How does it work?
The cell is the smallest unit of life. Our cells, like those of plants and fungi, are eukaryotic: they have a nucleus and many organelles wrapped in membranes. Each one has its own job, like the districts of a city.
From gene to protein
The recipe for a protein is written in DNA. In the nucleus a gene is copied into mRNA (transcription). The mRNA leaves through a nuclear pore, and a ribosome reads it and links amino acids together (translation). Proteins for export are made on the rough endoplasmic reticulum, get “labels” in the Golgi apparatus and leave the cell in vesicles (exocytosis). More about DNA itself: DNA Helix.
Energy: respiration and photosynthesis
Mitochondria “burn” glucose with oxygen and charge up ATP molecules — the cell's universal batteries. Chloroplasts do the reverse: using the energy of light, they build glucose from carbon dioxide and water.
A border with checkpoints
The membrane lets small molecules such as O2 and CO2 pass from where they are more concentrated to where they are less concentrated (diffusion). Ions go through protein channels. The sodium-potassium pump uses ATP to move ions “uphill”:
Why are cells small?
Everything a cell takes in and gets rid of passes through its surface. For a sphere of radius r:
Scale: from a virus to an egg cell
Logarithmic scale: the bar spans 10 nm to 1 mm, and each fifth of it is a tenfold increase in size. A human egg cell can be seen with the naked eye as a dot. 1 µm = 0.001 mm, 1 nm = 0.001 µm. More: Scale of the Universe.
What we simplified
This is a flat cross-section, and the organelles are placed at random. Ribosomes, nuclear pores, centrioles and membrane thickness are drawn many times larger so they are visible; the scale bar refers to the whole cell. The colors are arbitrary, like the dyes in fluorescence microscopy. The processes are greatly sped up and show only the most important stages.