Chapter One: Cell Structure

Overview of Chapter Content

• The chapter is divided into three parts:

  • Microscopy: Focus on different types of microscopes.

  • Calculation: Involves magnification and how to measure cells.

  • Cell Structure and Function: Builds on basic knowledge from IGCSE/O levels.

Microscopy

• Main Types of Microscopes:

  • Light Microscope: Utilizes visible light as a source of electromagnetic radiation.

  • Electron Microscope: Has two types:

    • Transmission Electron Microscope (TEM)

    • Scanning Electron Microscope (SEM)

Key Concepts in Microscopy

• Electromagnetic Radiation: Understanding the electromagnetic spectrum is crucial for microscopy. Different wavelengths are used for different microscopes.

• Magnification vs. Resolution:

  • These two terms are commonly misunderstood but are distinctly different.

Definitions

• Magnification:

  • Defined as the number of times an image is enlarged compared to the actual size of the object.

  • Represented with an "X" or "times" sign (e.g., 10x, 40x).

  • Common magnifications in laboratories: 10x or 40x.

• Resolution:

  • The ability to distinguish between two points clearly.

  • High resolution allows for clear distinction between points, while lower resolution results in blurriness, making it hard to distinguish points from each other or from the background.

  • Units of resolution are expressed in nanometers.

Relationship Between Magnification and Resolution

• An increase in magnification does not equate to an increase in resolution.

• Example:

  • Enlarge a picture using a camera: the image might seem closer, but increasing magnification can lead to blur if resolution is low.

Visual Examples of Resolution

• High Resolving Power: Clear and detailed images can show distinctions between points.

• Lower Resolving Power: Blurred images where points cannot be distinguished from each other or from the background.

Determinants of Resolution

• Electromagnetic Spectrum: Wavelength impacts resolution.

  • Shorter wavelengths provide higher resolving power.

  • Visible light has a longer wavelength than electrons, resulting in lower resolution for light microscopes.

Maximum Resolution

• Defined as half the wavelength used:

  • For visible light (400-700 nm), maximum resolution is 200 nm.

  • Smaller objects (e.g., ribosomes at 25 nm) cannot be seen with light microscopes due to being smaller than the lower resolution limit.

  • Larger objects (e.g., nucleus at 1 micrometer) can be viewed.

Light Microscope Details

• Source of Electromagnetic Radiation: Visible light (400 to 700 nm) used for imaging.

• Structure: Focused using mirrors and glass lenses to direct light into the observer's eye.

• Maximum Magnification: Generally can achieve 1500x but often used at 10x or 40x in exams.

• Observable Structures: Chloroplasts, nuclei, chromosomes, and occasionally mitochondria.

• Maximum Resolution: 200 nanometers, described as relatively low compared to electron microscopes.