A photograph of a cell is 45 mm wide. The magnification is ×1500. What is the actual width of the cell in µm?
- actual size = image size ÷ magnification
- actual size = 45 mm ÷ 1500 = 0.03 mm
- Change to micrometres: 0.03 × 1000 = 30 µm
Answer: 30 µm
Specialised cells, tissues and organs
Biology · Cells. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The idea
Cells in a large organism do not all look the same. A specialised cell has a shape and contents that suit one job. In an exam, always link the feature to the job: a root hair cell has a long extension, which gives a large surface area, so it absorbs water and ions quickly.
Cells are organised into levels. Similar cells working together make a tissue. Different tissues working together make an organ. Organs form organ systems, and systems make up the whole organism.
Cells are too small to measure directly, so we use magnification to find the real size from a drawing or photograph.
Rules to remember
- Order of size: cell → tissue → organ → organ system → organism.
- Tissue: a group of similar cells that work together to carry out the same function.
- Organ: several different tissues working together, e.g. stomach, heart, leaf.
- magnification = image size ÷ actual size
- actual size = image size ÷ magnification; image size = actual size × magnification
- 1 mm = 1000 µm. Put both sizes in the same unit before you divide.
- Examples: red blood cell (no nucleus, more room for haemoglobin), ciliated cell (cilia sweep mucus), xylem vessel (hollow dead tube, carries water), root hair cell (large surface area).
Common mistake
Dividing sizes that are in different units, such as millimetres by micrometres. Change both to the same unit first (multiply mm by 1000 to get µm), then divide.
Worked example
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