Biology | Variety of living organisms

Welcome to the World of Classification!

Hi there! Biology is full of life—literally! There are millions of different organisms on Earth, from tiny bacteria to giant redwood trees. Trying to study them all without organization would be a nightmare.
That’s why we have this chapter: Variety of living organisms. We are going to learn how scientists organize all life into neat groups, making it much easier to understand their characteristics, evolution, and relationships.

Don't worry if this seems like a lot of names at first. Think of classification like organizing a massive library. If every book was thrown randomly, you'd never find anything! Scientists use a universal filing system—and we're going to learn the rules.

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1. Why Do We Classify Living Organisms?

1.1 The Need for Organization (Classification)

Classification is the process of arranging organisms into groups based on similarities and differences. Why is this essential?

• To make sense of the world: It helps us manage the immense number of different species (biodiversity).
• To identify new organisms: If a scientist finds a new plant, classifying it tells them what existing group it belongs to.
• To show relationships: Organisms grouped together often share a common ancestor, helping us trace evolutionary history.
• To use universal names: We need a name that works globally, regardless of the local language.

Analogy: Imagine trying to find milk in a supermarket where food is organized randomly. Classification puts all the fruits together, all the cleaning products together, and all the dairy together. It just makes things logical!

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2. The Universal System of Classification

2.1 The Hierarchy of Classification

The classification system uses a hierarchy, which means levels that go from very general (like an entire country) down to very specific (like your exact house number). The biggest groups contain the most organisms but share the fewest features; the smallest groups contain the fewest organisms but share the most features.

There are seven main levels (or ranks) in this hierarchy, moving from the broadest (largest) to the narrowest (most specific):

1. Kingdom (The broadest group)
2. Phylum
3. Class
4. Order
5. Family
6. Genus
7. Species (The narrowest group)

Memory Aid (Mnemonic Trick!):
To remember the order, use this fun phrase:

King Philip Came Over For Good Soup

2.2 The Species Concept

The species is the fundamental unit of classification. Organisms are considered to belong to the same species if:

• They can reproduce (mate) naturally with each other.
• Their offspring are fertile (can also have babies).

Example: A horse and a donkey can mate to produce a mule, but the mule is usually sterile (infertile). Therefore, the horse and donkey belong to two different species.

2.3 Binomial Nomenclature (The Two-Name System)

Around 250 years ago, a Swedish botanist named Carl Linnaeus developed a formal naming system. This system is called Binomial Nomenclature (Bi = two, nomial = name).

Every single organism has a unique, two-part scientific name written in Latin. This is the universal name, regardless of where you are in the world.

The two parts are:

1. The Genus (written first, always starts with a capital letter).
2. The species (written second, always starts with a lowercase letter).

Important Formatting Rules:

• When typed, the name must be in italics (e.g., Homo sapiens).
• When hand-written, the name must be underlined (e.g., Homo sapiens).

Example: The domestic cat is named Felis catus. The domestic dog is Canis familiaris. This system ensures clarity—if a scientist anywhere in the world mentions Felis catus, everyone knows exactly which animal is being discussed.

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3. The Five Kingdoms

For IGCSE Biology, we focus on five major groupings, or Kingdoms, which cover all life forms based on their basic cell structure, complexity, and method of feeding.

3.1 Defining Key Cell Terms

• Eukaryotic: Cells that have a nucleus and other membrane-bound organelles (like plant, animal, fungal, and protoctist cells).
• Prokaryotic: Simple cells that lack a nucleus (bacteria).
• Autotrophic: Makes its own food, usually through photosynthesis (like plants).
• Heterotrophic: Must eat other organisms (or organic matter) to obtain energy (like animals).
• Saprotrophic: Feeds on dead or decaying matter by secreting digestive enzymes outside the body (like fungi).

3.2 The Characteristics of the Five Kingdoms

1. Kingdom Animalia (Animals)

• Cell Structure: Eukaryotic. Cells do not have a cell wall.
• Organization: Multicellular (made of many cells).
• Nutrition: Heterotrophic (must eat others).
• Example: Mammals, fish, insects, worms.

2. Kingdom Plantae (Plants)

• Cell Structure: Eukaryotic. Cells have a cell wall made of cellulose.
• Organization: Multicellular.
• Nutrition: Autotrophic (photosynthesis using chloroplasts).
• Example: Mosses, ferns, flowering plants.

