CDS Biology · Cell Biology

Kinds of Cells and the Microscope

How a compound microscope makes a cell look bigger, and how a prokaryotic cell such as a bacterium differs from the eukaryotic cells of plants and animals.

All Cell Biology notes

Why this matters

Seven CDS questions from 2019 (I) to 2026 (II). Five are about prokaryotes, two of them the same question on the nucleoid (2023 (II) and 2026 (I)). Two are about the microscope: what magnification means and the path light takes through it.

Concept 1 of 2: The compound microscope and magnification

Most cells are far too small to see with the eye, so we look at them through a compound microscope. Light from below is gathered onto the specimen, then two lenses enlarge it one after the other. Magnification simply says how many times bigger the image looks than the real thing.

Definition

The compound microscope:

Light comes from a mirror or lamp below, is focused onto the slide by the condenser, passes through the specimen to the objective lens, travels up the body tube, and is enlarged again by the eyepiece next to your eye.
Magnification = size of the image ÷ actual size of the object.
Total magnification = magnification of the objective × magnification of the eyepiece.
Cells are measured in micrometres (µm): 1 µm is one-thousandth of a millimetre.
Robert Hooke first saw and named cells (1665) in a thin slice of cork.

Magnification

\text{Magnification} = \dfrac{\text{size of the image}}{\text{actual size of the object}}

Size of the imagehow big the specimen looks through the microscope
Actual size of the objectthe real size of the specimen

Worked example

A cell 0.02 mm across looks 8 mm across under a microscope. What is the magnification?

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

CDS · 2019 · CDS (I) 2019 — General Knowledge · Q88Moderate

Example 1 · Cell Biology · Kinds of Cells and the Microscope

Which one of the following is the correct sequence of passage of light in a compound microscope?

Image size goes on top

Magnification is image ÷ object, so it is bigger than 1 for an enlarged image. Object ÷ image, or the difference or product of the two sizes, are the wrong options.

The condenser comes before the objective

Light meets the condenser first (below the stage), then the specimen and the objective, then the body tube, and the eyepiece last. An order that puts the eyepiece before the body tube is wrong.

Drill 1 more on the compound microscope and magnification

Concept 2 of 2: Prokaryotic and eukaryotic cells

A prokaryotic cell, such as a bacterium, is the simple kind. Its DNA lies loose in the middle of the cell with no membrane around it, and it has no membrane-bound organelles at all. It still has ribosomes, because every living cell must make proteins. Plant, animal and fungal cells are eukaryotic: they have a true nucleus wrapped in a membrane, and many organelles each in its own membrane.

Definition

Prokaryote against eukaryote:

Prokaryotes: bacteria and cyanobacteria (blue-green algae). No nuclear membrane: the DNA lies in a region called the nucleoid. No mitochondria, Golgi, ER or plastids. They do have a cell wall, a cell membrane, ribosomes (smaller, 70S) and often small extra rings of DNA called plasmids.
In photosynthetic bacteria the pigment sits on infoldings of the cell membrane (membranous vesicles called chromatophores), since there are no chloroplasts.
Eukaryotes: plants, animals, fungi and protists. A true nucleus with a nuclear membrane, membrane-bound organelles, and larger (80S) ribosomes.

Feature	Prokaryotic cell	Eukaryotic cell
Nucleus	No nuclear membrane; DNA in the nucleoid	True nucleus inside a nuclear membraneQ Asked twice, CDS 2023 (II) and 2026 (I): the region holding a bacterium's DNA is the nucleoid.
Mitochondria, Golgi, ER	Absent	Present
Ribosomes	Present (70S)	Present (80S)Q CDS 2025 (II): of mitochondria, a nucleus, ribosomes and Golgi bodies, only ribosomes are in a prokaryotic cell.
Photosynthetic pigment	On membranous vesicles of the cell membrane	In chloroplastsQ CDS 2019 (II): chlorophyll in photosynthetic bacteria is held on membranous vesicles.
Examples	Bacteria, cyanobacteria	Plants, animals, fungi, protists

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

CDS · 2026 · CDS (II) 2026 — General Knowledge · Q114Easy

Example 2 · Cell Biology · Kinds of Cells and the Microscope

Which one of the following is NOT found in a prokaryotic cell ?

Nucleoid, not nucleolus or nucleosome

The nucleoid is the bacterial DNA region with no membrane. The nucleolus is a body inside a true nucleus, and a nucleosome is DNA wound on proteins in eukaryotes.

Prokaryotes do have ribosomes and a wall

'No organelles' means no membrane-bound ones. Ribosomes, the cell wall, the cell membrane and plasmids are all found in bacteria; the nuclear membrane is not.

Drill 4 more on prokaryotic and eukaryotic cells

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

The compound microscope and magnification
Magnification
$\text{Magnification} = \dfrac{\text{size of the image}}{\text{actual size of the object}}$

Reference tables (1)

Prokaryotic and eukaryotic cells5 rows

Feature	Prokaryotic cell	Eukaryotic cell
Nucleus	No nuclear membrane; DNA in the nucleoid	True nucleus inside a nuclear membraneQ Asked twice, CDS 2023 (II) and 2026 (I): the region holding a bacterium's DNA is the nucleoid.
Mitochondria, Golgi, ER	Absent	Present
Ribosomes	Present (70S)	Present (80S)Q CDS 2025 (II): of mitochondria, a nucleus, ribosomes and Golgi bodies, only ribosomes are in a prokaryotic cell.
Photosynthetic pigment	On membranous vesicles of the cell membrane	In chloroplastsQ CDS 2019 (II): chlorophyll in photosynthetic bacteria is held on membranous vesicles.
Examples	Bacteria, cyanobacteria	Plants, animals, fungi, protists

Watch out for (4)

Image size goes on top
→ The compound microscope and magnification
The condenser comes before the objective
→ The compound microscope and magnification
Nucleoid, not nucleolus or nucleosome
→ Prokaryotic and eukaryotic cells
Prokaryotes do have ribosomes and a wall
→ Prokaryotic and eukaryotic cells

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