Notes on Characteristics and Classification of Living Organisms

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Theme 1: Characteristics and Classification of Living Organisms

• An introductory overview of how living things are defined by shared characteristics and how they are categorized into hierarchical groups (taxonomy) based on evolutionary relationships.

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1.1 Characteristics of Living Organisms

• Living organisms are defined by a set of key characteristics that distinguish living from non-living matter.

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7 Characteristics

• Movement

• Respiration

• Sensitivity

• Growth

• Reproduction

• Excretion

• Nutrition

Note: There are $7$ characteristics that are commonly used to describe living organisms.

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Movement

• Movement is an action by an organism or part of an organism that changes position or place in a controlled way.

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Respiration

• The release of energy from food substances (glucose) in all living cells, 24/7.

• Chemical equation for cellular respiration (simplified):

$\text{C}<em>6\text{H}</em>{12}\text{O}<em>6 + 6\,\text{O}</em>2 \rightarrow 6\,\text{CO}<em>2 + 6\,\text{H}</em>2\text{O} + \text{ATP}$

• Produces ATP (adenosine triphosphate) which powers cellular processes.

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Sensitivity

• The ability to detect stimuli (changes) in the environment and respond to them.

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Growth

• The permanent increase in the size and number of cells of an organism.

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Reproduction

• The ability to produce new similar offspring of the same species.

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Excretion

• The removal of waste products of metabolism (produced by body cells) from the body.

• Also the removal of products which are in excess in the body.

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Nutrition

• Obtaining organic and/or inorganic nutrients (food) from surroundings for energy, growth, repair, and health.

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1.2 Taxonomy

• Taxonomy is the science of classification and the use of a hierarchical system to classify living organisms.

• It deals with identification, naming, and classification of organisms into groups or taxa.

• Taxa (plural) are the groups within a classification system.

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Taxonomy: Hierarchical Levels (major ranks)

• Kingdom

• Phylum

• Class

• Order

• Family

• Genus

• Species

Mnemonic example (one common version):

• Kings Play Chess On Fine Glass Sets

• Alternative mnemonic included in slides: "Kings, Play, Chess, On, Family, Game, Shows" (Abbreviated to help recall order)

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Taxonomy: Nomenclature Overview

• The binomial system uses two Latin names: genus and species.

• The two parts together form the scientific name of an organism.

• The first part designates the genus and begins with a capital letter; the second part designates the species and is in lowercase.

• Example: $\textit{Homo\ sapiens}$ (genus first, species second).

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The Five Kingdoms

• Prokaryotes (Bacteria)

• Protoctista (e.g., malaria organism)

• Fungi (e.g., mushrooms)

• Plantae (Plants)

• Animalia (Animals)

Note: These kingdoms reflect major differences in cellular organization and metabolism.

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Hierarchical Levels by Kingdoms (illustrative labels)

• Kingdom (Prokaryotae, Protoctista, Fungi, Plantae, Animalia)

• Phylum (Phyla)

• Class (Classes)

• Order (Orders)

• Family (Families)

• Genus (Genera)

• Species

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Classification of Humans (example)

• Kingdom: Animalia

• Phylum: Chordata

• Class: Mammalia

• Order: Primates

• Family: Hominidae

  • Note: Animals in this order have flat nails rather than claws/hooves and relatively large brain mass relative to body size.

• Genus: Homo

• Species: sapiens

• Scientific name: $\textit{Homo sapiens}$

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Basis of Classification

• Common ancestors imply relatedness and evolution.

• Similarities in features (visible traits) help group organisms.

• Classification reflects evolutionary relationships.

• Species: a group of organisms that can reproduce to produce fertile offspring.

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Reasons for Classification (6)

• It is convenient when trying to identify unknown organisms.

• It provides insight into the evolution of species.

• It makes it easier to study organisms.

• It helps keep track of all organisms.

• It sorts organisms into an orderly framework.

• It facilitates global/international communication about organisms.

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Binomial System (continued)

• Scientists use an internationally agreed binomial system (two-part name).

• The binomial name is usually in Latin.

• The genus name (first part) designates the genus and begins with a capital letter.

• The species name (second part) is in lowercase.

• Example: $\textit{Homo sapiens}$ (genus then species).

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Conventions of Binomial Nomenclature

• The binomial name is conventionally typeset in italics.

• When handwritten, the name should be underlined separately.

