Cambridge IGCSE Marine Science 3: Marine organisms

Topic 3 develops a feature-based understanding of marine life, from cells to whole organisms. Cambridge 0697 limits this topic to named structures, classification groups, external adaptations, two life cycles and migration. Observation, drawing and dichotomous-key methods are owned by the separate practical hub.

Cells and their functions

Plant and animal cells contain a cell membrane, which controls exchange with the surroundings; cytoplasm, where many chemical reactions occur; a nucleus, which contains genetic material and controls cell activities; and mitochondria, where aerobic respiration releases usable energy.

Plant cells also have a cellulose cell wall for support, chloroplasts containing chlorophyll for photosynthesis, and a large vacuole containing cell sap. Not every plant cell contains chloroplasts, but photosynthetic cells commonly do.

Within this syllabus boundary, a bacterial cell has a cell wall, cell membrane and cytoplasm. It has no nucleus. Do not add organelles or molecular detail not required by the stated list when a question asks only for syllabus identification.

Reproduction

In asexual reproduction, one organism produces genetically exact copies of itself. It can increase numbers rapidly without finding a mate, but produces little inherited variation. In sexual reproduction, organisms produce male and female sex cells that fuse. Offspring receive characteristics from both parents, producing variation.

An organism may use both modes at different stages. Coral polyps, for example, bud asexually and also reproduce sexually by releasing gametes. The terms describe the mechanism, not whether the resulting organisms live alone or in a colony.

Classification and scientific names

Classification groups organisms by shared features. The three domains are Bacteria, Archaea and Eukarya. The marine eukaryotic kingdoms required here are animals, plants and protoctists.

The binomial system gives every species an internationally agreed two-part scientific name: genus followed by species, such as Orcinus orca. The genus begins with a capital letter, the species with a lower-case letter, and both are italicised when typed. Shared everyday names can vary, so binomials support precise communication.

Marine vertebrates

Classify the named vertebrate groups using skin covering, fertilisation and gas-exchange organs:

• marine mammals include cetaceans, pinnipeds and sirenians; they have hair at some life stage, use lungs and have internal fertilisation;
• marine birds include pelagic birds and shorebirds; feathers and lungs are diagnostic;
• marine reptiles include sea snakes and turtles; they have scales, lungs and internal fertilisation;