Glossary: Cell Biology and Organisation

This chapter's terms in context

The five core terms of Chapter 2, cell, organelle, nucleus, tissue and cell differentiation, build the vocabulary that every later human, animal and plant physiology chapter assumes a student already has. A cell is the smallest structural and functional unit of a living organism, capable of carrying out all the basic processes of life on its own; every specialised structure discussed later, from a red blood cell to a root hair cell, is still fundamentally a cell first. An organelle is a specialised structure inside a cell that performs a specific function, in the same way that an organ performs a specific function inside a whole body, the mitochondrion, ribosome, chloroplast and nucleus are all organelles, and confusing 'organelle' with 'organ' is a common vocabulary slip that costs marks in definition questions.

The nucleus deserves separate attention because it is both a named organelle and the site of genetic control, containing chromosomes made of DNA that direct every activity in the cell, including which proteins are synthesised. A cell that loses a functioning nucleus, such as a mature mammalian red blood cell, can no longer divide or repair itself. Students sometimes describe the nucleus only as 'the control centre' without adding what it controls or how, through coded genetic instructions on chromosomes, which loses the technical marks a full answer earns.

Tissue is the level of organisation directly above the single cell: a group of similar cells, of the same origin, working together to perform one function. Muscle tissue, xylem tissue and epithelial tissue are all named examples that recur across the syllabus, and the examiner expects a candidate to place a tissue correctly between the cell level below it and the organ level above it, rather than treating 'tissue' and 'organ' as interchangeable, which is one of the most frequent structural-hierarchy errors seen in scripts.

Cell differentiation explains how a single fertilised cell can eventually give rise to hundreds of different cell types, a process in which unspecialised cells change in structure and function to become suited to a specific role, such as a nerve cell adapted to conduct impulses or a root hair cell adapted to absorb water. Differentiation is why cells that started identical end up looking and behaving completely differently, and it links directly to the concept of stem cells introduced in the cell division chapter later in the syllabus.

Cell theory itself, that all living things are made of cells, that the cell is the basic unit of structure and function, and that new cells arise only from existing cells through division, sits behind every one of these five terms, and an SPM question that asks a candidate to justify why the cell is called the basic unit of life is really asking for this theory to be stated in the student's own words, connected to a concrete example such as a muscle cell or a leaf cell.

Paper 2 structured questions in this chapter often supply a diagram of an unfamiliar cell or organism and ask a candidate to use the five terms above to reason about it rather than recall a memorised list. A question might show a single-celled organism and ask why it does not need distinct tissues, or show a diagram of a plant organ and ask a candidate to identify which visible feature is a tissue and which is an organelle within one of its cells. Because these questions test transfer rather than recall, the most reliable preparation is not memorising organelle names in isolation but practising the underlying logic, cells with a shared origin and function form a tissue, and a structure within a single cell that performs one job is an organelle, so that the same five terms can be applied correctly to any new example an examiner introduces, familiar or not.