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B2.2B2. Biological Molecules and Enzymes

Enzymes — Combined Science 0653 - Biology Revision Notes & Topical Questions

5 min read 10 parts Lesson 3 of 6 1 quiz

📚 Combined Science 0653 - Biology Hub / Topical Revision
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Lesson summary

B2.2 Enzymes — COMBINED SCIENCE notes: Describe enzymes as biological catalysts and explain how temperature and pH affect their activity.Enzyme = a protein that acts as a biological catalyst, speeding up a chemical reaction without being changed or used up.Biological — enzymes ar…

  • Metabolic reactions
  • Active site
  • Specificity
  • Lock and key
  • Low temperature
  • Rising temperature

Part 1 of 8What enzymes are

Enzyme
Enzyme a protein that acts as a biological catalyst, speeding up a chemical reaction without being changed or used up.
Biological
  • Biological enzymes are made in living cells.
Metabolic reactions
  • Metabolic reactions enzymes keep the speed of all the reactions that keep an organism alive high enough to sustain life.
  • With digestive enzymes a meal takes 3 to 4 hours to digest, but without them it would take 2 to 3 weeks.

Part 2 of 8How enzymes work

Active site
  • Active site the region of the enzyme where the substrate attaches.
Specificity
  • Specificity each enzyme works on one substrate because its active site is a complementary shape to the substrate.
  • The shape comes from the enzyme's specific three-dimensional structure.
Lock and key
  • Lock and key the substrate fits the active site like a key in a lock.

Part 3 of 8Steps in enzyme action

1
The enzyme and its substrate collide randomly.
2
The substrate fits into the active site, forming an enzyme-substrate complex.
3
The reaction takes place and the product is made.
4
The product leaves the active site because it no longer fits.
5
The enzyme is unchanged and takes up another substrate.

Part 4 of 8Effect of temperature

Low temperature
  • Low temperature enzymes work slowly because the molecules have little kinetic energy, so there are fewer collisions.
  • Cold does not denature enzymes.
Rising temperature
  • Rising temperature the rate increases as molecules move faster and collide more often.
Optimum temperature
  • Optimum temperature the enzyme works fastest, which is 37 °C in the human body.
High temperature
  • High temperature bonds holding the enzyme's shape break and the active site changes shape.
  • The substrate no longer fits, and the enzyme is denatured.
Denaturation
  • Denaturation is irreversible, so activity stops.

Part 5 of 8Effect of pH

Optimum pH
  • Optimum pH most enzymes work best at pH 7.
  • Stomach enzymes have a low optimum of about pH 2.
  • Enzymes in the duodenum have a higher optimum of pH 8 or 9.
Wrong pH
  • Wrong pH a pH that is too high or too low breaks the bonds that hold the protein together.
  • The active site changes shape, the substrate does not fit, and the enzyme denatures.

Part 6 of 8Investigating temperature with amylase

1
Heat starch solution to a set temperature.
2
Put drops of iodine into the wells of a spotting tile.
3
Add amylase to the starch solution and mix.
4
Every minute add a drop of the mixture to a new well of iodine.
5
Continue until the iodine no longer turns blue-black, which shows all the starch is digested.
6
Record the time taken, then repeat at other temperatures.
  • Interpretation
    • the shorter the time, the faster the enzyme is working.

Part 7 of 8Investigating pH with amylase

1
Put drops of iodine in rows on a tile.
2
Place 2 cm³ of amylase and 1 cm³ of buffer solution in a tube.
3
Add 2 cm³ of starch solution and start the stopwatch.
4
Every 10 seconds put a drop of the mixture onto a fresh drop of iodine.
5
Continue until the iodine stays orange-brown.
6
Repeat with buffers of other pH values.
  • Interpretation
    • the less time taken, the better the enzyme works at that pH.

Part 8 of 8More from the specification

More from the specification
Investigating Catalase Activity with Hydrogen Peroxide
  • Adding pureed plant or liver tissue containing catalase enzyme to hydrogen peroxide produces oxygen gas,
    • where collecting gas volume per unit time measures the catalytic rate under varied temperatures.
Investigating pH Effects Using Buffer Solutions
  • Adjusting reaction media with specific buffer solutions determines the enzyme's optimum pH;
    • deviating from this optimum disrupts ionic and hydrogen bonds maintaining the active site configuration.
Graphical Interpretation of Rate Curves
  • A graph of enzyme reaction rate against substrate concentration displays an initial linear rise that plateaus as all catalytic active sites reach full saturation.

Key terms

TermMeaning
EnzymeA protein that acts as a biological catalyst.
CatalystA substance that speeds up a reaction without being used up.
SubstrateThe substance an enzyme acts on.
Active siteThe region of the enzyme where the substrate binds.
DenaturationPermanent loss of an enzyme's shape and activity.
OptimumThe temperature or pH at which an enzyme works fastest.
ProductThe substance made in the reaction.
Enzyme-substrate complexThe enzyme and substrate joined together at the active site.

Avoid these mistakes

1
Saying enzymes are used up in the reaction — they are unchanged.
2
Saying cold temperatures denature enzymes — they only slow them down.
3
Saying denaturation can be reversed — it is permanent.
4
Saying all enzymes work best at pH 7 — stomach enzymes prefer acid and duodenum enzymes prefer alkaline.

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