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Viruses and cellular defence
Structure of viruses
Viruses considered non-living as they exist in an inert state outside of a host cell
They consist of a strand of nucleic acid, either DNA or RNA, surrounded by a protective protein coat (the capsid)
Sometimes they have a further membrane of lipid, referred to as an envelope, surrounding the protein
Also have a tail section
Transmission of viruses
Viruses can be transmitted by travelling within a fluid, e.g. mucus droplets from a sneeze
Once they have entered a living host they begin the process of replication (replication process always follows a particular pattern)
Stage
Description
Diagram
Attachment
Virus binds to a specific receptor on the host cell surface.

Penetration
Viral nucleic acid enters the cell.

Synthesis of new components.
Viral nucleic acid takes over control of the cell metabolism stopping cell's normal nucleic acid and protein synthesis.
Viral nucleic acid is replicated using nucleotides from the host cell.
Protein coats are manufactured using the amino acids of the host cell.

Assembly
Whole virus particles are made when the nucleic acids are surrounded by the protein coats.

Release
Many viral particles are released when the cell bursts open (lysis) or by slow leakage.

Cellular defence
When an organism becomes infected with either a bacteria or virus it attempts to limit the damage that can be done to its tissues.
Animals
There are two types of white blood cells – phagocytes and lymphocytes.
Phagocytes engulf and breakdown foreign particles. The phagocyte detects the foreign object and moves towards it. The membrane of the phagocyte then projects out and encloses the foreign object in a vacuole.
The lysosomes (powerful enzyme containing vesicles) inside the phagocyte fuse with and release enzymes into the vacuole. The enzymes breakdown the foreign object and the products are either absorbed into the cell or released.
Lymphocytes produce antibodies in