3.5.1Additional lessons

Integration in Living Organisms — IB Biology SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 65 of 142

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Part 1 of 5Levels of Organisation

Multicellular organisms are built from a hierarchy of structures.

Specialised cells of one type, such as muscle cells, are grouped together to form a tissue that carries out a shared function.

Different tissues combine in an organ, for example the small intestine contains epithelial, muscle, nervous and connective tissue.

Organs that cooperate form an organ system, such as the digestive system, which breaks down and absorbs food.

Organ systems act together to maintain the whole organism, so a change in one system often affects others.

Emergent Properties and the Systems Approach

Emergent property

A property of a whole that arises from interactions of its parts and that the parts alone do not show.

Example

Individual cells cannot digest large macromolecules or move over long distances, but an organism with organ systems can do both.

Cooperation

Systems depend on each other, for instance the circulatory system supplies oxygen that the respiratory system takes in.

Systems approach

Studying how parts interact, not just each part alone, gives a better explanation of how the organism functions.

Nervous system

Uses electrical impulses along neurones and chemical neurotransmitters at synapses to give rapid, short-lived, localised responses.

Endocrine system

Uses hormones, secreted by glands into the blood, to give slower, longer-lasting responses in target organs.

Hormone

A chemical messenger carried in the blood that binds specific receptors on target cells and changes their activity.

Integration of organs

Hormones and nerves link organs, for example a pituitary hormone travels in blood to the ovaries during the menstrual cycle.

Feature	Nervous system	Endocrine system
Signal	Electrical impulse and neurotransmitter	Hormone in blood
Speed	Very fast	Slower
Duration of effect	Short	Longer
Target	Specific cells (localised)	Any cell with the receptor

Receptors and Targets

Specific binding

A hormone only affects cells with receptors that match its shape, so a hormone reaches the whole body but acts on a few targets.

Receptor position

Receptors may be on the cell surface membrane or inside the cell, depending on the hormone.

Waste removal

The kidneys excrete urea and other wastes, showing that the urinary system serves the whole organism.

Coordination

Both systems usually act together, for example a nervous signal can trigger hormone release.

Tissue A group of similar specialised cells working together.
Organ system A group of organs that work together to carry out a major function.
Emergent property A property of a whole system that its separate parts do not have.
Hormone A chemical messenger secreted by a gland and carried in the blood.
Target organ An organ with receptors for a hormone that responds to it.

Saying the endocrine system is faster than the nervous system;
- nerves act within milliseconds, hormones take longer.
Stating that hormones only reach their target organs;
- they travel everywhere in the blood but only targets have receptors.
Confusing an organ with an organ system, for example calling the stomach a system.
Giving an example of an emergent property that is just a sum of parts rather than a new function.

Last updated: 28 October 2026