Learning check
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The microscope
van Leeuwenhoek's microscope
Antonie van Leeuwenhoek (1632–1723).
Leeuwenhoek's microscope.
Antonie van Leeuwenhoek is credited with constructing the first microscope.
- Size: matchbox
- Held close to the eye
- Magnification: 400x
van Leeuwenhoek was the first to describe sperm and (probably) bacteria, which he called "animalcules" ("little animals").
Sperms, as van Leeuwenhoek saw them.
van Leeuwenhoeks drawings of "little animals" (animalcules).
Hooke's microscope
The light from an oil lamp (K) was focused on the specimen (M) with the help of a water-filled sphere (G).
Hooke also drew what he saw through his microscope.
- Publishes "Micrographia" with detailed drawings of the "little life".
- Coins the term "cell"
Hooke's drawing of a flea. Note the amount of detail in the image!
This sample of bark from cork oak was what made Hooke think of monastery cells.
In a sample from cork oak, Hooke sees what he describes looking like cells in a monastery. Although nobody at the time knew what a (biological) cell is, Hooke coined the term "cell".
A modern light microscope
The parts of a modern light microscope.
A modern light microscope works essentially the same way as Hooke's microscope, although the design has changed quite a bit.
What characterizes a cell?
All cells have
- a cell membrane,
- cytoplasm (fluid-like intracellular substance), and
- DNA (in one or several chromosomes).
Scanning electron microscope image of some E. coli bacteria. Each cell is about 2 μm long.
Epithelial cells stained with methylene blue. The cells are approx. 60 μm across.
Plant cells from yellow onion stained with methylene blue. The cells are approx. 200 μm long.
What the cell theory says
- All organisms consist of one or more cells.
- The cell is the fundamental living unit.
- New cells may only arise from other cells.
What do you need to build a cell?
Polysaccharides
Polysaccharides are made from sugar.
Examples:
- Cellulose
- Starch
- Glycogen
Proteins
Proteins are very large molecules.
Proteins are made of amino acids strung together in a long, long chain.
Lipids
Lipids are a diverse group of fat or fat-like molecules, insoluble in water.
Fats are made of glycerol and fatty acids.
Phospholipids have a hydrophilic head and two hydrophobic tails.
Cell membranes are made of phospholipids.
The cell membrane is a double layer of phospholipids.
Nucleic acids
Nucleic acids are made of nucleotides.
The nucleic acids are DNA and RNA.
Backbone
- Alternating sugar (S) and phosphate (P) residues.
Nucleobases
- A always base pairs with T (in RNA, T is replaced with U), and vice versa.
- G always base pairs with C
