HKDSE Biology Notes
Unit 2 The Cell as the Basic Unit of Life
2.1 Characteristics of Life
• Nutrition
- Obtain food for energy and to maintain life.
• Respiration
- Breakdown food to get energy for body activities.
• Movement
- Animals can move from place to place; Plants usually move by growing.
• Growth
- Increase un size and complexity.
• Irritability
- Sense internal and external environment and respond to it.
• Reproduction
- Produce new individuals (offsprings).
• Excretion
- Remove metabolic wastes.
2.2 Chemicals of Life
1. Inorganic
• Water
• Functions:__________________________________________________________________________
_________________________________________________________________________________
• Inorganic ions
• Types:_____________________________________________________________________________
_________________________________________________________________________________
2. Organic
• Carbohydrates
• Glucose, Starch, Glycogen, Cellulose
• Lipids
• Triglycerides, Phospholipid
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HKDSE Biology Notes
• Proteins
• Structural proteins, Enzymes, Hormones, Antibodies, Haemoglobin
• Nucleic acids
• Deoxyribonucleic acid, Ribonucleic acid
2.3 Basic Structure of a Cell
Common Cell Structures in Animal and Plant Cells:
Cell Structure
Function
Nucleus
• Surrounded by nuclear envelope.
• Contains DNA that carries genetic information.
• Controls cellular activities.
Cell Membrane
• Made up of lipids and proteins.
• Soft and selectively permeable
• Control substances in and out of the cell.
Cytoplasm
• Contains organelles.
• Aqueous solution of cell substances.
• Site for chemical reactions.
Mitochondrion
• Site for aerobic respiration to produce energy (ATP).
• Surrounded by a double membrane.
Vacuole
• Holds useful substances and sometimes wastes.
Rough Endoplasmic Reticulum
• Site for protein synthesis.
Smooth Endoplasmic Reticulum
• Site for hormones and lipids synthesis.
Unique Cell Structures in Plant Cells:
Cell Structure
Function
Cell Wall
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Chloroplast
• Contains chlorophyll to absorb light.
• Site for photosynthesis to produce food i.e. glucose.
• Surrounded by a double membrane.
Vacuole
• Holds useful substances, sometimes wastes, poison and pigments.
• Supports the plant when the vacuole is full of water.
Made up of cellulose.
Rigid and thick.
Supports and gives shape to cell.
Freely permeable.
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HKDSE Biology Notes
2.4Levels of Organisation
Organisms:
• Unicellular e.g. Amoeba
• Multicellular e.g. Human (made up of over 50 trillion of cells)
- Specialised cells have different shapes and structures to carry out different functions.
Level of Organisation:
• Cell —> Tissues —> Organ —> System —> Organism
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