Cell structure and organisation for O-Level Biology (SEAB 6093): the organelles of plant and animal cells, how plant and animal cells compare, using the light microscope and calculating magnification, and the levels of organisation from cell to organism

What this module covers

Cell structure and organisation is the starting point of O-Level Biology (SEAB 6093) and belongs to the Cells and Chemistry of Life theme. The cell is the basic unit of all living things, so you need to know the parts of a cell and what each does, how plant and animal cells differ, how to view cells under a light microscope and calculate their real size, and how cells build up into the whole organism. These ideas underpin almost every later topic, from transport across membranes to gas exchange and photosynthesis.

This overview links every dot point in the module; work through them in order, then check yourself with the questions at the end. See the full set at /sg-o-level/biology/syllabus.

The parts of a cell

Begin with the structures inside a cell. The page on the cell and its organelles names the main organelles of plant and animal cells and states the function of each: the cell membrane controls what enters and leaves, the cytoplasm is where reactions happen, the nucleus holds the genetic information, the mitochondria are the site of respiration, and in plant cells the cell wall gives support, the chloroplasts carry out photosynthesis and the vacuole stores cell sap. Learn each structure with its job, because Paper 2 often gives a diagram and asks you to label and explain.

Comparing plant and animal cells

Next, contrast the two cell types. The page on comparing plant and animal cells sets out what they share (cell membrane, cytoplasm, nucleus, mitochondria) and what only plant cells have (cell wall, large permanent vacuole, chloroplasts). It also relates specialised cells, such as the root hair cell and the red blood cell, to their functions, which is the application skill examiners reward.

Seeing cells: the light microscope

Cells are too small to see unaided, so you must know the tool. The page on using the light microscope covers the parts of the microscope, how to prepare and view a slide, and how to calculate magnification and actual size. This is examined in both the written and practical papers, so the magnification formula is essential.

From cell to organism

Finally, zoom out. The page on levels of organisation describes the sequence from cell to tissue to organ to organ system to organism, with examples and an explanation of division of labour. Knowing this hierarchy helps you place every later organ system (digestive, circulatory, respiratory) into context.

How this module is examined

• Structure with function. Naming an organelle is only half the mark; the examiner wants the matching function (for example, mitochondria are the site of respiration that releases energy).
• Compare cleanly. Plant and animal cell comparisons reward the three plant-only structures stated correctly, not a vague description.
• Calculate carefully. Magnification questions reward correct rearranging and consistent units, so always convert to the same unit before dividing and quote the answer with a unit.

Check your knowledge

A mix of recall, comparison and calculation questions. Attempt them, then check against the solutions.

1. Name two structures found in a plant cell but not an animal cell, and state the function of each. (2 marks)
2. State the function of the mitochondria and explain why a muscle cell contains many of them. (2 marks)
3. A cell drawing is 40 mm across and the magnification is 200. Calculate the actual width of the cell in micrometres. (2 marks)
4. List, in order, the levels of organisation from cell to organism. (2 marks)