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Cambridge IGCSE Biology · 0610
Chapter 1: Characteristics and classification of living organisms
Characteristics of living organisms
All living organisms undergo these seven processes:
- Movement
- An action by an organism or part of an organism causing a change of position or place. Animals move to find food or escape predators, while plants move their leaves to catch sunlight.
- Respiration
- Chemical reactions that occur in cells, leading to the breakdown of nutrient molecules to release energy for metabolism.
- Sensitivity
- The ability to detect or sense changes in the internal or external environment and to make appropriate responses.
- Growth
- A permanent increase in size and dry mass by an increase in cell number or cell size or both.
- Reproduction
- The processes that make more of the same kind of organism.
- Excretion
- Removal from organisms of the waste products of metabolism (chemical reactions in cells including respiration), toxic materials, and substances in excess of requirements.
- Nutrition
- Taking in of materials for energy, growth, and development. Animals require organic compounds, ions, and water. Plants require light, carbon dioxide, water, and ions for photosynthesis.
Examiner Report Insights
- Mark schemes expect respiration as chemical reactions in cells releasing energy — not breathing or gas exchange.
Exam Traps
- Students often confuse sensitivity with intelligence or conscious decision-making rather than detecting and responding to stimuli.
Concept and uses of classification systems
Organisms are classified into groups by the features that they share.
- Species
- A group of organisms that can reproduce to produce fertile offspring.
- Binomial system
- An internationally agreed system in which the scientific name of an organism is made up of two parts showing the genus and species. The first name refers to the genus and the second to the species (e.g., Homo sapiens).
Modern methods of classification
- Evolutionary relationships
- Classification systems aim to reflect how closely related organisms are.
- Traditional method
- Historically based on morphology (outward appearance) and anatomy (internal structure, like bone and organ arrangement).
- DNA sequences
- The sequences of bases in DNA are now used as a more accurate means of classification.
- Accuracy
- Groups of organisms which share a more recent ancestor have base sequences in DNA that are more similar than those that share only a distant ancestor.
As DNA codes for the amino acid sequence in proteins, proteins can also be studied to find these relationships.
Features in cells of all living organisms
The following features are used to help categorise organisms:
- Cytoplasm
- Jelly-like material within the cell in which reactions occur.
- Cell membrane
- Thin membrane surrounding the cell that controls the entry and exit of substances.
- DNA
- Genetic material that codes for proteins.
- Ribosomes
- Sites of protein synthesis.
- Enzymes
- Catalysts involved in reactions such as respiration.
The Five Kingdoms
Organisms are placed into one of five kingdoms based on their cell features:
- Animal
- Multicellular; has a nucleus; no cell wall; no chloroplasts.
- Plant
- Multicellular; has a nucleus; cell wall made of cellulose; contains chloroplasts.
- Fungus
- Usually multicellular; has a nucleus; cell wall made of chitin; no chloroplasts.
- Adaptation
- They have branched hyphae that release enzymes for external digestion and absorption.
- Prokaryote (Bacteria)
- Unicellular; no nucleus (circular loop of DNA); cell wall made of peptidoglycan; often contains plasmids.
- Protoctist
- Mostly unicellular; has a nucleus; may have cell walls or chloroplasts.
Exam Traps
- Protoctists are a diverse kingdom; a single cell may be plant-like or animal-like, so do not assume all are photosynthetic.
Main groups of Vertebrates (Animals with backbones)
- Mammals
- Body covered with fur/hair; have mammary glands; produce live young; have a pinna (external ear); warm-blooded; breathe via lungs.
- Birds
- Body covered with feathers; have wings and a beak; produce hard-shelled eggs; warm-blooded; breathe via lungs.
- Reptiles
- Dry skin with scales; produce rubbery waterproof eggs; cold-blooded; breathe via lungs.
- Amphibians
- Moist skin; produce jelly-covered eggs in water; cold-blooded; breathe via skin and lungs.
- Fish
- Body covered with scales; have fins and gills; have lateral lines to detect vibrations; cold-blooded.
Main groups of Arthropods (Invertebrates with jointed legs)
Arthropods are identified by their exoskeleton and segmented body:
- Insects
- 3 pairs of legs; body divided into head, thorax, and abdomen; 1 pair of antennae; 1 pair of compound eyes.
- Arachnids
- 4 pairs of legs; body divided into cephalothorax and abdomen; no antennae; several pairs of simple eyes.
- Crustaceans
- 5 or more pairs of legs; 2 pairs of antennae; 1 pair of compound eyes; hard chalky exoskeleton.
- Myriapods
- 10 or more pairs of legs (one pair per segment); 1 pair of antennae; simple eyes.
Main groups of Plants
- Ferns
- Non-flowering plants that do not produce seeds; reproduce via spores found on the underside of leaves.
- Flowering plants
- Reproduce via flowers and seeds produced in an ovary.
- Monocotyledons
- Identified by long narrow leaves with parallel veins; flower parts in threes; 1 cotyledon.
- Dicotyledons
- Identified by broad leaves with branching veins; flower parts in fives; 2 cotyledons.
Viruses
- Features
- Consist of a protein coat and genetic material.
- Classification
- Not classified as living because they cannot complete all seven life processes (e.g., they need a host cell to reproduce).
Dichotomous keys
Used to identify organisms based on identifiable features.
Consist of a series of choices (usually pairs) that lead the user to the name of the species.
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