Cell Theory, Prokaryotes and Eukaryotes: Quick Revision

A brief note on Cell Theory, Prokaryotes and Eukaryotes. Read it before a test. Try each question at the end before you open its answer.

The cell theory

• Hooke named cells in cork. Leeuwenhoek first saw live cells. Brown found the nucleus.
• Schleiden (plants) and Schwann (animals) gave the cell theory. Schwann noted the cell wall is unique to plants.
• Virchow: Omnis cellula-e cellula, every cell from a cell.
• Today: all organisms are made of cells and their products, and all cells arise from pre-existing cells.

Two kinds of cell

• Eukaryotic: a membrane-bound nucleus and membrane-bound organelles (protists, fungi, plants, animals).
• Prokaryotic: no nuclear membrane and no membrane-bound organelles (bacteria, blue-green algae, mycoplasma, PPLO).
• Every cell has a plasma membrane, cytoplasm and ribosomes.

Sizes and shapes

• Mycoplasma about $0.3\ \mu\text{m}$; bacteria $3$ to $5\ \mu\text{m}$; a red blood cell about $7.0\ \mu\text{m}$.
• Nerve cells are among the longest; the ostrich egg is the largest single cell.
• Shape suits the job. A cube of side $s$ has surface area to volume $\dfrac{6}{s}$, which falls as the cell grows.

The prokaryotic cell

• Envelope: glycocalyx (slime layer or capsule), cell wall, plasma membrane. Gram positive cells take up the stain; Gram negative ones do not.
• Nucleoid of naked DNA; plasmids carry traits such as antibiotic resistance.
• Mesosomes: infoldings of the membrane for wall formation, DNA copying, respiration and secretion. Chromatophores hold pigments.
• Flagellum: filament, hook, basal body. Pili and fimbriae do not move the cell; fimbriae help it stick.
• 70S ribosomes (50S and 30S) form polysomes. Inclusion bodies store phosphate, cyanophycean and glycogen granules.

A worked example

A cell has a cell wall, naked DNA, 70S ribosomes and no mitochondria. What kind of cell is it?

1. Naked DNA. There is no nuclear membrane.
2. No mitochondria. There are no membrane-bound organelles.
3. 70S ribosomes. These are the ribosomes of prokaryotes.
4. So it is a prokaryotic cell, most likely a bacterium.

Practice with new numbers

A program to try

The program prints the main differences between the two kinds of cell as a table.

Mistakes to avoid

• Thinking Hooke saw living cells. He saw the walls of dead cork cells.
• Putting the cell wall outermost in a bacterium. The glycocalyx lies outside it.
• Adding S units. 50S and 30S make a 70S ribosome.
• Giving mycoplasma a cell wall. They have none.

Check yourself

1. What did Virchow add to the cell theory?

2. Name two organisms with prokaryotic cells.

3. What is a polysome?

4. Which prokaryotic structure helps a bacterium stick to the tissues of a host?