# IGCSE Science/Section 1: The nature and variety of living organisms/Variety of living organisms

All organisms are made out of microscopic functional units, called cells. Multi-cellular organisms have many cells that make up the whole organism. These include plants and animals. Other cells are single-celled, which are just one cell. These are too small to see with just the human eye, so we have to use microscopes to view them.

### Plants

Plants are multicellular organisms. They use the sun's energy to create food, in a process called photosynthesis. To do this, in their cells they have chloroplasts, which contain chlorophyll. Chlorophyll is green, which gives plants their colour. Chlorophyll can convert solar energy (sunlight) to chemical energy (food). This food is stored as starch. An example of a plant is a tree. Their leaves contain chloroplasts, so their leaves are green, however, they will stop producing chloroplast and the chloroplast break down during the fall because the sunlight is less available so a brown color from carotenoids and xanthophyll pigments are noticeable.

### Animals

Animals are also multicellular organisms. They don't make their own food, instead they eat plants or other animals to get the energy they need. Animals usually have a nervous system for controlling their actions rapidly, whereas plants don't. Animals usually store excess food they eat as glycogen. Humans are a type of animal.

### Fungi

Fungi can be single-celled or multicellular. An example of a multicellular fungus is mushrooms, and an example of a single-celled fungus is yeast. Multicellular fungi are made of thread-like structures called hyphae. Fungi feed by releasing enzymes onto their food which breaks it down, so that they can absorb it. This is called saprotrophic nutrition. Like animals, fungi store food they don't use as glycogen.

### Bacteria

Bacteria are single-celled organisms. They are so small we cannot usually see them, so we need microscopes to look at them. Some special bacteria can carry out photosynthesis, but most bacteria feed off other organisms. Bacteria can cause disease, for example Pneumococcus, which causes pneumonia. However, some bacteria can be healthy. For exmaple, Bifidobacterium is used in Activia® yogurts. (You don't need to know the name of the bacteria, just that some can be good).

### Protoctists

Protoctists, also called protists are unicellular organisms. Some can perform photosynthesis and are more like plants, while others can't and are more like animal cells. Some protoctists are pathogenic, for example Plasmodium causes Malaria and is carried by moaquitoes. Some parasites have the characteristics of an animal cell , example amoeba whereas some protoctists have the characteristics of a plant cell , an example could be chlorella.

### Viruses

Viruses are extremely small, smaller than bacteria or protoctists. They always cause diseases. They infect cells of another organism and change the DNA or RNA to produce more copies of the virus. They come in lots of different shapes and sizes, but always contain DNA or RNA to change the cell which they have infected.
Examples are:
* the tobacco mosaic virus, which damaged tobacco plants by stopping chloroplasts being produced
* HIV, which causes AIDS
* influenza, which causes the flu
* the common cold
* ebola
* COVID-19 the Corona Virus which causes respiratory illness (like the flu) with symptoms such as a cough, fever, and in more severe cases, difficulty in breathing. You can protect yourself by washing your hands frequently, avoiding touching your face, and avoiding close contact.
There is some debate as to whether they are living organisms or not, as they do not carry out all the characteristics of living organisms.

### Pathogens

Pathogens are organisms that cause diseases. Fungi, bacteria and protoctists can be pathogens, and viruses are always pathogens.

### Cell structure

See cell structure for information about the insides of some cells.