IGCSE Biology: Characteristics and Classification of Living Organisms Study Guide

Characteristics of Living Organisms

• The Seven Characteristics of Life: All living organisms, regardless of whether they are single-celled (unicellular) or multicellular, plants or animals, exhibit seven specific characteristics. These can be remembered using the mnemonic MRS GREN.     * Movement: An action by an organism or part of an organism causing a change of position or place.     * Respiration: Describes the chemical reactions in cells that break down nutrient molecules and release energy for metabolism.     * Sensitivity: The ability to detect and respond to changes in the internal or external environment.     * Growth: A permanent increase in size and dry mass.     * Reproduction: The processes that make more of the same kind of organism.     * Excretion: The removal of waste products of metabolism and substances in excess of requirements.     * Nutrition: The taking in of materials for energy, growth, and development.

• Diversity of Life: There are approximately $8.7$ million species of organisms on Earth.

Classification Systems and Species Concepts

• Necessity of Classification: Biologists sort organisms into a meaningful order to organize knowledge and facilitate international agreement.

• Artificial vs. Natural Systems:     * Artificial systems: Grouping based on superficial similarities, such as "aquatic organisms" (grouping seaweed with porpoises) or "black and white creatures" (grouping magpies with zebras). These do not reflect deep biological commonalities.     * Natural systems: Grouping based on important shared features as determined by anatomy, physiology, and evolutionary history. These groupings change as new biological evidence is discovered.

• Species Definition: A species is a group of organisms that can reproduce to produce fertile offspring.     * Appearance: Members of a species often look very similar in anatomy and physiology, though variations exist.     * Human Intervention: Selective breeding can cause members of the same species to look different, such as different breeds of cats (e.g., American Longhair and Siamese), yet they can still breed successfully.

• Genus: Closely related species are grouped into a genus (plural: genera). For example, there are $45$ species of bronzeback snake within the genus Dendrelaphis.

Binomial Nomenclature

• The Binomial System: An internationally agreed-upon system where the scientific name of an organism consists of two parts.     * Generic Name: The first name, representing the genus, starting with a capital letter.     * Specific Name: The second name, representing the species, starting with a lowercase letter.     * Examples:         * Egyptian mongoose: Herpestes ichneumon         * Indian grey mongoose: Herpestes edwardsii

• Descriptive Latin Terms: Specific names often describe the organism:     * edulis: Edible.     * aquatilis: Living in water.     * bulbosus: Having a bulb.     * serratus: Having a jagged (serrated) edge.

• Avoiding Confusion: Local names vary (e.g., "Money Plant" and "Devil’s Ivy" for Epipremnum aureum), but the Latin binomial is universal.     * Case Study (The Robin):         * Indian Robin (Copsychus fulicatus): Male is mainly black with red-brown bottom feathers and a white shoulder flash. Length: approximately $17\,cm$.         * British Robin (Erithacus rubecula): Round body, bright orange-red breast, white belly, olive-brown upper feathers. Length: approximately $14\,cm$.

Dichotomous Keys

• Function: Keys are tools used to identify unfamiliar organisms by simplifying the identification process.

• Structure: Each key consists of pairs of contrasting features (dichotomous means "two branches"). They begin with general characteristics and move toward specific identifiers.

• Application: Identification can be based on major characteristics (like temperature regulation in vertebrates) or specific traits (like the presence of pincers or wing covers in compost heap invertebrates).

• Laboratory Example: Items of glassware can be identified using keys based on features like a pouring spout, broad base, sloping sides, or straight sides.

Classification and Evolutionary Relationships

• Common Ancestry: Classification reflects evolutionary links. If organisms share a recent common ancestor, they are classified more closely together.

• Homologous Structures: The pentadactyl limb in vertebrates (humans, lizards, birds, whales, bats) suggests a common ancestor. Despite different functions (flying, swimming, running), the bone arrangement is consistent:     * Top Bone: Humerus (with a ball and socket joint at the shoulder and hinge joint at the elbow).     * Lower Bones: Radius and ulna.     * Terminal Bones: Wrist bones followed by five groups of hand/finger bones (though some are reduced in birds).

• Misleading Appearances (Convergent Evolution):     * Pangolins: Covered in keratin scales, no teeth, long tongue to feed on ants, roll into a ball for protection.     * Armadillos: Armoured body with hard bony plates, long claws for digging, small enamel-less teeth to feed on grubs and insects, can roll into a ball.     * Classification: While they look similar, their anatomical differences (keratin vs. bone plates) mean they are not classified in the same group.

