Cell biology

The cell is the basic structural and functional unit of all living things. Some organisms consist of a single cell, while others, including plants and animals, are built from many billions of them. The cell theory holds that all living things are made of cells, that the cell is the smallest unit of life, and that new cells arise only from existing cells. Cells fall into two broad types: simpler prokaryotic cells, such as bacteria, which lack a nucleus, and more complex eukaryotic cells, found in plants, animals, fungi and protists.

A eukaryotic cell is enclosed by a cell membrane that controls what enters and leaves, and filled with a jelly-like cytoplasm in which the organelles are suspended. The nucleus holds the cell's DNA and directs its activities. Mitochondria release energy from nutrients through respiration; ribosomes and the endoplasmic reticulum build proteins; the Golgi apparatus modifies and packages them; and vesicles and vacuoles store and transport materials. Plant cells additionally have a rigid cell wall, a large central vacuole, and chloroplasts that carry out photosynthesis.

Cells reproduce by dividing. Mitosis produces two genetically identical cells and underlies growth, repair and asexual reproduction, while meiosis produces sex cells with half the chromosome number for sexual reproduction. Substances cross the cell membrane by diffusion, osmosis and active transport, allowing the cell to take in nutrients, expel waste and maintain the stable internal conditions it needs to function.

Frequently asked questions

What is the cell theory?

It states that all living things are made of cells, that the cell is the smallest unit of life, and that all cells come from pre-existing cells.

What is the difference between prokaryotic and eukaryotic cells?

Prokaryotic cells, such as bacteria, lack a nucleus and membrane-bound organelles. Eukaryotic cells, found in plants and animals, have both.

What structures do plant cells have that animal cells lack?

Plant cells have a rigid cell wall, a large central vacuole, and chloroplasts for photosynthesis.