The nature and variety of living organisms

The following sub-topics are covered in this section.

(a) Characteristics of living organisms

(b) Variety of living organisms

(a) Characteristics of living organisms

Students should:

1.1 understand how living organisms share the following characteristics:

• they require nutrition

• they respire

• they excrete their waste

• they respond to their surroundings

• they move

• they control their internal conditions

• they reproduce

• they grow and develop.

(b) Variety of living organisms

Students should:

1.2 describe the common features shown by eukaryotic organisms: plants, animals, fungi and protoctists

Plants: multicellular organisms with cells containing chloroplasts for photosynthesis and cellulose cell walls. They store carbohydrates as starch or sucrose. Examples include flowering plants like maize (a cereal) and herbaceous legumes like peas or beans.

Animals

Animals: multicellular organisms whose cells lack chloroplasts and cannot photosynthesize. They have no cell walls, enabling movement, which is typically coordinated by a nervous system. Animals often store carbohydrates as glycogen. Examples include mammals like humans and insects like houseflies or mosquitoes.

Fungi

Fungi: organisms that can be multicellular (e.g., Mucor with hyphal structures) or unicellular (e.g., yeast). Their cells have walls made of chitin and lack chloroplasts. Multicellular fungi consist of a mycelium made of hyphae, which are thread-like structures with multiple nuclei. They feed via saprotrophic nutrition by secreting enzymes onto food and absorbing nutrients. Fungi may store carbohydrates as glycogen.

Protists

Protists: mostly microscopic, single-celled organisms. Some, like Amoeba, resemble animal cells, while others, like Chlorella, have chloroplasts and are plant-like. Most algae are unicellular, but some, like seaweed, are multicellular. Some protists, such as Plasmodium, are pathogenic and cause diseases like malaria.

Prokaryotes (Bacteria)

Prokaryotes: microscopic, single-celled organisms lacking a nucleus, with DNA in a circular chromosome and sometimes plasmids. They have cell walls made of peptidoglycan and may have a protective slime layer. Some bacteria photosynthesize, but most feed on living or dead organisms. Examples include Lactobacillus bulgaricus (used in yogurt production) and Pneumococcus (causes pneumonia).

1.4 understand the term pathogen and know that pathogens may include fungi, bacteria, protoctists or viruses

Viruses: non-living particles smaller than bacteria. They are parasitic and reproduce only inside living cells, infecting all types of living organisms. Viruses lack a cellular structure, consisting of genetic material (DNA or RNA) enclosed in a protein coat. They have a variety of shapes and sizes. Examples include the tobacco mosaic virus (causes leaf discoloration by preventing chloroplast formation in tobacco plants), the influenza virus (causes flu), and HIV (causes AIDS).

Pathogens

Pathogens: microorganisms that cause diseases in other organisms. They include protists, fungi, bacteria, and viruses.