GCSE · Biology · Edexcel · Spec 1BI0

Effects of temperature, substrate concentration and pH on enzymes

Why does warming an enzyme speed it up, but heating it too much stop it for good? The answer is one tiny shape.

Slide along the curve: what are the molecules doing?

02.557.51002.557.510Temperature (arbitrary units) →Rate of reaction (arbitrary units) →(5, 3.6)

Temperature (arbitrary units) →: 5. Rate of reaction (arbitrary units) →: 3.6

Drag the point along the curve. On the way up, ask: how often is substrate hitting the active site (the slot on the enzyme that the substrate fits into)? At the peak, ask: how many active sites are busy? On the way down, ask: does the active site still have the right shape?

Exam line: Rising side: warmer molecules move faster, so enzyme and substrate collide more often and with more energy, and there are more successful collisions. The peak is the optimum temperature: every active site is full, so the rate is as high as it can be. Falling side: the enzyme's shape is breaking down, the substrate stops fitting, and the rate drops all the way to zero.
Watch out: The numbers on these axes are just a scale to slide along, not real temperatures. What matters is the shape of the curve, and the reason for each part of it.

Biology · Enzymes

Does this collision make product?

Will each collision turn substrate into product?

Still to sort

Successful: product is made (0)

The substrate fits into the active site and forms an enzyme-substrate complex.

Where the line is: Two things must both be true: it hits the active site, and its shape fits.

Unsuccessful: nothing happens (0)

No fit at the active site, so the reaction isn't catalysed.

Where the line is: A near-miss at the active site still counts as a miss if the shapes don't match.

5 of 5 still to sort.

An enzyme is a biological catalyst: it speeds a reaction up. But a molecule bumping into an enzyme isn't enough. Sort each collision and find out what it takes.

Watch out: Hitting the enzyme is not the same as hitting the active site. And reaching the active site only counts if the substrate actually fits.

Substrate concentration: predict, then check

Picture a graph of rate against substrate concentration. At first, adding substrate makes the rate climb: more substrate molecules means more frequent successful collisions. Then the line goes flat.

The line is flat. You tip in even more substrate. What happens to the rate?

Too hot, or the wrong pH

?

Reason it through

Why does the rate fall when an enzyme gets too hot, or when the pH moves away from its optimum?

Link 1 of 4

First link · your turn

An enzyme is a protein: a long chain of amino acids joined end to end and folded into a very specific shape. What holds that fold in place?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

Check your thinking

Can a cooked enzyme come back?

An enzyme is heated far above its optimum temperature, and the reaction stops. Then it is cooled back down to its optimum temperature.

What happens to the reaction now? Pick the idea closest to what you think.
How sure are you?

Biology · pH and enzymes

Where does each enzyme work best?

Every enzyme has an optimum pH, where it works fastest. Drag each enzyme to where you think its optimum sits on the pH scale. Hint: think about where it works.

Exam line: Plotted against pH, the rate is highest at the enzyme's optimum pH and lower on either side of it.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Three graphs, one tiny shape: the active site.

What you need to know

  • An enzyme is a biological catalyst. Its active site is specific to one substrate (lock and key), and the two form an enzyme-substrate complex.
  • The rate of reaction is the amount of change, such as product made or substrate broken down, per unit of time.
  • Substrate concentration: the rate rises as successful collisions become more frequent, then plateaus when all the active sites are full.
  • Temperature: the rate rises to a peak at the optimum as collisions become more frequent and energetic, then falls to zero as the enzyme denatures.
  • pH: the rate is highest at the optimum pH and lower above and below it. The optimum depends on where the enzyme works: about 7 in cells, about 1–2 in the stomach, about 8 in the small intestine.

The big picture

Enzymes are biological catalysts. A reaction only happens when a substrate collides with the enzyme's active site and fits it. Temperature and substrate concentration change how often those successful collisions happen, until every active site is full. Too much heat, or a pH away from the optimum, breaks the bonds that hold the enzyme's shape, so the active site changes shape, the enzyme is denatured and the rate falls.

Key points

1No fit, no reaction: only a substrate that collides with the active site and fits it can be turned into product.
2More substrate means more frequent successful collisions, until every active site is full. Then the rate plateaus, and only more enzyme will raise it.
3Below the optimum temperature, molecules move faster and with more energy, so more successful collisions happen.
4Above the optimum temperature, or away from the optimum pH, the bonds holding the enzyme's shape break, the active site changes shape and the substrate no longer fits.
5Denatured means a permanent change of shape that stops the enzyme working. Enzymes denature; they don't die.

Worked example

Problem

A student measured the rate of an enzyme-controlled reaction at six temperatures (units of product made per minute): 10 °C → 4, 20 °C → 9, 30 °C → 18, 40 °C → 27, 50 °C → 11, 60 °C → 0. Describe and explain these results.

⚠ Watch out

Reading the flat part of the substrate-concentration graph as 'the enzyme has stopped working'. It's the opposite. Every active site is full and the enzyme is working as fast as it can.

🧠

Memory hook

Two questions crack every enzyme graph: Are they meeting? Does it still fit? Meeting explains the rises and the plateau; fitting explains the falls.

✓

Check yourself

Of the three graphs (substrate concentration, temperature and pH), which ones fall to zero, and which one never falls? Say which of the two big ideas, collisions or shape, explains each.

Flashcards

(14)
What is an enzyme?
A biological catalyst: it speeds up the rate of a reaction.
What is the active site, and why is it called specific?
The part of the enzyme the substrate fits into. Its shape matches one substrate, like a lock and its key.
What is an enzyme-substrate complex?
The enzyme with its substrate fitted into the active site.
What makes a collision between substrate and enzyme 'successful'?
The substrate collides with the enzyme at the active site. Only then can the reaction be catalysed and product made.
Define rate of reaction.
The amount of change (substrate broken down or product made) per unit of time.
Why is the rate low at a low substrate concentration?
There are few substrate molecules, so successful collisions are infrequent and only a few active sites are full.
The substrate-concentration graph has levelled off. How could you make the rate rise again?
Add more enzyme. All the active sites are full, so more substrate won't help, but more active sites will.
Why does warming an enzyme towards its optimum speed the reaction up?
The molecules move faster and with more energy, collide more often and harder, so there are more successful collisions.
What is happening at the optimum temperature?
All the active sites are full, and the rate is as high as it can be.
What holds an enzyme in its folded shape?
Bonds between amino acids that are not next to each other in the chain.
How does a high temperature break an enzyme's shape?
The enzyme's molecules vibrate faster and with more force, which breaks the bonds holding its shape.
What does 'denatured' mean?
A permanent change in the shape of an enzyme that stops it working.
Describe a graph of enzyme rate against pH.
The rate is highest at the optimum pH and lower at pH values above and below it, reaching zero at extreme pH.
Optimum pH: enzyme in a cell, in the stomach, in the small intestine?
About 7, about 1–2 and about 8.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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