2.1.1 Levels of Organisation — Edexcel International GCSE Science Double Award (Modular) - Biology Unit 1 Revision Notes & Topical Questions

Part 1 of 8Key ideas

Order of organisation

Order of organisation organelle → cell → tissue → organ → organ system → organism.

A tissue contains similar cells; an organ contains different tissues.

Specialisation results from differentiation, controlled by which genes are expressed.

In animals most cells differentiate early and lose the ability to differentiate; plants keep meristems all their lives.

Embryonic stem cells can form almost any cell; adult stem cells can form only a few types.

Stem cells could be used to replace damaged cells (e.g. insulin-secreting cells in type 1 diabetes).

Example

Example the stomach (organ) contains muscle tissue, glandular tissue and epithelial tissue.

Size scale

1 m = 1000 mm; 1 mm = 1000 µm; 1 µm = 1000 nm.

Magnification = image size ÷ actual size (no units).

Part 2 of 8Learning objectives

Describe the levels of organisation in organisms

Describe the levels of organisation in organisms organelles, cells, tissues, organs and systems.

Give examples of each level for plants and animals.

Explain the importance of cell differentiation in the development of specialised cells.

Understand the advantages and disadvantages of using stem cells in medicine.

Part 3 of 8Essential knowledge

Essential knowledge

Organelles are sub-cellular structures with particular roles – e.g. mitochondria (aerobic respiration), ribosomes (protein synthesis), chloroplasts (photosynthesis).

Cells are the basic units of life; in multicellular organisms cells are specialised for particular functions.

Tissues are groups of similar cells working together: e.g. muscle tissue contracts; xylem transports water; epithelial tissue lines surfaces.

Organs contain several tissues

Organs contain several tissues the heart contains cardiac muscle, connective tissue and nervous tissue; a leaf contains epidermis, palisade mesophyll, spongy mesophyll, xylem and phloem.

Organ systems group organs with related functions

Organ systems group organs with related functions the digestive system includes the mouth, oesophagus, stomach, small and large intestine, liver and pancreas.

Differentiation is the process by which unspecialised cells develop specific structures; it happens because different genes are expressed in different cells, producing different proteins.

Examples of specialised cells

Examples of specialised cells red blood cells (no nucleus, biconcave, haemoglobin), root hair cells (large surface area for absorption), palisade cells (many chloroplasts), sperm cells (tail, many mitochondria).

Embryonic stem cells can differentiate into almost any cell type, so could be used to grow tissues to treat conditions such as diabetes or spinal injury.

Adult stem cells (e.g. in bone marrow) can only form a limited range of cells, such as the different types of blood cell; bone marrow transplants are already used to treat leukaemia.

Disadvantages of stem cell treatments

Disadvantages of stem cell treatments ethical objections to using embryos, risk of immune rejection, risk of uncontrolled division forming tumours, and possible transfer of viral infection.

Plant meristems at root and shoot tips remain able to differentiate throughout life, which is why plants can be cloned from cuttings.

Plant organ systems

Flowering plants have two main organ systems: the shoot system (stems, leaves, flowers, fruits) above ground and the root system below ground. Both contain xylem and phloem tissue, which links them for transport of water and food.

Unicellular vs multicellular

A unicellular organism carries out every life process within one cell, so it has no tissues or organs. Multicellular organisms divide the work between specialised cells organised into tissues, organs and organ systems.

Part 4 of 8Process & sequence

- Organelles such as the nucleus, ribosomes and mitochondria carry out specific functions within a cell.
- An unspecialised cell (e.g. a stem cell) divides by mitosis.
- Some genes are switched on and others off, so the cell differentiates and becomes specialised.
- Many similar specialised cells group together to form a tissue.
- Different tissues combine to form an organ.
- Organs with related functions form an organ system.
- Several organ systems working together make up the whole organism.
From stem cell to organismwrite these steps in this order; each step is a marking point.
1. Stem cell divides by mitosis.
2. Gene expression changes → cell differentiates.
3. Similar specialised cells form a tissue.
4. Different tissues form an organ.
5. Organs form an organ system, and systems make an organism.

Part 5 of 8Worked examples

Organ

Classify each as cell, tissue, organ or organ system: kidney, xylem, sperm, nervous system.

