Why a chunk of text is not a question
The usual approach is to cut material into pieces and call each piece a card: one sentence on the front, the next two on the back. Nothing about that split makes the front a prompt. You end up looking at a statement and trying to recall whatever happened to follow it in the source.
A definition is different, because the question is already inside it. “Enzymes are biological catalysts that lower the activation energy of a reaction” names a term and then explains it, so the halves separate cleanly into “What are enzymes?” and the explanation. Sentences without that shape are skipped rather than forced into a card.
The cost is a shorter deck. A descriptive 2,000-word chapter might give six cards where a definition-heavy one gives the full twenty. Six cards you can genuinely test yourself against are worth more than forty whose fronts either give the answer away or ask nothing at all.
Notes that produce cards, and notes that do not
The usual reason for a thin deck is narrative phrasing. “Bohr later proposed that electrons occupy fixed orbits” states a fact but never says what something is, so no card comes out. Rewritten as “The Bohr model is an atomic model in which electrons occupy fixed orbits”, the same fact produces a card immediately.
The linking verbs recognised are is, are, was, were, means, refers to, is called, is known as and is defined as. A leading “the”, “a” or “an” is stripped from the term, so “The nephron is the functional unit of the kidney” and “A nephron is the functional unit of the kidney” both give you the same front.
Number agreement comes from the verb rather than a guess at the noun, so “are” and “were” produce “What are ...?” and everything else produces “What is ...?”. Explanations longer than forty words are dropped, because a sentence that long is usually narrative that happens to contain “is” instead of a definition worth memorising.
Getting the cards into Anki
The export is one card per line, front and back separated by a tab, which is the layout Anki reads without configuration. In Anki, open File then Import, confirm that fields are separated by Tab, map the first field to Front and the second to Back, and choose the deck you want them added to.
Two things to watch. If your pasted notes contain tab characters, a line can split into three fields and the import will misalign, so paste from plain text where you can. Anki also flags a card as duplicate when its first field already exists, so revising the same chapter twice will not quietly double your deck.
This tool makes the cards; the spacing is Anki's job, and that scheduler is the part that produces retention. Anki is free on desktop and on Android, and only the iPhone app is paid, so the route from a chapter of notes to a working review schedule can cost nothing at all.