Cell StructureCambridge IGCSE Biology: Revision notes
Section 1
What are the key structures found in animal cells?
Animal cells are eukaryotic cells containing a true nucleus and multiple membrane-bound organelles. The main structures are:
- Cell membrane – a partially permeable barrier controlling what enters and leaves the cell
- Nucleus – contains genetic material (DNA) and controls cell activities
- Cytoplasm – gel-like substance where metabolic reactions occur
- Mitochondria – site of aerobic respiration; produces ATP (energy)
- Ribosomes – site of protein synthesis
- Vacuoles – small, temporary storage spaces (less prominent than in plants)
These structures work together to allow the cell to survive, grow, and carry out its functions.
Examiners expect you to describe the function of each structure, not just name it. Link structures to their roles: mitochondria = energy production, nucleus = control, ribosomes = protein synthesis.
Think of a cell like a factory: the nucleus is the office (management), mitochondria are the power plant (energy), ribosomes are the assembly line (production), and the cell membrane is the warehouse entrance (quality control).
Section 2
How do plant cells differ from animal cells?
Plant cells are also eukaryotic but contain additional structures absent in animal cells:
| Structure | Animal cells | Plant cells |
|---|---|---|
| Cell wall | Absent | Present (cellulose); provides rigidity and support |
| Vacuole | Small and temporary | Large, permanent; maintains turgor pressure |
| Chloroplasts | Absent | Present; site of photosynthesis |
| Shape | Round/irregular | Fixed, rectangular shape |
Common structures in both: nucleus, cytoplasm, cell membrane, mitochondria, ribosomes
Key differences explained:
- The cell wall is made of cellulose and sits outside the cell membrane, providing structural support
- The large vacuole occupies up to 90% of the plant cell volume and stores water, maintains cell turgor (firmness), and stores nutrients and pigments
- Chloroplasts contain chlorophyll and are the site of photosynthesis, allowing plants to produce their own food
When comparing cells, explicitly state which structures are present and which are absent. For example: 'Plant cells have a cell wall, but animal cells do not' gains more marks than just listing structures.
Students often confuse the cell wall with the cell membrane. Remember: the cell membrane is living and partially permeable; the cell wall is non-living and fully permeable (made of cellulose in plants).
Section 3
What is the structure of a bacterial cell?
Bacterial cells are prokaryotic cells – much smaller and simpler than eukaryotic cells. They lack a membrane-bound nucleus.
Key structures:
- Cell wall – surrounds the cell membrane; made of peptidoglycan (not cellulose); provides rigidity
- Cell membrane – controls entry and exit of substances
- Cytoplasm – contains ribosomes and genetic material
- Ribosomes – smaller than in eukaryotes; site of protein synthesis
- Circular DNA – the genetic material; floats freely in the cytoplasm (not enclosed in a nucleus)
- Plasmids – small, circular rings of DNA separate from the main chromosome; often carry useful genes (e.g., antibiotic resistance)
Key difference: Bacteria lack a true nucleus; their DNA is in a region called the nucleoid but is not enclosed by a membrane.
In exam questions, emphasise that bacterial DNA is free in the cytoplasm and not enclosed by a membrane, whereas eukaryotic DNA is enclosed in a membrane-bound nucleus. This is the defining difference.
When comparing cells: Plant cell has nucleus (DNA enclosed) + cell wall (cellulose) + chloroplasts. Bacterial cell has no nucleus (DNA free) + cell wall (peptidoglycan) + no chloroplasts. Both have ribosomes and cell membranes.
Section 4
How are specialised cells adapted to their functions?
Specialised cells are cells that have adapted structurally and functionally to perform a specific role. Here are the key examples for IGCSE:
| Specialised cell | Location | Adaptation | Function |
|---|---|---|---|
| Ciliated cell | Respiratory tract lining | Long, hair-like cilia; many mitochondria | Beat to move mucus and bacteria upwards out of lungs |
| Root hair cell | Plant root epidermis | Long extension increases surface area; large vacuole | Absorbs water and mineral ions from soil |
| Palisade mesophyll cell | Upper leaf tissue | Packed with chloroplasts; elongated shape | Carries out photosynthesis to produce glucose |
| Neurone | Nervous system | Long extensions (axon/dendrites); many mitochondria | Transmits electrical impulses over long distances |
| Red blood cell | Blood | No nucleus; biconcave disc shape; contains haemoglobin | Transports oxygen throughout the body |
| Sperm cell | Testis | Flagellum for movement; many mitochondria; small nucleus | Fertilises egg cell; carries male genetic material |
| Egg cell | Ovary | Large with stored nutrients; haploid nucleus | Provides cytoplasm and genetic material for new organism |
Key principle: Structure and function are closely linked. Cells are adapted by having more or fewer of certain organelles depending on their role.
