Plant and animal cells under the light microscope – Cell Structure and Specialisation (Senior 4 Biology) | Ganzaa

Cell Structure and Specialisation

Lessons

Senior 4 · Biology

Cell Structure and Specialisation

5 lessons

Lesson 1 of 5

Plant and animal cells under the light microscope

20 min

A leaf one cell thick

The leaf of a moss is so thin that you can put a whole leaf on a slide, add a drop of water, and see its cells without cutting or staining anything. Each cell is a little six-sided box of wall, lined with bright green chloroplasts. Next to it, a thin stained section of a mammal's liver shows a very different picture: soft, many-sided cells packed together, each with a large round nucleus, and no green at all.

In S1 you compared onion and cheek cells. This lesson asks A-level questions: exactly which structures can a light microscope show, which can it not, how big are they, and how do you decide whether an unknown cell came from a plant or an animal?

Definition

Cell: the smallest unit that can carry out all the processes of life, bounded by a cell surface membrane.

Organelle: a structure inside a cell with a particular function, such as the nucleus or a chloroplast.

Cell wall: in plants, a rigid layer of cellulose outside the cell surface membrane that supports the cell and stops it bursting.

Vacuole: a fluid-filled sac bounded by a membrane (the tonoplast); in many plant cells one large central vacuole holds cell sap.

Nucleolus: a dense, darkly staining region inside the nucleus where ribosomes are made.

What the light microscope shows

Figure 1 shows high-power drawings of the two cells described above. Compare them with the photographs below.

Structure	Plant cell	Animal cell	Seen with a light microscope?
Cell surface membrane	Present, pressed against the wall	Present, forms the boundary	Only as the edge of the cell; its structure needs an electron microscope
Cell wall (cellulose)	Present	Absent	Yes
Nucleus and nucleolus	Present	Present	Yes, especially when stained
Chloroplasts	In green parts only (leaves, young stems)	Absent	Yes, as green discs
Large central vacuole	Usually present	Absent (small vacuoles only)	Yes, as a clear or coloured space
Starch grains	Often present (stain blue-black with iodine)	Absent (animals store glycogen)	Yes
Mitochondria	Present	Present	Only as tiny dots with special stains
Ribosomes, endoplasmic reticulum, Golgi apparatus, lysosomes	Present	Present	No: too small, need an electron microscope
Centrioles	Absent in most flowering plants	Present	Rarely, only during division
Shape	Regular, fixed by the wall	Variable, often rounded	Yes

Worked example 1: measuring cells from a scale bar

On a screen, the 50 µm scale bar in the moss photograph below measures 30 mm. One cell measures 26 mm long and one chloroplast 2 mm across. Estimate the actual length of the cell and the diameter of the chloroplast.

The scale: 30 mm on the screen represents 50 µm, so 1 mm represents $\frac{50}{30} = 1.67\,\mu\mathrm{m}$ .
Cell: $26 \times \frac{50}{30} = 43\,\mu\mathrm{m}$ (to 2 significant figures).
Chloroplast: $2 \times \frac{50}{30} = 3.3\,\mu\mathrm{m}$ .
Check: plant cells of 20–100 µm and chloroplasts of a few micrometres are typical, so both answers are sensible. The 2 mm reading is imprecise, so the chloroplast size is only a rough estimate.

Living leaf cells of the moss Plagiomnium elatum at about ×400: six-sided cells with clear cell walls, each lined with many small green chloroplasts around a paler centre. The scale bar at the bottom right is 50 µm. Photo: Kristian Peters, CC BY-SA 3.0, via Wikimedia Commons

Worked example 2: plant or animal?

A cell from an unlabelled slide has a regular, box-like shape and a thick outer layer, no chloroplasts, and many oval grains that turn blue-black with iodine. Decide whether it came from a plant or an animal and suggest which organ.

