The levels of structural organization

Recall

The six levels of organization, simplest → most complex?

chemical → cellular → tissue → organ → organ system → organism

Each level is built from …?

the level below it

An element is …?

a unique pure substance

The common elements of the body?

hydrogen, oxygen, carbon, nitrogen, calcium, iron

The smallest unit of an element = …?

an atom

The three subatomic particles?

protons, neutrons, electrons

Two or more atoms bonded = …?

a molecule

The smallest independently functioning unit of life = …?

a cell

The cell's three basic features?

membrane, cytoplasm, organelles

A group of similar cells with a shared function = …?

a tissue

A structure of two or more tissue types = …?

an organ

A group of organs working together = …?

an organ system

The highest level of organization = …?

the organism

Understand

Why is a cell called the smallest 'independently functioning' unit?

It's the smallest unit that can carry out the functions of life on its own — a single bacterium is one cell doing exactly that.

How is an organelle like an organ?

Each organelle is a tiny unit inside the cell doing one specific job, just as each organ does a specific job for the whole body.

Why is an organ 'not just one tissue'?

An organ is made of two or more tissue types working together as one structure — a single tissue on its own isn't an organ.

What does 'each level is built from the one below' mean for tissues and organs?

A tissue is built from cells; an organ is built from tissues — every level is assembled from the level beneath it.

Why is putting an organ in a single system a simplification?

Most organs contribute to more than one system, so the boundaries between systems aren't rigid.

Put in containment order: cell, organ, tissue, organism, organ system.

organism ⊃ organ system ⊃ organ ⊃ tissue ⊃ cell (each level is made of the one below).

Apply

The stomach contains muscle, epithelial and nervous tissue working together. Which level of organization is the stomach?

Organ — an anatomically distinct structure of two or more tissue types (here muscle, epithelium and nervous tissue).

A pathologist sees a sheet of similar cells all doing one job. Which level is that?

Tissue — many similar cells working together for a specific function.

The heart, blood vessels and blood work together to move blood. Which level is that group?

Organ system (the cardiovascular system) — organs working together for a major function.

Rank simplest → most complex: organ, atom, cell, molecule, tissue.

atom → molecule → cell → tissue → organ (each built from the one before).

A single bacterium carries out all its own life functions. Which level of organization is it?

Cellular — a cell is the smallest independently functioning unit of life, and the bacterium is one cell.

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