Unit 1 notes for IB Biology HL students. I took my exams in July 2022 and got a 7 in Biology HL. The notes are a combination of information from different sources with a main focus on the IB Biology Textbook.
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1.2 Ultrastructure of Cells
Objectives:
❏ Understanding:
❏ Prokaryotes have a simple cell structure without compartments
❏ Eukaryotes have a compartmentalised cell structure
❏ Prokaryotes divide by binary fission
❏ Electron microscopes have a much higher resolution than light microscopes
❏ Applications:
❏ The structure and function of organelles within exocrine gland cells of the pancreas
❏ The structure and function of organelles within palisade mesophyll cells of the leaf
❏ Skills
❏ Drawing the ultrastructure of prokaryotic cells based on electron micrographs
❏ Drawing the ultrastructure of eukaryotic cells based on electron micrographs
❏ Interpretation of electron micrographs to identify organelles and deduce the function of
specialised cells
❏ Nature of Science
❏ Developments in scientific research follow improvements in apparatus: the invention of
electron microscopes led to greater understanding of cell structure
The invention of the electron microscope:
- There was a limit to the discoveries that could be made as light microscopes cannot produce clear images of
structures smaller than 0.2 micrometers (μm)
- Electron microscopes allowed images to be produced of things as small as 0.001 μm (200 times smaller
than with light microscopes)
- The electron microscopes revealed the ultrastructure of cells, including previously unknown features
The resolution of electron microscopes:
- Electron microscopes have a much higher resolution than light microscopes
- Resolution = Making the separate parts of an object distinguishable by eye
- The maximum resolution of a light microscope is 0.2 μm (200 nm)
- However powerful the lenses of a light microscope, the resolution cannot be higher than this
because it is limited by the wavelength of light (400-700 nm)
, - If we try to resolve smaller objects by making lenses with greater magnification, we find that it is
impossible to focus them properly and get a blurred image
- This is why the maximum magnification with light microscopes is usually x400
- Beams of electrons have a much shorter wavelength therefore electron microscopes have a much higher
resolution
- The resolution of a modern electron microscope is 0.001 μm or 1 nm
- Electron microscopes have a resolution that is 200 times greater than light microscopes
- Light microscopes reveal the structure of cells but electron microscopes reveal the ultrastructure
- This explains why light microscopes were needed to see bacteria with a size of 1 μm, but viruses
with a diameter of 0.1 μm could not be seen until electron microscopes had been invented
- Types of electron microscopes:
1. TEM (transmission electron microscope)
a. Generate high resolution cross-sections of objects
2. SEM (scanning electron microscope)
a. Display enhanced depth to map the surface of objects in 3D
Maximum resolution to see organelles as separate objects
Millimeters (mm) Micrometers (μm) Nanometers (nm)
Naked eye 0.1 100 100,000
Light microscope 0.0002 0.2 200
Electron microscope 0.000001 0.001 1
Light microscope Electron microscope
Uses light rays and focuses them with 2 or more Uses a beam of electrons and focuses them with
convex lenses to illuminate the object electromagnets to illuminate the object
Magnifies up to around x1500 Magnifies up to around x500,000
Low resolution (up to 200 nm) High resolution (up to 0.05 nm)
Wavelength of light is 400-700 nm Beams of electrons have a much shorter wavelength
Reveals nuclei, cell wall, cell organelles and chromatin Reveals details of cell organelles and cell structure (e.g.
etc cilia, flagella etc)
Portable, relatively inexpensive Not portable, very expensive
Can examine living tissue (thin) Specimen dead (in a vacuum)
Natural colours of sample preserved All images black and white unless artificially coloured
Prokaryotic cell structure:
- Prokaryotes have a simple cell structure without compartments
- Structure is simpler than in eukaryotic cell (still very complex in terms of the biochemicals that are
present)
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