Molecular structure and function

A biological molecule’s structure helps determine what it can do. The types of atoms it contains and the way they are bonded and arranged give the molecule its three-dimensional shape and chemical properties.

These features affect how the molecule interacts with other molecules. Its shape can determine whether it fits against another molecule, while its chemical properties affect whether the two molecules attract or repel each other. Such interactions allow biological molecules to carry out their functions in living organisms.

For example, an enzyme has a region called an active site, where a substrate binds. The active site’s shape and chemical properties help determine which substrate can bind, so they affect which reaction the enzyme can catalyse.

If a molecule’s structure changes, its shape or chemical properties may change too. This can alter its interactions with other molecules and therefore its function. For example, a change in an enzyme’s active site may make a substrate less able to bind, reducing the enzyme’s activity. Not every structural change has an effect on function.

Levels of biological organisation

A cell is a basic living unit. New cells are produced when existing cells divide. In a multicellular organism, many cells work together and are organised into increasingly larger structures.

A tissue is a group of cells working together. Different tissues work together to form an organ, and organs work together to form an organ system. An organism is an individual living thing made up of these coordinated parts.

The levels can be followed from the smallest to the largest:

Cell → tissue → organ → organ system → organism

Organisation in the human respiratory system

Ciliated cells form part of the tissue lining the trachea, or windpipe. The trachea is an organ made of different tissues working together. It is part of the respiratory system, an organ system within a human organism.