• Closely related organisms share the same genus; species differ.

• When a species is referred to repeatedly, the full name may be abbreviated after first use (e.g., $\textit{Homo sapiens}$ → $H.\ sapiens$).

• The binomial name is used globally by scientists.

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Examples of Binomial Names

• HUMAN = $\textit{Homo sapiens}$

• TIGER = $\textit{Panthera\ tigris}$

• HONEY BEE = $\textit{Apis\ mellifera}$

• GEMSBOk / Oryx = $\textit{Oryx\ gazella}$

• HOUSE CAT = $\textit{Felis\ catus}$

• AFRICAN ELEPHANT = $\textit{Loxodonta\ africana}$

• LION = $\textit{Panthera\ leo}$

• CHEETAH = $\textit{Acinonyx\ jubatus}$

• CAMEL THORN TREE = $\textit{Vachellia\ erioloba}$

• KUDU = $\textit{Tragelaphus\ strepsiceros}$

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Example: Tiny Classification

• T. rex = $\textit{Tyrannosaurus\ rex}$

• Genus: $\textit{Tyrannosaurus}$

• Species: $\textit{rex}$

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Use of Dichotomous Keys

• Based on pairs of descriptions.

• A series of questions about observable features.

• Each step presents two opposite descriptions; you choose the applicable one.

• The key directs you to the next pair of descriptions.

• Repeating steps lead to the identity of the unknown organism.

• Start with the organism you want to identify and read the first pair.

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Constructing Dichotomous Keys (Guidelines)

• Avoid using colour when constructing keys.

• Avoid size terms unless qualified and scaled for each organism.

• A key should have one fewer question than the number of organisms.

• Example: for six organisms, there should be five sets of questions.

• At the end, each final group should contain exactly one organism.

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Constructing Dichotomous Keys (More Guidelines)

• Use features that clearly divide specimens into two groups.

• Start the key with 1(a) and 1(b).

• Use visible and opposing differences.

• Features must be unambiguous and clearly recognizable (e.g., number of legs versus shape).

• Be precise; avoid subjective terms like many/few; use quantitative terms (e.g., more than ten, less than five).

• Do not group more than one feature at a time; focus on a single feature per decision when possible.

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Summary: Dichotomous Keys

• Based on pairs of descriptions (two questions per description).

• Move through the key by selecting the applicable description.

• Start with basic descriptions and progress to details.

• Always have one fewer question than organisms in the set.

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DNA Base Sequences in Classification

• DNA base sequences are used to assess evolutionary relationships.

• More closely related groups share more similar base sequences due to a more recent common ancestor.

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DNA Base Sequences (Practice Prompt)

• Example: If there are $10$ differences between species A and B, how many differences exist between species G and H? (Answer depends on provided data.)

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DNA-Based Identification

• Each species has a unique DNA/genes sequence.

• DNA obtained from an unknown sample is compared with DNA from known species.

• A match indicates a close relationship with few differences in base sequences.

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1.3 Features of Living Organisms (Core & Supplement)

Core (Key objectives)

• State the main features used to place animals and plants into the appropriate kingdoms.

• State the main features used to place organisms into groups within the animal kingdom, limited to:

  • (a) main vertebrate groups: mammals, birds, reptiles, amphibians, fish

  • (b) main arthropod groups: myriapods, insects, arachnids, crustaceans

• Classify organisms using the features identified in 1.3.1 and 1.3.2

Supplement

• State the main features used to place all organisms into one of the five kingdoms: animal, plant, fungus, prokaryote, protoctist.

• State the main features used to place organisms into groups within the plant kingdom, limited to ferns and flowering plants (dicotyledons and monocotyledons).

• Classify organisms using the features identified in 1.3.4 and 1.3.5.

• State the features of viruses, limited to a protein coat and genetic material.

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5 Kingdoms Overview (Diagrammatic Labels)

• Fungi, Prokaryote, Protoctista, Plantae, Angiophytes / Flowering

• Ferns, Dicotyledonae, Monocotyledonae

• Vertebrates / Invertebrates

• Animalia, Fish, Amphibia, Reptilia, Aves, Mammalia, Arthropoda, Insecta, Crustacea, Myriapoda, Arachnida

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KINGDOM → ANIMALIA

• Characteristics:

  • Multicellular (contains many cells)

  • Cells have a nucleus

  • Cells do not have cell walls

  • Cells do not have chloroplasts

  • Feeds on organic substances made by other living organisms

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CLASSIFICATION OF ANIMALS

• Animalia → Vertebrates / Invertebrates (non-chordata)

• Within Arthropoda: Insecta, Crustacea, Myriapoda, Arachnida

• Within Vertebrates: Osteichthyes (bony fish), Amphibia, Reptilia, Aves (birds), Mammalia, Chondrichthyes (cartilaginous fish)

• Diagrammatic ranks: Kingdom → Phylum → Class → etc.

