Describe the functions of cell structures in plant, animal and bacterial cells.

Cell Structure – IGCSE Biology 0610

Learning Objective

Describe the functions of the main cell structures in plant, animal and bacterial cells and compare the key differences between them.

Core Cell Structures (required by the syllabus)

• Cell wall
• Cell membrane (plasma membrane)
• Cytoplasm
• Nucleus
• Ribosomes
• Mitochondria
• Chloroplasts (plants only)
• Vacuoles (large central vacuole in plants; small vacuoles in animals)
• Capsule (bacterial – optional)
• Flagellum (bacterial – optional)

Supplementary Structures (enrichment only)

These organelles are not required for the IGCSE syllabus but are useful for deeper study.

• Nucleolus – site of ribosomal RNA synthesis.
• Endoplasmic reticulum (rough and smooth) – protein and lipid synthesis; detoxification (smooth).
• Golgi apparatus – modifies, sorts and packages proteins and lipids.
• Centrioles – organise spindle fibres during cell division (present in animal cells and in most plant cells only during mitosis).
• Pili (bacterial) – attachment and conjugation.

Useful Formulae & Cross‑References

• Magnification: Magnification = Image size ÷ Actual size.
  Example: If a cell appears 5 mm long on a microscope image but the real cell is 10 µm, the magnification is 5 mm ÷ 0.01 mm = 500×.
• See also the sections on diffusion, osmosis and active transport (energy‑requiring movement of substances against a concentration gradient) for the processes that occur across the cell membrane.

Plant Cells

• Cell wall: Rigid layer of cellulose; provides structural support, prevents the cell from bursting, and limits excessive water uptake.
• Cell membrane: Semi‑permeable barrier that regulates the entry and exit of substances.
• Cytoplasm: Gel‑like matrix where metabolic reactions occur and organelles are suspended.
• Nucleus: Enclosed by a nuclear envelope; contains DNA and directs cell activities.
• Ribosomes: Sites of protein synthesis (free in cytoplasm or attached to rough ER).
• Mitochondria: Site of aerobic respiration; produce ATP, generate heat and play a role in programmed cell death (apoptosis – optional).
• Chloroplasts: Contain chlorophyll; thylakoid membranes house the light‑dependent reactions of photosynthesis, while the stroma carries out the Calvin cycle.
• Large central vacuole: Stores water, ions, pigments, waste products and metabolites; maintains turgor pressure, which keeps the plant rigid.
• Cell division: New plant cells arise by mitotic division of existing cells.

Animal Cells

• Cell membrane: Controls movement of substances; provides protection and communication with the external environment.
• Cytoplasm: Medium for biochemical reactions; holds organelles in place.
• Nucleus: Stores genetic material and regulates cell function.
• Ribosomes: Protein synthesis.
• Mitochondria: Produce ATP through aerobic respiration; also generate heat.
• Vacuoles: Small, temporary storage compartments; may contain enzymes, waste or nutrients.
• Cell division: New animal cells are produced by mitosis of existing cells.

Bacterial Cells (Prokaryotes)

• Cell wall: Rigid peptidoglycan layer; gives shape and protects against osmotic pressure.
• Cell membrane: Regulates transport of nutrients and waste.
• Cytoplasm: Contains enzymes, ribosomes and the nucleoid region.
• Nucleoid: Region where the circular DNA molecule is located; not surrounded by a membrane.
• Ribosomes: Site of protein synthesis (70 S ribosomes).
• Capsule (optional): Species‑specific extra‑protective layer; helps prevent desiccation and evades host immune responses.
• Flagellum (optional): Provides locomotion for many bacteria.
• Cell division: Occurs by binary fission – the cell splits into two identical daughter cells.

Comparative Overview – Presence / Absence

Comparative Overview – Key Functions

Specialised Cells (examples and main functions)

• Root hair cell (plant): Increases surface area for water and mineral absorption.
• Guard cell (plant): Regulates opening and closing of stomata, controlling gas exchange and transpiration.
• Neurone (animal): Transmits electrical signals; specialised for rapid communication.
• Muscle fibre (animal): Contracts to produce movement.
• Red blood cell (animal): Transports oxygen using haemoglobin; lacks a nucleus.
• Sperm cell (animal): Motile, flagellated cell for fertilisation.

Sidebar: Constructing a Simple Dichotomous Key (Core 1.2)

Although not a cell‑structure topic, the syllabus expects students to use dichotomous keys to classify organisms. A brief example for three common leaf types:

1. Leaf shape:
   a) Simple (single blade) → go to 2
   b) Compound (multiple leaflets) → Acacia
2. Leaf margin:
   a) Smooth → Rosa
   b) Serrated → Betula

Students can adapt this format to classify cells (e.g., “Has cell wall? Yes → plant or bacterial; No → animal”).

Suggested Diagrams

• Detailed labelled plant cell showing cell wall, membrane, nucleus, nucleolus, chloroplasts, mitochondria, large central vacuole and cytoplasm.
• Detailed labelled animal cell highlighting membrane, nucleus, nucleolus, mitochondria, ribosomes and small vacuoles.
• Simplified bacterial cell diagram displaying cell wall, membrane, nucleoid, ribosomes, capsule (optional) and flagellum (optional).