How Living Organisms are Classified.

So far, scientists have identified over a million different kinds of organisms and new ones are constantly being discovered. With such a wide variety of plants and animals, obviously some system is needed to classify them and to group them according to similarities and differences. All living things are first classified into groups called kingdoms. This is the highest category into which organisms are classified. There are five main kingdoms – plants, animals, fungi, bacteria and protoctists. There is potentially a sixth – the viruses, but as we will see later, there is some argument as to whether they are really ‘alive’ due to their inability to perform any of the characteristics of life without a host organism.

Plants are multicellular organisms that contain chlorophyll and are photosynthetic.
Surrounded by cell walls made up of cellulose and contain a large permanent vacuole filled with sap.
Able to store carbohydrates.
Primitive plants reproduce via spores, more advanced forms such as flowering plants reproduce by producing seeds.
Herbaceous legumes produce seeds in a characteristic seed pod.

Animals are multicellular.
They are heterotrophic.
Carbohydrate is stored as glycogen.
Animals can be split in to vertebrates and invertebrates.

Fungi cannot photosynthesise and most feed by digesting dead organic material, extracellular digestion.
They secrete enzymes onto the food, called saprophytic.
Some fungi are parasitic.
They possess cell walls made of chitin.
Consist of filaments called hyphae, which contain many nuclei, and reproduce by releasing spores.

Bacteria are unicellular microorganisms.
Bacteria don't have a true nucleus instead they have DNA.
Cell walls made of peptidoglycan.
Plasmids (pieces of DNA involved in antibiotic resistance)within the cytoplasm.
Some bacteria are photosynthetic whilst others feed off living and dead organisms.
Microorganism is a single-celled organism.
Those which cause harm to human health are termed as pathogens.
Pneumococcus is a spherical bacterium that causes pneumonia so is regarded as a pathogen.

Protoctists are single-celled microbes.
Some have features like animal cells, while others have chloroplasts and photosynthesise like plants.
Malaria is a disease caused by the protoctist pathogen, Plasmodium.

Viruses can only be seen with the electron microscope.
Contain a core of either DNA or RNA surrounded by a protein coat called a capsid.
No cellular structure, and are smaller than bacteria.
Viral diseases multiply inside living host cells.
Host cells usually die as a result of infection.
Cannot reproduce on their own, need to enter host cell.
Viruses cause disease by the sum of the effects of damage to the host cells they invade.

Flu - Transmitted by droplet infection.
Sometimes the infection spreads down to the lungs, resulting in infections such as acute bronchitis or pneumonia.

Acquired Immune Deficiency Syndrome (Aids)  -caused by the human immunodeficiency virus (HIV). The leading cause of death among 25-45 year old men in the USA and up to 70% of people in African countries are infected.
Mostly transmitted sexually by blood or semen.

TMV (Tobacco Mosaic Virus).
infects tobacco plants resulting in discoloration of the leaves, causing significant loss in crop yield.