Comprehensive Biology Notes: Characteristics and Classification of Living Organisms

Characteristics of Living Organisms

The fundamental characteristics that define all living beings can be summarized using the mnemonic MRS GREN. Every organism must exhibit these seven processes to be classified as living.

• Movement: An action performed by an organism or a specific part of an organism that results in a change of position or place. While most animals can move their entire bodies from one location to another, plants generally move parts of themselves more slowly.

• Respiration: The set of chemical reactions occurring within cells to break down nutrient molecules and release energy for metabolism. This process is essential for all living organisms to obtain energy from food. Most organisms require oxygen for aerobic respiration, though some can respire anaerobically (without oxygen).

• Sensitivity: The ability to detect or sense stimuli (changes) in the internal or external environment and to execute appropriate responses to those stimuli.

• Growth: A permanent increase in size and dry mass. This occurs through an increase in cell number, cell size, or both.

• Reproduction: The biological processes responsible for producing more individuals of the same kind of organism. This involves complex mechanisms such as meiosis (occurring in the flower in plants), pollination, and fertilization (where the nucleus of the egg cell and sperm/pollen fuse to form an embryo).

• Excretion: The removal from organisms of the waste products of metabolism (chemical reactions in cells including respiration), as well as substances in excess of requirements.

• Nutrition: The taking in of materials for energy, growth, and development. These materials are utilized in chemical reactions to produce energy for repair and growth. Animals require organic compounds, ions, and water. Conversely, plants utilize light, carbon dioxide, water, and ions for photosynthesis.

Chemical Equation for Respiration

The chemical process for aerobic respiration is represented by the following equation:

$\text{Glucose} + \text{Oxygen} \rightarrow \text{Carbon dioxide} + \text{Water}$

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$

Comparison of Excretion and Egestion

It is critical to distinguish between these two processes:

• Excretion: Specifically refers to the removal of metabolic waste products (like $CO_2$ from respiration or urea) and substances in excess.

• Egestion: The passing out of undigested food as faeces through the anus.

The Science of Classification

Classification is the process of grouping different species together based on shared features.

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

• Evolutionary Relationships: Organisms are classified based on their evolutionary history. These relationships are determined by studying physical (morphological) characteristics and DNA base sequences.

• DNA Sequences: Classification systems aim to reflect evolutionary relationships. Groups of organisms sharing a more recent common ancestor are more closely related and possess DNA base sequences that are more similar than those who share a distant ancestor.

Taxonomic Hierarchy and Nomenclature

The biological classification system follows a specific sequence from the largest, most inclusive group to the smallest, most specific unit.

1. Kingdom: The largest unit (e.g., Animalia).

2. Phylum: (e.g., Chordata).

3. Class: (e.g., Mammalia).

4. Order: (e.g., Primates).

5. Family: (e.g., Hominidae).

6. Genus: A group of various species (e.g., Homo).

7. Species: The smallest unit of classification (e.g., sapiens).

Binomial Nomenclature: This is an internationally agreed system of naming species. The scientific name consists of two parts:

1. The Genus (always capitalized).

2. The Species (always lowercase).

Example: Humans are scientifically named Homo sapiens. This system was championed by Carolus Linnaeus (as seen in his work Systema Naturae). Organizations like the ICBN (International Code of Botanical Nomenclature) oversee these naming standards.

The Five Kingdom System

Proposed by R.H. Whittaker, all living beings are grouped into five kingdoms:

1. Monera (Prokaryotes): Primitive, unicellular organisms without a membrane-bound nucleus (e.g., bacteria).

2. Protista (Protoctista): Simple eukaryotic organisms, mostly unicellular (e.g., Amoeba).

3. Fungi (Mycota): Eukaryotic organisms that are typically saprophytic and non-motile.

4. Plantae: Multicellular phototropic organisms with cell walls made of cellulose.

5. Animalia: Multicellular heterotrophic organisms without cell walls.

Kingdom Monera

Monerans are prokaryotic organisms characterized by:

• Cell Structure: Unicellular; lacks a membrane-bound nucleus; DNA is double-stranded and suspended in the cytoplasm as a nucleoid. They lack membrane-bound organelles like mitochondria.

• Cell Wall: A rigid wall made of peptidoglycan is present.

• Survival: Can survive in extreme conditions (hot springs, acidic soils).

• Nutrition: Mostly heterotrophic.

• Circulation/Movement: Circulation via diffusion; movement aided by flagella.

• Reproduction: Mostly asexual via binary fission.

• Anatomy: Includes capsule, cell wall, membrane, cytoplasm, flagellum, pili, DNA, plasmid, and ribosomes.

Kingdom Protista (Protoctista)

Simple eukaryotic organisms that can be unicellular, colonial, or multicellular (like algae).

• Organelles: Possess mitochondria for respiration and some possess chloroplasts for photosynthesis.

• Movement: Often facilitated by flagella or cilia.

• Nutrition: Can be autotrophic or heterotrophic. Some use endocytosis to form food vacuoles by engulfing organisms.

• Reproduction: Primarily binary fission.

• Examples and Structures:

  • Paramecium: Features include cilia, pellicle, macronucleus, micronucleus, contractile vacuole with radiating canals, oral groove (vestibulum), cytostome (cell mouth), and cytoproct (anal pore).

