Comprehensive Study Notes on Living Organisms and Classification

Characteristics of Living Organisms

  • Growth: Living organisms grow by increasing in size and mass.
    • Single-celled organisms: Growth is achieved when the cell increases in volume and size.
    • Multicellular organisms: Growth involves an increase in both cell number and body size.
  • Excretion: Organisms remove harmful wastes from processes like respiration or digestion to avoid toxic buildup.
  • Nutrition: Organisms take in and use substances, including minerals, organic molecules, and water, for energy and raw materials needed for growth and development.
  • Sensitivity: Living organisms respond to changes in their internal and external environments; these changes are called stimuli. For example, sweating in response to hot weather.
  • Movement: Living organisms generally move by changing their position or location. Plants move parts of their bodies (e.g., phototropism).
  • Respiration: Cells of organisms carry out chemical reactions that break down nutrients and release energy.
  • Reproduction: Most organisms can produce new individuals similar to themselves, either sexually (two parents, two gametes) or asexually (one parent).

Classification Systems

  • Classification involves arranging living organisms into groups based on shared characteristics.
  • Ancient systems:
    • Aristotle’s system
    • Carolus Linnaeus’ system

Aristotle’s System

  • Developed over 2000 years ago by a Greek philosopher.
  • Classified organisms by physical characteristics or habitat.
  • Divided living things into plant and animal groups.
  • Animals were grouped by the presence or absence of red blood and subdivided by movement.
  • Plants were grouped according to size.

Requirements of a Good Classification System

  • Make it easier for scientists to understand and study large numbers of organisms.
  • Allow them to compare organisms easily and logically.
  • Help determine evolutionary relationships among organisms.
  • Aristotle’s system was useful but did not classify organisms according to their evolutionary history.

Carolus Linnaeus' System

  • Developed in the late 1700s by a Swedish scientist.
  • Based on physical characteristics and structural similarities of organisms.
  • Divided plants according to the number of male and female parts in their flowers.
  • This system reflects more of the evolutionary history of organisms.

Modern Systems of Classification

  • Based on anatomical and morphological characteristics:
    • Body shape
    • Structure of limbs
    • Study of DNA and protein sequences
  • DNA and protein sequencing has revolutionized classification.
  • Scientists compare the sequence of bases in the same gene from the DNA of different organisms.
  • Protein (amino acid) sequences can be used to determine relatedness between organisms.
  • Example: Red pandas are more closely related to raccoons than to giant pandas based on DNA and protein sequence analysis.

Common Ancestor

  • A common ancestor is the ancestor that several organisms have descended from.
  • Analysis shows red pandas and raccoons share more sequences with each other than either shares with giant pandas.
  • Organisms that share a more recent ancestor have more similar DNA base sequences and amino acid sequences than those that share only a distant ancestor.

Modern Classification Systems

  • Use systems with several levels of groupings, each level is referred to as a taxon.
  • Two widely used classification systems:
    • The Whittaker five-kingdom scheme (1969)
    • The three-domain scheme by Carl Woese (1990)
  • The three-domain system has an additional level called the domain, which is more inclusive than the kingdom.

Hierarchy of Classification

  • Taxonomy is the science of classifying organisms into categories according to a hierarchical system.
  • The Whittaker five-kingdom system consists of a hierarchy of seven taxa:
    • Kingdom (highest taxon, largest number of organisms)
    • Species (lowest taxon, smallest number of organisms)

Taxon: Species

  • A species is the lowest level of classification.
  • It consists of a group of organisms that can breed with one another and produce fertile offspring.
  • Members of a species typically resemble each other closely in appearance.

Levels of Classification

  • Species: Lowest level.
  • Genus: A group of closely related species that share certain traits.
  • Family: Genera with common features are grouped into a family.
  • Order: A group of related families that share common traits.
  • Class: A group of similar orders with common characteristics.
  • Phylum: A group of related classes with common features.
  • Kingdom: Consists of multiple phyla that share common features.

