Cambridge O Level Biology Notes

1.1 Cell Structure & Function

Animal & Plant Cells

• Animals:

  • Multicellular organisms.
  • Cells have a nucleus with a distinct membrane.
  • Lack cellulose cell walls and chloroplasts.
  • Feed on organic substances from other living things.
  • Store carbohydrates as glycogen.
  • Usually possess nervous coordination and mobility.

• Plants:

  • Multicellular organisms.
  • Cells have a nucleus, cellulose cell walls, and chloroplasts.
  • Capable of photosynthesis.
  • Store carbohydrates as starch or sucrose.
  • Lack nervous coordination.

Cell Structures

• Common to Animal and Plant Cells:

  • Nucleus: Contains genetic material (DNA).
  • Cytoplasm: Gel-like substance where chemical reactions occur.
  • Cell Membrane: Controls substance entry and exit.
  • Ribosomes: Site of protein synthesis.
  • Mitochondria: Site of aerobic respiration.

• Exclusive to Plant Cells:

  • Cell Wall: Made of cellulose, provides support.
  • Chloroplasts: Contain chlorophyll for photosynthesis.
  • Permanent Vacuole: Stores cell sap.

Visualizing Cells

• Light microscopes show larger structures; electron microscopes reveal smaller details like ribosomes and vesicles.

Biological Drawings

• Follow conventions for accurate depictions:
  • Title and magnification should be included.
  • Use plain white paper, clear single lines, no shading.
  • Label lines should be straight, not cross, and on one side.

Examining Cells Under the Microscope

Methods

• Use thin samples for light passage.
• Stains like methylene blue (animal cells) and iodine (plant cells) enhance visibility.
• Cheek cells and onion cells are common specimens.

Slide Preparation

• Liquid specimens: Use a pipette, cover with coverslip.
• Solid specimens: Cut thin layers, may require chemical treatment.

Microscope Use

• Start with the lowest power objective lens.
• Add water to prevent dehydration.

Bacterial Cells

Characteristics

• Microscopic, single-celled.
• Cell wall of peptidoglycan, no nucleus or membrane-bound organelles.
• May contain plasmids and flagella.

Examples

• Lactobacillus: Used in yogurt production.
• Pneumococcus: Causes pneumonia.

1.2 Specialised Cells, Tissues & Organs

Specialised Cells

• Differentiation controlled by genes.
• Examples include ciliated cells, nerve cells, red blood cells, sperm and egg cells.

Specialised Cells in Plants

• Root Hair Cell: Absorbs water/minerals, increased surface area.
• Xylem Vessel: Conducts water, provides support.
• Palisade Mesophyll Cell: Photosynthesis, contains chloroplasts.

Levels of Organisation

• Cells: Basic units.
• Tissues: Similar cells performing a function.
• Organs: Made from different tissues.
• Organ Systems: Organs working together.

Magnification Formula

Calculating Magnification

• Formula: Magnification = Image size / Actual size
• Units: No units for magnification; use millimeters for measurements.

Worked Example

• If an image is 30 mm, magnified by x3000, calculate actual size using the formula.

Examiner Tips

• Use millimeters for measurements and remember magnification has no units.

These notes provide a comprehensive overview of key concepts in cell biology, focusing on cell structure, specialized cells, and the practical use of microscopes. Use them as a study aid to reinforce understanding of biological principles.