Kidney = organ (many tissues);
- xylem = tissue (similar cells transporting water);
- sperm = cell;
- nervous system = organ system (brain, spinal cord, nerves).

×500

A cell with an actual length of 50 µm appears 25 mm long in a drawing. Calculate the magnification.

25 mm = 25 000 µm. Magnification = 25 000 ÷ 50 = ×500.

Suggest two advantages of using adult rather than embryonic stem cells in treating a patient.

1) Can be taken from the patient's own body, so there is little risk of rejection by the immune system.
- 2) No embryo is destroyed, so there are fewer ethical objections.

Part 6 of 8Summary

- Hierarchy: organelle → cell → tissue → organ → organ system → organism.
- A tissue is similar cells with a shared function.
- An organ is several different tissues working together.
- Specialised cells arise by differentiation of stem cells.
- Stem cells offer medical hope but raise ethical and safety issues.

Part 7 of 8Keep going

Levels of Organisation Edexcel O Level Biology: what to do next
These Levels of Organisation Edexcel O Level Biology notes covered the key ideas, worked examples and exam technique for this topic. Check your Levels of Organisation Edexcel O Level Biology understanding with the practice questions below, then reread any Levels of Organisation Edexcel O Level Biology point you were unsure about before moving on.
Levels of Organisation Edexcel O Level Biology: revision checklist
- Revise the main Levels of Organisation Edexcel O Level Biology definitions until you can write them from memory.
- Practise Levels of Organisation Edexcel O Level Biology exam questions under timed conditions.
- Link each Levels of Organisation Edexcel O Level Biology idea to a diagram or a worked example.
- List the common Levels of Organisation Edexcel O Level Biology mistakes and how to avoid them.
- Practice: Levels of Organisation Edexcel O Level Biology practice questions
- Previous lesson: 1.1 Characteristics of Living Organisms — O Level Notes
- Next lesson: 2.2 Cell Structure — O Level Notes
- Course: Edexcel O Level Biology — all units
- Official syllabus: Pearson Edexcel International GCSEs

Part 8 of 8Compare: Compare

Levels of organisation with examples
Level	Animal example	Plant example
Organelle	Mitochondrion	Chloroplast
Cell	Muscle cell	Palisade mesophyll cell
Tissue	Muscle tissue	Palisade mesophyll (tissue)
Organ	Heart	Leaf
Organ system	Circulatory system	Shoot system
Organism	Human	Sunflower

Embryonic vs adult stem cells vs plant meristems
Feature	Embryonic stem cells	Adult stem cells	Plant meristem cells
Source	Early embryos (e.g. spare IVF embryos)	Bone marrow, some other tissues	Root and shoot tips
Range of cells produced	Almost any cell type	Limited types (e.g. blood cells)	Any plant cell type
Medical / practical use	Potentially treat diabetes, paralysis	Bone marrow transplants	Cloning plants (cuttings, micropropagation)
Issues	Ethical objections; risk of rejection or tumours	Fewer ethical issues; limited ability	Few ethical issues

Key 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.

Exam technique

Keywords the mark scheme looks for in this lesson

Organelle · Cell · Tissue · Organ · Organ system · Organism · Differentiation · Stem cell · Embryonic stem cell · Adult stem cell · Meristem · Specialised cell

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)

1

Use specification keywords exactly (e.g. enzyme–substrate complex, active site, complementary).

2Link ideas

Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.

3Use the information in the question

“discuss/evaluate” answers must use the data provided and match the number of marks.

4

Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.

5Avoid vague words

“it”, “stuff”, “goes up”, “energy is made”. Name the substance and the process.

6

Extended (6-mark) responses are marked on content AND a logical, linked line of reasoning.

Words to avoid

1

a tissue is a group of different cellsa tissue is a group of similar cells working together to perform a function

2

an organ is a group of cellsan organ is a group of different tissues working together

3

stem cells can become anythingembryonic stem cells can differentiate into almost any cell type; adult stem cells only into a few types

4

differentiation is cells dividingdifferentiation is cells becoming specialised for a function

5

the leaf is a tissuethe leaf is an organ made of several tissues