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Edexcel A Level (IAL) Biology -3.6-3.7 Electron Microscopy of Animal Cells- Study Notes- New Syllabus

Edexcel A Level (IAL) Biology -3.6-3.7 Electron Microscopy of Animal Cells- Study Notes- New syllabus

Edexcel A Level (IAL) Biology -3.6-3.7 Electron Microscopy of Animal Cells- Study Notes -Edexcel A level Biology – per latest Syllabus.

Key Concepts:

  • 3.6 be able to recognise the organelles in 3.3 from electron microscope (EM) images
  • 3.7 (i) know how magnification and resolution can be achieved using light and electron microscopy
    (ii) understand the importance of staining specimens in microscopy

Edexcel A level Biology-Study Notes- All Topics

Recognising Eukaryotic Organelles in EM Images (3.3)

🌱 Introduction

Electron microscopes (EM) allow us to see ultrastructure of eukaryotic organelles at very high resolution. EM images show size, shape, density, and membranes, which help identify organelles.

Nucleus

  • EM Appearance: Large, round/oval, surrounded by double membrane (nuclear envelope). Nuclear pores visible as tiny openings. Contains chromatin (dark) and nucleolus (dense spot inside).
  • Recognition Tips: Biggest organelle. Nucleolus appears very dark inside nucleus.

Nucleolus

  • EM Appearance: Dense, spherical body inside nucleus.
  • Recognition Tips: Solid dark circle inside nucleus.

Ribosomes

  • EM Appearance: Tiny, dark dots (~20 nm) in cytoplasm or on RER.
  • Recognition Tips: Smallest visible particles. On membranes → RER.

Rough Endoplasmic Reticulum (RER)

  • EM Appearance: Flattened membranes studded with ribosomes, usually near nucleus.
  • Recognition Tips: “Studded membranes” → key. Ribosome dots differentiate from SER.

Smooth Endoplasmic Reticulum (SER)

  • EM Appearance: Tubular, smooth membranes, no ribosomes.
  • Recognition Tips: Lighter than RER. Tubular/network near RER.

Mitochondria

  • EM Appearance: Oval/rod-shaped, double membrane, inner membrane folded into cristae.
  • Recognition Tips: Look for cristae. Often scattered in cytoplasm.

Centrioles

  • EM Appearance: Cylindrical, microtubule 9×3 arrangement, usually in pairs near nucleus.
  • Recognition Tips: Pair of cylinders → near nucleus, forms spindle in division.

Lysosomes

  • EM Appearance: Small, spherical, densely stained vesicles.
  • Recognition Tips: Uniform dark round vesicles, smaller than mitochondria.

Golgi Apparatus

  • EM Appearance: Stacked, flattened cisternae with vesicles around. Cis face (receiving) vs trans face (shipping) may be visible.
  • Recognition Tips: Stacked flattened sacs with vesicles at edges, near nucleus, often next to RER.

🟢 Key Recognition Strategies

  • Size: Largest → nucleus, Smallest → ribosomes
  • Shape: Rod/oval → mitochondria, Flattened stacks → Golgi, Tubular → SER
  • Density: Dark/dense → nucleolus, lysosomes, Lighter → SER
  • Location: Near nucleus → RER, Golgi, centrioles, Scattered → mitochondria, lysosomes

⚡ Quick Recap
Nucleus → big, double membrane, contains nucleolus.
Nucleolus → dense dark spot inside nucleus.
Ribosomes → tiny dark dots, free or on RER.
RER → flattened membranes with ribosomes.
SER → smooth tubular membranes.
Mitochondria → oval/rod, inner cristae visible.
Centrioles → cylindrical, near nucleus, 9×3 microtubule pattern.
Lysosomes → small, dense, spherical vesicles.
Golgi → stacked cisternae, vesicles at edges.

Magnification, Resolution & Staining in Microscopy

🌱 Introduction

Microscopy allows us to see tiny structures in cells and tissues. Two main factors make microscopy effective: magnification and resolution. Staining is often necessary to improve visibility of specimens.

1. Magnification

  • Definition: Making an object appear larger than its actual size.
  • How it’s achieved:
    • Light Microscopy (LM): Uses lenses & visible light. Compound microscopes magnify up to ~1500×. Total magnification = Eyepiece × Objective lens.
    • Electron Microscopy (EM): Uses electron beams & magnetic lenses. TEM → up to ~1,000,000×. SEM → up to ~100,000×, shows surface structure.

2. Resolution

  • Definition: Ability to distinguish two points as separate, i.e., how clear the details are.
  • How it’s achieved:
    • Light Microscopy: Limited by wavelength (~400–700 nm). Best resolution ~200 nm.
    • Electron Microscopy: Electrons have smaller wavelength → higher resolution. TEM ~0.1 nm, SEM ~1–10 nm.
  • Key Point: High magnification without good resolution → blurry image. EM gives both high magnification and resolution.

3. Importance of Staining Specimens

  • Why staining is needed: Many cells and organelles are transparent → hard to see under LM. Stains improve contrast.
  • Common stains:
    • Methylene blue → highlights nuclei
    • Iodine → stains starch granules
    • Eosin → cytoplasm & connective tissues
    • Hematoxylin & eosin (H&E) → tissue sections
    • EM uses heavy metal stains (lead/uranium) → electrons scatter to produce contrast

📊 Summary Table

AspectLight MicroscopyElectron Microscopy
Magnification~1500×TEM: ~1,000,000×, SEM: ~100,000×
Resolution~200 nmTEM: ~0.1 nm, SEM: ~1–10 nm
Visible structuresLarger organelles, cellsUltrastructure of organelles
StainingColored dyes (LM)Heavy metals (EM)

⚡ Quick Recap
Magnification → makes objects appear larger.
Resolution → clarity, ability to see details.
Light microscope → good for cells, low resolution.
Electron microscope → ultrastructure, very high resolution.
Staining → improves contrast & visibility of transparent structures.

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