recognise organelles and other cell structures found in eukaryotic cells and outline their structures and functions, limited to: cell surface membrane, nucleus, nuclear envelope and nucleolus, rough endoplasmic reticulum, smooth endoplasmic reticulum

Cambridge A‑Level Biology 9700

Topic 1 – Cells as the Basic Units of Living Organisms

Learning Objective

Recognise organelles and other cell structures found in eukaryotic cells and outline their structures and functions (AO1). Be able to link structure to function and predict the outcome of experimental observations (AO2).

1. The Microscope in Cell Studies

  • Specimen preparation

    • Wet mounts – live cells in a drop of water; useful for observing movement and flagella.

    • Permanent slides – heat‑fixed, stained (e.g., iodine, methylene blue) to reveal internal detail.

  • Eyepiece graticule & stage micrometer

    • Stage micrometer: slide with a known division length (e.g., 0.01 mm).

    • Calibrate the eyepiece graticule by counting how many graticule divisions correspond to a set number of micrometer divisions.

    • Once calibrated, the graticule gives a quick way to measure cells or organelles directly on the microscope image.

  • Magnification – total magnification = ocular (eyepiece) magnification × objective magnification (e.g., 10× × 40× = 400×).

  • Resolution vs. magnification

Resolution limits

• Light microscope: ≈0.2 µm (cannot resolve structures smaller than this).

• Electron microscope: ≈0.1 nm (allows visualisation of macromolecular complexes).

Increasing magnification beyond the resolution limit only enlarges a blurry image.

  • Practice problem

    Question: A stage micrometer has divisions of 0.01 mm. After calibration, 5 graticule divisions correspond to one micrometer division. If a cell spans 12 graticule divisions, what is its actual length?

    Answer: 12 ÷ 5 = 2.4 µm (since 1 µm = 0.001 mm, the cell is 2.4 µm long).

  • Electron‑microscopy preparation (brief)

    • Fixation with glutaraldehyde (primary) and osmium tetroxide (secondary) to preserve membranes.

    • Dehydration through an ethanol series, then embedding in resin.

    • For scanning EM – sputter‑coat the block with a thin layer of gold or platinum to make the surface conductive.

2. Key Eukaryotic Organelles (required list)

Organelle / StructureStructure (key features)Primary Function(s)AO1 / AO2 relevance
Cell surface membrane (plasma membrane)Fluid‑mosaic: phospholipid bilayer with embedded proteins, cholesterol, glycolipids.Selective permeability, cell‑cell communication, maintenance of cell shape, active transport of ions and nutrients.AO1 – identify components; AO2 – explain how fluidity and protein channels enable transport.
Cell wall (plants & fungi)Rigid layer external to plasma membrane; cellulose fibres in plants, chitin in fungi.Provides mechanical support, protection against osmotic burst, determines cell shape.AO1 – recognise wall in diagram; AO2 – predict effect of removing the wall on turgor pressure.
NucleusLarge spherical organelle surrounded by a double nuclear envelope; contains nucleoplasm, chromatin and a nucleolus.Stores DNA, coordinates gene expression, controls cell cycle.AO1 – locate nucleus; AO2 – relate nuclear pores to RNA export.
Nuclear envelopeTwo concentric lipid bilayers perforated by nuclear pores (~50 nm diameter).Regulates traffic of macromolecules between nucleus and cytoplasm.AO2 – explain why large proteins require nuclear‑localisation signals.
NucleolusDense, non‑membranous region; contains fibrillar centres, dense fibrillar component and granular region.Site of rRNA transcription and ribosomal subunit assembly.AO1 – identify in nucleus; AO2 – link rRNA synthesis to protein‑synthesis capacity.
Rough endoplasmic reticulum (RER)Network of flattened sacs & tubules studded with ribosomes on the cytoplasmic face.Synthesises membrane‑bound and secretory proteins; initiates folding and N‑linked glycosylation.AO2 – predict effect of RER damage on secretion of insulin.
Smooth endoplasmic reticulum (SER)Tubular membrane network lacking ribosomes.Lipid synthesis (phospholipids, sterols), detoxification of drugs/toxins, calcium‑ion storage.AO2 – explain why liver cells have abundant SER.
MitochondrionDouble‑membrane organelle; inner membrane folded into cristae, matrix contains enzymes of the TCA cycle.Site of aerobic respiration; produces ATP via oxidative phosphorylation.AO1 – recognise cristae; AO2 – relate ATP demand to increased mitochondrial number in muscle.
ChloroplastDouble‑membrane organelle; internal thylakoid stacks (grana) surrounded by stroma; contains own DNA.Photosynthesis – light‑dependent reactions in thylakoids, Calvin cycle in stroma.AO1 – identify chloroplast in plant cell; AO2 – predict impact of herbicide that blocks photosystem II.
LysosomeMembrane‑bound vesicle containing hydrolytic enzymes (acid hydrolases) active at pH ≈ 5.Intracellular digestion of macromolecules, autophagy, recycling of organelles.AO2 – explain why lysosomal storage diseases arise from enzyme deficiencies.
Centrioles (animal cells)Pair of orthogonal, barrel‑shaped structures made of microtubule triplets.Organise the mitotic spindle during cell division; nucleate cilia/flagella.AO2 – predict consequence of centriole loss on mitosis.
Cilia (animal cells)Hair‑like projections of the plasma membrane containing a core of nine doublet microtubules (9+2 arrangement).Movement of extracellular fluid (e.g., respiratory epithelium) or locomotion in protozoa.AO2 – relate ciliary dysfunction to respiratory disease.
Microvilli (intestinal epithelial cells)Finger‑like extensions of the plasma membrane supported by a core of actin filaments.Increase surface area for absorption and secretion.AO2 – calculate how a 10‑fold increase in surface area enhances nutrient uptake.
Tonoplast (vacuolar membrane)Membrane surrounding the central vacuole; contains transport proteins and proton pumps.Regulates movement of ions, metabolites and water into/out of the vacuole; generates turgor pressure.AO2 – explain how the tonoplast contributes to stomatal opening.
Large central vacuole (plant cells)Membrane‑bound sac occupying up to 90 % of cell volume; filled with cell sap (water, ions, pigments).Storage of nutrients and waste, maintenance of cell rigidity (turgor), pH buffering.AO2 – predict effect of vacuole loss on leaf wilting.
Plasmodesmata (plant cells)Microscopic channels traversing the cell wall, linking cytoplasms of adjacent cells; contain a desmotubule (derived from ER).Facilitate symplastic transport of ions, signalling molecules and viruses.AO2 – discuss why viruses exploit plasmodesmata to spread.