Link adaptations to function explicitly: 'Ciliated cells have many mitochondria because movement requires energy' or 'Palisade cells have many chloroplasts so that photosynthesis can occur efficiently.' This shows understanding.
Don't just list adaptations; explain why each adaptation helps the cell's function. For example, saying 'root hair cells are long' gets fewer marks than 'root hair cells have long extensions to increase surface area for faster water absorption'.
Section 5
How are cells organised into tissues, organs, and systems?
Cells do not work in isolation. They are organised into increasingly complex levels of organisation:
Definitions:
- Cell – the smallest living unit that can carry out all life processes
- Tissue – a group of similar cells working together to perform the same function (e.g., muscle tissue, nervous tissue, epithelial tissue)
- Organ – a structure made of different tissues working together to perform a specific function (e.g., heart, lung, brain, leaf)
- Organ system – a group of organs working together to perform a major body function (e.g., circulatory system, respiratory system, digestive system)
- Organism – a complete individual with all organ systems (e.g., a plant, animal, or human)
Hierarchy (from smallest to largest):
- Cell
- Tissue
- Organ
- Organ system
- Organism
Example in a plant: Palisade mesophyll cells → Palisade tissue → Leaf (organ) → Photosynthetic system → Plant (organism)
Example in a human: Ciliated cells → Ciliated epithelial tissue → Trachea (organ) → Respiratory system → Human (organism)
Describe the heart as an organ: Heart = organ made of cardiac muscle tissue (contractile) + nervous tissue (control) + epithelial tissue (lining). Heart is part of the circulatory system → circulatory system is part of the organism.
When asked to 'describe the organisation of cells,' examiners want to see the hierarchy explicitly stated with examples. Always use the correct terminology: never say 'system of cells' when you mean 'tissue'.
Section 6
How are new cells produced?
New cells are produced by the division of existing cells. This is a fundamental principle in biology.
Key points:
- All new cells arise from the division of pre-existing cells
- In multicellular organisms, growth occurs through mitosis – cells divide to produce two identical daughter cells
- In unicellular organisms (e.g., bacteria), division produces new individuals
- Sexual reproduction involves the production of gametes (sperm and egg cells) via meiosis, which reduces chromosome number
- Asexual reproduction in plants can occur through budding, fragmentation, or vegetative propagation – all produce genetically identical copies
Significance for growth and repair:
- Mitotic division allows organisms to grow from a single cell (zygote) to a multicellular organism
- In adults, mitotic division replaces damaged or worn-out cells (e.g., red blood cells, skin cells)
- Without cell division, organisms cannot grow or repair themselves
The specification states 'new cells are produced by division of existing cells.' In exam answers, use this exact phrase and explain that growth depends on mitosis, while sexual reproduction involves meiosis and gamete formation.
Must Know
- Animal cells contain: nucleus, cytoplasm, cell membrane, mitochondria, ribosomes, and small vacuoles. Plant cells additionally have: cell wall (cellulose), large vacuole, and chloroplasts.
- Bacterial cells are prokaryotic: no true nucleus; DNA is circular and free in the cytoplasm; have plasmids; cell wall made of peptidoglycan.
- Specialised cells have structures adapted to their function: ciliated cells (cilia for movement), root hair cells (large surface area for absorption), palisade cells (many chloroplasts for photosynthesis), neurones (long extensions for impulse transmission), red blood cells (no nucleus, haemoglobin for oxygen transport), sperm and egg cells (for reproduction).
- Organisation hierarchy: Cell → Tissue → Organ → Organ system → Organism. Each level performs increasingly complex functions.
- New cells are produced by division of existing cells: Mitosis produces identical cells for growth and repair; meiosis produces gametes for sexual reproduction.
- Describe functions, not just structures: Link every organelle to its role (e.g., mitochondria = energy production, ribosomes = protein synthesis, chloroplasts = photosynthesis).
That's the notes covered.
Carry on to the next subtopic.