Cambridge IGCSE Biology Notes 2: Organisation of the Organism
Cambridge IGCSE Biology 0610 and 0970 notes on plant, animal and bacterial cells, organelle functions, specialised cells, levels of organisation, magnification and size conversion.
Topic 2 of Cambridge IGCSE Biology 0610 and 0970 connects cell structures to their functions, then builds from specialised cells to tissues, organs, organ systems and organisms. It also requires magnification and size calculations. Core candidates work in millimetres; Extended candidates also convert between millimetres and micrometres.
2.1 Cell structure
Structures shared by plant and animal cells
The cell membrane controls movement of substances into and out of the cell. It is a boundary with selective transport roles, not a rigid supporting wall.
Cytoplasm is where many chemical reactions occur. It contains enzymes and surrounds the cell structures.
The nucleus contains genetic material and controls cell activities by controlling which proteins are produced.
Ribosomes are the sites of protein synthesis. Mitochondria are the sites of aerobic respiration, where energy is released from nutrient molecules for cellular processes.
Plant-cell structures
The cellulose cell wall supports the cell, helps maintain shape and prevents bursting when water enters. It is freely permeable and lies outside the cell membrane.
Chloroplasts contain chlorophyll, which absorbs light energy for photosynthesis. They occur in photosynthetic plant cells but are not present in every plant cell, for example most root cells.
A large permanent vacuole contains cell sap, a solution of sugars and salts. Water entering the vacuole contributes to turgor pressure against the cell wall and helps support non-woody tissues.
Plant cells also have a cell membrane, cytoplasm, nucleus, ribosomes and mitochondria.
Animal cells
Animal cells have a cell membrane, cytoplasm, nucleus, ribosomes and mitochondria. They do not have a cellulose cell wall or chloroplasts. Small temporary vacuoles may occur, but the large permanent cell-sap vacuole is a characteristic plant-cell feature in this comparison.
The absence of a wall allows varied cell shapes and movement, but support and shape can still come from the cytoskeleton, surrounding tissues or extracellular material beyond this syllabus detail.
Bacterial cells
A bacterial cell has a cell wall, cell membrane, cytoplasm and ribosomes. Its genetic material includes a circular DNA molecule and may include plasmids, which are smaller DNA loops.
Bacteria have no nucleus. Their circular DNA lies free in the cytoplasm. They also lack mitochondria and chloroplasts; relevant reactions occur using the cell membrane and cytoplasm.
The bacterial wall is not described as cellulose in this syllabus boundary. Do not transfer the composition of plant walls to all cell walls.
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