Intracellular and extracellular enzymes
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Question
What is an intracellular enzyme?
Answer
An enzyme that catalyses a reaction **inside** the cell that produced it (e.g. a **respiration** enzyme).
Question
What is an extracellular enzyme?
Answer
An enzyme that is **secreted** and catalyses a reaction **outside** the cell (e.g. a **digestive** enzyme).
Question
What does it mean to 'secrete' an enzyme?
Answer
To **release** the enzyme out of the cell, through the membrane, into the surroundings.
Question
Give an example of an intracellular enzyme.
Answer
A **respiration** enzyme (or catalase breaking down hydrogen peroxide inside the cell).
Question
Give an example of an extracellular enzyme.
Answer
A **digestive** enzyme such as **amylase, protease or lipase** — or the enzymes a decomposer secretes onto dead matter.
Question
Why does a cell secrete a digestive enzyme instead of keeping it inside?
Answer
Because a large food molecule (e.g. starch, protein) is **too big to cross the cell membrane**; it must be broken into small soluble subunits first.
Question
What does an extracellular digestive enzyme do to a large food molecule?
Answer
It **hydrolyses** it into **small, soluble subunits** (e.g. starch → maltose/glucose) that **can be absorbed**.
Question
How do decomposers (saprotrophs) feed?
Answer
They **secrete extracellular enzymes** onto dead matter, digest it **externally**, then **absorb** the small soluble products.
Question
Does being secreted change how an enzyme works?
Answer
**No** — both intracellular and extracellular enzymes are globular proteins that **lower activation energy**, are **specific** and are **reusable**. Only the location differs.
Question
In a data question, what tells you an enzyme is extracellular?
Answer
Its **activity appears outside the cell** (e.g. in the surrounding liquid / culture medium), acting on a substrate the cell has not absorbed.
Question
What is a metabolic pathway?
Answer
A linked series of enzyme-controlled reactions where the **product of one reaction is the substrate of the next** — run by intracellular enzymes.
Question
What do 'intra-' and 'extra-' mean?
Answer
**Intra** = inside; **extra** = outside — so intracellular acts **inside** the cell and extracellular acts **outside** it.
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Topic 3.1 hub
Enzymes and metabolism
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