DNA Sequencing in Classification

• Chemical Basis: DNA (deoxyribonucleic acid) consists of sequences of bases that code for amino acids and proteins. Every species has a unique number of chromosomes and a unique DNA base sequence.

• Morphological Similarity: DNA is particularly useful for distinguishing between species that look nearly identical (morphologically and anatomically).

• Human and Primate Evolution: Traditional classification (family Pongidae for apes) was based on anatomy. Genetic evidence clarifies actual relationships:     * Humans are related to Chimpanzees with only a $1.2\%$ genome difference.     * Humans are related to Gorillas with a $1.6\%$ genome difference.     * Humans are related to Orangutans with a $3.1\%$ genome difference.     * Chimpanzees are more closely related to humans than to gorillas.     * Bonobos: Identified as a different species from chimps in $1929$; they share the same percentage difference from humans as chimps do.     * Chromosome Counts: Humans have $46$ chromosomes, while Orangutans, Gorillas, Chimpanzees, and Bonobos all have $48$.

Taxonomic Schemes

• Two-Kingdom Scheme (Linnaeus): Categorized all life into Animal and Plant.

• Five-Kingdom Scheme (Whittaker): Includes Animal, Plant, Fungus, Prokaryote, and Protoctist.

• Six-Kingdom/Three-Domain System (Woese): Based on ribosomal RNA structure.     * Archaea Domain: Ancient prokaryotes without nuclear membranes; unique biochemistry.     * Eubacteria Domain: Standard prokaryotic organisms.     * Eukarya Domain: All organisms with a membrane-bound nucleus (Protoctist, Fungus, Plant, Animal).

The Animal Kingdom

• Definition: Multicellular organisms with cells that lack cell walls and chloroplasts. Most ingest solid food and digest it internally.

• Invertebrates: Animals without a vertebral column (a convenient, though not natural, group).     * Arthropods: Characterized by jointed limbs and a hard external skeleton (cuticle) made of segments and flexible joints. Regions: Head, thorax, and abdomen.         * Crustacea: (e.g., crabs, lobsters, woodlice). Two pairs of antennae, compound eyes, jointed limbs on each segment (some modified into mouthparts).         * Insects: (e.g., butterflies, beetles). Three pairs of legs, wings (usually two pairs), one pair of antennae, compound eyes. Body segments: Head, thorax, abdomen. Cuticle prevents water loss.         * Arachnids: (e.g., spiders, mites, scorpions). Four pairs of legs, no antennae, several pairs of simple eyes. Body regions: Cephalothorax (combined head/thorax) and abdomen. Have pedipalps for reproduction or biting.         * Myriapods: (e.g., centipedes, millipedes). Head and segmented body (not divided into thorax/abdomen). One pair of antennae, simple eyes. Centipedes are carnivores; millipedes are herbivores. Millipedes appear to have two pairs of legs per segment due to fused abdominal segments.

• Vertebrates: Animals with a vertebral/spinal column made of vertebrae that protect the spinal cord. Includes a skull protecting the brain and jaws with teeth.     * Body Temperature:         * Variable (Cold-blooded): Body temperature depends on surroundings (Fish, Amphibians, Reptiles).         * Constant (Warm-blooded): Internal regulatory mechanisms maintain high body temperature regardless of environment (Birds, Mammals).     * Class Fish: Streamlined shape, overlapping scales, fins, gills (protected by operculum), external fertilisation via jelly-covered eggs.     * Class Amphibia: "Double life" (land/water). Moist skin for gas exchange, four limbs (webbed feet in some), carnivores, external fertilisation (e.g., frogs, toads, newts).     * Class Reptiles: Dry, scaly skin to reduce water loss, internal fertilisation, eggs with tough paper-like shells (e.g., lizards, snakes, turtles).     * Class Birds: Feathers (down, contour, and quill), scales on legs, wings, beak, internal fertilisation, hard-shelled eggs in nests (e.g., pigeons, sparrows).     * Class Mammals: Hair/fur, mammary glands for suckling young with milk, diaphragm for breathing, pinna (external ear flaps), various tooth types, give birth to live young (e.g., humans, gerbils, dogs).

The Plant Kingdom

• General Features: Multicellular organisms with cellulose cell walls and chloroplasts for photosynthesis.

• Ferns: Land plants with stems (often underground rhizomes), leaves (several cells thick, containing chloroplasts), and roots. They do not produce seeds; they reproduce via single-celled spores produced in sporangia on the underside of leaves.

• Flowering Plants: Reproduce by seeds formed in flowers and enclosed in an ovary. Often divided by cotyledon (embryonic leaf) count:     * Monocotyledons: One cotyledon, long narrow leaves, parallel veins, flower parts in threes (e.g., grasses, lilies).     * Dicotyledons: Two cotyledons, broad leaves with branching network veins, flower parts in fives (e.g., trees, shrubs, geraniums).

