Cells and Cell Structure

What you'll learn

This revision guide covers the fundamental building blocks of life examined in WJEC GCSE Biology. You will master the structures found in animal and plant cells, understand the differences between prokaryotic and eukaryotic cells, and examine how specialisation enables cells to perform specific functions. These concepts form the foundation for understanding tissues, organs and organ systems in later topics.

Key terms and definitions

Cell membrane — a partially permeable barrier that controls the movement of substances in and out of the cell and separates the cell contents from the environment.

Cytoplasm — the jelly-like substance inside a cell where most chemical reactions take place, containing enzymes and other dissolved substances.

Mitochondria (singular: mitochondrion) — organelles where aerobic respiration occurs, releasing energy from glucose for the cell's activities.

Chloroplasts — organelles found only in plant cells and algae that contain chlorophyll and are the site of photosynthesis.

Prokaryotic cell — a simple cell type that lacks a true nucleus and membrane-bound organelles, such as bacterial cells.

Eukaryotic cell — a complex cell type containing a nucleus and membrane-bound organelles, including all animal, plant, fungal and protist cells.

Specialised cell — a cell that has differentiated to perform a particular function, with specific adaptations in its structure.

Cell wall — a rigid structure made of cellulose (in plants) that surrounds the cell membrane, providing support and preventing the cell from bursting.

Core concepts

Animal and plant cell structures

Both animal and plant cells are eukaryotic and share several common structures:

Nucleus — contains genetic material (DNA) organised into chromosomes. The nucleus controls all cell activities and contains the instructions for making proteins.

Cell membrane — controls what enters and leaves the cell through partially permeable pores. Small molecules like oxygen, carbon dioxide and water can pass through easily, whilst larger molecules require active transport.

Cytoplasm — a water-based gel containing dissolved nutrients, salts and enzymes. Most metabolic reactions occur here, including the first stages of respiration (glycolysis).

Mitochondria — double-membraned organelles with a folded inner membrane (cristae) that increases surface area for aerobic respiration. Cells that require lots of energy (muscle cells, sperm cells) contain more mitochondria.

Ribosomes — tiny structures found free in the cytoplasm or attached to the endoplasmic reticulum. They are the site of protein synthesis, where amino acids are joined together following instructions from DNA.

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

Cell wall — made from cellulose fibres that provide structural support. Unlike the cell membrane, the cell wall is fully permeable and allows all dissolved substances through.

Permanent vacuole — a large, sap-filled cavity that maintains internal pressure (turgor) to keep the cell rigid. The vacuole also stores dissolved sugars, mineral ions and pigments.

Chloroplasts — contain the green pigment chlorophyll, which absorbs light energy for photosynthesis. Chloroplasts contain internal membrane stacks called grana where light-dependent reactions occur. Only cells in green parts of plants contain chloroplasts.

Prokaryotic cells

Bacterial cells are prokaryotic and structurally simpler than eukaryotic cells. Key features include:

No true nucleus — genetic material exists as a single circular chromosome of DNA that floats freely in the cytoplasm. There is no nuclear membrane separating it from the rest of the cell.

Plasmids — small, circular loops of DNA separate from the main chromosome. Plasmids often carry genes for antibiotic resistance and can be transferred between bacteria.

Cell wall — present but not made from cellulose (bacterial cell walls contain peptidoglycan instead). This provides structural support and protection.

Flagella (singular: flagellum) — long, whip-like structures used for movement. Some bacteria have multiple flagella; others have none.

Pili — hair-like projections that help bacteria attach to surfaces and transfer DNA between cells.

Prokaryotic cells are significantly smaller than eukaryotic cells (typically 0.2–2.0 μm in diameter compared to 10–100 μm for eukaryotic cells). They lack mitochondria, chloroplasts and other membrane-bound organelles.

