1b Variety of Living Organisms¶

Part of 1 The Nature and Variety of Living Organisms.

Organisms are grouped here by their shared cell features, modes of nutrition and overall body plan. The important comparisons are between eukaryotes, prokaryotes and viruses, and between the main eukaryotic groups themselves.

Learning Objectives¶

ID	Official specification wording	Main teaching sections
`1b-lo-1`	1.2 describe the common features shown by eukaryotic organisms: plants, animals, fungi and protoctists Plants: these are multicellular organisms; their cells contain chloroplasts and are able to carry out photosynthesis; their cells have cellulose cell walls; they store carbohydrates as starch or sucrose. Examples include flowering plants, such as a cereal (for example, maize), and a herbaceous legume (for example, peas or beans). Animals: these are multicellular organisms; their cells do not contain chloroplasts and are not able to carry out photosynthesis; they have no cell walls; they usually have nervous co-ordination and are able to move from one place to another; they often store carbohydrate as glycogen. Examples include mammals (for example, humans) and insects (for example, housefly and mosquito). Fungi: these are organisms that are not able to carry out photosynthesis; their body is usually organised into a mycelium made from thread-like structures called hyphae, which contain many nuclei; some examples are single-celled; their cells have walls made of chitin; they feed by extracellular secretion of digestive enzymes onto food material and absorption of the organic products; this is known as saprotrophic nutrition; they may store carbohydrate as glycogen. Examples include Mucor, which has the typical fungal hyphal structure, and yeast, which is single-celled. Protoctists: these are microscopic single-celled organisms. Some, like Amoeba, that live in pond water, have features like an animal cell, while others, like Chlorella, have chloroplasts and are more like plants. A pathogenic example is Plasmodium, responsible for causing malaria.	Eukaryotic Groups
`1b-lo-2`	1.3 describe the common features shown by prokaryotic organisms such as bacteria Bacteria: these are microscopic single-celled organisms; they have a cell wall, cell membrane, cytoplasm and plasmids; they lack a nucleus but contain a circular chromosome of DNA; some bacteria can carry out photosynthesis but most feed off other living or dead organisms. Examples include Lactobacillus bulgaricus, a rod-shaped bacterium used in the production of yoghurt from milk, and Pneumococcus, a spherical bacterium that acts as the pathogen causing pneumonia.	Prokaryotes
`1b-lo-3`	1.4 understand the term pathogen and know that pathogens may include fungi, bacteria, protoctists or viruses Viruses: these are not living organisms. They are small particles, smaller than bacteria; they are parasitic and can reproduce only inside living cells; they infect every type of living organism. They have a wide variety of shapes and sizes; they have no cellular structure but have a protein coat and contain one type of nucleic acid, either DNA or RNA. Examples include the tobacco mosaic virus that causes discolouring of the leaves of tobacco plants by preventing the formation of chloroplasts, the influenza virus that causes ‘flu’ and the HIV virus that causes AIDS.	Pathogens and Viruses

Eukaryotic Groups¶

Eukaryotic cells contain genetic material enclosed within a nucleus and have membrane-bound organelles. The four main eukaryotic groups are plants, animals, fungi and protoctists.

Plants — for example, cereal crops such as maize and herbaceous legumes such as peas: - Multicellular organisms whose cells contain chloroplasts, which are the site of photosynthesis. Chlorophyll pigments within the chloroplast absorb light energy from the sun. - Cells have a cellulose cell wall, which gives structural strength. - Possess a permanent vacuole that stores cell sap and helps maintain rigidity. - Store carbohydrates as starch or sucrose.

Animals — for example, mammals such as humans and insects such as flies: - Multicellular, but with no cell wall and no chloroplasts, so they cannot photosynthesise. - Typically co-ordinate movement using a nervous system. - Store carbohydrates as glycogen.

Fungi — for example, Mucor (hyphal) and yeast (single-celled): - The fungal body is usually organised into a network of thread-like structures called hyphae, which together form a mycelium. Each hypha contains many nuclei. Some fungi are single-celled. - Cell walls are made of chitin, not cellulose. - Feed by saprotrophic nutrition: digestive enzymes are secreted onto food outside the cell, breaking it into soluble molecules that can then be absorbed. - May store carbohydrates as glycogen.

Protoctists — for example, Amoeba and Chlorella: - Usually microscopic single-celled organisms. - Some, such as Amoeba, resemble animal cells and live in pond water. - Others, such as Chlorella, possess chloroplasts and behave more like plant cells.

Prokaryotes¶

Prokaryotic cells differ fundamentally from eukaryotic cells because they have no nucleus and no membrane-bound organelles.

Bacteria — for example, Lactobacillus bulgaricus (rod-shaped, used in yoghurt production) and Pneumococcus (spherical, causes pneumonia): - Single-celled and very small. - Possess a cell wall, cell membrane and cytoplasm. - Carry circular chromosomes of DNA free in the cytoplasm rather than inside a nucleus; many also possess smaller circular DNA loops called plasmids. - Some bacteria are capable of photosynthesis; most feed from living or dead organic matter.

The main comparison to remember is that bacteria are structurally simpler than eukaryotes even when they perform similar life processes.

Pathogens and Viruses¶

A pathogen is a disease-causing microorganism or infectious agent. In this course pathogens can include bacteria, fungi, protoctists and viruses.

Viruses — for example, tobacco mosaic virus, influenza and HIV: - Much smaller than bacteria. - Not living organisms in the conventional sense because they have no cellular structure. - Consist of a single type of nucleic acid (either DNA or RNA) enclosed in a protein coat. - Obligate parasites: they can only reproduce inside living cells, hijacking the host cell's machinery to produce many copies of themselves before the cell bursts, releasing new viral particles. - Can infect all types of living organism.

Bacterial pathogens — for example, Salmonella (food poisoning): - Reproduce rapidly by binary fission. - Often cause disease by producing toxins that damage host cells.

Protoctist pathogens — for example, Plasmodium (causes malaria): - Parasitic, using animals as hosts.

Fungal pathogens — for example, athlete's foot: - Produce spores that can spread in the air or by contact between people. - Can be treated with fungicide drugs.

Common Confusions¶

Fungi vs plants: Fungi may have cell walls like plants, but they do not photosynthesise and their walls are made of chitin rather than cellulose.
Viruses vs bacteria: Viruses are much smaller and have no cell structure. Antibiotics target bacteria, not viruses.
Yeast is a fungus: Yeast is unusual because it is single-celled, but it is still a fungus, not a protoctist.
Protoctists are not plants, animals or fungi: They are a diverse group that do not fit neatly into the other categories.

Key Terms¶

Eukaryote: an organism whose cells contain genetic material enclosed in a nucleus.
Prokaryote: an organism made of cells that do not contain a nucleus.
Protoctist: a mostly single-celled eukaryotic organism that does not fit into the plant, animal or fungal groups.
Hypha: a thread-like fungal filament containing many nuclei.
Mycelium: the network of hyphae that forms the body of many fungi.
Saprotrophic nutrition: feeding by extracellular digestion followed by absorption of the soluble products.
Pathogen: an organism or agent that causes disease.
Virus: a non-cellular infectious particle made of nucleic acid inside a protein coat.
Plasmid: a small circular loop of DNA found in bacterial cells, separate from the main chromosome.
Binary fission: asexual reproduction in bacteria in which a single cell divides to produce two identical cells.
Chitin: the material from which fungal cell walls are made.