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Biology AS CIE

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  1. 1-cell-structure
    10 主题
  2. 2-biological-molecules
    19 主题
  3. 3-enzymes
    13 主题
  4. 4-cell-membranes-and-transport
    16 主题
  5. 5-the-mitotic-cell-cycle
    8 主题
  6. 6-nucleic-acids-and-protein-synthesis
    9 主题
  7. 7-transport-in-plants
    11 主题
  8. 8-transport-in-mammals
    16 主题
  9. 9-gas-exchange
    6 主题
  10. 10-infectious-diseases
    6 主题
  11. 11-immunity
    10 主题
课 1, 主题 1
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1-2-cells-as-the-basic-units-of-living-organisms AS viruses

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Exam code:9700

Key features of viruses

  • Viruses are non-cellular particles that infect living cells

    • Note that viruses are not cells, and they are not considered to be living organisms, so are referred to as ‘particles’

  • They are much smaller than prokaryotic cells, with a diameter of 20-300 nm

  • Structurally they have

    • A nucleic acid core made of either DNA or RNA

    • A protein coat called a capsid

  • Some viruses have an outer layer called an envelope; this forms from the membrane phospholipids of the host cell in which they were produced

  • Viruses can only reproduce by infecting living cells and using their protein-building machinery to produce new viral particles

    • Viruses use attachment proteins on their surface to bind to and infect their host cells

Diagram of a virus with labelled parts: green capsid, protein molecules as projections, and red DNA or RNA genetic code inside.
Basic virus structure includes a protein capsid and a nucleic acid core

Examiner Tips and Tricks

Never describe viruses as cells or living organisms in exam answers.

Examiners often include this as a trap — viruses are non-cellular particles that rely on host cells to reproduce. Make sure to refer to them as “virus particles”, not “cells”, and state that they cannot carry out life processes independently.

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