All living organisms have to reproduce and look for nourishment based on functions that take place at the cellular level. Basic cell functions include growing, splitting and carrying out specific operations such as movement or synthesizing essential substances.

Depending on the cell, these functions either take place either throughout the cell or within specialized cell submodules.

Cells can produce energy in several different ways, but the most common are photosynthesis and cellular respiration.

In green plant photosynthesis, cells change light to starches and sugars that can be stored and used to power other basic cell functions.

In animal cells, glucose from food is broken down to produce energy and carbon dioxide during cell respiration. Both types of cells store the energy in the form of adenosine triphosphate (ATP) molecules.

Where energy production takes place depends on the kind of cell. Primitive cells such as those of single-cell prokaryotes have a simple cell structure and produce energy in the cytoplasm of the cell.

Plants produce energy from photosynthesis in chloroplasts while both plant and animal cells produce and store energy in specialized organelles called mitochondria.

Cells use the produced energy for growing and for splitting. Cells grow larger individually and split to make their tissues grow or to make the overall organism grow bigger. Before it can split, a cell has to grow large enough so it can form two viable daughter cells.

A cell grows by absorbing nutrients, breaking them down into the components it needs and synthesizing proteins. It uses small complexes called ribosomes to create many of its proteins, and it uses lipids and sugars from nutrients to build additional cell structures and add to its cell membrane.

When the cell is large enough, it will divide if more of its type of cell are needed.

For example, nerve cells in higher animals often don't divide at all while skin cells divide frequently. When it is ready to divide, a cell duplicates its DNA, elongates and splits. The two daughter cells each have a complete copy of the DNA and a share of the ribosomes. If the cell had organelles, approximately equal numbers are left in each daughter cell.

Simple cells such as those of bacterial cells all have a basic cell structure that doesn't change. They have a cell wall, a cell membrane and ribosomes scattered throughout the cell. Their DNA is coiled near the cell membrane and the cells can't undertake specialized functions.

The cells of higher plants and animals have a more complicated structure with a nucleus containing DNA and organelles such as mitochondria for specific purposes.

Depending on what basic cell function they carry out, they may have special shapes, structures or capabilities. Contrary to the cells of simpler organisms, cells in more complex organisms often look completely different, and their basic functions are adapted to special tasks.

Specialized cells such as muscle and gland cells use basic cell functions to carry out specific tasks.

Muscle cells have a large number of mitochondria because they need extra energy to produce motion. The ATP molecules in muscle cells make the cells contract when a muscle shortens and expand as the muscle relaxes again.

Cells in glands use energy from mitochondria to synthesize enzymes produced by the gland. These specializations allow organisms to carry out more complicated activities.