Cells

What you'll learn

Cells form the foundation of all living organisms and represent approximately 15-20% of Paper 2 content in CIE IGCSE Biology examinations. This topic covers the structure and function of animal and plant cells, specialised cells, levels of organisation from cells to organisms, and the use of microscopes. Questions frequently appear as labelling diagrams, calculating magnification, comparing cell types, and explaining adaptations of specialised cells.

Key terms and definitions

Cell — the basic structural and functional unit of all living organisms, containing cytoplasm and enclosed by a membrane.

Organelle — a specialised structure within a cell that performs a specific function, such as the nucleus or mitochondria.

Magnification — the number of times larger an image appears compared to the actual size of the object, calculated as image size ÷ actual size.

Resolution — the ability to distinguish between two separate points; higher resolution produces clearer, more detailed images.

Differentiation — the process by which cells become specialised for particular functions during development.

Tissue — a group of similar cells working together to perform a specific function.

Organ — a structure composed of different tissues working together to perform a specific function.

System — a group of organs working together to perform a particular function in an organism.

Core concepts

Animal and plant cell structure

Both animal and plant cells are eukaryotic cells, meaning they contain a nucleus and membrane-bound organelles. The key structures found in animal cells include:

• Cell membrane — controls the movement of substances into and out of the cell; composed of lipids and proteins with a partially permeable structure
• Cytoplasm — jelly-like substance where chemical reactions occur; contains enzymes and dissolved nutrients
• Nucleus — contains genetic material (DNA) organised into chromosomes; controls cell activities and cell division
• Mitochondria (singular: mitochondrion) — site of aerobic respiration; produces ATP energy from glucose and oxygen
• Ribosomes — site of protein synthesis; where amino acids are assembled into proteins

Plant cells contain all the structures found in animal cells, plus three additional structures:

• Cell wall — made of cellulose; provides structural support and prevents the cell from bursting when water enters by osmosis
• Chloroplasts — contain chlorophyll; site of photosynthesis where light energy is converted to chemical energy in glucose
• Permanent vacuole — contains cell sap (a solution of sugars and salts); maintains turgor pressure to keep the cell rigid

Exam questions frequently require you to identify these structures on diagrams and state their functions. Remember that not all plant cells contain chloroplasts (e.g., root cells are underground and do not photosynthesise).

Bacterial cell structure

Bacterial cells are prokaryotic cells — they lack a nucleus and membrane-bound organelles. This fundamental difference distinguishes bacteria from animal and plant cells. Key features include:

• Cell wall — made of peptidoglycan (not cellulose); provides shape and protection
• Cell membrane — controls entry and exit of substances
• Cytoplasm — contains ribosomes but no mitochondria or chloroplasts
• Circular DNA — genetic material not enclosed in a nucleus; lies free in the cytoplasm
• Plasmids — small circular DNA molecules separate from the main bacterial chromosome; often carry genes for antibiotic resistance
• Flagella (singular: flagellum) — tail-like structures used for movement

Bacterial cells are typically much smaller than plant and animal cells (1-10 μm compared to 10-100 μm). Questions often ask you to compare prokaryotic and eukaryotic cells or identify bacterial structures.

Specialised cells

Specialised cells have structures adapted to their specific functions. CIE IGCSE examinations commonly test the following examples:

Ciliated epithelial cells line the airways (trachea and bronchi). Adaptations:

• Tiny hair-like cilia on the cell surface that beat in a coordinated rhythm
• Move mucus containing trapped dust and pathogens up towards the throat
• Numerous mitochondria provide energy for ciliary movement

Root hair cells absorb water and mineral ions from soil. Adaptations:

• Long extension (root hair) increases surface area for absorption
• Thin cell wall for short diffusion distance
• Large permanent vacuole maintains concentration gradient for water uptake
• Numerous mitochondria provide energy for active transport of mineral ions

Palisade mesophyll cells carry out photosynthesis in leaves. Adaptations:

