Cell structure and organisation
Section: Biology | Syllabus: Cambridge IGCSE Combined Science (0653)
Plant and Animal Cell Structures
All living organisms are made of cells. Cells are the basic units of life. Plant and animal cells share several structures but also have important differences.
Animal Cells
- Animal cells contain:
- Cell membrane - controls the movement of substances into and out of the cell; partially permeable
- Nucleus - controls cell activities; contains DNA which carries genetic information and controls protein synthesis
- Cytoplasm - jelly-like fluid filling the cell; site of many chemical reactions of metabolism
- Mitochondria - site of aerobic respiration; release energy for the cell's activities
- Ribosomes - site of protein synthesis (translation of mRNA into protein)
- Animal cells have no cell wall, no chloroplasts and no large permanent vacuole - these are found only in plant cells
A typical animal cell has a cell membrane, nucleus, cytoplasm, mitochondria and ribosomes, but no cell wall, chloroplasts or permanent vacuole
Plant Cells
- Plant cells contain the same structures as animal cells (cell membrane, nucleus, cytoplasm, mitochondria, ribosomes), plus three additional structures:
- Cell wall - made of cellulose; provides rigid support and shape; fully permeable to water and dissolved substances
- Chloroplasts - contain chlorophyll which absorbs light energy; site of photosynthesis (only present in green parts of the plant)
- Permanent vacuole - large, contains cell sap; maintains turgor pressure to support the cell
A typical plant cell has all the structures of an animal cell, plus a cell wall, chloroplasts and a large permanent vacuole
Not all plant cells have chloroplasts - for example, root cells are not green and contain no chloroplasts.
Bacterial Cell Structure
Bacteria are prokaryotes - their cells lack a membrane-bound nucleus. Despite having a wide variety of shapes and sizes, all bacteria share the same biological characteristics:
- They are microscopic, single-celled organisms, much smaller than plant or animal cells
- They possess a cell wall (made of peptidoglycan, not cellulose), a cell membrane, cytoplasm and ribosomes
- They lack a nucleus, but contain a single circular chromosome of DNA that floats freely in the cytoplasm
- Plasmids are often present - small, circular loops of DNA, separate from the main chromosome, that may carry extra genes such as those for antibiotic resistance
- They lack mitochondria, chloroplasts and other membrane-bound organelles found in plant and animal cells
Some bacteria also have one or more flagella (singular: flagellum) - long, thin, whip-like tails attached to the cell that allow it to move.
- Examples of bacteria include:
- Lactobacillus - a rod-shaped bacterium used in the production of yoghurt from milk
- Pneumococcus - a spherical bacterium that acts as the pathogen causing pneumonia
A bacterial cell has no nucleus - its circular DNA lies free in the cytoplasm
Common Mistakes
- ❌ Thinking a bacterial cell has a nucleus, since it has DNA
- ✓ the defining feature of a prokaryote is that it has no membrane-bound nucleus. Its DNA is a single, circular chromosome lying free in the cytoplasm. Eukaryotes (animals, plants, fungi, protoctists) all have a nucleus surrounded by a nuclear membrane
Cell, Tissue, Organ, Organ System and Organism
In multicellular organisms, cells are organised into increasingly complex levels, each built from the one below it.
| Level | Definition | Human example | Plant example |
|---|---|---|---|
| Cell | The basic structural and functional unit of all living organisms | Muscle cell, red blood cell | Palisade mesophyll cell, root hair cell |
| Tissue | A group of cells with similar structure and function working together | Muscle tissue, blood | Xylem, phloem, palisade mesophyll |
| Organ | A structure made of different tissues working together to perform a specific function | Heart, stomach, lung | Leaf, root, flower, stem |
| Organ system | A group of organs working together to carry out a major body function | Digestive system, circulatory system | Root system, shoot system |
| Organism | A complete, individual living thing made up of organ systems working together | A human being | A flowering plant |
Common Mistakes
- ❌ Thinking a tissue and an organ are the same thing
- ✓ a tissue is made of similar cells all doing the same job. An organ is made of different tissues working together - for example, the heart contains muscle tissue, nervous tissue and connective tissue
Specialised Cells
In a multicellular organism, cells become specialised (differentiated) to carry out specific functions. Each specialised cell's structure is adapted to its role - more mitochondria where more energy is needed, more surface area where more absorption occurs, and so on.
Root Hair Cells
- Function: absorption of water and mineral ions from the soil
- Key adaptations:
- Long, hair-like extension greatly increases surface area for absorption
- Thin cell wall and large vacuole for rapid water uptake
The long hair-like extension increases the surface area available for absorbing water and mineral ions
Palisade Mesophyll Cells
- Function: photosynthesis
- Key adaptations:
- Many chloroplasts to absorb maximum light
- Tall, column-shaped cells packed together near the top of the leaf to intercept light
Packed with chloroplasts and positioned near the leaf surface, palisade cells absorb the maximum light for photosynthesis
Red Blood Cells
- Function: transport of oxygen
- Key adaptations:
- Biconcave disc shape increases surface area for gas exchange
- No nucleus - more space for haemoglobin
- Flexible membrane to squeeze through capillaries
The biconcave disc shape increases surface area for gas exchange, and having no nucleus leaves more room for haemoglobin
Worked Example: Linking Structure to Function
- Question: red blood cells have no nucleus. Explain how this adaptation helps the red blood cell carry out its function.
- Answer: having no nucleus leaves more space inside the cell for haemoglobin, allowing the cell to carry more oxygen. Always explain why an adaptation helps the cell's function, not just state the feature
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