compare the structure of typical plant and animal cells

Objective (AO1, AO2, AO3)

To compare the structure of a typical plant cell with that of a typical animal cell, recognising common and distinctive features, relating each structure to its function, and linking observations to the Cambridge International AS & A Level Biology (9700) syllabus.

1. The Microscope in Cell Studies (Syllabus 1.1) (AO1, AO2)

1.1 Preparing a temporary (wet‑mount) slide

• Place a small drop of water (or a suitable stain such as iodine or methylene blue) on a clean glass slide.
• Transfer a thin fragment of fresh tissue (e.g., onion epidermis or cheek cells) onto the drop.
• Cover gently with a cover‑slip, avoiding air bubbles.
• Secure the slide on the stage and bring the specimen into focus using the coarse then the fine adjustment knobs.

1.2 Measuring with an eyepiece graticule

• Place the calibrated eyepiece reticle (usually 0.1 mm divisions) over the specimen.
• Count the number of divisions that span a known distance on a stage micrometer (e.g., 1 mm = 10 divisions).
• Calculate the field of view (FOV) at the chosen magnification:
  

  FOV = (stage‑micrometer distance) ÷ (number of eyepiece divisions)

1.3 Calculating total magnification

Total magnification = (objective magnification) × (eyepiece magnification) × (any additional zoom).

Worked example (40× objective, 10× eyepiece, 0.5× head zoom):

Total magnification = 40 × 10 × 0.5 = 200×

1.4 Resolution versus magnification

• Resolution – ability to distinguish two points as separate; limited by wavelength of light and numerical aperture (NA) of the objective.
• Increasing magnification alone does not improve resolution unless the NA is sufficiently high.

Check‑your‑understanding (AO2)

Question: A 0.2 mm feature occupies 4 divisions on the eyepiece graticule when using a 40× objective and a 10× eyepiece. What is the actual size of the feature?

Answer: Each division represents 0.2 mm ÷ 4 = 0.05 mm on the slide; therefore the feature is 0.2 mm, confirming the calibration.

2. Overview of Cell Types (Syllabus 1.2) (AO1)

2.1 Prokaryotic cells (e.g., bacteria)

• Size: 0.2–2 µm (generally smaller than eukaryotes).
• Cell wall: Peptidoglycan (Gram‑positive) or thin peptidoglycan + outer membrane (Gram‑negative).
• Nucleoid region with a single circular DNA molecule – no true nucleus.
• No membrane‑bound organelles; metabolic reactions occur in the cytoplasm or at the plasma membrane.
• Ribosomes: 70 S (smaller than eukaryotic 80 S).
• Reproduction: binary fission.

2.2 Eukaryotic cells (plant and animal) (AO1)

All eukaryotes possess a true nucleus and a suite of membrane‑bound organelles that compartmentalise biochemical processes.

3. Common Eukaryotic Organelles – Functions (AO1)

4. Plant‑Specific Structures – Functions (AO1)

• Cell wall (cellulose) – Provides rigidity, defines shape, and protects against osmotic lysis.
• Chloroplasts – Contain thylakoid membranes with chlorophyll; conduct photosynthesis (light reactions → ATP/NADPH, Calvin cycle → glucose).
• Large central vacuole – Stores water, ions and metabolites; generates turgor pressure that supports the plant body.
• Plasmodesmata – Cytoplasmic channels that permit direct exchange of solutes and signalling molecules between adjacent cells.
• Starch granules (cytoplasmic inclusion) – Store photosynthetic product as a polysaccharide.

Practical activity (AO2)

Starch test: Place a thin slice of onion epidermis in iodine solution. Iodine turns dark blue‑black where starch granules are present, confirming the function of the chloroplast‑derived product.

5. Animal‑Specific Structures – Functions (AO1)

• Lysosomes – Contain hydrolytic enzymes for intracellular digestion and autophagy.
• Centrosome (with a pair of centrioles) – Organises microtubules; forms the mitotic spindle during cell division.
• Small, numerous vacuoles – Involved in transport, storage of metabolites and regulation of ion balance.
• Cell junctions

  • Tight junctions – Create a seal between epithelial cells.
  • Adherens & desmosomes – Provide mechanical adhesion.
  • Gap junctions – Allow passage of ions and small molecules for intercellular communication.

• Glycogen granules (cytoplasmic inclusion) – Short‑term energy reserve.

Practical activity (AO2)

Glycogen test: Fix cheek cells, treat with periodic acid‑Schiff (PAS) reagent, and observe magenta staining of glycogen granules under the microscope.