• Packed with chloroplasts containing chlorophyll to absorb light energy
• Positioned near the upper surface of the leaf to receive maximum light
• Cylindrical shape allows cells to be tightly packed
• Large permanent vacuole pushes chloroplasts to the edge of the cell

Red blood cells transport oxygen from lungs to respiring tissues. Adaptations:

• Biconcave disc shape increases surface area for oxygen diffusion
• No nucleus provides more space for haemoglobin
• Contains haemoglobin protein that binds reversibly with oxygen
• Flexible shape allows movement through narrow capillaries

Sperm cells deliver male genetic material to egg cells. Adaptations:

• Tail (flagellum) for swimming towards the egg
• Numerous mitochondria in the middle section provide energy for movement
• Acrosome contains enzymes to digest through the egg cell membrane
• Haploid nucleus contains half the genetic information

Egg cells (ova) are fertilised by sperm and develop into embryos. Adaptations:

• Large cytoplasm contains nutrients (yolk) for early embryo development
• Haploid nucleus contains half the genetic information
• Cell membrane changes after fertilisation to prevent entry of additional sperm
• Jelly-like coating protects the cell

Exam questions typically provide a diagram or description and ask you to explain how specific features relate to the cell's function. Always link structure to function explicitly.

Levels of organisation

Multicellular organisms show hierarchical organisation:

1. Cells — basic units (e.g., palisade cell, muscle cell)
2. Tissues — groups of similar cells (e.g., xylem tissue, epithelial tissue, muscle tissue)
3. Organs — structures containing different tissues (e.g., leaf, heart, stomach, brain)
4. Organ systems — groups of organs working together (e.g., digestive system, circulatory system, respiratory system)
5. Organisms — complete living things

Examples of tissues in plants:

• Xylem tissue — transports water and mineral ions; provides support
• Phloem tissue — transports dissolved sugars (translocation)
• Epidermis — protective outer covering of leaves and roots

Examples of tissues in animals:

• Epithelial tissue — covers body surfaces and lines organs
• Muscle tissue — contracts to produce movement
• Nervous tissue — carries electrical impulses
• Connective tissue — supports and binds other tissues

The leaf is an example of a plant organ containing multiple tissues: epidermis (protection), palisade mesophyll (photosynthesis), spongy mesophyll (gas exchange), xylem and phloem (transport).

Using microscopes

Microscopes magnify specimens and allow observation of cell structures. Understanding magnification calculations is essential for CIE IGCSE Biology.

Light microscopes:

• Use visible light and glass lenses
• Magnify up to approximately ×1500
• Lower resolution — cannot see structures smaller than 200 nm
• Can observe living specimens
• Used to view cells, tissues, and larger organelles like nuclei and chloroplasts

Electron microscopes:

• Use beams of electrons instead of light
• Magnify up to ×500,000 or more
• Much higher resolution — can see structures as small as 0.5 nm
• Can only view dead specimens
• Reveal detailed ultrastructure including ribosomes, mitochondrial membranes, and cell membrane structure

Magnification calculation:

The formula used in examinations is:

Magnification = Image size ÷ Actual size

Alternatively: Image size = Magnification × Actual size

Or: Actual size = Image size ÷ Magnification

Remember to convert units to ensure both measurements use the same unit (usually micrometres, μm, or millimetres, mm).

Conversion factors:

• 1 mm = 1000 μm
• 1 μm = 1000 nm
• 1 m = 1000 mm

Worked examples

Example 1: Magnification calculation

Question: A student observes a plant cell under a microscope. The cell measures 50 mm in the student's diagram. The actual length of the cell is 0.05 mm. Calculate the magnification.

