The living cell

Pebble plants (also known as living stones) are plants that can be found in the dry areas of Southern Africa. As their name suggests, they look very much like small stones. This makes it hard to distinguish between these plants and the stones that surround them. However, pebble plants are living organisms, whereas the stones that surround them are lifeless.

What kinds of things does life need in order to exist? The existence of life requires:

• water in liquid form.
• elements that render life possible, such as carbon, hydrogen, nitrogen, oxygen and phosphorus
• a source of energy, such as the Sun.

There can be no life without energy. Energy is needed to make different things happen. All life on planet Earth receives most of its energy from the Sun.

Plants, algae and some bacteria are capable of photosynthesis. This means that they can make use of the energy contained in sunlight to produce chemical energy. Photosynthetic organisms store this energy in chemical compounds. All other living organisms are dependent on the chemical energy produced by these organisms.

The solar energy bound by green plants and other producers moves through the food chain and is used by consumers, such as herbivores and predators.

All life on planet Earth is organic. This means that it is based on different carbon compounds. Carbon is an element that can easily form different kinds of molecular structures, such as chains and wheels.

When carbon chains are joined together with different elements, such as hydrogen, nitrogen or oxygen, the result is an organic compound. Living organisms need and create these kinds of compounds, which include carbohydrates, fats, and proteins. In addition to the elements and compounds listed above, living organisms also need other elements in order to thrive. These elements include phosphorus, potassium, calcium, magnesium and iron.

All living organisms are made up of one or multiple cells. Cells contain large amounts of water. Therefore, water is essential for all living cells.

Animal cells

Animal cells contain the following parts:

• The cell is surrounded by a cell membrane. It regulates the movement of substances to and from the cell.
• The cell is filled with cytoplasm, where various important reactions take place.
• The nucleus is the “brain” of the cell. It contains the cell's genome in DNA form.
• The Golgi apparatus is involved in the production of substances within the cell.
• The mitochondrion is where the cell produces energy. Muscle cells, for example, contain several mitochondria.
• Unlike plant cells, animal cells do not contain cell walls or vacuoles.

Plant cells are also made up of different parts:

• The nucleus comprises the "brain" of the cell. It contains the cell's genome in DNA form.
• The cell membrane regulates what substances can move to and from the cell. 
• The cytoplasm is where various important reactions take place.
• The cell wall of a plant cell provides protection and support.
• The vacuole of the plant cell is used to store water.
• The chloroplasts of the plant cell are where photosynthesis takes place.
• Like the animal cell, the plant cell also has mitochondria.

Both plant and animal cells can join together to form larger structures. These structures are known as cell tissues. For example, human muscle cells combine to form muscular tissue, whereas the superficial cells of a plant's leaves combine form superficial tissue.