Variety of living organisms

Plants

All plants are multicellular eukaryotic organisms whose cells contain chloroplasts, enabling them to carry out photosynthesis — the process of converting light energy into glucose. Because plant cells photosynthesise, plants can make their own food, which means they do not need to consume other organisms.

Plant cells have cellulose cell walls, which provide structural support and maintain the rigid shape of each cell.

Plants store carbohydrates as starch (in structures like tubers and seeds) or as sucrose (transported in the phloem).

Flowering plants are the most common group studied. Examples include a cereal such as maize and a herbaceous legume such as peas or beans.

Animals

All animals are multicellular eukaryotic organisms whose cells lack chloroplasts and cell walls, which means they cannot carry out photosynthesis and must obtain food by consuming other organisms.

Animals usually have nervous co-ordination, allowing them to detect and respond rapidly to changes in their environment. Most animals are able to move from one place to another, which helps them find food, escape predators, and locate mates.

Animals often store carbohydrate as glycogen, mainly in the liver and muscles. Examples include mammals such as humans and insects such as the housefly and mosquito.

MisconceptionStudents sometimes state that animals have "no cell membrane" because they have no cell wall. All animal cells have a cell membrane; they simply lack the rigid outer wall that plant, fungal, and bacterial cells possess.

Exam TipNever confuse cell wall (absent in animals) with cell membrane (present in all cells).

Fungi

Fungi are eukaryotic organisms that cannot carry out photosynthesis. Unlike plants, they have no chloroplasts, so they must obtain nutrients from other sources.

The body of a typical fungus is organised into a mycelium [a network of thread-like filaments], which is made from structures called hyphae. Each hypha contains many nuclei rather than being divided into individual cells with one nucleus each.

Fungal cell walls are made of chitin, not cellulose.

Fungi feed by saprotrophic nutrition: they secrete digestive enzymes extracellularly [outside the cell] onto food material, then absorb the organic products of digestion. This makes them vital decomposers in ecosystems.

Fungi may store carbohydrate as glycogen, just like animals.

Some fungi are single-celled. Mucor is a typical example with a hyphal structure, while yeast is a single-celled fungus used widely in baking and brewing.

Examiner InsightQuestions frequently ask you to describe how fungi feed. The two-step process (secrete enzymes externally, then absorb products) must both be stated for full marks.

Exam TipAlways name the feeding method as saprotrophic nutrition and describe both secretion and absorption.

Protoctists

Protoctists are microscopic, single-celled eukaryotic organisms. They are a diverse group, and different species can resemble either animal cells or plant cells in their features.

Amoeba lives in pond water and has features like an animal cell — it has no cell wall and no chloroplasts, and it moves by changing its shape.

Chlorella, by contrast, contains chloroplasts and can photosynthesise, making it more like a plant cell.

A pathogenic example is Plasmodium, the protoctist responsible for causing malaria. Plasmodium is transmitted to humans through the bite of infected mosquitoes.

MisconceptionStudents often classify protoctists as either plants or animals. Protoctists are their own separate kingdom. Some share features with plants and others with animals, but they belong to neither group.

Exam TipIn classification questions, always place protoctists in their own group — never under plants or animals.

Bacteria

Bacteria are prokaryotic organisms — microscopic, single-celled, and fundamentally different from the eukaryotic groups above. The key distinction is that bacteria lack a true nucleus. Instead, their genetic material exists as a single circular chromosome of DNA that floats freely in the cytoplasm.

Bacterial cells have a cell wall, a cell membrane, and cytoplasm, much like other cells. However, they also contain small rings of extra DNA called plasmids, which can carry genes for useful traits such as antibiotic resistance. Some bacteria can carry out photosynthesis, but most feed off other living or dead organisms.

Lactobacillus bulgaricus is a rod-shaped bacterium used in the production of yoghurt from milk. Pneumococcus is a spherical bacterium that acts as the pathogen causing pneumonia. These two examples illustrate that bacteria vary in shape and function.

Examiner InsightA common question asks you to compare prokaryotic and eukaryotic cells. Remember to mention: no nucleus, circular DNA, plasmids present, and no membrane-bound organelles in prokaryotes.

Exam TipIf asked for differences, state what prokaryotes lack and what they have instead — e.g. "no nucleus; DNA is a circular chromosome in the cytoplasm."

Pathogens and Viruses

A pathogen is an organism that causes disease. Pathogens may include fungi, bacteria, protoctists, or viruses. Each of the major groups studied in this section contains at least one pathogenic example, so diseases can be caused by very different types of organism.

Viruses are not classified as living organisms. They are extremely small particles, smaller than bacteria, with no cellular structure. A virus consists of a protein coat surrounding a core of nucleic acid, which is either DNA or RNA — never both. Viruses are parasitic: they can only reproduce inside living cells, hijacking the host cell's machinery to make copies of themselves. They infect every type of living organism.

Examples include the tobacco mosaic virus (TMV), which causes discolouring of tobacco plant leaves by preventing the formation of chloroplasts; the influenza virus, which causes flu; and HIV, which causes AIDS.

MisconceptionStudents often describe viruses as living organisms. Viruses have no cellular structure, cannot reproduce on their own, and carry out no metabolic reactions. They are not considered alive.

Exam TipIf asked why viruses are not living, state: no cellular structure, cannot reproduce independently, no metabolism.

MnemonicFBPV — Fungi, Bacteria, Protoctists, Viruses (pathogen types).