The Five Kingdoms

  • Kingdom Animalia: Multicellular, eukaryotic, heterotrophic, cells lack a cell wall.
  • Kingdom Plantae: Multicellular, eukaryotic, autotrophic (photosynthesis), cells have a cell wall made of cellulose.
  • Kingdom Fungi: Heterotrophic, eukaryotic, reproduce using spores, cells have a cell wall made of chitin, may be unicellular or multicellular.
  • Kingdom Prokaryotae: Unicellular, prokaryotic, may be autotrophic or heterotrophic.
  • Kingdom Protoctista: Eukaryotic, cannot be classified as animals, plants, nor fungi; may be multicellular or unicellular; may be autotrophic or heterotrophic.
  • Viruses are not included in any kingdom because they are not independent living organisms; they reproduce within host cells.

Binomial System of Nomenclature

  • The scientific name = the binomial nomenclature, meaning “two-term naming”.
  • The first term identifies the genus, and the second term identifies the species.
  • Example: Helix pomatia (Roman Snail)
  • The genus name is capitalized, and the species name starts with a lowercase letter. If handwritten, both names should be underlined; if printed, they are written in italics.
  • The genus may be abbreviated (e.g., P. domesticus).
  • The specific name is usually descriptive (e.g., hirsutum = hairy, aquatilis = living in water, bulbosus = having a bulb).

Kingdom Prokaryotae (Bacteria)

  • Unicellular organisms.
  • They may have additional structures like a capsule surrounding the cell wall or flagella for movement.
  • They also have ribosomes used to produce proteins.

Kingdom Protoctista

  • Includes unicellular and multicellular organisms.
  • Most are unicellular.
  • All are eukaryotes.
  • Cells have nuclei and membrane-enclosed organelles.
  • Cell wall may or may not be present.
  • Some have chloroplasts and can produce their own food by photosynthesis.
  • Others feed by engulfing food particles and digesting them.

Kingdom Fungi

  • Includes multicellular and unicellular organisms.
  • Most are multicellular.
  • Mushrooms and molds are multicellular fungi, whereas yeast is unicellular.
  • Fungi have a nucleus.
  • Possess a cell wall made of chitin.
  • Do not have chloroplasts and cannot perform photosynthesis.
  • Fungi can feed by:
    • Saprophytic nutrition: breaking down large organic molecules and absorbing them.
    • Parasitic nutrition: obtaining nutrients directly from other organisms, causing harm to the host.

Multicellular Fungi

  • Have interwoven fine threadlike structures called hyphae, which are collectively known as mycelium.

Kingdom Animalia

  • Multicellular organisms.
  • Lack cell walls or chloroplasts.
  • Heterotrophic: Cannot make their own food.
  • Ingest solid food particles and digest them internally.
  • Most animals are motile.
  • Includes multicellular organisms that may or may not have a vertebral column.

Kingdom Animalia: Invertebrates

  • Do not have a vertebral column or backbone.
  • Include:
    • Arthropods (Phylum Arthropoda)
    • Nematodes (Phylum Nematoda)
    • Annelids (Phylum Annelida)
    • Mollusks (Phylum Mollusca)

Arthropods (Phylum Arthropoda)

  • Invertebrates with:
    • A hard outer covering called an exoskeleton: Supports and protects the body.
    • Segmented body: the division of the body into identical parts or segments allows the animal to move easily.
    • Jointed legs.

Main Groups of Arthropods

  • Crustaceans (Class Crustacea): Mainly aquatic, breathe through gills, have a cephalothorax, two or more pairs of antennae, and five pairs of legs.
  • Insects (Class Insecta): Have three pairs of jointed legs and sometimes wings; bodies divided into head, thorax, and abdomen.
  • Arachnids (Class Arachnida): Have four pairs of appendages, no antennae, and a cephalothorax; include spiders, scorpions, ticks, and mites.
  • Myriapods (Class Chilopoda, Class Diplopoda): Have segmented bodies with one or two pairs of legs attached to each segment; include centipedes and millipedes.

Nematodes (Phylum Nematoda)

  • Worms that are non-segmented and cylindrical (roundworms).
  • Have a slender body that is tapered at both ends.
  • Example: parasitic roundworm (Ascaris lumricoides), which can infect humans.

Annelids (Phylum Annelida)

  • Worms with long, cylindrical bodies divided into ring-like segments.
  • Have soft, moist skin.
  • Live in water or moist soil.
  • Examples: earthworms, leeches, and tube worms.