Other Kingdoms

• The Fungus Kingdom: Mostly composed of thread-like hyphae forming a mycelium. They possess cell walls (non-cellulose) and vacuoles but no chloroplasts. May be saprotrophs (decomposers) or parasites (living on a host).     * Examples: Mushrooms, toadstools, moulds, and yeasts (single-celled).

• The Prokaryote Kingdom: Single-celled bacteria and blue-green algae. Chromosomes are not organized into a nucleus.

• The Protoctist Kingdom: Single-celled organisms with a true nucleus (chromosomes in a nuclear membrane).     * Plant-like: Euglena and Chlamydomonas (possess chloroplasts, use flagellum).     * Animal-like: Amoeba (moves via cytoplasm flow) and Paramecium (uses cilia).

Viruses

• Biological Status: Not considered living organisms because they lack cell membranes, cytoplasm, ribosomes, and the characteristics of nutrition, respiration, excretion, and growth.

• Reproduction: Occurs only inside living host cells using the host's materials.

• Structure: Consists of a central core of nucleic acid (DNA or RNA) surrounded by a protein coat called a capsid (made of capsomeres). Some, like the influenza virus, have an outer membrane.

Questions & Discussion

• Question 1: In the mnemonic MRS GREN, what do the letters N, S, and M stand for?     * Response: Nutrition, Sensitivity, and Movement.

• Question 2: Explain the meaning of the term binomial system and give an example.     * Response: It is an internationally agreed system where an organism's scientific name has two parts: the genus and the species. Example: Copsychus fulicatus.

• Question 3: Identify invertebrates X and Y from a compost heap using Figure $1.4$.     * Response: (Based on the dichotomous key provided in the transcript).

• Question 4 (Challenge): Create your own key for laboratory glassware.     * Response: (Instructional prompt for students).

• Question 5: DNA questions/answers:     * a. Describe what a chromosome is made from: Chromosomes are made of DNA.     * b. State the name of the chemical that forms genes: DNA.     * c. What do bases code for?: Bases code for amino acids and proteins.     * d. i) How is DNA used for identification?: Each species has a unique sequence of bases. ii) When is this useful? When species are very similar morphologically.     * e. Relatedness of humans and chimps vs. gorillas: Humans share a lower percentage difference in genome ($1.2\%$) with chimpanzees than with gorillas ($1.6\%$).

• Question 11: Would Lamium album and Lamium purpureum cross-pollinate successfully?     * Response: They are different species in the same genus; generally, they cannot produce fertile offspring.

• Question 12: Why does bracken (fern) grow well after a fire?     * Response: The stem is underground and protected from the fire, allowing it to send up new leaves later.

The Seven Characteristics of Life

All living organisms, regardless of whether they are single-celled (unicellular) or multicellular, plants or animals, exhibit seven specific characteristics. These can be remembered using the mnemonic MRS GREN:

1. Movement
   • An action by an organism or part of an organism causing a change of position or place.
2. Respiration
   • Describes the chemical reactions in cells that break down nutrient molecules and release energy for metabolism.
3. Sensitivity
   • The ability to detect and respond to changes in the internal or external environment.
4. Growth
   • A permanent increase in size and dry mass.
5. Reproduction
   • The processes that make more of the same kind of organism.
6. Excretion
   • The removal of waste products of metabolism and substances in excess of requirements.
7. Nutrition
   • The taking in of materials for energy, growth, and development.

Diversity of Life

There are approximately $8.7$ million species of organisms on Earth.

Classification Systems and Species Concepts

Necessity of Classification

Biologists sort organisms into a meaningful order to organize knowledge and facilitate international agreement.

Artificial vs. Natural Systems

• Artificial systems: Grouping based on superficial similarities, such as "aquatic organisms" (grouping seaweed with porpoises) or "black and white creatures" (grouping magpies with zebras). These do not reflect deep biological commonalities.
• Natural systems: Grouping based on important shared features as determined by anatomy, physiology, and evolutionary history. These groupings change as new biological evidence is discovered.

Species Definition

A species is a group of organisms that can reproduce to produce fertile offspring.

• Appearance: Members of a species often look very similar in anatomy and physiology, though variations exist.
• Human Intervention: Selective breeding can cause members of the same species to look different, such as different breeds of cats (e.g., American Longhair and Siamese), yet they can still breed successfully.

Genus

Closely related species are grouped into a genus (plural: genera). For example, there are $45$ species of bronzeback snake within the genus Dendrelaphis.