Metabolism and Enzymes
Metabolism is the sum of all chemical reactions in a cell, building up (anabolism) and breaking down (catabolism). Almost all of these reactions are controlled by enzymes.
This chapter covers what enzymes are, how they work, the factors that change their activity, and their uses in daily life.
Two other factors also change the rate of an enzyme-catalysed reaction: substrate concentration and enzyme concentration. Increasing either one raises the rate because more collisions between enzyme and substrate can occur, but only up to the point where all the available active sites are already occupied at any instant, after which the rate levels off.
Enzymes are named mostly by adding the suffix '-ase' to the name of their substrate or the reaction they catalyse, such as amylase acting on starch or protease acting on protein. Recognising this naming pattern helps you work out what an unfamiliar enzyme does directly from its name in an exam question.
Anabolic reactions include photosynthesis and protein synthesis, both of which build larger molecules and require an input of energy; catabolic reactions include respiration and digestion, both of which break large molecules into smaller ones and release energy that the cell can use.
Digestive enzymes illustrate how optimum pH varies between locations in the body: pepsin in the stomach works best in strongly acidic conditions, while trypsin in the small intestine works best in mildly alkaline conditions, showing that 'optimum pH' is a property of each individual enzyme rather than a single fixed value for all enzymes.
A rate of reaction is calculated from a graph as the gradient of the curve, or more simply as the amount of product formed, or substrate used up, divided by the time taken; SPM questions often ask for this rate to be calculated directly from a results table rather than only described in words.
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Content standards in this chapter
Key concepts
- Metabolism
- Anabolism builds large molecules from small ones (uses energy); catabolism breaks large molecules down (releases energy).
- Enzymes as biological catalysts
- Enzymes are proteins that speed up reactions without being used up, and are specific to one substrate.
- Lock-and-key model
- A substrate fits the enzyme's active site like a key in a lock, forming an enzyme-substrate complex.
- Effect of temperature
- Rate rises to an optimum (around body temperature) then falls sharply as the enzyme denatures at high temperature.
- Effect of pH
- Each enzyme has an optimum pH; away from it the rate drops, and extreme pH denatures the enzyme.
- Applications
- Enzymes are used in detergents, food processing, brewing and medicine.
- Effect of substrate concentration
- As substrate concentration increases, the rate of reaction increases because more substrate molecules collide with the available active sites; beyond a certain point the rate plateaus because every active site is already occupied at any moment.
- Effect of enzyme concentration
- As enzyme concentration increases, with excess substrate present, the rate of reaction increases because there are more active sites available; the rate eventually levels off once substrate becomes the limiting factor instead.
- Naming enzymes
- Most enzyme names end in '-ase' and are built from the substrate they act on or the reaction they catalyse, such as amylase for starch, protease for protein and lipase for fat.
- Enzymes in daily life
- Biological detergents contain protease and lipase to remove protein and fat stains at lower washing temperatures; the food industry uses enzymes such as pectinase to clarify fruit juice and rennin in cheese-making.
- Active site and tertiary structure
- An enzyme is a globular protein folded into a precise three-dimensional shape; the active site is a small region of this shape whose contours and chemical groups are complementary to one specific substrate.
- Immobilised enzymes
- Enzymes can be fixed onto or within an inert support, such as alginate beads, so they can be reused repeatedly and easily separated from the product, which is common in industrial applications like producing lactose-free milk.
- Calculating rate from data
- Rate of reaction can be found by dividing the change in the amount of product or substrate by the time taken, or by finding the gradient of the steepest part of a graph of amount against time.
How this chapter is examined
SPM Biology is assessed over three papers. Paper 1 has 40 objective questions (40 marks) in 1 hour 15 minutes; Paper 2 carries 100 marks across Sections A, B and C in 2 hours 30 minutes; Paper 3 is the practical, testing science process skills. Content from this chapter can appear in any of them, so lessons drill recall for Paper 1 and structured answers for Paper 2.
Common exam angles
- Interpreting a graph of enzyme activity against temperature or pH.
- Explaining denaturation in terms of the active site.
- Designing or evaluating an experiment on enzyme activity (Paper 3).
- Sketching and explaining a graph of rate of reaction against substrate concentration.
- Identifying an enzyme from its name and stating the substrate it acts on.
- Explaining why biological detergents work well at lower washing temperatures.
- Explaining why different digestive enzymes have different optimum pH values.
- Describing an industrial or medical use of immobilised or purified enzymes.
- Distinguishing metabolism from digestion when asked to define the term precisely.
Common mistakes
What students write: Saying enzymes are 'killed' at high temperature.
What earns the mark: Enzymes are not alive; at high temperature they are denatured, their active site changes shape and no longer fits the substrate.
What students write: Writing that a low temperature denatures enzymes.
What earns the mark: Low temperature only slows enzymes down; activity returns when warmed. Only high temperature and extreme pH denature them.
What students write: Treating enzymes as used up in a reaction.
What earns the mark: Enzymes are catalysts and are unchanged after a reaction, so one enzyme molecule can be reused many times.
What students write: Saying an enzyme works on any substrate.
What earns the mark: Each enzyme is specific, its active site fits only one substrate or a group of similar substrates.
What students write: Saying the rate of reaction keeps rising forever as substrate concentration increases.
What earns the mark: The rate plateaus once all the enzyme's active sites are occupied at any given moment, so adding more substrate beyond this point has no further effect.
What students write: Assuming adding more enzyme always speeds up a reaction.
What earns the mark: Once substrate becomes the limiting factor, adding more enzyme no longer increases the rate, because there is not enough substrate for the extra active sites to bind.
What students write: Writing that lipase breaks down protein.
What earns the mark: Lipase breaks down lipids (fats) into fatty acids and glycerol; protease is the enzyme that breaks down protein.
What students write: Describing enzymes only as substances that 'help' reactions.
What earns the mark: Enzymes are biological catalysts: they lower the activation energy needed for a reaction and speed it up without being permanently changed themselves.
What students write: Assuming every enzyme in the body has the same optimum pH.
What earns the mark: Different enzymes have different optimum pH values suited to where they work, such as pepsin in the acidic stomach and trypsin in the mildly alkaline small intestine.
What students write: Describing an enzyme's shape as fixed and rigid at every temperature.
What earns the mark: An enzyme's active site can flex slightly at its optimum temperature, but a large rise in temperature permanently distorts this shape and denatures the enzyme.
What students write: Reading the rate of reaction off a graph as the final value reached.
What earns the mark: The initial rate is usually taken from the steepest part of the graph near the start, since this is when substrate concentration, and therefore rate, is highest.
What students write: Confusing metabolism with digestion, treating the two words as interchangeable.
What earns the mark: Digestion is only one catabolic process that breaks down food; metabolism is the much broader term covering every chemical reaction in the body, both anabolic and catabolic.
Study this chapter
Processes in this chapter
Experiments in this chapter
Frequently asked questions
Why does enzyme activity fall at high temperature?
What is the lock-and-key model?
Are enzymes used up when they work?
Why does the rate of an enzyme reaction level off as substrate concentration increases?
How do biological detergents use enzymes?
Why do pepsin and trypsin have different optimum pH values?
What is an immobilised enzyme and why is it useful?
How do you calculate the rate of an enzyme-catalysed reaction from a graph?
What is the difference between anabolism and catabolism?
Source:SRC-DSKP-EN, SRC-FORMAT
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