Solution:

Step 1: Identify the values

• Image size = 50 mm
• Actual size = 0.05 mm

Step 2: Apply the formula Magnification = Image size ÷ Actual size Magnification = 50 ÷ 0.05 Magnification = ×1000

[2 marks: 1 mark for correct formula/working, 1 mark for correct answer with multiplication sign]

Example 2: Specialised cell adaptation

Question: Explain how the structure of a root hair cell is adapted for its function. [3 marks]

Solution:

• The root hair extension increases surface area [1] for absorption of water/mineral ions [1]
• Contains many mitochondria [1] to provide energy for active transport of mineral ions [1]
• Thin cell wall [1] provides a short diffusion pathway [1]

[Any 3 marks from the points above — note that each structural feature must be linked to its function]

Example 3: Comparing cell types

Question: State two differences between bacterial cells and plant cells. [2 marks]

Solution:

• Bacterial cells have circular DNA/no nucleus whereas plant cells have DNA in a nucleus [1]
• Bacterial cells have plasmids whereas plant cells do not [1]
• Bacterial cell walls are made of peptidoglycan whereas plant cell walls are made of cellulose [1]
• Bacterial cells have no mitochondria/chloroplasts whereas plant cells have these organelles [1]

[Any 2 marks — each difference must be comparative, stating both cell types]

Common mistakes and how to avoid them

• Mistake: Stating that plant cells do not have mitochondria because they have chloroplasts. Correction: Plant cells have both chloroplasts (for photosynthesis) and mitochondria (for respiration). Photosynthesis only occurs in the light, but respiration occurs continuously to release energy from glucose.

• Mistake: Confusing the cell wall and cell membrane, or stating that animal cells have cell walls. Correction: The cell membrane is present in all cells and controls substance movement. The cell wall is only in plant and bacterial cells (with different compositions) and provides structural support.

• Mistake: Forgetting to convert units in magnification calculations or omitting the multiplication sign (×) in the answer. Correction: Always convert to the same unit before calculating. Express magnification as "×1000" not just "1000" to gain full marks.

• Mistake: Describing what an organelle does without explaining how the structure enables that function. Correction: Always link structure to function. For example, "mitochondria release energy" is incomplete; write "mitochondria contain enzymes for aerobic respiration which releases energy from glucose."

• Mistake: Stating that the nucleus "controls the cell" without further detail. Correction: The nucleus contains chromosomes made of DNA, which carry genes that code for proteins. These proteins determine cell structure and control chemical reactions.

• Mistake: Confusing resolution with magnification. Correction: Magnification refers to how much larger an image appears. Resolution refers to the ability to distinguish between two separate points. Electron microscopes have both higher magnification and higher resolution than light microscopes.

Exam technique for Cells

• Labelling diagrams: Use a ruler to draw straight label lines that touch the structure being identified. Do not use arrowheads. Write labels horizontally outside the diagram. Common structures tested include nucleus, cytoplasm, cell membrane, cell wall, chloroplast, vacuole, and mitochondria.

• "State" and "Name" questions (1 mark each): Give brief, factual answers without explanation. For example, "State the function of ribosomes" requires only "protein synthesis" not a detailed description of translation.

• "Explain" questions (2-3 marks): Provide reasoning that links cause and effect. For specialised cell questions, identify a structural feature and explain how it enables the specific function. Each distinct point usually earns one mark.

• Calculation questions: Show your working clearly. Even if the final answer is incorrect, marks are often awarded for correct method. Always include units in your final answer. For magnification, remember the multiplication sign (×).

Quick revision summary

Animal and plant cells are eukaryotic with a nucleus and organelles (mitochondria, ribosomes). Plant cells additionally have a cellulose cell wall, chloroplasts, and a permanent vacuole. Bacterial cells are prokaryotic with circular DNA, no nucleus, and plasmids. Specialised cells have adaptations for specific functions: root hair cells (absorption), red blood cells (oxygen transport), ciliated cells (mucus movement). Organisation levels: cells → tissues → organs → systems → organisms. Magnification = image size ÷ actual size. Electron microscopes have higher magnification and resolution than light microscopes.