A mitochondrion is 2 µm long. In an electron micrograph it is drawn 30 mm long. What is the magnification?
- Change the image size to µm: 30 mm × 1000 = 30 000 µm.
- magnification = image size ÷ actual size = 30 000 ÷ 2.
- = 15 000.
Answer: ×15 000
Cell theory, microscopy, cell wall and plasma membrane
Biology · The cell. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The idea
The cell theory says that all organisms are made of one or more cells, that the cell is the structural and functional unit of life, and that new cells come only from cells that already exist.
Cells are studied with microscopes. Magnification is how much larger the image is; resolution is the ability to see two close points as separate. The electron microscope has far greater resolution than the light microscope. Cell parts are separated by cell fractionation.
A plant cell has a rigid wall outside its plasma membrane. The plasma membrane is described by the fluid mosaic model: a double layer of phospholipids with protein molecules set in it. It controls what enters and leaves the cell.
Rules to remember
- Hooke (1665) saw cells in cork; Robert Brown (1831) the nucleus; Schleiden (1838) plants are made of cells; Schwann (1839) animals are made of cells; Virchow (1855) "omnis cellula e cellula".
- magnification = size of image ÷ actual size of object; 1 mm = 1000 µm.
- Cell fractionation: cells are broken open and centrifuged at increasing speeds; nuclei settle first, then mitochondria and chloroplasts, then ribosomes.
- Cell wall from outside to inside: middle lamella (calcium and magnesium pectates) → primary wall (cellulose with pectin and hemicellulose) → secondary wall (cellulose, lignin and other deposits).
- Fluid mosaic model (Singer and Nicolson, 1972): phospholipid bilayer, hydrophilic heads outwards and hydrophobic tails inwards, with proteins embedded.
- Diffusion, facilitated diffusion and osmosis go down the concentration gradient and need no energy; active transport goes against the gradient and uses ATP and carrier proteins.
- Endocytosis takes material in: phagocytosis for solids, pinocytosis for liquids; exocytosis sends material out.
Common mistake
In magnification questions students divide millimetres by micrometres. Change both sizes to the same unit first (multiply mm by 1000 to get µm).
Worked example
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