GCSE Biology - AQA

1.1.1 - Introduction to Cells

Introduction to Cells

All living things are made up of tiny structures called cells. Cells are the basic building blocks of all living organisms.

Unicellular organisms

Some organisms (living things) are made up of just one cell. These are called unicellular organisms.

Examples of unicellular organisms include:

  • Bacteria
  • Some species of algae
  • Some species of fungi (unicellular fungi are called yeasts)
  • Some other organisms, such as amoebas
Photos of unicellular organisms. Text at the top says, "Examples of Unicellular Organisms". Then there is a photo of bacterial cells, labelled "Bacteria", a photo of algal cells, labelled "Some species of algae", a photo of some fungal cells, labelled "Some species of fungi", and a photo of an amoeba, labelled "Amoebas".

Unicellular organisms are organisms made up of a single cell. Examples include bacteria, some types of algae, some types of fungi and amoebas. (Algae image: CSIRO on Wikimedia Commons; Fungi image: Ran Yuping et al. on Wikimedia Commons; Amoeba image: SmallRex on Wikimedia Commons)

Multicellular organisms

Other organisms are made up of multiple cells. These are called multicellular organisms.

Examples of multicellular organisms include:

  • Animals
  • Plants
  • Some species of fungi
  • Some species of algae
Photos of multicellular organisms. Text at the top says, "Examples of Multicellular Organisms". Then there is a photo of an octopus, labelled "Animals", a photo of some Rhododendrons, labelled "Plants", a photo of some mushrooms growing out of a tree, labelled "Some species of fungi", and a photo of some kelp, labelled "Some species of algae".

Multicellular organisms are organisms made up of multiple cells. Examples include plants, animals, some species of fungi and some species of algae.

Sub-cellular structures

Within each cell there are many small structures. These are called sub-cellular structures. Each sub-cellular structure has its own specific function.

Examples of sub-cellular structures include the nucleus, the cell membrane and ribosomes. The functions of these and other sub-cellular structures will be explored later in the course.

Labelled diagram of an animal cell. The cell is a big blob. The outer boundary of the cell is the cell membrane. It contains a large circular structure called the nucleus. The fluid within the cell is the cytoplasm. There are small dots within the cell called ribosomes. There are also small red structures called mitochondria.

Diagram of an animal cell, with its sub-cellular structures labelled.

Microscope image of onion cells. The background is light blue and in front of these the cell walls and nuclei of the cells are visible in a darker blue/purple colour. The cells are all different shapes and sizes.

Onion cells viewed through a microscope. Each cell's nucleus and cell wall are visible (note: the cells have been stained to make these structures visible). Image adapted from 'Onion skin cells under a microscope' by Katekor1 on Wikimedia Commons


Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.


What are cells?


What are organisms that are made up of one cell called?


What are organisms that are made up of multiple cells called?


What are sub-cellular structures?


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