Range of Sizes, Features and Complexity of Living Oragnisms

Living organisms can range greatly in size, from the minute microorganisms like bacteria that are only a few micrometres in length, to the gigantic blue whales that can reach up to 30 metres long.
Features of organisms can be physical (size, appearance), genetic (DNA structure, number of chromosomes) or behavioural (feeding habits, interaction with other species etc.).
The simplest living organisms are called unicellular organisms, because they consist of only one cell. Examples include bacteria and yeast.
Multicellular organisms are more complex, consisting of many cells working together. These range from simple multicellular organisms like algae and fungi, to the more complex animals and plants.
Multicellular organisms have cells that are organised into tissues, organs and organ systems. For instance, the human body has a circulatory system, digestive system, reproductive system etc.
Organisms can also be categorised based on their feeding habits. Autotrophs, such as plants, can make their own food using sunlight, whereas heterotrophs, like animals, must consume other organisms for sustenance.
The complexity of an organism can also be seen in its genetic structure. The human genome consists of about 3 billion base pairs of DNA, whereas the genome of a simple bacterium may only consist of a few million base pairs.
Organisms can reproduce sexually or asexually, adding another layer of complexity. Sexual reproduction involves the combination of genetic material from two parents, whereas asexual reproduction involves a single parent and usually doesn’t involve the joining of genetic material.
Organisms can be classified into five main kingdoms: Monera (bacteria), Protista (protozoa and algae), Fungi, Plantae and Animalia.
Classification of organisms, from Kingdom down to Species, allows scientists to identify the diversity and relationships amongst them. For example, organisms within the same family will share more characteristics than those in the same class.
Biodiversity is a measure of the variety of life in a particular habitat or on the planet as a whole. This includes diversity within and between species, as well as diversity of ecosystems.