GCSE · Biology · AQA · Spec 8461

Principles of organisation

You are not one thing — you are four levels of building, each one made from the one below.

How living things are built

The ladder of organisation

Showing 6 layers: Ladder, Cell, Tissue, Organ, Organ system, Organism

Layers

Explore

start at the bottom and climb

The whole ladder: cells build tissues, tissues build organs, organs build organ systems, and those systems together make the organism.

Start at the bottom rung and climb: each level is built from the one below it.

Relationship matrix

Tap any cell to reveal it. Tap a column header to read one property down every item.

	The wording to learn	One real example
CellLevel 1
TissueLevel 2
OrganLevel 3
Organ systemLevel 4

Each cell hides a short answer and the reason behind it. Predict before you tap.

The line that is hardest to see

TissuevsOrgan

These two rungs sit next to each other, and telling them apart is the whole skill.

Focus

What it is built from

Tissue

Cells grouped together because they have a similar structure and function

Organ

Two or more different tissues gathered into one structure

The insight

What it is built from is the give-away: cells grouped by a similar structure and function means tissue, several different tissues packed together means organ.

How many jobs are going on inside it

Tissue

One job, which its cells carry out because they have a similar function

Organ

Several different jobs at once, adding up to one whole task

A real example

Tissue

Muscle tissue

Organ

The heart

Where it sits on the ladder

Tissue

Level 2 — one rung above the single cell

Organ

Level 3 — one rung above tissue

Your turn: place this structure

Read what it is built from before you choose.

Skin covers the whole body. It is built from a tough outer covering, a layer underneath holding blood vessels and nerve endings, and a layer of fat below that. Which level of organisation is skin?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How every living thing is built in levels — cells, tissues, organs, organ systems.

What you need to know

Cells are the basic building blocks of all living organisms.
A tissue is a group of cells with a similar structure and function.
Organs are aggregations of tissues performing specific functions.
Organs are organised into organ systems, which work together to form organisms.
A structure's level is decided by what it is built from, not by its size or how important its job is.

The big picture

Living things are not built in one piece. Cells are the basic building blocks; cells with a similar structure and function are grouped into tissues; and different tissues are combined into organs that perform specific functions. Organs in turn are organised into organ systems, and those systems work together to form the whole organism. The skill it asks for is the same each time: deciding which of these levels a structure sits on, by asking what it is built from.

Key points

1Four levels, one order: cell, then tissue, then organ, then organ system.

2Each level is built from the level below it, and can do a job that level could not do alone.

3Cells with a similar structure and function grouped together? Tissue. Several different tissues combined? Organ.

4Level is decided by how something is organised — not by its size, its importance, or whether it is solid.

5Organ systems working together are what make up the whole organism.

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Worked example

Problem

A leaf has a waxy outer covering, a layer of cells packed with the green pigment that traps light, and tubes running through it that carry water. Which level of organisation is a leaf, and how can you tell?

⚠ Watch out

Deciding the level from how important or how big something is. Blood does a vital job all over the body and is still counted as a tissue — because it is not several different tissues combined, which is what an organ is.

🧠

Memory hook

Cells, Tissues, Organs, Systems — CTOS. Each rung is simply a team made of the rung below it.

✓

Check yourself

Pick any structure in your own body and decide its rung by asking one question: what is it built from?

Flashcards

(7)

What are cells?

The basic building blocks of all living organisms — every level above is built from them.

What is a tissue?

A group of cells with a similar structure and function.

What is an organ?

An aggregation of tissues performing specific functions.

What is an organ system?

Organs organised to work together; organ systems then work together to form the organism.

Put the four levels in order, smallest unit first.

Cell, then tissue, then organ, then organ system.

Why is blood counted as a tissue rather than an organ?

An organ is an aggregation of tissues performing specific functions, and blood is not several different tissues combined into one structure.

Give an example of each level, all from the same part of the body.

Muscle cell, then muscle tissue, then the heart, then the circulatory system.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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