From Cells to Organisms: Mechanism

Mechanism

Follow the mechanism step by step and distinguish what causes the change from what is merely observed.

This extension applies that lens specifically to From Cells to Organisms.

Cells rarely work alone. In any large organism they are organised into five levels, each built from the one before:

• A tissue is a group of similar cells doing the same job — muscle tissue, nerve tissue, xylem tissue in a plant stem.
• An organ is several different tissues working together as a unit — the heart, the stomach, a leaf, a root.
• An organ system is a group of organs that together carry out one large function — the digestive system, the circulatory system, the respiratory system.
• An organism is the complete living thing, with all its systems working at once.

Worked example — place the heart in the hierarchy.

Start at the smallest and build upwards:

1. A single cardiac muscle cell — one cell that can contract.
2. Many of them together form cardiac muscle tissue.
3. That tissue, plus nerve tissue, blood vessels and connective tissue, forms the heart — an organ.
4. The heart together with the arteries, veins and blood forms the circulatory system.
5. That system, working alongside the digestive, respiratory and other systems, makes up the human being — the organism.

Do the same for a plant: palisade cell → palisade tissue → leaf → shoot system → maize plant.