Characteristics of Living Organisms: A Comprehensive Guide

What Makes a Living Thing ‘Living’?

Non-Living Things

Non-living things are made up of inorganic matter. The most abundant chemical elements that make up inorganic matter are: oxygen (O), silicon (Si), aluminum (Al), and iron (Fe).

Living Things

All living things, also called organisms, are made up of the same chemical elements.

The elements that form part of living matter are known as bioelements.

The six most abundant bioelements are: carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), and sulfur (S). They make up more than 90% of living matter. The atoms and molecules of bioelements combine with each other through chemical reactions, producing biomolecules. Biomolecules may be:

• Inorganic, such as water and mineral salts.
• Organic (unique to living things), such as carbohydrates, lipids, proteins, and nucleic acids.

In order to classify something as living, it must:

• Be composed of bioelements.
• Be made up of small units called cells.
• Perform the three vital functions of nutrition, reproduction, and interaction.
• Grow and develop.
• Adapt to its environment in order to survive.
• Move in some way.

Autotrophic Nutrition

Autotrophic organisms (autotrophs) capture energy from their environment and use it to transform inorganic substances, such as water, carbon dioxide, and mineral salts, into organic matter. The process through which most autotrophs make their own food is called photosynthesis.

Plants, algae, and some bacteria are autotrophs.

Heterotrophic Nutrition

Heterotrophic organisms (heterotrophs) cannot produce their own food. They obtain organic matter by feeding on other organisms. Heterotrophs need to consume organic matter in order to get the nutrients they need.

Animals, protozoans, and fungi are heterotrophs.

Heterotrophs can:

• Obtain energy from eating plants (herbivores).
• Obtain energy from eating other animals (carnivores).
• Eat both plants and animals (omnivores).
• Obtain nutrients from dead organic matter (saprotrophs).

Fungi are saprotrophic organisms.

Interaction

Changes in the environment of an organism, such as changes in temperature and humidity, or in the organism itself, such as hunger, are called stimuli. The adaptations or reactions to those changes are called responses. Responses can vary considerably.

Reproduction

Reproduction is the capacity of organisms to form new individuals similar to themselves. It can be asexual or sexual.

Asexual Reproduction

Two or more identical individuals are produced from a single parent.

Sexual Reproduction

Two individuals of different sexes are required. These organisms produce specialized cells known as gametes. These gametes join together to produce new organisms which are similar, but not identical, to their parents.

Types of Cells

There are two types of cells: prokaryotic cells and eukaryotic cells.

Prokaryotic Cells

This is the simplest type of cell. Prokaryotic means ‘before a nucleus’. This is because prokaryotic cells do not have a defined nucleus and the DNA is dispersed in the cytoplasm. This is the cell type found in single-celled organisms such as bacteria.

• Cell wall: A strong protective structure around the cell membrane. As it is rigid, it gives shape to the cell.
• Cell membrane
• Genetic material
• Flagellum
• Cytoplasm

Eukaryotic Cell

Eukaryotic literally means ‘with a true nucleus’. In these cells, the genetic material is protected by a double membrane that separates it from the cytoplasm. Eukaryotic cells are more complex than prokaryotic cells because they have specialized organelles. These organelles play different roles in the cell.