Sample Notes: Cells

O Level and IGCSE Biology — Detailed Notes

Chapter 1.1: Cell Structure and Function

Chapter 1.2: Specialised Cells, Tissues and Organs

Microscopy of Animal and Plant Cells

Preparation and Observation

• Cells are stained to improve visibility under the microscope.
• Common stains:
  • Methylene blue for animal cells (stains the nucleus).
  • Iodine solution for plant cells (stains starch grains and enhances cell wall visibility).
• Sources of cells:
  • Animal: cheek cells scraped using a clean cotton bud.
  • Plant: onion epidermal peel or Elodea leaf.

Microscopy Procedure

1. Place the specimen on a clean slide.
2. Add a drop of the appropriate stain.
3. Gently lower the coverslip using a mounted needle to avoid air bubbles.
4. Use low power objective first, then switch to high power.
5. Adjust focus and illumination for clarity.

Organelle Identification in Animal Cells

Visible Structures

• Nucleus: Controls cell functions and stores genetic material.
• Cytoplasm: Site of most chemical reactions; jelly-like substance.
• Cell membrane: Controls what enters and exits the cell.
• Mitochondria: Site of aerobic respiration; produces ATP.
• Ribosomes: Small granules responsible for protein synthesis.

Organelle Identification in Plant Cells

Additional Structures in Plant Cells

• Chloroplasts: Contain chlorophyll; site of photosynthesis.
• Sap vacuole: Contains cell sap; maintains turgor pressure.
• Cell wall (cellulose): Provides structure and protection.

Shared Structures

• Also contain nucleus, cytoplasm, mitochondria, ribosomes, and a cell membrane like animal cells.

Structure of a Bacterial Cell

Key Components

• Cell wall: Rigid outer layer (not cellulose); made of peptidoglycan.
• Cell membrane: Controls movement of substances in/out.
• Cytoplasm: Contains enzymes and nutrients.
• Ribosomes: Site of protein synthesis (smaller than in eukaryotes).
• Circular DNA: Located in nucleoid region; controls cell activities.
• Plasmids: Small rings of extra DNA that often carry antibiotic resistance.

Functions of Cell Structures

Comparison Table

Comparison of Cell Types

Summary Table

Cell Specialisation

Meaning and Importance

• Cells in multicellular organisms become specialised to perform specific functions.
• Structure is adapted to their role — allows division of labour.

Examples

• Red blood cells: No nucleus, biconcave shape, haemoglobin → carries oxygen.
• Root hair cells: Long extension increases surface area → absorbs water and minerals.
• Muscle cells: Long, fibrous, many mitochondria → contraction and movement.
• Ciliated epithelial cells: Hair-like structures → move mucus in respiratory tract.
• Xylem vessels: Dead, hollow tubes → transport water, support plant.
• Sperm cells: Tail for motility, mitochondria, nucleus → fertilisation.
• Egg cells: Large, nutrient-rich → support embryo after fertilisation.

Biological Organization

Definitions

• Cell: Basic structural and functional unit of life.
• Tissue: Group of similar cells performing a specific function (e.g., muscle tissue).
• Organ: Group of tissues working together for a common function (e.g., stomach).
• Organ system: Group of organs performing a major function (e.g., respiratory system).
• Organism: An individual living being made of one or more organ systems.

Magnification Formula

Formula

• Magnification = Image size / Actual size

Notes

• Units must be consistent (e.g., both in µm or mm).
• 1 mm = 1000 µm
• To calculate actual size, rearrange:
  • Actual size = Image size / Magnification

Example

• Image of a cell = 60 mm
• Actual size = 30 µm
• Convert: 60 mm = 60,000 µm
• Magnification = 60,000 / 30 = 2000×

Practical Applications

• Identify and label organelles from diagrams or microscope images.
• Draw diagrams of cells based on actual microscopic observations.
• Match specialised cells with their adaptations.
• Perform magnification calculations.
• Differentiate between prokaryotic and eukaryotic structures.