IGCSE Biology Notes: Characteristics and Classification of Living Organisms, Cells

Characteristics & Classification of Living Organisms

1.1 Characteristics of Organisms

Biology is the study of organisms. An organism is a complete living thing. All organisms share seven characteristics:

1. Movement: The ability to change position (animals move their whole body, plants move parts of themselves).
2. Respiration: Breaking down glucose to release energy.
3. Sensitivity: Detecting and responding to changes in the environment (internal or external).
4. Growth: Permanent increase in size and dry mass.
5. Reproduction: Making more organisms of the same kind.
6. Excretion: Removal of waste products of metabolism.
7. Nutrition: Taking in substances for energy and growth.

Some non-living things may have some of these characteristics, but only living organisms possess all seven.

• Respiration: Organisms obtain energy from nutrients, usually glucose. It's a chemical reaction inside every living cell. Respiration is a part of metabolism, which encompasses all chemical reactions in cells.

• Sensitivity: Detecting changes in the internal (e.g., blood temperature) or external (e.g., sunlight intensity) environment. Example: Venus flytrap closing on a fly.

• Growth: A permanent increase in size and dry mass (mass after all water is removed). This is different from temporary size changes like a pufferfish inflating with water to deter predators.

  • Dry mass measurement: Killing and drying an organism, then measuring its mass without water.

• Excretion: Removing waste products from chemical reactions inside cells. Also removes substances in excess, like extra water.

• Nutrition: Taking in materials to build cells and for energy.

1.2 The Biological Classification System

Classification is putting things into groups. Biologists classify organisms by how closely related they are, tracing back to a common ancestor (a species in the past that gave rise to multiple species today, e.g., the common ancestor of all mammals).

When classifying, shared features suggest relatedness. For instance, if a new animal has hair and produces milk for its young, it's classified as a mammal.

• Species: The smallest group in biological classification. Organisms within a species can reproduce with each other to produce fertile offspring (offspring that can also reproduce).
  • Example: Horses (Equus caballus) can reproduce with each other to produce more horses.
  • Donkeys (Equus asinus) can reproduce with each other to produce fertile donkeys. Horses and donkeys can reproduce together, but this produces infertile offspring (Mules).

The Binomial Naming System

Scientific names for species consist of two words (binomial system).

1. The first word is the genus, a group of related species (e.g., horses and donkeys are in the same genus).
2. The second word indicates the specific species.

The genus name is capitalized, the species name is in lowercase. The binomial name is italicized (or underlined when writing).

1.3 Keys

Dichotomous keys are used to identify organisms. "Dichotomous" means branching into two. The key presents two descriptions at a time, leading to the organism's identification through a series of choices.

1.4 Kingdoms

The largest groups into which living organisms are classified are kingdoms. There are five kingdoms:

1. Animal kingdom
2. Plant kingdom
3. Fungus kingdom
4. Prokaryote kingdom
5. Protoctist kingdom

Animal Kingdom

• Animals move actively to find food.
• Their cells have a nucleus but no cell walls or chloroplasts.
• They feed on organic substances made by other living organisms.

Plant Kingdom

• Plants contain the green pigment chlorophyll, enabling them to perform photosynthesis (using sunlight to make sugars).
• Their cells have a nucleus and cell walls made of cellulose, and they often contain chloroplasts.
• Plants feed by photosynthesis.
• They may have roots, stems, and leaves.

Fungus Kingdom

• Fungi do not have chlorophyll and do not photosynthesize.
• They feed saprophytically or parasitically on organic material.
• Many fungi are decomposers, breaking down waste and dead organisms to return nutrients to the soil.
• They reproduce by forming spores.
• Usually multicellular, but some like yeast are unicellular.
• Have nuclei and cell walls (not made of cellulose).

Protoctist Kingdom

• Kingdom Protoctista contains a mixed collection of organisms.
• They all have cells with a nucleus, but some have plant-like cells with chloroplasts and cellulose cell walls, while others have animal-like cells without these features.
• Most protoctists are unicellular but some, such as seaweeds, are multicellular.

Prokaryote Kingdom

• Bacteria belong to the prokaryote kingdom.
• Prokaryotic cells do not have a nucleus.
• They have cell walls, not made of cellulose.
• They have no mitochondria.
• They have a circular loop of DNA, which is free in the cytoplasm.
• They often have plasmids.

1.5 Groups within the Animal and Plant Kingdoms

Vertebrates (Animals with Backbones)

• Fish
  • Scaly skin.
  • Gills throughout life.
  • Fins.
  • Eggs have no shells.
• Amphibians
  • Skin with no scales.
  • Eggs have no shells, laid in water.
  • Tadpoles (larvae) live in water, adults often on land.
  • Tadpoles have gills, adults have lungs.
• Reptiles
  • Scaly skin.
  • Lay eggs with soft, waterproof shells.
• Birds
  • Feathers (sometimes scales).
  • Beak.
  • Front limbs are wings.
  • Lay eggs with hard shells.
• Mammals
  • Hair on skin.
  • Young develop in a uterus, attached to the mother by a placenta.
  • Females have mammary glands (produce milk).
  • Different kinds of teeth (incisors, canines, premolars, molars).
  • Pinna (ear flap).
  • Sweat glands in skin.
  • Diaphragm.

