Cell Biology and Organisation

Structure and function are always tested together, so naming an organelle correctly is only half of what an examiner wants; the other half is explaining what that structure actually does inside the cell.

It also compares how life processes such as nutrition, respiration, excretion and response are carried out. In a unicellular organism like Amoeba, one cell alone performs every process, absorbing food, releasing energy and removing waste through its own cell surface.

In a multicellular organism such as a human, the same processes are shared out among specialised cells, tissues and organs that depend on one another to keep the whole body functioning, a division of labour a single free-living cell can never achieve.

A further content standard builds the levels of organisation from the ground up: similar cells grouped together form a tissue, different tissues working as a unit form an organ, related organs cooperating form a system, and all the systems together make up the complete organism. Each level exists only because the level below it is correctly structured, which is why examiners often ask you to place named human examples, such as muscle tissue, the heart and the circulatory system, in the right order, from the smallest level to the largest.

Across the whole chapter, the real skill being tested is linking a structure's shape to its role. A root hair cell is long and thin because that increases the surface area available for absorbing water, and a red blood cell is biconcave and has no nucleus so that it can carry more oxygen.

Memorising organelle names without this structure-function reasoning is the most common reason marks are lost in this chapter, more than any single organelle name forgotten under exam pressure.

Content standards in this chapter

1. 2.1 Cell Structure and Function
2. 2.2 Living Processes in Unicellular Organisms
3. 2.3 Living Processes in Multicellular Organisms
4. 2.4 Levels of Organisation in Multicellular Organisms

Key concepts

Cell structure

A generalised cell has a nucleus that controls its activities, cytoplasm where chemical reactions take place, mitochondria that release energy through respiration, and ribosomes that build proteins. A plant cell adds three structures an animal cell lacks: a rigid cellulose cell wall for support, chloroplasts that trap light energy, and one large permanent vacuole that keeps the cell turgid.

Animal vs plant cells

Plant cells have a cellulose cell wall outside the plasma membrane, chloroplasts for photosynthesis, and a large permanent vacuole for water storage and turgor; animal cells have none of these three structures. Both cell types still share a nucleus, cytoplasm, mitochondria, ribosomes and a plasma membrane that controls what enters and leaves.

Living processes in unicellular organisms

Organisms such as Amoeba and Paramecium consist of a single cell that must carry out nutrition, respiration, excretion, growth, movement and response entirely on its own, without any other cell to share the work. Food, oxygen and waste all cross the same cell surface, and the cell has no separate organs to help, unlike the many cells of a multicellular animal.

Living processes in multicellular organisms

In a multicellular organism, no single cell performs every life process alone. Specialised cells, tissues, organs and systems divide the work between them, nutrients absorbed by the digestive system are transported by the circulatory system to cells all over the body, which could not happen inside one isolated cell.

Levels of organisation

Multicellular organisms are built as a hierarchy: similar cells form a tissue, different tissues working together form an organ, related organs form a system, and every system together forms the organism. A fault at one level, such as damaged cells in a tissue, can disrupt the organ and system built from it. Examiners often test this hierarchy by asking you to place a named human example, such as a muscle cell, muscle tissue, the heart and the circulatory system, at the correct level.

Specialised cells

Cell structure matches function: a red blood cell is biconcave and lacks a nucleus so it can carry more oxygen; a root hair cell is long and thin to increase the surface area for absorbing water; a nerve cell has a long fibre to carry electrical impulses quickly over a distance. A sperm cell has a tail for swimming towards an egg, and a guard cell changes shape to open or close a leaf's stomata, showing that specialisation applies equally to plant and animal cells.

Tissues in animals and plants

A tissue is a group of similar cells working together for one function, such as muscle tissue that contracts or xylem tissue that transports water in a plant. Recognising a named tissue is the link between the cell level and the organ level in the hierarchy of organisation. Other examples include epithelial tissue that lines surfaces such as the gut, and phloem tissue that transports dissolved food in a plant.

Cell as the basic unit of life

Every reaction that keeps an organism alive, including respiration, protein synthesis and waste removal, happens inside cells. Because a cell is the smallest unit that can carry out these processes on its own, it is called the basic structural and functional unit of life, and damage at the cell level ultimately affects the whole organism.

Selective permeability of the plasma membrane

The plasma membrane surrounds every cell and controls which substances enter and leave, letting in useful molecules such as oxygen and glucose while allowing waste products to pass out. This control keeps the cell's internal conditions suitable for the reactions it carries out, and the same basic membrane structure appears in every type of cell, from bacteria to human neurons.

The nucleus and genetic control

The nucleus contains chromosomes made of DNA, carrying the genetic instructions that direct every activity inside the cell, including which proteins it builds. A cell without a working nucleus, such as a mature red blood cell, cannot divide or repair itself, and eventually breaks down and is replaced. Chromosome number is also characteristic of a species; human body cells, for instance, normally contain 23 pairs.

How this chapter is examined

SPM Biology is assessed over three papers. Paper 1 has 40 objective questions (40 marks) in 1 hour 15 minutes; Paper 2 carries 100 marks across Sections A, B and C in 2 hours 30 minutes; Paper 3 is the practical, testing science process skills. Content from this chapter can appear in any of them, so lessons drill recall for Paper 1 and structured answers for Paper 2.

Common exam angles

• Labelling a fully drawn animal or plant cell diagram and stating the precise function of each organelle shown.
• Comparing animal and plant cells side by side in a table of shared and unique structures.
• Arranging a mixed list of structures, cell, tissue, organ, system, organism, into the correct order of organisation.
• Explaining how a named specialised cell's structure suits its one main function, using correct biological terms.
• Comparing how the same life process, such as respiration or excretion, is carried out in a unicellular organism and in a human organ system.
• Identifying an unfamiliar organelle correctly from a written description of its function alone, without seeing a labelled diagram.

Common mistakes

Study this chapter

Structures in this chapter

• Animal cell structure
• Plant cell structure
• Structure of a bacterium
• Mitochondrion structure and function
• Chloroplast structure and function
• Nucleus structure and function

Experiments in this chapter

• Prepare Slides of Animal and Plant Cells
• Using a Light Microscope
• Preparing a Temporary Slide (Wet Mount)
• Calculating Magnification

Key terms

CellOrganelleNucleusTissueCell differentiation

Frequently asked questions

Source:SRC-DSKP-EN, SRC-FORMAT