6. Comparative Table (AO2)

7. Functional Implications of Structural Differences (AO2)

1. Photosynthesis vs. heterotrophy – Chloroplasts enable plants to capture light energy and fix CO₂, whereas animal cells rely entirely on mitochondria to oxidise organic nutrients.
2. Mechanical support – The rigid cell wall together with turgor pressure from the central vacuole allows plants to grow upright without a skeletal system.
3. Cell‑division mechanisms – Animal cells use centrosomes to organise the mitotic spindle; plant cells form a pre‑prophase band, a phragmoplast and a cell plate to replace the missing centrosome.
4. Intercellular communication – Plasmodesmata provide direct cytoplasmic continuity in plants, while animals employ gap junctions and specialised adhesion complexes.
5. Digestive and recycling processes – Lysosomes mediate autophagy and macromolecule turnover in animal cells; plant vacuoles perform analogous functions, often storing hydrolytic enzymes.
6. Energy‑storage strategy – Starch (plant) and glycogen (animal) are polymeric glucose reserves that can be mobilised when required.

8. Biological Molecules (Syllabus 2) (AO1, AO2, AO3)

8.1 Carbohydrates

• General formula: (CH₂O)n.
• Monosaccharides – α‑glucose and β‑glucose (draw both anomers).
• Disaccharides – maltose (α‑1,4‑linkage), sucrose (α‑1,β‑2‑linkage), lactose (β‑1,4‑linkage).
• Polysaccharides – starch (amylose + amylopectin, plant storage), glycogen (animal storage), cellulose (β‑1,4‑glucose polymer, plant cell wall).
• Key reactions: hydrolysis (e.g., starch → maltose) and condensation (formation of glycosidic bonds).

Practical test (AO2)

Benedict’s test – Reducing sugars turn orange‑red on heating. Use glucose solution as a positive control and starch as a negative control.

8.2 Lipids

• Major classes: fatty acids, triglycerides, phospholipids, sterols.
• Amphipathic nature of phospholipids – hydrophilic head (phosphate + choline) + two hydrophobic fatty‑acid tails.
• Structure of a typical membrane phospholipid (draw schematic).
• Functions: energy storage (fatty acids), membrane formation, signalling (steroid hormones).

Practical test (AO2)

Lipid stain (Sudan III or Oil‑Red O) – Stains neutral lipids red in animal adipose tissue; demonstrates the hydrophobic nature of lipids.

8.3 Proteins

• Composed of amino‑acid residues linked by peptide bonds.
• Four levels of structure: primary, secondary (α‑helix, β‑sheet), tertiary, quaternary.
• Key functional groups: –NH₂ (amine), –COOH (carboxyl), side‑chain R groups.
• Denaturation – loss of secondary/tertiary structure (heat, pH, urea) – illustrated by egg‑white experiment.

Practical test (AO2)

Biuret test – Peptide bonds give a violet colour with copper(II) sulphate in alkaline solution.

8.4 Water

• Polarity, hydrogen bonding, high specific heat, high heat of vaporisation, cohesion, adhesion, surface tension.
• Biological relevance: temperature regulation, transport in xylem, solvent for biochemical reactions.

9. Enzymes (Syllabus 3) (AO1, AO2, AO3)

9.1 Mode of action

• Enzyme + substrate ⇌ enzyme‑substrate complex → enzyme + product.
• Active site – specific three‑dimensional pocket; lock‑and‑key vs. induced‑fit models.
• Enzyme specificity (e.g., amylase hydrolyses α‑1,4‑glycosidic bonds).

9.2 Factors affecting activity (AO2)

• Temperature – optimum ≈ 37 °C for most human enzymes; denaturation above optimum.
• pH – optimum varies (pepsin ≈ 2, alkaline phosphatase ≈ 9).
• Substrate concentration – Michaelis–Menten kinetics; Vmax and Km.
• Enzyme concentration – reaction rate proportional to enzyme amount (provided substrate is not limiting).
• Inhibitors – competitive (bind active site) and non‑competitive (bind elsewhere).

9.3 Practical investigation (AO2, AO3)

Catalase activity – Place a fixed amount of liver (source of catalase) in a test tube, add H₂O₂ of varying concentrations, and measure the volume of O₂ gas released in 30 s (gas‑collecting syringe). Plot rate of reaction vs. substrate concentration to illustrate Vmax and Km.

10. Cell Membranes & Transport (Syllabus 4) (AO1, AO2, AO3)

10.1 Fluid‑mosaic model

• Phospholipid bilayer with embedded proteins (integral, peripheral, channel, carrier, receptor).
• Role of cholesterol – modulates fluidity.
• Glycocalyx – carbohydrate chains on the extracellular face.

