EDEXCEL
IGCSE
BIOLOGY
REVISION GUIDE
, Topic 1: The Nature and
Variety of Living
Organisms
1.1 Characteristics of living organisms
All living organisms share seven life processes. A useful mnemonic is MRS GREN:
Movement — an action by an organism causing a change of position or place
Respiration — the chemical reactions in cells that break down nutrient molecules and release
energy
Sensitivity — the ability to detect and respond to changes in the environment
Growth — a permanent increase in size
Reproduction — processes that make more of the same kind of organism
Excretion — removal from organisms of the waste products of metabolism, toxic materials, and
substances in excess of requirements
Nutrition — taking in of materials for energy, growth and development
1.2 Cell structure
Animal and plant cells share some structures but differ in others. This is a very common exam
comparison question.
Nucleus: Contains genetic material (DNA) that controls the cell's activities
Cytoplasm: Gel-like substance where most chemical reactions happen
Cell membrane: Controls what enters and leaves the cell
Mitochondria: Site of aerobic respiration — releases energy
Ribosomes: Site of protein synthesis
Plant cells additionally have:
Cell wall (made of cellulose) — provides structural support
Chloroplasts — contain chlorophyll, site of photosynthesis
Permanent vacuole — contains cell sap, helps maintain turgor pressure
1.3 Classification
Organisms are classified into groups based on shared features. The hierarchy, from largest to
smallest group, is:
Kingdom → Phylum → Class → Order → Family → Genus → Species
IGCSE
BIOLOGY
REVISION GUIDE
, Topic 1: The Nature and
Variety of Living
Organisms
1.1 Characteristics of living organisms
All living organisms share seven life processes. A useful mnemonic is MRS GREN:
Movement — an action by an organism causing a change of position or place
Respiration — the chemical reactions in cells that break down nutrient molecules and release
energy
Sensitivity — the ability to detect and respond to changes in the environment
Growth — a permanent increase in size
Reproduction — processes that make more of the same kind of organism
Excretion — removal from organisms of the waste products of metabolism, toxic materials, and
substances in excess of requirements
Nutrition — taking in of materials for energy, growth and development
1.2 Cell structure
Animal and plant cells share some structures but differ in others. This is a very common exam
comparison question.
Nucleus: Contains genetic material (DNA) that controls the cell's activities
Cytoplasm: Gel-like substance where most chemical reactions happen
Cell membrane: Controls what enters and leaves the cell
Mitochondria: Site of aerobic respiration — releases energy
Ribosomes: Site of protein synthesis
Plant cells additionally have:
Cell wall (made of cellulose) — provides structural support
Chloroplasts — contain chlorophyll, site of photosynthesis
Permanent vacuole — contains cell sap, helps maintain turgor pressure
1.3 Classification
Organisms are classified into groups based on shared features. The hierarchy, from largest to
smallest group, is:
Kingdom → Phylum → Class → Order → Family → Genus → Species
