Every fact in the three cells lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Simplifications to be aware of: the younger lessons sort cell parts into 'plant only', 'animal only' and 'both'. This is a simplification: the textbook says animal cells generally have centrosomes and lysosomes and most plant cells lack lysosomes. The numbers for the human body (about 30 trillion cells, about 200 cell types, about 330 billion cells replaced a day, roughly one bacterial cell for each human cell) are scientific estimates for an average adult, not exact counts. Cell sizes are typical ranges; some cells, such as nerve cells, are far longer. The field-of-view method in the 15–16 experiment gives only an estimate: the uncertainty is easily 10–20%.
All sources
[1] Biology 2e, 4.1 Studying Cells
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
With few exceptions we cannot see individual cells with the naked eye, so scientists use microscopes. Magnification is enlarging an object in appearance; resolving power is the ability to distinguish two adjacent structures as separate. A typical human red blood cell is about 8 µm across (about 250 fit across a pinhead). Lab light microscopes magnify up to about 400 times; electron microscopes give higher magnification and resolution. Stains help show transparent cells but usually kill them. Robert Hooke coined the word 'cell' in 1665 for box-like structures in cork; van Leeuwenhoek saw single-celled 'animalcules'. Cell theory (Schleiden and Schwann, late 1830s; Virchow): all living things are made of cells, the cell is the basic unit of life, and new cells come from existing cells.
All cells share four parts: a plasma membrane, cytoplasm, DNA and ribosomes. Prokaryotes (Bacteria and Archaea) are mostly single-celled, have no nucleus or other membrane-bound organelles, keep their DNA in the nucleoid and mostly have a cell wall. Prokaryotic cells are 0.1–5.0 µm in diameter, eukaryotic cells about 10–100 µm. For a sphere, surface area = 4πr² and volume = 4πr³/3, so the surface-area-to-volume ratio falls as a cell grows; a cell that is too large may not have enough membrane for diffusion. Cells cope by dividing, becoming thin or elongated, folding membranes or having organelles.
Eukaryotic cells have a membrane-bound nucleus and many membrane-bound organelles. The plasma membrane controls what moves in and out; the cytoplasm is the region between the membrane and the nucleus. The nucleus stores DNA; mitochondria make ATP by cellular respiration ('powerhouses'); ribosomes make proteins. Plant cells (not animal cells) have a cell wall made mainly of cellulose, chloroplasts for photosynthesis and a large central vacuole. Animal cells generally have a centrosome and lysosomes; most plant cells lack lysosomes.
'The total magnification is the product of the ocular magnification times the objective magnification': a 10× eyepiece with a 40× objective gives 400×. Eyepieces typically magnify 10×, objectives 4× to 100×. The resolution of light microscopes is limited by the wavelengths of visible light; above about 1000× extra magnification adds size but not detail.
National Institute of General Medical Sciences (NIH), Biomedical Beat, 2024 · Reliability: High
Type: Government science agency
An adult has about 30 trillion human cells; about 330 billion cells die and are replaced each day, mostly blood cells and cells lining the gut; the body has about 200 cell types (for example bone, fat, stem and reproductive cells); stem cells divide and develop into new cell types; a nerve cell's axon can be about a metre long; roughly one bacterial cell for every human cell; about 70% of a cell's mass is water.