Cells: the basic unit of life
Every living thing is made of cells, from a single bacterium to the 30 trillion or so cells in your body. This lesson covers what cells are and how the two big kinds differ.
Cell theory
Cell theory has three parts: all living things are made of one or more cells; the cell is the smallest unit that carries out all the functions of life; and new cells come only from existing cells. It grew out of microscope work in the 1600s-1800s and is one of biology's unifying ideas.
Why cells are small
Cells exchange food, oxygen, and waste across their surface, but their needs grow with their volume. As a cell grows, volume rises faster than surface area, so a large cell could not supply its interior fast enough. Being small, or flattened, or folded, keeps the surface-to-volume ratio high.
Prokaryotic cells
Prokaryotes (bacteria and archaea) are small, usually 1-10 micrometres, and have no nucleus: their DNA sits in a region called the nucleoid, often as one circular chromosome. They have ribosomes, a cell membrane, usually a cell wall, and sometimes flagella for swimming. They were the only life on Earth for over a billion years.
Eukaryotic cells
Eukaryotes (animals, plants, fungi, and protists) have a nucleus enclosing their DNA and many membrane-bound organelles, each a specialised compartment. They are typically 10-100 micrometres across. Compartments let incompatible chemical processes run side by side, like separate rooms in a factory.
Plant versus animal cells
Plant cells have a rigid cellulose cell wall, chloroplasts for photosynthesis, and a large central vacuole that stores water and keeps the cell firm. Animal cells lack these but have centrioles and often lysosomes. Both have a nucleus, mitochondria, ribosomes, endoplasmic reticulum, and Golgi apparatus.
Microscopes
Light microscopes magnify up to about 1000 times and can view living cells. Electron microscopes use electron beams to resolve far smaller details, such as ribosomes and membranes, but samples must be specially prepared and are usually dead. Fluorescent stains let researchers light up specific molecules inside living cells.
Unicellular and multicellular life
Some organisms are a single cell that does everything; others are multicellular, with cells specialised into tissues, organs, and organ systems. In a multicellular body, a muscle cell, nerve cell, and skin cell share the same DNA but use different genes, which gives them their distinct shapes and jobs.
Worked example
A cube-shaped cell 1 unit across has surface area 6 and volume 1, a ratio of 6. A cube 3 units across has area 54 and volume 27, a ratio of 2. Tripling the width cut the ratio to a third, which is why large cells struggle to exchange materials.
Key takeaways
All life is made of cells, which come from other cells. Prokaryotes lack a nucleus and organelles; eukaryotes have both. Plant cells add a wall, chloroplasts, and a central vacuole. Small size keeps the surface-to-volume ratio high. Multicellular organisms specialise cells by using different genes.
Flashcard
All life is cells; cells from cells
Check your understanding
Which structure would you find in a bacterium?