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VERTEBRATES

• Characteristics:

  • Have backbones (vertebral column)

  • Distinct, well-differentiated head

  • Internal skeleton made of bone

• Most familiar classes:

  • Fish

  • Amphibians (Amphibia)

  • Reptiles (Reptilia)

  • Birds (Aves)

  • Mammals (Mammalia)

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CLASS → FISH

• All live in water (salt or fresh); some can come onto land briefly (e.g., mudskippers)

• Characteristics:

  • Scaly skin

  • Gills throughout life

  • Covered by operculum for bony fish; no operculum for cartilaginous fish

  • Fins with bony rays

  • Eggs have no shells and are laid in water

  • Streamlined bodies

  • Lateral line system

  • Ectothermic

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Fish Anatomy (Terminology Reference)

• Operculum, Dorsal Fin, Caudal Fin, Pectoral Fin, Anal Fin, Pelvic Fin, Lateral Line

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CLASS → AMPHIBIANS

• Most adults live on land; must return to water to breed

• Larvae are tadpoles; undergo metamorphosis

• Frogs, toads, salamanders

• Characteristics:

  • Smooth, moist skin with no scales

  • Eggs have no shells and are laid in water

  • Tadpoles live in water and have gills

  • Adults live on land and have lungs

  • Ectothermic

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CLASS → REPTILES

• Dry skin with scales; do not return to water to breed

• Eggs have soft, waterproof shells (prevents drying out)

• Examples: crocodiles, lizards, snakes, turtles, tortoises

• Characteristics:

  • Tough, dry, scaly skin

  • Eggs with leathery shells

  • Ectothermic

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CLASS → BIRDS

• Evolved from dinosaurs; only animals with feathers

• Characteristics:

  • Body covered in feathers

  • Feet covered in scales

  • Have a toothless beak

  • Front limbs modified into wings

  • Not all birds can fly

  • Eggs with hard shells

  • Heart has 4 chambers

  • Endothermic

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CLASS → MAMMALS

• Give birth to live young (also some fish/reptiles do this)

• Distinctive features across mammal reproduction and care:

  • Body covered by hair

  • Young develop in uterus, attached to mother by placenta

  • Females have mammary glands and produce milk

  • Differentiated dentition: incisors, canines, premolars, molars

  • Pinna (external ear flap)

  • Sweat glands

  • Diaphragm

  • Endothermic

  • Heart with 4 chambers

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Classification of Animals (Recap)

• Animalia → Vertebrates / Invertebrates (non-chordata)

• Within Vertebrates: Fish, Amphibia, Reptilia, Aves, Mammalia

• Classification framework summarized via Kingdom, Phylum, Class, etc.

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INVERTEBRATES

• Characteristics:

  • No backbone

  • External skeleton (exoskeleton)

• Largest group in the animal kingdom

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PHYLUM – ARTHROPODA

• Characteristics:

  • Jointed legs/limbs

  • Segmented bodies

  • Waterproof exoskeleton (chitin) outside of body

  • Supports body and prevents drying out when living on land

• Note: More arthropods exist than all other animal groups combined

• Examples: Insects, crustaceans, spiders, scorpions, centipedes

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CLASS → INSECTS

• Very successful due to exoskeleton and tracheae (air tubes that convey air directly to tissues)

• Adapted for life in dry places; mainly terrestrial

• Characteristics:

  • Three pairs of jointed legs

  • Two pairs of wings (may be vestigial)

  • Body divided into three segments: head, thorax, abdomen

  • One pair of antennae

  • One pair of compound eyes

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Class → Insects ( continued )

• Tracheae for respiration; body covered with an exoskeleton that reduces water loss

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CLASS → CRUSTACEANS

• Breath through gills; mostly aquatic or wet habitats

• Examples: crabs, lobsters, woodlice

• Characteristics:

