Levels of cell organisation

Understand the idea

Cells did not appear fully modern. Early Earth may have received small molecules from meteorites, deep-sea vents, or lightning. RNA is often thought to be an early self-copying molecule because it can store information and speed reactions. Lipid molecules can form tiny bags in water, a step toward membranes. Cells are thought to be at least 4.0–4.3 billion years old.

Inside a body, organisation climbs. The chemical level is atoms and molecules such as amino acids. The cellular level is the working unit, including specialised cells such as egg, sperm, muscle, and nerve cells. A tissue is a group of cells with one job. Humans have epithelial, muscle, nervous, and connective tissues.

An organ is two or more tissues doing one function (heart, liver, kidney). A system is organs with a shared job: digestive, nervous, endocrine, circulatory, respiratory, urinary, reproductive. Cellular physiology is activity inside the cell; organ physiology is how organs work together.

What to remember

• Chemical → cell → tissue → organ → system.
• Four human tissues: epithelial, muscle, nervous, connective.
• Organ = two or more tissues; system = organs with one function.

Recap

Life likely started with simple molecules and membranes. In the body, molecules build cells, cells build tissues, tissues build organs, and organs build systems.