01
From cells to systems
A single cell can only do so much, so in a multicellular organism cells specialise and work together. Similar specialised cells form a tissue; different tissues combine into an organ; organs working together form an organ system; and the systems together make the organism. This division of labour lets the organism do far more than any single cell could.
02
Cell specialisation
A specialised cell has a shape and structure suited to its job. A red blood cell is a biconcave disc packed with haemoglobin (no nucleus) to carry oxygen; a nerve cell is long and thin to carry impulses; a root hair cell has a large surface area to absorb water; a sperm cell has a tail to swim. Structure fits function throughout biology.
03
Plant and animal tissues
In animals, the main tissue types are epithelial (covering/lining), muscle (movement), nervous (impulses) and connective (support). In plants, key tissues include the epidermis (protection), xylem (carries water up), phloem (carries food) and mesophyll (photosynthesis).
| Organ system | Main job |
|---|
| Digestive | break down and absorb food |
| Circulatory | transport blood and materials |
| Respiratory | gas exchange |
| Nervous | coordination and response |
04
A worked example
Worked example
A root hair cell has a long, thin extension. Explain how this feature suits its function.
- Its job is to absorb water and mineral ions from the soil.Identify the function first.
- The long, thin extension increases the surface area.A projecting shape adds area.
- A larger area means faster absorption by osmosis and active transport.More membrane allows more exchange.
The extension increases surface area, speeding absorption of water and mineral ions — structure fitting function.
05
Stem cells and specialisation
Every specialised cell starts as an unspecialised stem cell. Through differentiation, a stem cell switches on the particular genes it needs and develops the shape and contents for one job — becoming a nerve cell, a muscle cell or a red blood cell. In animals, stem cells allow growth and the repair of tissues; in plants, unspecialised cells at the tips (meristems) keep this ability for life, which is why a cutting can grow a whole new plant.
06
Organ systems working together
No organ system works alone. Digesting a meal and delivering its energy needs the digestive system to break food down and absorb it, the circulatory system to carry the nutrients to every cell, the respiratory system to supply the oxygen for respiration, and the nervous and hormonal systems to coordinate it all. This cooperation is what makes a large, complex organism possible.
Levels build on each other
Specialised cells → tissues → organs → organ systems → organism. Each level depends on the ones below, and the systems constantly work together — no part of the body is truly independent.
Practice questions
6 questions · 21 marks · full working on every one
Try each one on paper first, then open the working. The marks are shown where they are actually awarded, because that is where they are actually lost.
Short questions
4 · 10 marksTwo marks each, in the style of the short-question section of the paper. Answer in two or three lines.
SQ1[2 marks]
Put these in order of increasing complexity: organ, cell, organism, tissue, organ system.
Model answer
Cell → tissue → organ → organ system → organism.
Examiner tip. Cells are smallest; the organism is the whole.
SQ2[3 marks]
Explain how a red blood cell is adapted to its function.
Model answer
A biconcave disc gives a large surface area for absorbing oxygen; it is packed with haemoglobin to carry oxygen; and it has no nucleus, leaving more room for haemoglobin.
Examiner tip. Link each feature to carrying oxygen.
SQ3[2 marks]
Explain the role of stem cells in growth and repair.
Model answer
Stem cells are unspecialised and can keep dividing; their daughter cells differentiate into the specialised cells a tissue needs, allowing the organism to grow and to repair or replace damaged tissue.
Examiner tip. Unspecialised + can differentiate = source of new specialised cells.
SQ4[3 marks]
Describe how three organ systems work together when you digest and use a meal.
Model answer
The digestive system breaks the food into small soluble molecules and absorbs them; the circulatory system carries these nutrients (and oxygen) to the cells; the respiratory system supplies the oxygen needed to release the food's energy in respiration.
Examiner tip. Digest and absorb → transport → supply oxygen.
Long questions
2 · 11 marksTheory and numerical together, as they appear in the long-question section.
LQ1[6 marks]
Multicellular organisms rely on specialised cells and division of labour.
- Explain what is meant by a specialised cell. [2]
- Name two specialised cells and one adaptation of each. [4]
Mark scheme
- A cell whose structure/shape[1]
- is adapted to carry out one particular function[1]
- Nerve cell — long and thin to carry impulses[1]
- (valid adaptation)[1]
- Root hair cell — long extension for a large absorbing area[1]
- (or red blood cell — biconcave/haemoglobin)[1]
A specialised cell is shaped for one job; e.g. nerve cell (long, carries impulses) and root hair cell (large area, absorbs water).
Examiner tip. Always link the adaptation to the cell's function.
LQ2[5 marks]
Multicellular organisms are organised into levels.
- List the levels of organisation from cell to organism. [2]
- Explain why cells specialise (division of labour). [3]
Mark scheme
- Cell → tissue → organ[1]
- → organ system → organism[1]
- Different cells become specialised for particular jobs[1]
- each doing its task efficiently[1]
- so the organism as a whole can do far more than a single unspecialised cell could[1]
Cell→tissue→organ→organ system→organism; specialisation (division of labour) lets each cell do its job efficiently so the organism achieves more.
Examiner tip. Give the full ladder; division of labour = efficiency.