  • 5 or more pairs of jointed legs

  • Two pairs of antennae

  • Exoskeleton

  • Body divided into cephalothorax and abdomen

  • Compound eyes on stalks

  • Carapace

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CLASS → ARACHNIDS

• Generally terrestrial

• Examples: spiders, ticks, scorpions

• Characteristics:

  • Four pairs of jointed legs

  • No antennae

  • Body divided into cephalothorax and abdomen

  • Some have chelicerae

  • Leathery exoskeleton

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CLASS → MYRIAPODS

• Terrestrial

• Examples: centipedes and millipedes

• Characteristics:

  • Body consists of many similar segments (metameric)

  • Each segment has jointed legs

  • Centipedes: 1 pair of legs per segment

  • Millipedes: 2 pairs of legs per segment

  • One pair of antennae

  • Body divided into head and abdomen

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CLASS → MYRIAPODS (Centipedes)

• Characteristics:

  • Flattened (dorsi-ventrally) bodies

  • 18+ segments

  • Each segment has one pair of legs

  • One pair of long antennae

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CLASS → MYRIAPODS (Millipedes)

• Characteristics:

  • Cylindrical, round bodies

  • Segmented into many parts

  • Each segment bears two pairs of legs (four legs per segment)

  • One pair of short antennae

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CLASSIFICATION OF FLOWERING PLANTS

• Phylum: Angiophyta (flowering plants)

• Kingdom: Plantae

• Classes: Monocotyledonae, Dicotyledonae

• Ferns: Ferns (Pteridophyta) also included in plant kingdom.

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KINGDOM → PLANTAE

• All plants share green pigment (chlorophyll) and photosynthesis ability.

• They absorb energy from sunlight and use it to make sugars (photosynthesis).

• Most familiar group is flowering plants; plants have leaves, stems, roots, and sometimes flowers.

• Ferns and mosses do not have flowers.

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KINGDOM → PLANTAE (More Characteristics)

• Plants are generally stationary, adapted to remain in one place to capture sunlight.

• Under microscope, plant cells have cell walls made of cellulose and may contain chloroplasts and a nucleus.

• Characteristics:

  • Cells have a nucleus

  • Cell walls made of cellulose

  • May contain chloroplasts

  • Feed by photosynthesis

  • May have roots, stems, leaves (some plants lack leaves)

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PHYLUM → FERNS

• Leaves are called fronds

• Do not produce flowers

• Reproduce by spores located on underside of fronds

• Some ferns can reach considerable height (up to about 20 m)

• Characteristics:

  • Roots, stems, and leaves (fronds)

  • No flowers

  • Reproduce by spores

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Fern Anatomy (Illustrative)

• frond, sporangia with spores, rhizome (underground stem), roots

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PHYLUM → FLOWERING PLANTS

• Characteristics:

  • Plants with roots, stems, and leaves

  • Reproduce via flowers and seeds

  • Seeds produced inside an ovary in flowers

  • Divided into two main groups based on seed structure:

  • Dicotyledons (two cotyledons)

  • Monocotyledons (one cotyledon)

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CLASS → MONOCOTS

• Characteristics:

  • Seeds with one cotyledon

  • Roots grow from stem (adventitious root system)

  • Leaves with parallel veins

  • Flower parts in multiples of three

  • Vascular bundles in stem are scattered (not in a distinct ring)

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MONOCOTS: Visuals (Summary)

• Seeds: one cotyledon

• Leaves: long, narrow, strap-shaped

• Veins: parallel

• Flowers: parts in multiples of three

• Roots: adventitious, fibrous

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CLASS → DICOTS

• Characteristics:

  • Seeds with two cotyledons

  • Main root with side roots (tap root system)

  • Leaves with branched network of veins

  • Flower parts in multiples of four or five

  • Vascular bundles in stem arranged in a ring

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DICOTS: Visuals (Summary)

• Seeds: two cotyledons

• Leaves: broad leaves with branched veins

• Veins: branched network

• Flowers: parts in multiples of four or five

• Roots: tap root system

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KINGDOM → FUNGI

• Historically classified with plants, but now treated separately.

• Main body of most fungi is made of microscopic threads called hyphae; a network is called mycelium.

• Do not photosynthesize.

• Common examples: mushrooms and toadstools; yeasts are unicellular.