A thick outer layer and fixed shape suggest a cellulose cell wall: a plant cell.
Blue-black with iodine means starch, which plants store and animals do not (animals store glycogen, which does not give this colour).
No chloroplasts means it is not from a green part of the plant: a storage organ that grows away from light, such as a potato tuber or a cassava root.
The absence of chloroplasts alone would not prove "animal": most cells in roots and tubers have none.

Microbehunter: "Observing moving CELL ORGANELLES of an ONION under the microscope". Watch the small particles streaming in the cytoplasm of living cells, something an electron microscope can never show.

Warning

Common mistakes

Saying plant cells have a cell wall "instead of" a cell membrane. They have both; the membrane lies just inside the wall.
Labelling ribosomes, endoplasmic reticulum or Golgi on a light-microscope drawing. They cannot be seen at that resolution.
Assuming every plant cell has chloroplasts. Root, tuber and onion-bulb cells have none.
Shading or colouring a biological drawing. Use clear, single lines and ruled label lines that do not cross.

Tip

Exam technique. To identify a cell as plant or animal, use features that are always present: a cellulose wall and, usually, a large permanent vacuole for plants; no wall and centrioles for animals. Chloroplasts are good evidence for a plant when present, but their absence proves nothing.

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Practice: cells under the light microscope

List three structures visible in a plant cell, but not in an animal cell, under a light microscope.
Explain why the endoplasmic reticulum cannot be seen with a light microscope.
Why are liver cells stained before viewing, while moss leaf cells can be viewed without stain?
A scale bar of 20 µm measures 16 mm on a photograph. A cell on the same photograph measures 52 mm. Find its actual length.
A cell is 80 µm long. How long will it be in a drawing at a magnification of ×500? Give your answer in mm.
A cell has no cell wall, a nucleus, and many small dark granules that do not stain with iodine. Identify it as plant or animal, with reasons.
Evaluate the statement: "Animal cells are smaller than plant cells."

Show answersHide answers

Cell wall, chloroplasts (in green tissue), large central vacuole; also starch grains.
Its membranes are about 7 nm thick and its spaces far closer together than 200 nm, the limit of resolution of light, so it cannot be resolved.
Liver cell contents are almost colourless, so stains are needed for contrast; moss cells have their own colour in their chloroplasts, and the leaf is only one cell thick, so light passes through it.
$52 \times \frac{20}{16} = 65\,\mu\mathrm{m}$ .
$I = A \times M = 80 \times 500 = 40\,000\,\mu\mathrm{m} = 40\,\mathrm{mm}$ .
Animal: no cell wall, and the granules are not starch (they could be glycogen, which animals store).
Generally true but not always. Many plant cells (20–100 µm or more) are larger than typical animal cells (10–30 µm), partly because the large vacuole takes up space. But some animal cells are very large: a human egg cell is about 0.1 mm, and some nerve cells are over a metre long. It is a trend, not a rule.

Quick check

Which structure could you label on a drawing made with a school light microscope at ×400?

Key points

A light microscope shows the cell wall, membrane boundary, cytoplasm, nucleus and nucleolus, chloroplasts, vacuoles and starch grains, but not ribosomes, ER, Golgi or lysosomes.
Plant cells: cellulose wall, often a large vacuole, chloroplasts in green tissues, starch. Animal cells: no wall, variable shape, centrioles, glycogen.
Use a scale bar to find actual size: actual size = image size × (scale-bar value ÷ scale-bar length).

Sources and credits

2.1.4: Eukaryotic Cells, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for the structures of plant and animal cells and how they differ. Adapted by Ganzaa.
Figure 1, the worked examples, practice and quick check: written by Ganzaa.
Photos: Kristian Peters, CC BY-SA 3.0; Arya the Ace, CC BY 4.0; Mikael Häggström, CC0; all via Wikimedia Commons.
Video: "Observing moving CELL ORGANELLES of an ONION under the microscope", Microbehunter, embedded from YouTube.

1A leaf one cell thick

2What the light microscope shows

A leaf one cell thick

What the light microscope shows