1.2Unit 1 · Living Organisms & Cells
Levels of Organisation — Edexcel IGCSE Biology Modular - UNIT 1 Revision Notes & Topical Questions
5 min read 7 parts Lesson 3 of 6 1 quiz
Lesson summary
Describe the levels of organisation in multicellular organisms.Orderorganelles → cells → tissues → organs → organ systems → organism.The stomach contains muscular, glandular and epithelial tissue.The heart is an organ in the circulatory system.Plant organs include roots, stems, l…
- Cells are the basic building blocks of all organisms
- This organisation allows division of labour
- Organ systems and their organs
- Explain why the stomach is described as an organ rather than a tissue
- Specialised cell
- Give / State / Name
Section 1 of 7Levels of Organisation
Cells are the basic building blocks of all organisms
Cells are the basic building blocks of all organisms every living organism is made of one or more cells, and all cells come from existing cells. Each cell carries out life processes using its organelles.
Organelles
Organelles sub-cellular structures such as the nucleus, mitochondria, ribosomes and chloroplasts, each with its own job.
Cells
Cells in multicellular organisms cells are specialised, e.g. red blood cells for carrying oxygen, sperm cells for fertilisation.
Tissues
Tissues groups of similar cells with the same function, e.g. muscle tissue contracts, glandular tissue secretes enzymes and hormones, epithelial tissue covers organs.
Organs
Organs several tissues working together, e.g. the stomach uses muscle tissue to churn, glandular tissue to secrete digestive juice and epithelium to line it. In plants, the leaf is an organ made of epidermis, mesophyll, xylem and phloem.
Organ systems
Organ systems groups of organs with a shared role, e.g. the digestive system (mouth, stomach, intestines, liver, pancreas), circulatory, nervous and reproductive systems.
This organisation allows division of labour
This organisation allows division of labour each part is adapted to its job, making the whole organism more efficient.
Organ systems and their organs
Respiratory systemtrachea and lungs.
- Excretory systemkidneys, lungs and skin.
- Nervous systembrain, spinal cord and nerves.
- Reproductive systemovaries and uterus or testes.
- In plants, the shoot system includes leaves, stem and flowers, built from tissues such as epidermis, mesophyll, xylem and phloem.
Section 2 of 7Process & sequence
1
Organelles carry out specific jobs inside the cell.
2
Cells differentiate to become specialised.
3
Similar cells group to form a tissue.
4
Different tissues combine to form an organ.
5
Organs work together as an organ system.
6
Organ systems together make up the organism.
Levels of organisationwrite these steps in this order; each step is a marking point.
- Organelle
- Cell
- Tissue
- Organ
- Organ system
- Organism
Section 3 of 7Compare
| Level | Animal example | Plant example |
|---|---|---|
| Organelle | Mitochondrion | Chloroplast |
| Cell | Muscle cell | Palisade cell |
| Tissue | Muscle tissue | Palisade mesophyll |
| Organ | Stomach | Leaf |
| Organ system | Digestive system | Shoot system |
Section 4 of 7Worked examples
Explain why the stomach is described as an organ rather than a tissue.
- The stomach is made of several different tissues (muscular, glandular, epithelial) working together to digest food.
- A tissue consists of only one type of similar cell, so the stomach is an organ.
- The stomach is made of several different tissues (muscular, glandular, epithelial) working together to digest food.
Section 5 of 7Key terms
Section 6 of 7Command terms
Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.
Section 7 of 7Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Organelle · Cell · Specialised cell · Tissue · Organ · Organ system · Organism · Epithelial tissue
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
How to get full marks (examiner guidance)
Use specification keywords exactly (e.g. eukaryotic/prokaryotic, enzyme–substrate complex, active site, complementary).
Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.
Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
Do not extrapolate a graph line beyond the data or to zero unless asked.
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.
Marked practiceQuizzes for this lesson
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