2.2.1Structure and Function in Living Organisms
Cell Structures
10 min read 8 parts Lesson 3 of 55 1 quiz
Section 1 of 8Cell Structures
Essential knowledge
Cell structures
Cell structures the nucleus holds DNA and controls the cell; cytoplasm is the site of most reactions; the cell membrane controls entry and exit; mitochondria release energy by aerobic respiration; ribosomes make proteins.
- Plant cells also have a cellulose cell wall for support, chloroplasts for photosynthesis and a vacuole containing cell sap.
Similarities and differences
Similarities and differences animal and plant cells share nucleus, cytoplasm, membrane, mitochondria and ribosomes. Only plant cells have a cell wall, chloroplasts and a large permanent vacuole. Animal cells usually have irregular shapes; plant cells are more regular due to the wall.
Importance of cell differentiation
Importance of cell differentiation multicellular organisms need different cells for different jobs. During development, cells differentiate by expressing only certain genes, producing specialised cells such as nerve cells (long axons) or palisade cells (many chloroplasts). This division of labour makes the organism more efficient.
Most animal cells lose the ability to differentiate early, while many plant cells keep this ability throughout life in meristems at shoot and root tips.
Stem cells in medicine — advantages
Stem cells in medicine — advantages embryonic stem cells can form almost any cell type and could replace damaged cells, e.g. insulin-producing cells for diabetes or nerve cells after spinal injury. Adult stem cells from bone marrow are already used to treat leukaemia.
Stem cells — disadvantages
Stem cells — disadvantages using embryos raises ethical concerns because the embryo is destroyed; stem cells may divide uncontrollably and form tumours; transplanted cells may be rejected by the immune system; cultures could transfer viral infections; adult stem cells can make only a limited range of cells.
Light vs electron microscope
The nucleus, cell wall, chloroplasts and vacuole can be seen with a light microscope, but ribosomes and the detail of mitochondria are too small and need an electron microscope.
Adaptations of specialised animal cells
Ciliated cells in the airways have cilia that beat to sweep mucus and trapped particles towards the throat. Nerve cells are very long to carry impulses over large distances. Red blood cells are biconcave with no nucleus, giving more room for haemoglobin.
Adult stem cells in bone marrow
These can divide to form only blood cells: red cells, white cells and the cells that make platelets. Transplants can treat leukaemia, anaemia, clotting disorders or a weakened immune system.
Donor vs patient's own stem cells
Donor cells carry different antigens, so the patient must take immunosuppressant drugs to stop rejection. Cells grown from the patient's own stem cells have the same genes and antigens, so they are not rejected.
Section 2 of 8Process & sequence
- A zygote divides by mitosis to form a ball of identical unspecialised cells.
- Cells receive signals that switch particular genes on or off.
- Only certain genes are expressed, so specific proteins are made.
- The cell changes shape and gains structures suited to its function.
- Specialised cells group into tissues, then organs.
From stem cell to organwrite these steps in this order; each step is a marking point.
- Unspecialised stem cell
- Genes switched on or off
- Specialised cell forms
- Cells form tissue
- Tissues form organ
Section 3 of 8Compare
| Structure | Animal cell | Plant cell |
|---|---|---|
| Nucleus, cytoplasm, membrane | Yes | Yes |
| Mitochondria and ribosomes | Yes | Yes |
| Cellulose cell wall | No | Yes |
| Chloroplasts | No | Yes (in green parts) |
| Large permanent vacuole | No (small temporary vacuoles only) | Yes |
| Source | Can form | Issues |
|---|---|---|
| Embryonic stem cells | Almost any cell type | Ethical objections; risk of rejection |
| Adult stem cells (e.g. bone marrow) | Limited range of cell types | Fewer ethical issues; limited use |
| Plant meristems | Any plant cell type | Used for cloning plants |
Section 4 of 8Worked examples
1Explain why a root hair cell does not contain chloroplasts.
- Root hair cells are underground and receive no light, so they cannot photosynthesise.
- The genes for chloroplast development are not expressed, as the cell has differentiated to absorb water and minerals instead.
2
Give one argument for and one against using embryonic stem cells to treat paralysis.
- For: embryonic stem cells can differentiate into nerve cells, which could replace damaged cells and restore movement.
- Againstthe embryo is destroyed, which many people think is unethical, and there is a risk of tumour formation or rejection.
Section 5 of 8Keep going
Cell Structure Edexcel IGCSE Biology 4BI1: what to do next
These Cell Structure Edexcel IGCSE Biology 4BI1 notes covered the key ideas, worked examples and exam technique for this topic. Check your Cell Structure Edexcel IGCSE Biology 4BI1 understanding with the practice questions below, then reread any Cell Structure Edexcel IGCSE Biology 4BI1 point you were unsure about before moving on.
Cell Structure Edexcel IGCSE Biology 4BI1: revision checklist
- Revise the main Cell Structure Edexcel IGCSE Biology 4BI1 definitions until you can write them from memory.
- Practise Cell Structure Edexcel IGCSE Biology 4BI1 exam questions under timed conditions.
- Practice: Cell Structure Edexcel IGCSE Biology 4BI1 practice questions
- Previous lesson: 2.1 Levels of Organisation — Notes
- Next lesson: 2.3 Biological Molecules — Notes
- Course: Edexcel International Gcse Biology 4BI1 — all units
- Official syllabus: Pearson Edexcel International GCSEs
Section 6 of 8Key terms
Section 7 of 8Command 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 8 of 8Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Nucleus · Cytoplasm · Cell membrane · Mitochondrion · Ribosome · Chloroplast · Cell wall · Vacuole · Differentiation · Stem 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)
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
Cambridge 9700 AS Biology
Core Formula
Magnification Triangle & Unit Conversion
Formula: M = I / A ⇔ A = I / M ⇔ I = M × A
Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.