Arthropods (Animals with Jointed Legs, No Backbone)

• Exoskeleton.
• Insects
  • Three pairs of jointed legs.
  • Two pairs of wings.
  • Breathe through tracheae.
  • Body divided into head, thorax, abdomen.
  • One pair of antennae.
• Crustaceans
  • More than four pairs of jointed legs.
  • Two pairs of antennae.
• Arachnids
  • Four pairs of jointed legs.
  • No antennae.
  • Body divided into cephalothorax and abdomen.
• Myriapods
  • Many similar body segments.
  • Each segment has jointed legs.
  • One pair of antennae.

Ferns

• Plants with roots, stems, and leaves (fronds).
• Do not produce flowers.
• Reproduce by spores on the underside of fronds.

Flowering Plants

• Plants with roots, stems, and leaves.

• Reproduce using flowers and seeds.

• Seeds produced inside an ovary in the flower.

• Dicotyledons (Dicots)

  • Seeds with two cotyledons.
  • Main root with side roots.
  • Network of veins in leaves.
  • Flower parts in multiples of four or five.

• Monocotyledons (Monocots)

  • Seeds with one cotyledon.
  • Roots grow directly from the stem.
  • Parallel veins in leaves.
  • Flower parts in multiples of three.

1.6 Viruses

Viruses are not considered living organisms because they cannot perform the seven characteristics of life on their own. They need to invade a host cell to reproduce. Viruses are made of genetic material (RNA or DNA) surrounded by a protein coat.

2 Cells

2.1 Animal and Plant Cells

• All organisms are made of cells.
• Multicellular: Made of millions of cells.
• Unicellular: Made of a single cell (e.g., bacteria, yeast).
• New cells are formed from existing cells.

Microscopes

• Light microscope: Uses light and glass lenses to magnify images (up to approximately 1500 times).
• Electron microscope: Uses a beam of electrons and can magnify up to 500000 times.
  • Photomicrograph: A photograph taken using a light microscope.
  • Electron micrograph: A picture taken with an electron microscope.

Cell Structures and Functions

• Cell membrane: A thin layer of protein and fat around every cell; controls what enters and leaves the cell (partially permeable).
• Cell wall: A tough layer made of cellulose around plant cells; provides support and prevents bursting (fully permeable).
• Cytoplasm: A clear, jelly-like substance that fills the cell; contains dissolved substances and is where metabolic reactions take place (approximately 70\% water).
• Vacuole: A fluid-filled space inside a cell, surrounded by a membrane. Plant cells have large, permanent vacuoles containing cell sap, while animal cells have much smaller vacuoles (vesicles).
• Nucleus: Contains the genetic information (DNA) stored on chromosomes; controls protein synthesis.
• Chloroplasts: Found in plant cells; contain chlorophyll for photosynthesis; may contain starch grains.
• Mitochondria: Found in almost all plant and animal cells; where aerobic respiration (energy release from glucose) takes place. Muscle cells have many mitochondria.
• Ribosomes: Tiny structures where proteins are made; use instructions from DNA.

2.2 Bacterial Cells

• Bacterial cells are prokaryotic (do not have a nucleus).
• Cell wall: Made of peptidoglycan (not cellulose); supports the cell.
• Cell membrane: Controls what enters and leaves the cell.
• Cytoplasm: Contains ribosomes.
• DNA: A circular chromosome (bacterial chromosome).
• Plasmids: Smaller circles of DNA used in genetic modification.

2.3 Specialised Cells

• Multicellular organisms have cells with specific functions and modified structures (specialised cells).
• Tissue: A group of similar cells working together to perform a particular function (e.g., cells lining the stomach).
• Organ: A group of different tissues carrying out a function together (e.g., the stomach).
• Organ system: Several organs working together (e.g., the digestive system).

Examples of Specialised cells:

See Table 2.1 in source document for details

• Ciliated cell
• Neurone
• Red blood cell
• Sperm cell
• Egg cell
• Root hair cell
• Palisade mesophyll cell

2.4 Sizes of Specimens

• Cells are very small, requiring microscopes for observation.
• Magnification: How many times larger an image is than the actual object.

magnification = \frac{size : of : image}{size : of : actual : object}

• Ensure units are the same before calculating magnification.
• Magnification is written with a multiplication sign (e.g., x5) and has no units.
• Use micrometres (μm) for small objects: 1 : μm = 1 \times 10^{-6} m , 1 : m=10^6 μm