Principles of Organisation Notes

AQA GCSE Biology: Revision notes

Key facts

  • Organisms are organised as cells → tissues → organs → organ systems.
  • A tissue is a group of similar cells; an organ is several different tissues working together.
  • The digestive system is an organ system: several organs work together to digest and absorb food.
  • Enzymes are protein catalysts; each fits one substrate at its active site (lock and key).
  • Extreme temperature or pH denatures an enzyme by changing the shape of its active site.

Levels of organisation

Cells make tissues, tissues make organs and organs make organ systems, each level built from the one below.

Multicellular organisms are built in a hierarchy from the smallest working unit to the whole organism.

  • Cells are the basic building blocks.
  • A tissue is a group of cells with a similar structure and function.
  • An organ is made of different tissues performing a specific function.
  • An organ system is a group of organs working together for one function of the whole organism.
  1. 1

    Cells

    the basic building blocks of all organisms

  2. 2

    Tissues

    a group of cells with a similar structure and function, working together

  3. 3

    Organs

    different tissues performing specific functions

  4. 4

    Organ systems

    groups of organs working together for the whole organism

The hierarchy of organisation

Which statement correctly describes an organ?

The digestive system

The digestive system is an organ system: several organs, each made of different tissues, work together to digest and absorb food.

The digestive system includes the stomach, small intestine, large intestine, liver and pancreas.

Each organ contains different tissues doing different jobs, such as muscular tissue that churns or moves food, glandular tissue that makes digestive juices, and epithelial tissue that covers surfaces. All contribute to one overall function: digestion and absorption of food.

Labelled diagram of the human digestive system
The digestive system is an organ system: stomach, small intestine, large intestine, liver and pancreas work together.

Stomach

  • Muscular tissue churns the food
  • Glandular tissue makes acid and protease
  • Epithelial tissue covers the surfaces

Small intestine

  • Glandular tissue makes enzymes
  • Epithelial tissue absorbs digested food
  • Muscular tissue moves food along

Why is the digestive system called an organ system?

Enzymes

Enzymes are protein catalysts; each fits one substrate at its active site, and its activity depends on temperature and pH.

Enzymes are biological catalysts made of protein. Each catalyses a specific reaction because of the shape of its active site.

The lock and key model: only a substrate with a complementary shape (the key) fits the active site (the lock).

Activity rises with temperature up to an optimum, then falls sharply as the enzyme is denatured, because the active site changes shape. Each enzyme also has an optimum pH.

1020304050607020406080100xyoptimumenzyme activity
Enzyme activity against temperature: the shape of the curve (not real data)

What happens when an enzyme denatures?

Other organ systems

Yes: the heart is an organ in the circulatory system, which carries blood around the body.

The same hierarchy applies across the body. The heart is an organ made largely of muscle tissue that pumps blood as part of the circulatory system.

This is a double circulatory system: the right side pumps blood to the lungs, and the left side pumps blood around the rest of the body.

Individual organs, each built from several tissues, combine into an organ system that carries out one overall process for the whole organism.

Labelled diagram of the human heart
The human heart, showing the chambers, valves and main blood vessels.

In a double circulatory system, where does the right side of the heart pump blood?

Try an exam question

Explain why the stomach is an organ and not a tissue.

[3 marks]

That's the notes covered.

Carry on to the next subtopic.