7/27/2019 Cellular Defence

1/4

The nature of viruses

Viruses are very small obligate parasites. This means that they cannotsurvive without taking over a host cell.

Viruses contain DNA or RNA ( nucleic acid) surrounded by a coat whichis usually protein.

After a virus invades a host cell it forces the cell to replicate viralDNA/RNA, make copies of its protein coat and self-assembles resultingin the release of large numbers of viruses.

Tobacco mosaic virus HIV Bacteriophage

Infects plants Infects Humans Infects Bacteria

DNA or RNA

Protein Coat

DNA or RNA

Protein Coat

Structure of Viruses

7/27/2019 Cellular Defence

2/4

Invasion of a cell by a virusDNA or RNA

Protein Coat

1. Viral DNA is

injected

2. Viral DNA

replicated

3. Viral mRNA for

protein coat

transcribed by host

4. Virus protein coats

translated by host ribosomes

5. Viruses self-assemble

6. Lysosome enzyme

used to help virus burstout of cell

Host

Cell

Nucleus with

host DNA

Host cell supplies virus with:

Nucleotides

Amino Acids

ATP Enzymes

Invasion of a cell by a virusDNA or RNA

Protein Coat

DNA or RNA

Protein Coat

1. Viral DNA is

injected

2. Viral DNA

replicated

3. Viral mRNA for

protein coat

transcribed by host

4. Virus protein coats

translated by host ribosomes

5. Viruses self-assemble

6. Lysosome enzyme

used to help virus burstout of cell

Host

Cell

Nucleus with

host DNA

Host cell supplies virus with:

Nucleotides

Amino Acids

ATP Enzymes

7/27/2019 Cellular Defence

3/4

Cellular response in defence in animals

Any invading foreign particles are known as antigen. These include bacteria,viruses, fungi and cells foreign to the body.

White bolld cells known as phagocytes engulf and digest antigen by the processofphagocytosis.

The antigen is enclosed in a vacuole within the cell

Lysosomes containing powerful protein digesting enzymes fuse with thevacuole

The antigen is digested and the breakdown compounds diffuse into cytoplasm

Lymphocytes are a second type of white blood cell that recognise foreignantigen and produce special proteins calledantibodies which bind with anddestroy the antigen.

The combined effects of phagocytes and lymphocytes is a particular problemduring tissue transplantation. The foreign tissue is treated as an antigen andrejected by the body. It is necessary to use

Donors which closely match tissue type of recipient

Use immuno-suppressors which reduce inhibit immune system.

TOP TIP - Try the data interpretation comparing primary and secondaryresponses to immunisation injections

Phagocytotic WBC

AntigenWBC engulfsantigen trappingit in a vacuole

Lysosomes containing

powerful protein digestingenzymes fuse with the vacuole

Enzymesreleased into the

vacuole

Antigen digestedDigested parts diffusesinto cytoplasm

Phagocytotic WBC

AntigenWBC engulfsantigen trappingit in a vacuole

Lysosomes containing

powerful protein digestingenzymes fuse with the vacuole

Enzymesreleased into the

vacuole

Antigen digestedDigested parts diffusesinto cytoplasm

7/27/2019 Cellular Defence

4/4

Cellular defence mechanisms in plants.

Plants dont have an immune system but must protect themselves from herbivores

Plants can protect themselves by producing a variety of toxic compounds

Tannins- These are acidic chemicals produced when a parasite such as insect orfungus penetrates the plant. They are produced inside swellings called galls whichhelp isolate the parasite

Cyanide Hydrogen cyanide is a poison which blocks a herbivores cytochromesystem and is produced by some plants (e.g. white clover) in response to tissuedamage. The production of hydrogen cyanide is known as cyanogenesis

Nicotine This is a poison produced by tobacco plants which over stimulates thecentral nervous system of herbivores

a) Plants can protect themselves by isolating injured areas using barrier substancessuch as

Resins These are glassy substances such as amber which exude from plants and

trees and help isolate possible routes by which the plant may be invaded e.g. xylemand phloem vessels

Lignin Lignin is normally part of the xylem vessels but if a vessel is invaded byfungi extra lignin is produced to clog the vessel completely

This photographs shows

swellings called galls which are

produced by the leaf after aninsect lays eggs on it. This

isolates the leaf from the egg.

When the insect hatches it eats

its way down towards the leaf

but gets a mouthful of acidic

tannins which help to stop it

getting any further.