Size and scale

Understanding the relative sizes of cells and subcellular structures is essential:

• Most animal cells: 10–30 μm in diameter
• Most plant cells: 10–100 μm in diameter (larger due to vacuole)
• Bacterial cells: 0.2–2.0 μm in diameter
• Nucleus: approximately 10 μm
• Mitochondria: 1–10 μm
• Chloroplasts: 3–10 μm
• Ribosomes: approximately 20 nm (0.02 μm)

The unit conversions you need to know:

• 1 millimetre (mm) = 1,000 micrometres (μm)
• 1 micrometre (μm) = 1,000 nanometres (nm)
• 1 metre = 1,000,000 μm

Microscopes are essential for viewing cells. Light microscopes can magnify up to ×2000 and are used to view whole cells and larger organelles. Electron microscopes can magnify up to ×2,000,000 and reveal internal details of organelles and even ribosomes.

Specialised cells in animals

Differentiation is the process by which cells become specialised for particular functions. Once differentiated, most animal cells cannot change function.

Red blood cells (erythrocytes) — transport oxygen from lungs to tissues:

• Biconcave disc shape increases surface area for oxygen absorption
• No nucleus, maximising space for haemoglobin
• Contain haemoglobin, which binds reversibly to oxygen
• Flexible to squeeze through narrow capillaries

Nerve cells (neurones) — transmit electrical impulses around the body:

• Long axon carries impulses over long distances
• Dendrites create branched connections with other neurones
• Myelin sheath insulates the axon and speeds up transmission
• Many mitochondria provide energy for active transport of ions

Sperm cells — male gametes for sexual reproduction:

• Tail (flagellum) enables swimming towards the egg
• Many mitochondria in the middle section provide energy for movement
• Acrosome in the head contains enzymes to digest the egg membrane
• Haploid nucleus contains half the genetic information (23 chromosomes)

Egg cells (ova) — female gametes for sexual reproduction:

• Much larger than sperm, containing nutrient reserves in the cytoplasm
• Haploid nucleus (23 chromosomes)
• After fertilisation, the cell membrane changes to prevent entry of other sperm

Ciliated epithelial cells — line airways in the respiratory system:

• Hair-like cilia beat in a coordinated rhythm
• Sweep mucus containing trapped dust and bacteria towards the throat
• Many mitochondria provide energy for ciliary movement

Specialised cells in plants

Plant cells can retain the ability to differentiate throughout the plant's life, particularly in meristems (growth regions).

Root hair cells — absorb water and mineral ions from soil:

• Long extension (root hair) increases surface area for absorption
• Thin cell wall for short diffusion distance
• Many mitochondria provide energy for active transport of mineral ions
• Large permanent vacuole speeds up water movement by osmosis

Palisade mesophyll cells — main site of photosynthesis in leaves:

• Columnar shape allows many to be tightly packed in upper leaf surface
• Very high chloroplast density maximises light absorption
• Large permanent vacuole pushes chloroplasts to cell edges, closer to light
• Position near upper leaf surface where light intensity is highest

Xylem vessels — transport water and mineral ions up the plant:

• Cells join end-to-end with no end walls, forming continuous tubes
• Cell walls thickened with lignin for strength and support
• No cytoplasm or subcellular structures when mature (cells are dead)
• Lignin deposited in spirals or rings allows flexibility

Phloem sieve tube elements — transport dissolved sugars (translocation):

• Joined end-to-end with perforated sieve plates allowing flow
• Very little cytoplasm and no nucleus when mature (cells are alive)
• Companion cells alongside provide metabolic support
• Mitochondria in companion cells provide energy for active loading of sugars

Worked examples

Example 1: Comparing cell types

Question: The table shows features of three different cell types. Complete the table using ticks (✓) and crosses (✗). [3 marks]

Feature	Bacterial cell	Plant cell	Animal cell
Cell wall present	✓
Mitochondria present			✓
Chloroplasts present		✓

Mark scheme answer:

Feature	Bacterial cell	Plant cell	Animal cell
Cell wall present	✓	✓	✗
Mitochondria present	✗	✓	✓
Chloroplasts present	✗	✓	✗

Examiner guidance: Award 1 mark for each fully correct row. Remember that bacterial cells have cell walls but these are not made of cellulose. Bacterial cells lack membrane-bound organelles including mitochondria and chloroplasts.

