Protoctist kingdom

Protoctists are a diverse group of organisms whose cells have nuclei. Most protoctists are single-celled: one cell carries out the activities needed for life.

Protoctists differ in how they obtain food. Some have chloroplasts and make their own food by photosynthesis. Others obtain food from other organisms or from organic material.

The ability to photosynthesise does not, by itself, mean that an organism belongs to the plant kingdom. A single-celled organism with a nucleus may photosynthesise and still be classified as a protoctist.

Fungus kingdom

Fungi obtain nutrients from organic material rather than making their own food by photosynthesis. Their cells have cell walls, but they do not have chloroplasts, so they cannot photosynthesise.

How fungi obtain nutrients

A fungus releases digestive substances onto organic material outside its body. These break large molecules into small, soluble products, which the fungus then absorbs. This is called external digestion.

The structure of many fungi

Many fungi are made of fine, thread-like structures called hyphae. The hyphae grow through or over the material from which the fungus obtains nutrients. Together, they form a network called a mycelium.

Fungi, plants and animals

Fungi and plants both have cell walls, but photosynthetic plants have chloroplasts and can make food by photosynthesis. Fungi do not have chloroplasts and obtain nutrients by external digestion and absorption.

Animals obtain food by ingestion: they take it into their bodies before digesting it. Fungi instead digest organic material outside their bodies and absorb the soluble products.

Plant and animal kingdoms

Plants and animals are placed in different kingdoms because their cells have different structures and they obtain food in different ways.

Plant kingdom

Plant cells have a cellulose cell wall around the cell membrane and a large permanent vacuole. Many plant cells also contain chloroplasts, where photosynthesis takes place. Not every plant cell has chloroplasts: for example, root cells generally do not.

Plants make their own food by photosynthesis, using light energy to make sugars. Organisms that make their own food are described as autotrophic.

Animal kingdom

Animal cells do not have cell walls or chloroplasts. Animals cannot make food by photosynthesis; they obtain it by feeding on other organisms. Organisms that obtain food this way are described as heterotrophic.

Most animals can move and have nervous systems that coordinate rapid responses to changes around them.

Distinguishing plants from animals

When classifying an organism at this level, look at both its cell structures and how it obtains food. A plant has cellulose cell walls and makes food by photosynthesis, using chloroplasts in its photosynthesising cells. An animal lacks cell walls and chloroplasts and obtains food by feeding.