DNA sequence take place when these cells develop.

▶️ Answer/Explanation

Answer: B. Different genes are expressed in each cell type.

Explanation: In multicellular organisms (including plants), nearly all cells contain the same DNA. The differences among cell types arise from differential gene expression—distinct sets of genes are turned on or off in each cell type. This selective expression produces different proteins, which in turn confer the unique structures and functions of xylem, phloem, parenchyma, sclerenchyma, etc., even within the same organ.

Evaluating the options:

A. Incorrect — Cells express many genes, not just one. The pattern (profile) of expressed genes differs by cell type.

B. Correct — Different cell types arise because different genes are expressed, despite identical genomes.

C. Incorrect — Cells do not delete “unused” genes; they retain the full genome and regulate genes primarily via expression control.

D. Incorrect — The DNA sequence generally does not change during normal differentiation; what changes is which genes are active.

IB DP Biology- A2.2 Cell structure - IB Style Questions For SL Paper 1A -New Syllabus

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