Mollusks (Phylum Mollusca)

  • Have soft, non-segmented bodies.
  • The soft body is protected by:
    • One shell (e.g., snails)
    • Two shells (e.g., mussels and oysters)
  • Examples without an external shell: slugs, squid, and octopus.
  • Many mollusks are aquatic animals with gills; some live on the seashore and on land.

Kingdom Animalia: Vertebrates

  • Animals that have a vertebral column.
  • Belong to the phylum Chordata.
  • Include:
    • Amphibians
    • Reptiles
    • Birds
    • Mammals
    • Fish

Amphibians (Class Amphibia)

  • Vertebrates with moist skin lacking scales.
  • Adults live on land and breathe with lungs; young live in water and breathe with gills.
  • Ectotherms (cold-blooded organisms).
  • Adults have four legs.
  • Must return to water to lay eggs.
  • Examples: frogs, toads, and salamanders.

Reptiles (Class Reptilia)

  • Vertebrates with dry, scaly, and waterproof skin.
  • Most live on land and breathe with lungs.
  • Ectotherms.
  • Typically have four legs, but some have no legs.
  • Lay eggs on land, near, or in water.
  • Examples: snakes, lizards, crocodiles, turtles, and tortoises.

Birds (Class Aves)

  • Vertebrates that have feathers, a beak, two scaly legs, a pair of wings used for flying.
  • Endotherms (maintain a constant internal body temperature).
  • Have a complex respiratory system that also includes lungs.
  • Eggs have a hard shell.
  • Examples: vultures, penguins, and ostriches.

Mammals (Class Mammalia)

  • Vertebrates that have hair or fur, four limbs, and mammary glands.
  • Breathe with lungs.
  • Move by walking, flying, or swimming.
  • Some live on land, and others live in water.
  • Most embryos develop inside their mothers, and after birth, they feed on milk from mammary glands.
  • Examples: deer, bats, and whales.

Fish (Class Agnatha, Class Chondrichthyes, and Class Osteichthyes)

  • Aquatic organisms that breathe with gills, have scaly skin and fins for swimming.
  • Lay eggs in water.
    • Class Agnatha:
      • Lampreys and hagfish.
    • Class Chondrichthyes:
      • Sharks and rays.
    • Class Osteichthyes:
      • Salmon and trout.

Kingdom Plantae

  • Multicellular organisms able to synthesize their own food through photosynthesis.
  • All plant cells have a nucleus and cell walls made of cellulose.
  • Classification uses the same taxa as animals, but botanical classification may use the term division instead of phylum.

Non-Vascular Plants

  • Small in size.
  • Lack vascular tissues, flowers, or seeds.
  • Example: mosses.
  • Live in wet environments.
  • Have simple structures resembling roots and leaves but lack vascular tissues.
  • Reproduce by producing spores.

Vascular Plants: Ferns

  • Seedless vascular plants.
  • The root system anchors the plant in soil.
  • Roots draw water and nutrients from the soil into the plant’s vascular system.
  • The stem supports the growth of leaves, which make food through photosynthesis.
  • Do not produce flowers.
  • Reproduce with spores that form on the underside of their leaves.
  • Leaves are called fronds, and stems are usually called rhizomes.

Vascular Plants: Flowering Plants

  • Monocot:
    • Leaves with parallel vein patterns.
    • Fibrous root system (many equal-sized roots).
    • Examples: Grasses, corn, tulips, daffodils, and palm trees.
  • Dicot:
    • Leaves with branching vein patterns.
    • Tap root system (lateral roots grow from a main root).
    • Examples: Oak, ash, fruit trees, lupins, wallflowers, roses, sunflowers, potatoes, tomatoes, lettuce, and peas.

Dichotomous Key

  • A tool used to identify organisms.
  • Consists of a series of paired statements; each statement is either the opposite of or completely different from the other.
  • Dichotomous means two branches; each step has two possibilities with opposing characteristics.
  • Selecting one statement leads to another pair of statements until the organism is identified.
  • During development, it is best to choose characteristics that differ greatly among the studied organisms.