Cell Structure and Specialisation

Topic: Cell Biology and Biodiversity — Subject: Biology — Target age: 15 (Kenyan context)

Specific Learning Outcomes

• a) Differentiate between light and electron microscopes in the study of cell structure.
• b) Prepare temporary slides for observation and estimate cell size using a light microscope.
• c) Describe the structure and functions of plant and animal cells as seen in electron micrographs (ultrastructure).
• d) Relate the structures of specialised cells in plants and animals to their functions.
• e) Appreciate the cell as the basic unit of life.

1. Overview — Why cells matter

All living organisms are made of cells. The cell is the basic unit of life. Studying cell structure explains how living things grow, obtain energy, transport materials and reproduce. At this level we use light microscopes for classroom work and electron microscopes for detailed ultrastructure seen in micrographs.

2. Light vs Electron Microscope — key differences

3. Preparing temporary slides (practical steps)

Choose either onion epidermis (plant) or cheek (buccal) cells (animal). Materials available in many Kenyan schools: onion, clean slide, cover slip, dropper, water, iodine (Lugol's) or methylene blue, forceps, scissors, paper towel, microscope.

1. Onion epidermis:
   1. Peel a thin transparent layer (epidermis) from an onion scale using forceps.
   2. Place a drop of water on the centre of a clean slide and lay the epidermis flat on it.
   3. Add one drop of iodine solution (stains starch and cell walls) or methylene blue (for animal cells); iodine also stains the nucleus faintly.
   4. Gently lower a cover slip at an angle to avoid air bubbles.
   5. Remove excess stain with a paper towel and place the slide on the stage.
2. Cheek cells:
   1. Rub the inside of the cheek gently with a clean cotton bud; smear the material on a slide.
   2. Add one drop of saline or water, then one drop of methylene blue to stain nuclei.
   3. Place a cover slip and observe.
3. Microscopy steps:
   1. Start with the low-power objective (×4 or ×10) to locate the sample; then switch to ×40 (high) and ×100 (if oil immersion is available and you are trained).
   2. Use coarse focus first on low power, then fine focus on higher powers.
   3. Clean lenses before and after use; follow school safety rules.

4. Estimating cell size with a light microscope

Two common methods: (A) use an ocular (eyepiece) micrometer calibrated with a stage micrometer; (B) estimate using field of view (FOV).

Method A — Ocular micrometer calibration (recommended)

1. Place a stage micrometer (a slide with known scale, e.g., 1 mm divided into 100 divisions = 10 μm/division) under the microscope.
2. At each objective, count how many ocular divisions correspond to a known stage micrometer distance. Calculate calibration: 1 ocular division = (stage micrometer distance ÷ number of ocular divisions) μm.
3. Now measure a cell: count how many ocular divisions the cell spans; multiply by calibration value to get actual cell size (in μm).

Method B — Using field of view

1. Measure diameter of the field of view at low power using stage micrometer (or find it in microscope notes).
2. Estimate cell length = (cell length in image ÷ diameter of field) × actual field diameter.
3. Example: if FOV = 2 mm (2000 μm) and cell appears to fill ¼ of FOV, cell ≈ 2000 × 1/4 = 500 μm.

Notes: 1 mm = 1000 μm (micrometres). Record magnification used. In Kenyan schools, stage micrometers can be shared across classes for calibration.

5. Ultrastructure of plant and animal cells (as seen in electron micrographs)

The electron microscope shows internal organelles and fine structures. Below are essential organelles with structure and function.

6. Relating structure to function: specialised cells

Specialised cells have structures adapted to their functions. Examples below include plant and animal cells commonly covered in the Kenyan syllabus.

7. Simple visual: schematic plant cell (stylised)

(A simple diagram can help memorise organelles.)

8. Suggested learning experiences (Kenyan classroom-friendly)

• Practical lab: prepare onion and cheek cell temporary mounts; observe and draw labeled diagrams at ×40 and ×400. Estimate cell size using stage or ocular micrometer.
• Group activity: compare classroom light microscope images with printed electron micrographs (from textbooks/online). Identify organelles visible with each microscope.
• Demonstration/field visit: arrange a demo or short visit to a nearby university or research lab (if available) to see electron microscope images and discuss its uses.
• Inquiry task: give pupils different specialised cells (images or prepared slides). Ask them to identify structural features and explain how these suit the cell's function.
• Project: create a poster or short presentation showing "A Day in the Life of a Cell" highlighting organelles and their roles; include Kenyan-relevant examples (e.g., maize leaf palisade cells, human red blood cells).
• Assessment ideas: practical test on slide preparation; short questions requiring calculations of cell size; compare and contrast essay on light vs electron microscopes.

9. Safety and good practice

• Handle knives, scalpels and glass slides carefully; use forceps for small tissues.
• Use stains (iodine/methylene blue) with care — wear gloves and do not ingest.
• Always carry microscopes with two hands; clean lenses only with lens paper or tissue supplied.
• Dispose biological material and stains according to school rules or rinse down sink with plenty of water if allowed.

10. Summary — appreciation

Cells are the fundamental units of life. Light microscopes let us view living cells and learn basic cell structure and size. Electron microscopes reveal the fine details of organelles that explain cell function. Understanding specialised cells links structure to function and explains how organisms survive, grow and reproduce. These ideas form a foundation for further study in biology and life sciences.

Prepared for Kenyan secondary classroom use (Form 3 / age ~15). Adapt activities to available resources and safety rules of your school.