Characteristics & classifications of living organisms - Biology

September 19, 2025

Characteristics & classifications of living organisms - Biology - Naba, Aaliya, and Maryam

Hello there! This is such an overdue post but it's finally here!
This is notes for the very first chapter in biology.

Characteristics & classification of living organisms

1.1 Characteristics of Organisms

An organism is a living thing and there are many different kinds of them on earth. Some kinds of organisms include humans, animals, plants, and bacteria.

They are pretty different when it comes to their characteristics because, seriously, you don’t look like a bacterium.

However, there are seven characteristics that every living thing possesses, and they are:

Growth – all of them grow larger by adding new cells and growing them.

Growth is the permanent increase in size and dry mass. Growth is not the ability of a pufferfish that makes itself larger for a period of time while it is under threat.

Dry mass is the mass of an organism after it has been killed and has all of its water removed. Scientists observe the dry mass of the species before determining its average mass.

Movement – all of them are able to move, whether it's a small distance or just parts of themselves.

Some plants have the ability to move quickly. Their chloroplasts also move inside them.

Sensitivity – all of them react to the changes in their environment.

Sensitivity can be a result of internal or external factors, such as sunlight and noise. Plants sense light and turn towards it.

Excretion – all of them remove toxic/unwanted waste from their bodies.

Since there are a lot of chemical reactions inside cells, the products of some of these reactions must be removed from the body because they aren’t useful to the cells or just because there is an excess of them.

Reproduction – all of them are able to produce more of their kind.

Not every single one of the organisms can reproduce, but many of their kinds can.

Nutrition – all of them use substances from around them to provide energy or make new cells (FOR GROWTH!).

All of the organisms need chemicals to build their cells and need a source of energy. Plants and animals have very different sources of nutrition.

Respiration – all of them break down glucose inside their cells and release energy that they can use.

Respiration is a chemical reaction in cells that breaks down nutrient molecules and releases energy for metabolism. Usually, they obtain the nutrients from glucose. These chemical reactions are called metabolism.

1.2 The binomial classification system

Classification means to put things into groups, and there are many ways to classify organisms. Biologists have come up with a method of classification based on their relation with each other.

They also look at a potential common ancestor — a species that lived in the past which is thought to have given rise to several species alive today.

A useful way of classifying organisms is by comparing their features. For example, any animal that gives milk to its young is considered to be a mammal.

The smallest group that biologists classify as organisms is a species. When two of the same species reproduce, their offspring are fertile, which means they are able to reproduce.

Horses, for example, belong to a species called Equus caballus (ekwis - cabaloose) that can reproduce amongst themselves to produce more horses. Donkeys, however, belong to Equus asinus, which also reproduces amongst itself and has fertile offspring. BUT, if a donkey and a horse reproduced, they would have an infertile offspring (one that can not reproduce).

The binomial naming system is a system in which certain species are written in a special way. (Look at the names of the species of horses and donkeys.)

This name contains two parts: a genus and a species. A donkey and a horse share the same ancestor, and hence their genus is the same. But their features are quite obviously different, and so their species is different.

The genus is generally written with a first capital letter, and the species is written in all small letters. They are also always written in italics during printing.

1.3 Keys

A way of identifying organisms is through the dichotomous key, a key that leads you through options containing a pair of descriptions that you may choose from, and it continues leading you on until you find a name.

Over here, we have a lovely little creature and we aren’t aware about who he may be, hence, we consult the key.

We are going to look at our handsome young organism and compare it to the first pair of statements. Does it have a jointed limb? Yup! So we go to the next step.
Does it have more than 5 pairs of jointed arms? Nope, doesn’t look like it, he’s got 3 jointed legs on each side.
So in 3, we can see that our beautiful being has a pair of large claws, so it is a… crab! How marvellous!

There are two opposing statements as pairs and so you have to choose between them carefully.

1.4 Kingdoms

The species are the smallest group when it comes to classification. A bigger group of classification is the Kingdom

We are well aware of the animal and plant kingdoms.

Some characteristics of the animal kingdom include:

Cells with a nucleus but no chloroplasts or cell walls.
Feed on organic substances made by other living beings.

The plant kingdom includes two major types of plants: the flowering and the non-flowering.

All of the plants have a green colour caused by a pigment called chlorophyll that absorbs light, which will be combined with carbon dioxide and water to make glucose. It is a major part of the photosynthesis process.

Some characteristics of the plant kingdom include:

They have cell walls made of cellulose and have a nucleus and they contain chloroplasts
They feed through photosynthesis
Most of them have roots, stems, and leaves.

Alongside plants and animals, there are 3 more kingdoms: namely, fungi, prokaryotes, and protists.