3. Kingdom Fungi (Fungi)

• Cell Structure: Eukaryotic. Cells have a cell wall, but it is made of chitin (not cellulose).
• Organization: Mostly multicellular (made of filaments called hyphae), though yeast is unicellular.
• Nutrition: Saprotrophic (feeds on dead or decaying matter). They do not photosynthesise.
• Example: Mushrooms, molds, yeast.

4. Kingdom Protoctista (Protoctists)

(This is sometimes called the "catch-all" kingdom, as it includes everything eukaryotic that doesn’t neatly fit into Animals, Plants, or Fungi.)

• Cell Structure: Eukaryotic. Often have a cell wall, but structure varies.
• Organization: Mostly unicellular (single-celled), but some are multicellular (like large algae).
• Nutrition: Can be autotrophic (like algae) or heterotrophic (like Amoeba).
• Example: Amoeba (often moves using pseudopods), Paramecium, Algae.

5. Kingdom Prokaryotae (Bacteria/Prokaryotes)

• Cell Structure: Prokaryotic (lacks a nucleus and mitochondria).
• Organization: Always unicellular (single-celled).
• Nutrition: Can be autotrophic or heterotrophic.
• Example: Lactobacillus (used to make yoghurt), Pneumococcus (causes pneumonia).

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4. Classification within the Animal Kingdom

Once we identify an organism as an animal, we continue classifying it down the hierarchy (Phylum, Class, Order, etc.).

The Animal Kingdom is first broadly divided into two main groups:

1. Vertebrates: Animals that possess a backbone (internal skeleton).
2. Invertebrates: Animals that do not possess a backbone.

4.1 Characteristics of Vertebrate Classes

Vertebrates are further divided into five main Classes. It is crucial to know the distinguishing features of each.

A note on temperature control:

• Ectotherms (Cold-blooded): Body temperature relies on the external environment (Fish, Amphibians, Reptiles).
• Endotherms (Warm-blooded): Body temperature is regulated internally and kept constant (Birds, Mammals).

| Class | Skin Feature | Respiration (Breathing) | Reproduction | Temperature Control | Examples |

|----------------------|-----------------------|-----------------------|-------------------------|---------------------------|----------------------------------|

| Fish | Scales | Gills | Lay eggs in water | Ectotherm (Cold-blooded) | Trout, Shark |

| Amphibians | Moist, permeable skin | Gills (when young), Lungs/Skin (as adult) | Lay soft eggs in water | Ectotherm (Cold-blooded) | Frogs, Newts, Salamanders |

| Reptiles | Dry, waterproof scales | Lungs | Lay hard/leathery shelled eggs on land | Ectotherm (Cold-blooded) | Snakes, Lizards, Crocodiles |

| Birds | Feathers | Lungs | Lay hard-shelled eggs on land | Endotherm (Warm-blooded) | Eagle, Penguin |

| Mammals | Hair or fur; sweat glands | Lungs | Give birth to live young; feed young milk | Endotherm (Warm-blooded) | Humans, Whales, Bats, Dogs |

4.2 Key Features of Invertebrates

Invertebrates make up over 97% of all animal species! The most successful invertebrate group you need to know about is the Arthropods.

The Phylum Arthropoda

Arthropods are characterized by:

• A hard outer skeleton (exoskeleton).
• A segmented body (body divided into sections).
• Jointed limbs/legs.

Arthropods are further divided into four key classes:

1. Insects:
   • 3 pairs of legs (6 legs total).
   • 3 body segments (head, thorax, abdomen).
   • Usually have 1 or 2 pairs of wings.
   • Examples: Beetle, fly, bee.
2. Crustaceans:
   • 5 or more pairs of legs.
   • 2 main body segments (cephalothorax and abdomen).
   • Often aquatic (live in water).
   • Examples: Crab, shrimp, lobster.
3. Arachnids:
   • 4 pairs of legs (8 legs total).
   • 2 main body segments.
   • Do not have antennae or wings.
   • Examples: Spider, scorpion, tick.
4. Myriapods:
   • Many pairs of legs (sometimes hundreds!).
   • Many body segments.
   • Examples: Centipede, millipede.