Page 316 - Biology_F5

Gas exchange and respiration

a phosphate group to fructose-6-phosphate diffusing out of the cell because charged
to form fructose 1, 6-biphosphate. The new molecules cannot easily cross membranes.
hydroxyl group on C-1 is phosphorylated by The entry of sugars into glycolysis is
ATP. Another purpose for phosphorylation controlled at this step through regulation
is to prevent any later product from of the enzyme phosphofructokinase.

o FOR ONLINE READING ONLY
Fructose-6-phosphate (C H O P) + ATP ⎯⎯⎯⎯⎯⎯ ADP + Fructose-1,6-
phosphofructokinase
→
6 11 6
bisphosphate (C H O P ).
6 10 6 2
H H H H
P O CH O HC OH O CH O HC 1 O P
6
PFK 5 2 + ADP + H
H H OH OH + ATP Mg H H OH OH
4 3
OH H OH H

Fructose -6-phosphate Fructose 1-6-bisphosphate

The phosphofructokinase catalysed reaction known as glyceraldehyde-3-phosphate
is irreversible under cellular conditions, and or 3-phosphoglyceraldehyde (3-PGAL).
therefore, it is the first committed step in Only the glyceraldehyde 3-phosphate
glycolysis. can proceed immediately through
glycolysis. Dihydroxyacetone phosphate
(b) Lysis must be isomerised to glyceraldehyde-3-
In this stage the phosphorylated 6-carbon phosphate by the enzyme phosphotriose
sugar is broken down with the help isomerase to continue with the next steps
of enzymes into two 3-carbon sugar of the glycolytic pathway.
phosphate. The enzyme aldolase splits
fructose 1, 6-biphosphate into two sugars Fructose 1, 6-biphosphate (C H O P ) +
6 10 6 2
that are isomers of each other. These two aldolase ↔ Dihydroxyacetone phosphate
sugars are dihydroxyacetone phosphate (C H O P) + Glyceraldehyde phosphate
3
3
5
and glyceraldehyde phosphate also (C H O P)
3 5 3
o O O
6
o P o CH 2 O CH 2 o P o 1 CH 2 O P o 4 C H
1
o 5 OH 2 o Aldolase 2 C O o + H 5 C OH O
OH P o
4 3 3 CH OH 6 CH 2 O
OH 2 o
Fructose-1-6-bisphosphate Dihydroxyacetone Glyceraldehyde-3-phosphate
phosphate

Form Five Student’s Book
309