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a thick layer inside the primary cell wall. It consists mainly of cellulose, hemicellulose and lignin, and makes the cell walls rigid and waterproof.
an atom or a group of atoms that has gained or lost one or more electrons and so has an electric...
a molecule that accelerates (speeds up) a chemical reaction, by lowering its activation energy,...
a molecule that is formed when oxygen and haemoglobin combine chemically.
a scientist who studies fossils.
the hard 'roof' of the front of the mouth.