describe and interpret photomicrographs, electron micrographs and drawings of typical plant and animal cells

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology – Cells as the Basic Units of Living Organisms

Objective

Describe and interpret photomicrographs, electron micrographs and drawings of typical plant and animal cells.

Key Concepts

1. Types of Microscopy

  • Light (photomicroscopy) – uses visible light; suitable for observing whole cells, organelles such as vacuoles, chloroplasts and the nucleus.

  • Transmission electron microscopy (TEM) – electrons pass through a thin section; reveals internal ultrastructure (e.g., ribosomes, endoplasmic reticulum, mitochondrial cristae).

  • Scanning electron microscopy (SEM) – electrons scan the surface; provides three‑dimensional surface detail of cell walls, microvilli and cilia.

2. Typical Plant Cell – Main Features

A typical plant cell is characterised by the following structures (see suggested diagram):

  • Rigid cell wall composed of cellulose, providing structural support.

  • Large central vacuole that stores water, ions and metabolites; contributes to turgor pressure.

  • Numerous chloroplasts containing thylakoid stacks (grana) for photosynthesis.

  • Peripheral cytoplasm with a dense network of actin filaments and microtubules.

  • Typical animal‑type organelles: nucleus, mitochondria, Golgi apparatus, endoplasmic reticulum.

Suggested diagram: labelled drawing of a typical plant cell showing cell wall, plasma membrane, large central vacuole, chloroplasts, nucleus, mitochondria and cytoskeleton.

3. Typical Animal Cell – Main Features

Key structures of a typical animal cell (see suggested diagram):

  • Flexible plasma membrane without a rigid cell wall.

  • Prominent nucleus with a nucleolus and nuclear envelope.

  • Numerous mitochondria for aerobic respiration.

  • Well‑developed endoplasmic reticulum (rough and smooth) and Golgi apparatus for protein processing and transport.

  • Centrioles within the centrosome, important for cell division.

  • Various lysosomes, peroxisomes and ribosomes.

Suggested diagram: labelled drawing of a typical animal cell showing nucleus, nucleolus, mitochondria, ER, Golgi, centrosome with centrioles, lysosomes and plasma membrane.

Interpreting Photomicrographs

Photomicrographs are obtained with light microscopes and are useful for visualising overall cell shape and large organelles. When analysing a photomicrograph, consider the following criteria:

FeatureWhat to Look ForTypical Plant Cell ExampleTypical Animal Cell Example
Cell outlinePresence of a distinct wall or membraneThick, uniform cell wall surrounding the cellIrregular, flexible plasma membrane
VacuoleLarge, clear space occupying most of the cell interiorLarge central vacuole (often appears as a clear zone)Small or absent vacuoles
ChloroplastsGreen, disc‑shaped organellesNumerous, often clustered near the cell peripheryAbsent
NucleusDarkly stained region with a clear nucleolusUsually central but may be displaced by vacuoleTypically central, often spherical
Cell junctionsVisible connections between adjacent cellsPlasmodesmata (hard to resolve at low magnification)Tight junctions, desmosomes, gap junctions (may appear as lines)

Interpreting Electron Micrographs

Electron micrographs provide high‑resolution detail of sub‑cellular structures. Interpretation focuses on ultrastructural features that are not visible in light microscopy.

Microscopy TypeResolution (nm)Key Structures \cdot isualisedTypical Plant Cell ExampleTypical Animal Cell Example
TEM (thin section)≈ 1–2Organelle membranes, ribosomes, nucleoplasm, thylakoid lamellaeChloroplast envelope, stacked grana, starch granulesMitochondrial cristae, nuclear pores, centrioles
SEM (surface)≈ 5–10Cell wall texture, microvilli, cilia, pollen grain exineCell wall ridges, stomatal poresMicrovilli on intestinal epithelium, ciliary rows on protozoa

Comparative Summary of Plant vs. Animal Cells

FeaturePlant CellAnimal Cell
Cell wallPresent (cellulose)Absent
Central vacuoleLarge, occupies up to 90 % of volumeSmall or absent
ChloroplastsPresent (photosynthesis)Absent
LysosomesRareCommon
CentriolesUsually absentPresent (in centrosome)
Plasmodesmata vs. Gap junctionsPlasmodesmata for cytoplasmic continuityGap junctions for intercellular communication
Surface‑area‑to‑volume ratio (SA:V)\$\displaystyle SA:V = \frac{6}{d}\$ (larger cells have lower SA:V, mitigated by extensive vacuole)\$\displaystyle SA:V = \frac{6}{d}\$ (smaller cells maintain higher SA:V for rapid exchange)

Practical Tips for Exam Questions

  1. Identify the microscopy technique from clues in the question (e.g., “electron‑dense” suggests TEM).

  2. Look for hallmark organelles: chloroplasts → plant; centrioles → animal.

  3. Note the relative size of vacuoles and presence/absence of a cell wall.

  4. When interpreting a drawing, check that each labelled structure matches the typical location (e.g., nucleus near the centre of an animal cell, peripheral in many plant cells).

  5. Use the SA:V relationship to explain why cells are generally small and why large plant cells develop a large vacuole.