10.2 Transport mechanisms

10.3 Surface‑area‑to‑volume (SA:V) implications (AO2)

Calculate SA:V for a sphere (SA = 4πr², V = 4/3πr³). Discuss why diffusion limits cell size and how microvilli increase SA in intestinal epithelial cells.

Practical activity (AO2)

Dialysis‑tube diffusion – Fill a dialysis tube with a 0.5 M sucrose solution, place it in distilled water, and measure the change in mass over time. Relate the rate to the concentration gradient and membrane permeability.

11. The Mitotic Cell Cycle (Syllabus 5) (AO1, AO2, AO3)

11.1 Phases of the cell cycle

• Interphase – G₁ (cell growth), S (DNA synthesis), G₂ (pre‑mitotic growth).
• Mitosis – Prophase, Metaphase, Anaphase, Telophase.
• Cytokinesis – Division of the cytoplasm (cleavage furrow in animal cells; cell plate formation in plant cells).

11.2 Key structures

• Centrosome & centrioles (animal cells) – nucleate spindle fibres.
• Pre‑prophase band (plant) – predicts future cell‑plate site.
• Spindle apparatus – microtubules attach to kinetochores on chromosomes.
• Chromosome movements – congression, alignment at the metaphase plate, sister‑chromatid separation.

11.3 Functional significance (AO2)

Accurate segregation prevents aneuploidy; checkpoints (G₁‑S, G₂‑M) ensure DNA integrity.

Practical investigation (AO2, AO3)

Observe onion root tip cells stained with aceto‑orcein. Identify cells in each mitotic stage, record the proportion of cells in mitosis, and discuss the relationship between growth rate and mitotic index.

12. Nucleic Acids & Protein Synthesis (Syllabus 6) (AO1, AO2, AO3)

12.1 DNA structure

• Double helix: antiparallel strands, deoxyribose‑phosphate backbone, nitrogenous bases (A, T, G, C).
• Base‑pairing rules: A ↔ T (2 H‑bonds), G ↔ C (3 H‑bonds).
• Major & minor grooves – sites of protein binding.

12.2 RNA structure

• Single‑stranded; ribose sugar; uracil replaces thymine.
• Types: mRNA (coding), tRNA (adapter), rRNA (ribosomal component).

12.3 Central Dogma – From DNA to protein (AO1)

1. Transcription – In the nucleus, RNA polymerase synthesises a complementary mRNA strand from a DNA template (5'→3').
2. RNA processing (eukaryotes) – 5' cap, poly‑A tail, intron removal (splicing).
3. Translation – In ribosomes, mRNA codons are read; each codon (three bases) specifies an amino‑acid delivered by a specific tRNA.
4. Polypeptide folding – Chaperone proteins assist formation of secondary, tertiary, and quaternary structures.

12.4 Genetic code (AO2)

• Degenerate – most amino acids are encoded by more than one codon.
• Non‑overlapping, start codon (AUG = Met), stop codons (UAA, UAG, UGA).

Practical activity (AO2, AO3)

DNA extraction from strawberries – Demonstrates cell‑lysis, precipitation of nucleic acids, and visualisation of DNA as a white, stringy mass. Follow with a gel‑electrophoresis simulation to discuss fragment size separation.

13. Why It Matters – Linking Structure to Key Concepts (AO3)

Understanding cellular structure underpins three core syllabus concepts:

• Cells as the units of life – Differences such as cell walls, chloroplasts, and lysosomes determine whether an organism is autotrophic or heterotrophic, influencing ecosystem roles.
• Biochemical processes – Mitochondria and chloroplasts illustrate how cells capture and convert energy (respiration vs. photosynthesis), linking to the flow of energy through living systems.
• Observation & experiment – Microscopy, staining techniques, and biochemical assays allow students to visualise structures and test functional hypotheses, fulfilling the practical requirements of the syllabus.

14. Summary (AO1, AO2, AO3)

Plant and animal cells share the fundamental eukaryotic architecture—nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, ribosomes and cytoskeleton—but differ markedly in specialised organelles that reflect their ecological roles. Plant cells are equipped for photosynthesis, structural rigidity and large‑volume water storage, whereas animal cells possess features that support mobility, rapid cell division and intracellular digestion. Mastery of these structural differences, together with a solid grounding in biological molecules, enzymes, membrane transport, the cell cycle and nucleic‑acid chemistry, equips students to interpret cellular function in the broader context of organismal biology and the Cambridge International AS & A Level Biology syllabus.