Structures and Functions in Living Organisms · 6 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 18% of your exam marks.
Cell structure and organelle function appear on nearly every paper; one of the highest-frequency topics.
All living organisms are built from one or more cells. A single-celled organism such as an amoeba is one cell on its own. A multicellular organism such as a human is built from trillions of cells working together.
Inside every cell is a jelly-like substance called the cytoplasm, surrounded by a thin cell membrane that separates the cell from the outside world. Eukaryotic cells contain small sub-cellular structures called organelles suspended in the cytoplasm; each organelle does a specific job.
In a complex multicellular organism, structure is built up in five tidy levels:
| Level | What it is | Example |
|---|---|---|
| Organelle | A specialised sub-cellular structure inside a cell | Mitochondrion (site of aerobic respiration) |
| Cell | The basic structural and functional unit of life | Red blood cell |
| Tissue | A group of similar cells working together to perform one function | Muscle tissue, xylem tissue |
| Organ | Different tissues working together to perform a particular function | Heart, leaf, kidney |
| Organ system | A group of organs working together to perform a major function | Circulatory system, digestive system |
A handy way to remember the order: organelles → cells → tissues → organs → organ systems.