Cell Structure and Organisation

This guide covers Topic 1, Cells, in full — subtopics 1.1 Cell structure and function and 1.2 Specialised cells, tissues and organs — for Cambridge O Level Biology 5090, 2026–2028 series.

Where this fits in 5090

Cells are the starting point of the whole syllabus: the structures named here (membrane, cytoplasm, nucleus and so on) are assumed knowledge in every later topic, starting with Movement into and out of Cells, and the molecules described in Biological Molecules are what those structures are built from.

Syllabus coverage

CAMBRIDGE O LEVEL BIOLOGY 5090

• Examine animal and plant cells from suitable material under the microscope, using an appropriate temporary staining technique such as methylene blue or iodine solution, and draw diagrams of the observations (1.1)
• Identify, on diagrams, photomicrographs or electron micrographs, the ribosomes, mitochondria, nucleus, cytoplasm and cell membrane in an animal cell (1.1)
• Identify, on diagrams, photomicrographs or electron micrographs, the ribosomes, mitochondria, chloroplasts, nucleus, sap vacuole, cytoplasm, cell membrane and cellulose cell wall in a plant cell (1.1)
• Describe the structure of a bacterial cell, limited to: ribosomes, circular DNA and plasmids, cytoplasm, cell membrane and cell wall (1.1)
• Describe the functions of these structures in animal, plant and bacterial cells (1.1)
• Understand that cells can become specialised, with structure related to function, as illustrated by examples in the syllabus (1.2)
• Understand the terms cell, tissue, organ, organ system and organism (1.2)
• State and use the formula magnification = image size ÷ actual size (1.2)

5090 is not tiered — every candidate covers all of the above.

Animal, plant and bacterial cell structures

Structure	Animal cell	Plant cell	Bacterial cell
Cell membrane	✓	✓	✓
Cytoplasm	✓	✓	✓
Ribosomes	✓	✓	✓
Mitochondria	✓	✓	—
Nucleus	✓	✓	— (circular DNA + plasmids instead)
Cell wall	—	✓ (cellulose)	✓
Chloroplasts	—	✓	—
Sap vacuole	—	✓	—

Each structure has a specific function: the cell membrane controls what enters and leaves the cell; cytoplasm is where many chemical reactions occur; ribosomes are the site of protein synthesis; mitochondria release energy through respiration; the nucleus contains DNA and controls the cell’s activities; the cell wall (plant and bacterial) gives structural support and shape; chloroplasts are the site of photosynthesis; the sap vacuole stores cell sap and helps maintain the cell’s turgor.

Bacterial cells differ fundamentally from animal and plant cells: they have no true nucleus (their DNA is circular and free in the cytoplasm, plus separate small circles of DNA called plasmids) and no mitochondria or chloroplasts.

Magnification

Magnification relates the size of an image to the actual size of the object it represents:

magnification = image size / actual size

Worked example. A photomicrograph shows a cell 40 mm long. Its actual length is 0.02 mm. Find the magnification.

magnification = image size / actual size = 40 / 0.02 = 2000×

Rearranging this same equation lets you find actual size or image size instead, given the other two quantities — always keep the units of image size and actual size consistent before dividing.

Specialised cells, tissues and organs

Cells become specialised — developing a structure suited to a specific function — rather than remaining generic. A red blood cell, for instance, is packed with haemoglobin and has no nucleus, maximising its oxygen-carrying capacity; a root hair cell has a long, thin extension, maximising surface area for water uptake.

This specialisation builds into a hierarchy of organisation:

cell → tissue → organ → organ system → organism

A tissue is a group of similar cells performing the same function; an organ is a structure made of several tissues working together for a particular function; an organ system is a group of organs working together; the organism is the complete individual.

Common mistakes

• Listing chloroplasts or a cell wall for an animal cell, or vice versa listing a nucleus for a bacterial cell — the structural differences between the three cell types are a common and specific exam focus.
• Confusing nucleus and nucleolus, or vacuole and mitochondria on a labelled diagram — know exactly which structure each named term refers to.
• Mixing up image size and actual size in the magnification formula, producing a wildly wrong answer — actual size is always the smaller number for a magnified image.
• Not converting units before calculating magnification — image size and actual size must be in the same units before dividing.
• Treating “specialised” as just “different-looking” rather than explaining how the structure relates to the cell’s specific function.

Quick revision checklist

• The structures present in animal cells, plant cells and bacterial cells, and which are unique to each
• The function of each named structure
• magnification = image size ÷ actual size, and rearranging it
• Cell specialisation, and the cell → tissue → organ → organ system → organism hierarchy

Related resources

• Biological Molecules — what the structures described here are chemically built from
• Enzymes — the biological catalysts active within these cells
• Cambridge O Level Biology subject hub

Written against Cambridge O Level Biology 5090, 2026–2028 series. Always check the current syllabus for your examination year.