Levels of organisations

• Most animals and plants consist of different types of cells organised as tissues, organs and systems.
• The human respiratory system is a body system adapted for efficient gas exchange.
• In order of increasing complexity, multicellular organisms are made of: cells → tissues → organs → organ systems.
• A cell is the basic building block of all living organisms.
• A tissue is a group of cells working together to perform a shared function, and often with similar structure.
• An organ is a structure made up of groups of different tissues, working together to perform specific functions.
• An organ system is a group of organs with related functions, working together to perform certain functions within the body.
• An organelle is a specialised unit within a cell which performs a specific function.