AQA A Level Biology复习笔记2.1.5 Structure of Prokaryotic Cells

Prokaryotic Cells

 

  • Animal and plant cells are types of eukaryotic cells, whereas bacteria are a type of prokaryote
  • Prokaryotic cells are much smaller than eukaryotic cells (between 100 - 1000 times smaller)
  • They also differ from eukaryotic cells in having:
    • A cytoplasm that lacks membrane-bound organelles
    • Their ribosomes are structurally smaller (70 S) in comparison to those found in eukaryotic cells (80 S)
    • No nucleus (instead they have a single circular DNA molecule that is free in the cytoplasm and is not associated with proteins)
    • A cell wall that contains murein (a glycoprotein)

     

  • In addition, many prokaryotic cells have a few other structures that differentiate the species from others and act as a selective advantage, examples of these are:
    • Plasmids
    • Capsules
    • Flagellum

     

  • Plasmids are small loops of DNA that are separate from the main circular DNA molecule
    • Plasmids contain genes that can be passed between prokaryotes (e.g. genes for antibiotic resistance)

     

  • Some prokaryotes (e.g. bacteria) are surrounded by a final outer layer known as a capsule. This is sometimes called the slime capsule
    • It helps to protect bacteria from drying out and from attack by cells of the immune system of the host organism

     

  • Flagellum (plural = flagella) are long, tail-like structure that rotate, enabling the prokaryote to move (a bit like a propeller)
    • Some prokaryotes have more than one

     

 

Additional structures unique to prokaryotic cells

Additional-structures-unique-to-prokaryotic-cells-table

 

 

Cell-Components-Prokaryotic-cell

Prokaryotic cells are often described as being ‘simpler’ than eukaryotic cells, and they are believed to have emerged as the first living organisms on Earth

 

Exam Tip

Make sure you learn the typical structures and organelles found in prokaryotic cells, as well as their functions. You also need to know all the differences between prokaryotic and eukaryotic cells. Remember - the features in the table above are not present in all prokaryotes so keep this in mind when answering exam questions.

 

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