12. More examples of enzyme controlled reactions

Enzymes and the synthesis of carbohydrates, proteins and lipids

Three examples of 'small' molecules to 'big' molecules!

The previous section on digestion was all about enzymes breaking large molecules down to small molecules to aid digestion and absorption of nutrients.

Here we consider the opposite biochemistry - building up essential large molecules from small substrate molecules.

The molecules so formed are examples of naturally occurring polymers.

Some examples of 'big molecule' synthesis reactions - ALL catalysed by specific enzymes

(Most are the opposite of the digestive chemistry described above)

 

1. Protein synthesis

Several enzyme catalysts join amino acids together to form proteins - its a complicated process!

Proteins are an example of a natural polymer.

Enzyme reaction word equation: proteins == enzymes ==> protein

The synthesis of proteins from amino acids in the ribosomes.

For more details see DNA structure and Protein Synthesis  (gcse biology revision notes)

 

2. Lipid synthesis

Again, it takes multiple enzymes to synthesise lipid fats from glycerol and long chain fatty acids.

Enzyme reaction word equation: glycerol + 3 fatty acids === enzymes ===> lipid oil or fat

The synthesis of lipids from fatty acids and glycerol.

The molecular details of lipid oils and fats are not required for GCSE biology, but you should recognise aspects of their structure from your GCSE chemistry course on the equation diagrams below.

e.g. the functional groups: carboxylic acid -COOH, unsaturated' alkene group >C=C< and alcohol C-OH.

 Lipids are NOT polymers

 

3. Complex carbohydrate synthesis

Complex carbohydrates are synthesised by combing together lots of small sugar molecules such as glucose into long polymer molecules such as glycogen (in animals) and starch and cellulose in plants.

The enzyme glycogen synthase (the name says it all), can join lots of simple glucose molecules into long chain glycogen molecules - a natural polymer.

Enzyme reaction word equation: glucose == glycogen synthase ==> glycogen

The synthesis of complex carbohydrates like glycogen from smaller sugar molecules.

Glycogen is used by animals, like ourselves, as a chemical energy store.

Other enzymes can rapidly convert glycogen back to glucose to fuel respiration - to provide energy for cells to perform all their functions.

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Key points Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

Key points about enzymes synthesising bigger molecules from small molecules

Enzyme-Catalysed Reactions That Build Large Molecules

In human biology, anabolic reactions are chemical processes that build larger, complex molecules from smaller ones.

These reactions are crucial for growth, repair, and energy storage.

Enzymes catalyse these reactions by lowering the activation energy, allowing them to occur efficiently at body temperature.

Here are three key examples:

1. Protein Synthesis

• Substrates: Amino acids

• Product: Proteins (polypeptides)

• Enzyme Role: Enzymes inside ribosomes (e.g. peptidyl transferase) link amino acids via peptide bonds to form proteins.

• Importance: Proteins are essential for structure (e.g. muscle fibres), transport (e.g. haemoglobin), and enzymatic control of metabolism.

2. Carbohydrate Formation (Glycogenesis)

• Substrates: Glucose molecules

• Product: Glycogen (storage carbohydrate)

• Enzyme Role: Enzymes like glycogen synthase catalyse the joining of glucose into glycogen.

• Importance: Glycogen is stored in the liver and muscles, providing a quick energy reserve.

3. Lipid Formation

• Substrates: 3 fatty acids + 1 glycerol

• Product: Triglyceride (a fat/lipid molecule)

• Enzyme Role: Enzymes catalyse ester bond formation between glycerol and fatty acids.

• Importance: Lipids are used for energy storage, insulation, and forming cell membranes (phospholipids).

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Why These Reactions Matter in Human Biology

• They allow the body to build its own tissues, enzymes, and hormones.

• They support growth and cellular repair.

• They enable the body to store energy efficiently for later use.

• They maintain homeostasis and metabolic balance.

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Summary of learning objectives and key words or phrases

Be able to describe how enzymes use small molecules to synthesise big molecules e.g. protein synthesis from amino acids, lipid synthesis from fatty acids, carbohydrate synthesis to make starch (plants) or glycogen (animals) from small sugar molecules like glucose.

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