The fungus kingdom

Fungi used to be classified as plants, but they aren’t plants due to their very different features. They have cell walls (not made out of cellulose) but do not photosynthesise.

Mushrooms and toadstools are examples of fungi. They are made of microscopic threads called hyphae.

They feed on faeces, human foods, dead plants and dead animals. They are also decomposers sometimes and break down waste material.
They reproduce with spores.

Some useful fungi include mushrooms and yeast that we consume.

However, there are also many harmful fungi that cause diseases.

The protoctist kingdom

All of them have a nucleus; some have plant-like cells (chloroplasts and cellulose cell walls), and others have animal-like cells.
Most are unicellular, while some are multicellular, like seaweed
They feed off photosynthesis, or others feed on organic substances from other organisms.

The prokaryotic system

Bacteria belong to this kingdom.
Their cells are very different compared to other organisms, mainly because they don’t have a nucleus or mitochondria. They do have a cell wall, but it isn't made out of cellulose. They also have a circular loop of DNA and often have plasmids.
Some bacteria are harmful, such as the bacteria that cause cholera while some are useful like those used in biotechnology.
Some carry out photosynthesis.
Most of them are unicellular.

1.5 Groups within the animal and plant kingdoms

Within kingdoms, biologists classify organisms into groups.

Within the Animal Kingdom – they are classified as:

Vertebrates
Arthropods

Vertebrates are animals that have backbones:

Fish
Amphibians
Reptiles
Birds
Mammals

Fish

They have scaly skin, gills, and fins.
Their eggs have no shells and are laid in water

Amphibians

They undergo a change called metamorphosis where they change from a larva to an adult with a different body form
They have no scales
Their eggs are laid in water – they don’t have shells.
Tadpoles live in water and have gills but adults live on land with lungs.
Examples are frogs, toads, and salamanders.

Reptiles

They have scaly skin
They lay eggs with soft shells.
Some examples are crocodiles, lizards, snakes, turtles and tortoises.

Birds

They have feathers (sometimes they have scales).
They have a beak.
Their front two limbs are wings. Not all of them can fly with it.
They lay eggs with hard shells.
Examples: chickens, ostrich, kiwis.

Mammals

They have hair on their skin
The young females develop uteruses and have mammary glands that produce milk to feed the young.
They have different kinds of teeth.
They have a pinna, an ear flap on the outside of the body.
They have sweat glands in the skin.
They have a diaphragm, it is a muscle that separates the chest cavity from the abdominal cavity in mammals and helps with breathing.
Examples: Humans, bears, lions.

Arthropods are animals with jointed legs and no backbone. They have a waterproof exoskeleton, a supportive structure on the outside of the body.

They have several pairs of jointed legs
They have exoskeleton.

Insects

They have 3 pairs of jointed legs
They have 2 pairs of wings (some may be vestigial; too small and useless.)
They breathe through trachea.
Their body is divided into 3: head, thorax, abdomen
They have a pair of antennae.
Examples: cockroaches, grasshoppers, and dragonflies.

Crustaceans

They have more than 4 pairs of jointed legs
They have two pairs of antennae
Examples: spiders, ticks, and scorpions.

Arachnids

They have 4 pairs of joined legs
No antennae
They have 2 parts of the body, a cephalothorax and abdomen.

Myripods

Their body has many similar segments and each segment has a pair of jointed legs
They have a pair of antennae.
Some examples are centipedes and millipedes.

Within the Plant Kingdom they are classified as:

Ferns
Flowering plants

Ferns

They are plants with roots, stems, leaves, but no flowers
They reproduce using spores produced on the underside of their fronds (leaves)

Flowering plants

They are plants with roots, stems, and leaves.
They reproduce using flowers and seeds and the seeds are produced inside the ovary of the flower.
They are divided into two main groups, dicotyledons and monocotyledons. This refers to the structure of their seeds.
Monocotyledons have only one seed leaf while dicotyledons have two seed leaves.

Dicotyledons (dicots)

Have seeds with two cotyledons.
They usually have a main root with side roots coming out of it.
Leaves have a network of veins.
Have flower parts in multiples of four or five.

Monocotyledons (monocots)

Have seeds with one cotyledon
Roots grow out directly from the stem.
Leaves have parallel veins.
Have flower parts in multiples of three.
They have vascular bundles in the stem.

1.6 Viruses

Viruses cause many illnesses and diseases such as cold, influenza, or serious ailments like aids.
Viruses aren’t considered living organisms because they can only just exist until they get inside a living cell and take over and continue multiplying itself until they burst out of the cell and evade others and continue the process. Generally, the host cell dies when a virus evades it.
Viruses can not move, excrete, show sensitivity, grow or reproduce on their own.