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.
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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