Identify the cell structures in diagrams and images of plant, animal and bacterial cells.

2.1 Cell Structure

This section equips you to identify and name the main structures in plant, animal and bacterial cells – a core requirement of the Cambridge IGCSE Biology (0610) syllabus (AO1). It also provides the concise structure‑function statements needed for AO2.

Core vs. Supplementary Structures

Key Facts Boxes – Syllabus Wording

Each organelle below is summarised in a “Key Fact” box that mirrors the exact phrasing used in the syllabus. Optional details are clearly marked as Extended content – enrichment.

Common Cell Components (present in all three cell types)

• Cell membrane (plasma membrane)

  Flexible phospholipid bilayer that controls the movement of substances in and out of the cell.

• Cytoplasm

  Jelly‑like matrix that suspends the organelles and contains the cytosol.

• Ribosomes

  Sites of protein synthesis; appear as tiny dots.

  Extended content – enrichment: 80 S in eukaryotes, 70 S in prokaryotes.

• DNA (genetic material)

  Stored in a membrane‑bound nucleus in eukaryotes or in the nucleoid region of prokaryotes.

Plant Cell (Eukaryote)

• Cell wall (cellulose)

  Rigid layer outside the plasma membrane that gives shape and protection.

• Plasma membrane

  Lies just inside the cell wall.

• Nucleus (with nucleolus)

  Membrane‑bound organelle containing the cell’s DNA; nucleolus is the site of ribosomal RNA synthesis.

• Chloroplast

  Double‑membrane organelle containing chlorophyll; site of photosynthesis.

• Large central vacuole

  Big fluid‑filled space that stores water, ions and metabolites and maintains turgor pressure.

• Mitochondrion

  Site of aerobic respiration (cellular respiration).

• Endoplasmic reticulum (ER)
  • Rough ER

    Studded with ribosomes; synthesises proteins destined for membranes, secretion or lysosomes.

  • Smooth ER

    Lacks ribosomes; synthesises lipids and detoxifies harmful substances.

• Golgi apparatus

  Modifies, sorts and packages proteins and lipids for transport to their final destinations.

• Plasmodesmata (optional)

  Microscopic channels that link the cytoplasm of adjacent plant cells, allowing transport of water and solutes.

Animal Cell (Eukaryote)

• Plasma membrane

  Flexible outer covering.

• Nucleus (with nucleolus)

  Membrane‑bound organelle containing the cell’s DNA.

• Centrioles

  Pair of barrel‑shaped structures that organise the spindle fibres during mitosis.

• Lysosome

  Membrane‑bound vesicle containing hydrolytic enzymes for intracellular digestion.

• Mitochondrion

  Site of aerobic respiration.

• Endoplasmic reticulum (rough & smooth)

  Rough ER – protein synthesis; Smooth ER – lipid synthesis and detoxification.

• Golgi apparatus

  Modifies, sorts and packages proteins and lipids.

• Centrosome (contains centrioles)

  Microtubule‑organising centre.

• Cell‑junctions (optional)
  • Tight junctions – seal adjacent cells to prevent leakage.
  • Desmosomes – provide mechanical strength.
  • Gap junctions – allow communication between cells.

Bacterial Cell (Prokaryote)

• Cell wall (peptidoglycan)

  Gives shape and protects against osmotic pressure.

• Plasma membrane

  Lies just inside the cell wall.

• Nucleoid region

  Irregularly shaped area containing a single circular DNA molecule.

• Ribosomes (70 S)

  Site of protein synthesis.

• Capsule (optional)

  Gelatinous outer layer that protects against desiccation and the host immune system.

• Flagellum (optional)

  Long, whip‑like appendage that provides locomotion.

Comparison of Plant, Animal and Bacterial Cells

Structure – Function Links (AO2)

How to Identify Structures in Diagrams and Microscope Images

1. Determine the outermost layer
   • Thick rigid wall → plant cell.
   • Thin flexible membrane, no wall → animal cell.
   • Uniform thin wall with a grainy texture → bacterial cell.
2. Look for chloroplasts – green, double‑membrane organelles with internal stacks (plant only).
3. Locate a large clear space – occupies most of the interior in plant cells (large central vacuole).
4. Identify the nucleus – membrane‑bound, often with a darker nucleolus. Absence indicates a prokaryote.
5. Search for centrioles and lysosomes – paired barrel structures and small vesicles, respectively (animal cells).
6. Observe ribosome distribution – numerous tiny dots; in bacteria they are uniformly small (70 S).
7. Check external features
   • Capsule – smooth outer coating.
   • Flagellum – long filament extending from the cell surface.
8. Optional features
   • Plasmodesmata – thin channels crossing the plant cell wall.
   • Cell‑junctions (tight, desmosomes, gap) – specialised contacts between animal cells.

Cell Division – Relevance to Cell Structure

All living organisms are composed of cells that arise from pre‑existing cells. In eukaryotes, the nucleus and centrioles play a central role in mitosis, ensuring that each daughter cell receives a complete set of chromosomes and the appropriate complement of organelles.

Magnification – Mathematical Requirement (AO1)

Formula: Magnification = (image size ÷ actual size)

Remember to convert units so they match before dividing.

Practice problem (cell‑image context)

A microscope slide shows a bacterial cell that is 2 mm long on the picture. The real length of the bacterium is 2 µm. Calculate the magnification.

Solution:

1. Convert 2 mm to µm: 2 mm = 2000 µm.
2. Magnification = 2000 µm ÷ 2 µm = 1000×.

Practice Questions

1. Label the structures shown in the provided plant cell diagram.
2. Which organelle is responsible for photosynthesis and where would you find it?
3. Explain why a bacterial cell does not have a nucleus.
4. Compare the function of the cell wall in plant cells with that in bacterial cells.
5. Identify three structures that are present in animal cells but absent in plant cells.
6. Calculate the magnification of a microscope image where a 2 mm‑wide bacterial cell is shown as 4 cm on the picture.
7. State why the presence of centrioles indicates that the cell is animal (or specialised eukaryotic) rather than plant.
8. Describe one way in which the large central vacuole contributes to a plant’s ability to remain upright.

Suggested Diagrams (for classroom use)