3. Plant vs. Animal Cells – Quick Comparison

FeaturePlant CellAnimal Cell
Cell wallCellulose‑based, rigidAbsent
Large central vacuoleProminent, up to 90 % volumeSmall, often absent
ChloroplastsPresent (photosynthesis)Absent
PlasmodesmataPresentAbsent
CentriolesAbsent (spindle organised by other MTOCs)Present (pair of centrioles)
LysosomesFew; digestive functions often performed by vacuoleNumerous
Cilia / FlagellaRare (except in some algae)Common on epithelial cells & sperm
MicrovilliOften on root epidermal cellsCommon on intestinal epithelium

4. Prokaryotic Cells – Brief Outline (required for the syllabus)

  • Size: 0.1–5 µm.

  • Cell envelope: thick peptidoglycan wall (Gram‑positive) or thin peptidoglycan + outer membrane (Gram‑negative).

  • Genetic material: single circular chromosome in a nucleoid; often plasmids.

  • Ribosomes: 70S (30S + 50S) – smaller than eukaryotic 80S.

  • Reproduction: binary fission; no mitosis or meiosis.

  • Examples relevant to the syllabus: Escherichia coli (Gram‑negative) and Staphylococcus aureus (Gram‑positive).

5. Viruses – What Students Need to Know

  • Non‑cellular infectious particles; nucleic acid (DNA or RNA) enclosed in a protein capsid; some have a lipid envelope derived from host membranes.

  • Obligate intracellular parasites – cannot carry out metabolism or reproduce outside a host cell.

  • : attachment → penetration → uncoating → replication → assembly → release (lysis or budding).

  • Typical syllabus examples: influenza virus (enveloped, RNA) and bacteriophage T4 (non‑enveloped, DNA).

6. Summary of Functional Themes

  • Boundary & Communication: plasma membrane, cell wall, plasmodesmata, cilia, microvilli.

  • Genetic Control: nucleus, nucleolus, mitochondrial & chloroplast DNA.

  • Protein Synthesis & Processing: ribosomes, rough ER, smooth ER, Golgi (not required but useful for context).

  • Energy Conversion: mitochondria (cellular respiration) and chloroplasts (photosynthesis).

  • Structural Support & Storage: cell wall, central vacuole, tonoplast, cytoskeleton (microtubules, actin filaments).

  • Digestion & Recycling: lysosomes, vacuoles.

7. Suggested Diagrams for Revision

  1. Labelled animal eukaryotic cell showing: plasma membrane, nucleus (envelope + nucleolus), mitochondrion, rough ER, smooth ER, lysosome, centrioles, cilia, microvilli.

  2. Labelled plant eukaryotic cell showing the same organelles plus: cell wall, large central vacuole (tonoplast), chloroplast, plasmodesmata.

  3. Simple schematic of a typical prokaryotic cell (no nucleus, single circular chromosome, cell wall, flagellum).

  4. Typical virus diagram (capsid, nucleic acid, optional envelope, surface spikes).

8. Key Points for Revision (AO1 & AO2)

  1. Double‑membrane nature of the nucleus and the role of nuclear pores in macromolecular traffic.

  2. The nucleolus is the site of rRNA synthesis; it lacks a surrounding membrane.

  3. Rough ER – ribosome‑studded; synthesises proteins destined for membranes, secretion or lysosomes.

  4. Smooth ER – ribosome‑free; synthesises lipids, detoxifies xenobiotics, stores Ca²⁺.

  5. Mitochondria generate ATP by oxidative phosphorylation; more mitochondria are present in high‑energy tissues.

  6. Chloroplasts conduct photosynthesis; thylakoid membranes house the light‑dependent reactions.

  7. Lysosomes contain acid hydrolases; deficiency leads to storage diseases.

  8. Centrioles organise the mitotic spindle; loss impairs chromosome segregation.

  9. Cilia and flagella share the 9+2 microtubule arrangement; defects cause respiratory or reproductive problems.

  10. Microvilli increase absorptive surface area; a 10‑fold increase in area can dramatically boost nutrient uptake.

  11. Cell wall provides rigidity; removal makes plant cells prone to osmotic lysis.

  12. Large central vacuole maintains turgor pressure; loss leads to wilting.

  13. Plasmodesmata enable symplastic transport; viruses exploit them to move cell‑to‑cell.

  14. Resolution limits of microscopes: do not confuse magnification with the ability to see finer detail.

  15. Active transport across the plasma membrane requires ATP – produced mainly in mitochondria (animals) or chloroplasts (plants).