New cells only come from the division of existing cells. In a multicellular organism those cells then specialise, each with a structure suited to one function, and they are organised into tissues, organs and organ systems.
New cells are produced by division of existing cells. Specialised cells have specific functions: ciliated cells move mucus in the trachea and bronchi, root hair cells absorb water and mineral ions, palisade mesophyll cells photosynthesise, neurones conduct electrical impulses, red blood cells transport oxygen, and sperm and egg cells (gametes) are for reproduction. Similar cells form a tissue, tissues form an organ, organs form an organ system, and organ systems together form an organism.
Fig. 1: the five levels of organisation, for a human and for a plant
Section 01
New cells from old.
Press Divide every cell and watch the order of events. Then press Keep dividing and count the cells after each division until it stops.
The rule: new cells are produced by the division of existing cells. A cell never appears from nothing.
Nucleus first: the nucleus divides first so that each new cell receives a full, identical copy of the genetic material. Then the cytoplasm divides and two cells form.
Grow, then divide again: the new cells grow to full size before they divide again.
Doubling: one cell gives 2, 4, 8, 16, 32, then 64 cells after six divisions. This is how a whole organism grows from a single fertilised egg, and how damaged tissue is repaired.
Section 02
Cells built for one function.
Choose each cell in the simulator and watch it at work. For each one, link a feature of its structure to the job it does.
Specialised cell
Function
Where
How the structure helps
ciliated cell
movement of mucus
trachea and bronchi
cilia beat to sweep mucus, with trapped dust and pathogens, up and out of the airways
root hair cell
absorption
root surface
a long, thin extension gives a large surface area to absorb water and mineral ions
palisade mesophyll cell
photosynthesis
upper part of a leaf
packed with chloroplasts to absorb as much light as possible
neurone
conduction of electrical impulses
brain, spinal cord, nerves
a very long fibre carries impulses over long distances
red blood cell
transport of oxygen
blood
full of haemoglobin; no nucleus, leaving more room for it; biconcave shape gives a large surface area
sperm cell
reproduction (male gamete)
testes
flagellum to swim; many mitochondria release energy for swimming
egg cell
reproduction (female gamete)
ovaries
large, with energy stores in its cytoplasm for the early embryo
Examiner trap
Learn the function in the syllabus words. A red blood cell's function is the transport of oxygen, not just carrying blood; a ciliated cell moves mucus, it does not trap dust itself (the mucus, made by other cells, does the trapping).
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
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How are new cells produced?
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One cell keeps dividing, and every new cell divides too. How many cells are there after six divisions?
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What is the function of ciliated cells in the trachea and bronchi?
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Why does a red blood cell have no nucleus?
Section 03
Cell, tissue, organ, system.
Pick the human, then step up through the five levels. Repeat for the plant and compare the example at each level.
Cell: the basic unit of a living organism, for example a muscle cell or a palisade mesophyll cell.
Tissue: a group of cells with similar structures, working together to perform a shared function, for example muscle tissue or palisade mesophyll tissue.
Organ: a structure made of a group of tissues, working together to perform specific functions, for example the heart, the stomach or a leaf.
Organ system: a group of organs with related functions, working together to perform body functions, for example the circulatory system, the digestive system or the shoot system of a plant.
Organism: a whole living thing, made of organ systems working together, for example a human or a bean plant.
Examiner trap
An organ contains several different tissues; a tissue is made of similar cells. A leaf is an organ (epidermis, palisade and spongy mesophyll, xylem and phloem tissues), not a tissue. The heart is an organ, not an organ system: it is one part of the circulatory system.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which of these is an organ?
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Which list shows the levels of organisation from smallest to largest?
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How is a root hair cell adapted for its function?
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Which statement describes a tissue?
Skill unlocked
Specialised cells and levels of organisation, mastered.
This skill is now recorded on your profile. Keep the chain going.