  • Euglena: Features include flagellum, stigma, reservoir, chloroplast, nucleus, nucleolus, and pellicle.

  • Amoeba: Features include pseudopodia (false feet), ectoplasm, plasmasol, plasmagel, contractile vacuole, and food vacuoles.

Kingdom Fungi (Mycota)

• Cellular Nature: Eukaryotic, non-vascular, and typically non-motile.

• Structure: Can be unicellular or composed of microscopic threads called hyphae.

• Cell Wall: Composed of chitin.

• Nutrition: Saprophytic; they use enzymes to digest food externally before ingestion. They store food reserves as glycogen.

• Reproduction: Produce sexual or asexual spores.

Kingdom Plantae

The plant kingdom is divided into Cryptogams (non-seed bearing) and Phanerogams (seed-bearing).

Cryptogams

• Bryophyta (e.g., Mosses, Liverworts): Small plants growing close to the ground; body not well-defined into true roots/stems; lack vascular tissue; reproduce via spores.

• Pteridophyta (e.g., Ferns, Horsetails): Well-differentiated body (roots, stems, leaves); possess vascular tissue; reproduce via spores.

Phanerogams

• Gymnosperms (e.g., Pine, Cycas): Vascular plants producing "naked" seeds (lacking an outer fruit), usually in cones. They have male cones (producing pollen) and female cones (producing ovules).

• Angiosperms: Flowering plants that produce seeds enclosed in fruit. They are categorized into two groups:

Comparison of Monocotyledons and Dicotyledons:

Kingdom Animalia: Invertebrates

Animals without a backbone are classified into several phyla:

• Porifera (Sponges): Asymmetrical; pores throughout the body; no mouth or anus.

• Cnidaria (Jellyfish, Anemones): Radial symmetry; mouth but no anus; tentacles with stinging cells.

• Platyhelminthes (Flatworms): Bilateral symmetry; flat, soft body; mouth but no anus.

• Nematoda (Roundworms): Bilateral symmetry; cylinder-shaped body; mouth but no anus. Often parasites (e.g., pinworms).

• Annelida (Segmented Worms): Bilateral symmetry; segmented body; possess both mouth and anus (e.g., earthworms, leeches).

• Mollusca (Squids, Snails): Bilateral symmetry; muscular foot and mantle; often have a shell.

• Echinodermata (Starfish): Penta-radial symmetry; radiating arms; tube feet with suction pads.

• Arthropoda: Bilateral symmetry; jointed appendages; exoskeleton.

Phylum Arthropoda: Detailed Classification

Kingdom Animalia: Vertebrates (Phylum Chordata)

Vertebrates possess a backbone and are divided into five main classes:

• Pisces (Fish): Aquatic; scales; cold-blooded; 2-chambered heart; breathe via gills; use fins for locomotion; oviparous (lay eggs).

• Amphibia: Land and water; moist, naked skin with mucus glands; cold-blooded; 3-chambered heart; gills (larvae) and lungs/skin (adults); return to water to reproduce; oviparous.

• Reptilia: Terrestrial; waterproof skin with scales; cold-blooded; 3-chambered heart (except crocodiles which have 4); internal fertilization; produce amniote eggs with leathery shells.

• Aves (Birds): Terrestrial/Aerial; feathers; warm-blooded; 4-chambered heart; hollow bones and wings for flight; beaks with no teeth; produce amniote eggs.

• Mammalia: Terrestrial/Aquatic; hair/fur; warm-blooded; 4-chambered heart; mammary glands produce milk; heterodont teeth (incisors, canines, premolars, molars); external pinna and diaphragm; viviparous (give birth to live young).

Viruses

Viruses are not classified as living organisms because they cannot complete the seven life processes (MRSGREN) independently.

• Structure: Non-cellular; consist of genetic material (DNA or RNA) surrounded by a protein coat.

• Mechanism: They inject genetic material into a target cell, which then replicates the virus, harming the host cell.

• Examples: Influenza, Coronavirus, Hepatitis viruses.

Cladistics

Cladistics is a method of classification that identifies shared characteristics derived from a common ancestor.

• Clade: A group of biological taxa (such as species) that includes all descendants of one common ancestor (e.g., the great ape and human clade).

• Three Domains of Life: Archaea (Archaebacteria), Bacteria, and Eukarya (Eukaryotes).

Dichotomous Keys

A tool used to identify items in the natural world through a series of choices.

Steps to Construct a Key:

1. List the characteristics of the specimens.

2. Organize characteristics in a logical order.

3. Divide the specimens into two groups based on a major characteristic.

4. Continue dividing into smaller subgroups.

5. Draw the diagram/flowchart.

6. Test the key for accuracy.

Best Practices:

• Only consider one characteristic at a time.

• Use morphological (observable) features.

• Use contrasting statements (e.g., "Has feathers" vs. "Does not have feathers").

• Use specific questions leading to yes/no answers.

Syllabus Summary

• Core Concepts: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition. Binomial naming (Genus species). Species as fertile-offspring-producing groups. DNA base sequences as the basis for modern classification.

• Supplemental Concepts: Five kingdoms (Animal, Plant, Fungus, Prokaryote, Protoctist). Plant groups (Ferns, Monocots, Dicots). Identifying features of vertebrate and arthropod classes. Viral structure (protein coat and genetic material).