Structure
Eukaryotic cell
Prokaryotic cell
nucleus
usually present, surrounded by a nuclear envelope and containing chromosomes and a nucleolus
no nucleus, and therefore no nuclear envelope or nucleolus
mitochondria
usually present
never present
chloroplasts
present in some plant cells
never present
endoplasmic reticulum
usually present
never present
ribosomes
relatively large, about 30 nm in diameter, or 80S
relatively small, about 20 nm in diameter, or 70S
chromosomes
DNA arranged in long strands, DNA present, not associated associated with histone with proteins, circular proteins plasmids may also be present
cell wall
always present in plant cells, made of cellulose, never present in animal cells
always present, made of peptidoglycan
flagella
sometimes present
some have flagella, but these have a different structure from those in eukaryotic cells
Table 1.4 Differences between prokaryotic and eukaryotic cells. The unit ‘S’ is a Svedberg unit, used to compare sizes of cell organelles.
Nature of science Scientific advance follows technical innovation – the electron microscope A typical animal cell is 10–20 μm in diameter, which is about one-fifth the size of the smallest particle visible to the naked eye. Robert Hooke was the first scientist to see and describe cells, although he didn’t know what they were. Later, Anton van Leeuwenhoek, who built one of the first microscopes in 1674, was able to see living cells of Spirogyra and bacteria.
Can we believe our eyes? Our own perception is a crucial source of knowledge. The way we see things depends on the interaction between our sense organs and our mind, and what we perceive is a selective interpretation. When studying material that has been prepared for microscopic examination, we must always bear in mind that staining and cutting cells will alter their appearance. Interpreting images requires care, and what we perceive in a particular image is likely to be influenced by these techniques as well as our own expectations.
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Questions to consider
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Consider the shapes of mitochondria in Figure 1.12. Why do some mitochondria appear cylindrical and others circular? Plant cells have a single central vacuole. Examine the plant cell in Figure 1.14. How many vacuoles can you see? How can you explain this?