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FUNGI (Structural Features)

• sporangium (spore-containing structure)

• spores

• aerial hypha

• cap and gills (with spores)

• mycelium

• feeding hypha

• bread as an illustrative growth medium

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KINGDOM → FUNGI (Ecological Roles)

• Feed saprophytically or parasitically on organic material (e.g., faeces, spoiled foods, dead plants/animals).

• Many are decomposers; return nutrients to soil.

• Reproduce by forming spores.

• Tiny groups of cells with protective outer covering.

• Spread by wind or animals and grow to form new fungi.

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FUNGI: Practical Notes

• Different fungi include edible mushrooms; yeast is used to make ethanol and bread.

• Some fungi produce antibiotics (e.g., penicillin).

• Some are harmful, causing food decay or diseases (e.g., ringworm, athlete’s foot).

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KINGDOM → FUNGI (Characteristics)

• Usually multicellular (yeast is unicellular)

• Have nuclei (eukaryotic)

• Cell walls made of chitin (not cellulose)

• Do not have chlorophyll or chloroplasts

• Saprotrophic or parasitic; feed by digesting organic material and absorbing it

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KINGDOM → PROTOCTISTA

• Mixed collection of organisms; all have cells with a nucleus.

• Some have plant-like cells (chloroplasts and cellulose walls); others have animal-like cells (without these features).

• Most are unicellular; some are multicellular (e.g., certain seaweeds).

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Protoctista (Concrete Example)

• A pond water sample showing diverse microscopic organisms.

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KINGDOM → PROTOCTISTA (Characteristics)

• Multicellular or unicellular

• Have a nucleus

• May or may not have cell walls or chloroplasts

• Some feed via photosynthesis (if chloroplasts present); others feed on organic substances produced by other organisms

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KINGDOM → PROKARYOTE

• Contains a huge number of organisms; bacteria belong to this kingdom.

• Cells lack a true nucleus and other membrane-bound organelles.

• Prokaryotes are extremely diverse.

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PROKARYOTES (Imaging Context)

• Electron microscopy reveals bacteria; example image shows bacteria in the digestive tract.

• Magnification around x10,000 (illustrative).

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KINGDOM → PROKARYOTES (Characteristics)

• Usually unicellular.

• No nucleus.

• Cell walls not made of cellulose but peptidoglycan (murein).

• No mitochondria.

• Circular DNA (plasmids) free in cytoplasm.

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PROKARYOTES (Genetic/Cellular Features)

• Circular DNA; often have plasmids.

• Distinctive features relate to their cellular structure rather than complex organelles.

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VIRUSES

• Not normally considered living organisms.

• Cause diseases (e.g., influenza, HIV/AIDS).

• Cannot move, feed, excrete, respond to stimuli, grow, or reproduce on their own.

• They require a living host cell to reproduce.

• Outside a host, viruses exist as inert particles.

• They do not have cells; they consist of genetic material (RNA or DNA) surrounded by a protein coat (capsid).

• They can only reproduce by infecting a host cell and hijacking its machinery.

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VIRUSES (Structure & Behavior)

• Basic scale: about 10 nanometres in many diagrams.

• Can line up many thousands of viruses in a tiny space.

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SUMMARY

• All organisms show seven characteristics: $Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition$.

• Organisms are classified into groups based on shared features; classification reflects evolutionary relationships.

• The binomial naming system shows genus and species, with the genus capitalized and species lowercase, typically written in italics: $\textit{Genus\ species}$.

• Dichotomous keys identify organisms using paired descriptions; start with the most basic features and progress to details, ensuring decisions can be made based on visible traits.

• Plants: kingdom features include cellulose-containing cell walls and chloroplasts in some cells; animals lack these plant-specific features.

• Vertebrates are classified into five main groups: fish, amphibians, reptiles, birds, and mammals, each with distinctive features.

• Arthropods are the largest animal phylum and are characterized by an exoskeleton and jointed legs; major subgroups include insects, arachnids, crustaceans, and myriapods.

• The five kingdoms extend beyond animals/plants to include fungi, protoctista, and prokaryotes; the fungal and protoctist kingdoms differ markedly in cellular structure and metabolism.

• Ferns and flowering plants represent two major plant groups, with monocotyledons and dicotyledons differentiated by seed structure, leaf venation, root systems, and vascular bundle arrangement.

• Viruses are not considered living organisms because they lack cellular structure and independent metabolic processes; they rely on host cells for replication.