Biology 9700/12 — May/June 2018

Background Concept

All cells share some basic features, but the structure of plant cells (eukaryotic) and prokaryotic cells differs in important ways. Despite these differences, both cell types must carry out the same fundamental processes — enclosing their contents, synthesising proteins, and providing structural support — so they share certain features:

• Cell surface membrane: Every cell, whether prokaryotic or eukaryotic, is bounded by a plasma (cell surface) membrane. In prokaryotes, this is the only membrane system present (apart from infoldings such as mesosomes in some species); in plant cells, it lies just inside the rigid cell wall.
• Ribosomes: Both cell types contain ribosomes, the sites of protein synthesis. Prokaryotic ribosomes are 70S (smaller), while eukaryotic (plant) ribosomes are 80S (larger), but the basic function is identical.
• Cell wall: Plant cells have a cellulose cell wall that provides rigidity and prevents the cell from bursting as water enters by osmosis. Prokaryotic cells also have a cell wall, but it is made of peptidoglycan (murein), not cellulose. The wall gives the bacterium its shape and protects it from osmotic lysis.

The other structures that often differ include: a true membrane-bound nucleus (absent in prokaryotes — their DNA is in a nucleoid region), mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, and other membrane-bound organelles (all absent in prokaryotes).

Understanding the Question

The question lists three cellular features and asks which are found in both plant cells and prokaryotic cells. The correct answer must identify the features shared by these two very different cell types, not features unique to one.

Approach

Go through each numbered feature and decide whether it is present in (i) plant cells and (ii) prokaryotic cells. Only features present in both cell types should be selected.

Step-by-Step Reasoning

• Feature 1 — cell wall: Plant cells ✓; prokaryotic cells ✓ (peptidoglycan wall). → Include.
• Feature 2 — cell surface membrane: Plant cells ✓; prokaryotic cells ✓ (plasma membrane enclosing cytoplasm). → Include.
• Feature 3 — ribosomes: Plant cells ✓; prokaryotic cells ✓ (70S ribosomes for protein synthesis). → Include.

All three features are common to plant and prokaryotic cells, so the answer is A (1, 2 and 3).

Key Takeaways

• Despite huge structural differences between plant and prokaryotic cells, all cells need a plasma membrane, ribosomes for protein synthesis, and (in walled cells) a cell wall.
• The chemistry of the cell wall differs: cellulose in plants, peptidoglycan in bacteria — but the presence of a wall is shared.
• A common misconception is to think prokaryotes have no cell wall, or to confuse cell wall with cell surface membrane. They are distinct structures: the membrane is the inner phospholipid layer, and the wall (when present) sits outside it.

Common Mistakes

• Selecting D (2 and 3 only): forgetting that bacteria have a cell wall, often confused by the different chemistry (peptidoglycan vs cellulose).
• Selecting B (1 and 2 only): wrongly thinking prokaryotes lack ribosomes — every cell needs ribosomes to make protein.
• Selecting C (1 and 3 only): wrongly thinking prokaryotes do not have a cell surface membrane. All cells are bounded by a plasma membrane.

Things to Be Careful About

• "Cell wall" and "cell surface membrane" are not the same structure; the wall is external to the membrane.
• "Prokaryotic" includes bacteria (and archaea); both groups have a cell wall, although archaeal walls are chemically distinct from bacterial peptidoglycan walls.
• The question asks about plant cells and prokaryotic cells — features found in only one cell type (e.g. large central vacuole, chloroplasts, nucleus) must be excluded.