Pearson Science Double Award Biology 1: Nature and Variety of Living Organisms

Living things share processes, but they do not all have the same cellular organisation or method of nutrition. Pearson Science Double Award 4SD0 Topic 1 asks you to recognise evidence for life and classify unfamiliar examples by features rather than appearance alone.

These notes follow Issue 4 of the linear 4SD0 specification. The topic is assessed in Biology Paper 1B alongside the rest of Double Award Biology.

Characteristics of living organisms

Living organisms require nutrition, release usable energy by respiration, remove metabolic waste by excretion, respond to their surroundings, move, control internal conditions, reproduce, and grow and develop. No single item is a perfect test of life. A dormant seed may show little visible movement, while fire spreads and uses oxygen without being a cellular organism.

Respiration is the chemical process that releases energy in cells. Breathing is ventilation of gas-exchange surfaces. Excretion removes products of metabolism, such as carbon dioxide or urea; egestion removes undigested food and is not excretion.

Eukaryotic groups

Eukaryotic cells contain a nucleus. Pearson requires these comparisons:

• Plants are multicellular, have cellulose cell walls and chloroplasts, photosynthesise, and commonly store carbohydrate as starch or sucrose. Maize and peas or beans are representative examples.
• Animals are multicellular, lack cell walls and chloroplasts, usually show nervous coordination and movement, and often store carbohydrate as glycogen. Humans, houseflies and mosquitoes illustrate the group.
• Fungi do not photosynthesise. Most form a mycelium of hyphae, often containing many nuclei. Their walls contain chitin. They secrete digestive enzymes onto food and absorb soluble products, called saprotrophic nutrition. Mucor is filamentous, while yeast is single-celled. Fungi may store glycogen.
• Protoctists are mostly microscopic and single-celled. Amoeba has animal-like features, Chlorella has chloroplasts, and Plasmodium is the pathogen that causes malaria. The group is diverse, so one example does not describe every protoctist.

Prokaryotes, pathogens and viruses

Bacteria are prokaryotes. A bacterial cell has cytoplasm, a cell membrane, a cell wall, a circular chromosome and often plasmids, but no nucleus. Some photosynthesise, while many feed on living or dead material. Lactobacillus bulgaricus helps produce yoghurt; Pneumococcus can cause pneumonia.

A pathogen is an organism or agent that causes disease. Pathogens can include fungi, bacteria, protoctists and viruses. The word does not mean that every member of the group causes disease.

Viruses are not living organisms in this specification. They are non-cellular particles with a protein coat and one type of nucleic acid, either DNA or RNA. They reproduce only inside living host cells. Tobacco mosaic virus damages tobacco leaves, influenza virus causes flu, and HIV can lead to AIDS.