Example 2: Explaining specialisation

Question: Explain how the structure of a sperm cell is adapted to its function. [4 marks]

Mark scheme answer:

Any four from:

• The tail/flagellum allows the cell to swim/move towards the egg (1)
• Many mitochondria in the middle section release energy/ATP for movement (1)
• The acrosome contains digestive enzymes to break down/penetrate the egg membrane (1)
• Streamlined/pointed head reduces resistance when swimming (1)
• Haploid nucleus contains genetic information/half the chromosomes to fuse with the egg (1)
• Large number of mitochondria because swimming requires a lot of energy (1)

Examiner guidance: Link each structural feature to its specific function. Avoid vague statements like "mitochondria provide energy" — specify what the energy is used for. Quality of written communication matters in extended answers worth 4+ marks.

Example 3: Calculation involving magnification

Question: A student observes a plant cell using a light microscope. The image of the cell is 4.5 mm wide. The actual width of the cell is 30 μm. Calculate the magnification. Show your working. [3 marks]

Mark scheme answer:

Step 1: Convert measurements to same units 4.5 mm = 4,500 μm (1)

Step 2: Apply magnification formula Magnification = image size ÷ actual size (1) Magnification = 4,500 ÷ 30 = ×150 (1)

Examiner guidance: Always convert to the same units first. Show all working even if you use a calculator. Include the multiplication sign (×) before the final answer. Common error: forgetting to convert mm to μm.

Common mistakes and how to avoid them

• Confusing cell wall and cell membrane — The cell wall (plants only) is outside the cell membrane and made of cellulose. The cell membrane is present in all cells and controls what enters/exits. They are separate structures with different functions.

• Stating mitochondria "produce energy" — Mitochondria do not create energy. They release energy from glucose through aerobic respiration. Energy cannot be created or destroyed, only transferred.

• Claiming all plant cells contain chloroplasts — Only cells in green parts of plants contain chloroplasts. Root cells, internal stem cells and most flower parts lack chloroplasts as they are not exposed to light.

• Describing prokaryotes as "not having DNA" — Bacterial cells do contain DNA, but it is not enclosed in a nucleus. The DNA exists as a circular chromosome in the cytoplasm plus smaller plasmid loops.

• Forgetting to convert units in magnification calculations — Always convert to the same units before calculating. Show the conversion step in your working to secure method marks even if your final answer is incorrect.

• Writing incomplete answers about specialisation — Always link structure to function. "The cell has many mitochondria" scores no marks; "The cell has many mitochondria to release energy for active transport" scores full marks.

Exam technique for "Cells and Cell Structure"

• Command word focus — "Describe" requires you to state features without explanation. "Explain" requires you to link structure to function using linking words like "so that," "which allows" or "therefore." "Compare" means you must make direct comparisons, not describe each item separately.

• Drawing and labelling — Use a sharp pencil and ruler for straight label lines. Lines must touch the structure without crossing. Label lines should not have arrowheads. Write labels horizontally for neatness.

• Table completion questions — Read column and row headers carefully. Check whether the question requires ticks/crosses or written responses. If one box is completed as an example, match that format exactly (e.g., full sentences vs. single words).

• Calculations — Show all working for full method marks. State formulae, convert units, and check your answer is sensible (e.g., magnification should typically be between ×40 and ×2000 for light microscopes). Include units in your final answer.

Quick revision summary

All cells have a cell membrane, cytoplasm and genetic material. Eukaryotic cells (plants and animals) contain a nucleus and membrane-bound organelles like mitochondria. Plant cells additionally have a cellulose cell wall, permanent vacuole and often chloroplasts. Prokaryotic cells (bacteria) are smaller and simpler, lacking a nucleus and organelles. Specialised cells have structural adaptations that suit their specific functions. Understanding size scales and magnification calculations is essential for practical work and exam questions.