Glycolysis is a metabolic pathway that breaks down glucose to produce energy. It occurs in the cytoplasm through 10 steps involving various enzymes. During glycolysis, two ATP molecules are used to produce two pyruvate molecules, two NADH molecules, and two ATP molecules, providing energy for cellular functions. Glycolysis is the first step of cellular respiration and an important source of energy during anaerobic conditions like exercise when oxygen is limited.

1. GLYCOLYSIS
Dr. Poonam Bansal
Assistant Professor
Maharishi Markandeshwar (Deemed to Be University)
Mullana

2. Key Points
• Glycolysis is a metabolic pathway and an anaerobic energy source.
• Also known as Embden-Meyerhof pathway (in honor of the major
contributors towards its discovery and understanding).
• It doesn't require oxygen, hence its purpose in anaerobic respiration, it is
also the first step in cellular respiration.
• The process entails the oxidation of glucose molecules that is the single
most crucial organic fuel in plants, microbes, and animals.
• Most cells prefer glucose (although there are exceptions, such as acetic acid
bacteria that prefer ethanol).
Glycolysis

3. • Glycolysis produces two molecules of pyruvate, two molecules of ATP, two
molecules of NADH, and two molecules of water.
• The pyruvate can be used in the citric acid cycle or serve as a precursor for
other reactions.
• Glycolysis takes place in the cytoplasm.
• There are 10 enzymes involved in breaking down sugar. The 10 steps of
glycolysis are organized by the order in which specific enzymes act upon the
system.

4. Entry Points
Substrates can enter the glycolysis pathway via three different ways, which are
referred to as ‘entry points’. These are:
1.Dietary glucose – glucose is directly absorbed into the blood stream from the
gastrointestinal tract and enters the pathway.
2.Glycogenolysis – glucose is released from hepatic stores of glycogen and
enters the pathway.
3.Other monosaccharides – galactose and fructose enter the glycolysis
pathway at various levels via common intermediates.

5. Phases of Glycolysis
Glycolysis can be considered as a two part process. Firstly, energy is consumed to
generate high energy intermediates, which then go on to release their energy during
the second phase.
•Energy investment or preparatory phase – requires two ATP molecules to
produce high energy intermediates.
•Energy pay out phase – The intermediate is metabolized, producing four ATP
molecules and two NADH molecules.

6. Preparatory phase

7. Step 1
The enzyme hexokinase phosphorylates or adds a
phosphate group from ATP to glucose in a
cell’s cytoplasm producing glucose 6-phosphate or
G6P. One molecule of ATP is consumed during this
phase and is spontaneous and irreversible.
It is regulated by product inhibition; higher concentrations of G6P inhibit hexokinase and slow the
reaction.
In the liver, glucokinase also catalyzes this reaction. It has a higher Km than hexokinase, and therefore
works at greater concentrations of serum glucose.
Galactose can enter glycolysis here through its conversion into G6P, via galactose-1-phosphate and
glucose-1-phosphate.

8. Step 2
The enzyme phosphoglucomutase isomerizes
G6P into its isomer fructose 6-phosphate or
F6P. Isomers have the same molecular
formula as each other but different atomic
arrangements.
This provides an entry point for fructose into
glycolysis.

9. Step 3
The phosphofructokinase uses another ATP
molecule to transfer a phosphate group to F6P in
order to form fructose 1,6-bisphosphate or FBP.
This creates an unstable molecule that will split
spontaneously to form two 3 carbon molecule
and consumes the second molecule of ATP.
This is a key regulatory step of glycolysis. It is allosterically inhibited by ATP and activated by
AMP. Furthermore, phosphofructokinase is inhibited by glucagon, while insulin activates the enzyme.
This ensures that when there is high blood glucose, and therefore high circulating insulin, the speed of
glycolysis increases.

10. Step 4
By this step, the energy consumption of the
‘investment phase’ is complete and two ATP
molecules have been consumed.
The enzyme aldolase splits fructose 1,6-
bisphosphate into a ketone and an aldehyde
molecule viz. dihydroxyacetone phosphate
(DHAP) and glyceraldehyde 3-phosphate (GAP).

11. Step 5
The enzyme triose-phosphate isomerase rapidly
converts DHAP into GAP (these isomers can inter-
convert).
Both molecules of GA3P then enter the second stage of
glycolysis, the payout phase.

12. Payout phase
In the payout phase, a
molecule of NADH and two
molecules of ATP are
produced per molecule of
GA3P entering the pathway.
As first molecule of glucose
has generated two
molecules of GA3P, the
total payout from the
payout phase is 2 NADH +
4 ATP.
2 ATP are used in
the investment phase, the
net gain from our first
molecule of glucose is 2
NADH and 2 ATP.

13. Step 6
GA3P is converted into 1,3-bisphosphoglycerate
(1,3-BPG) by glyceraldehyde phosphate
dehydrogenase.
This yields a molecule of NADH, formed by the
reduction of NAD+.
The enzyme glyceraldehyde 3-phosphate
dehydrogenase (GAPDH) serves two functions in
this reaction.
First, it dehydrogenates GAP by transferring one of
its hydrogen (H⁺) molecules to the oxidizing
agent nicotinamide adenine dinucleotide (NAD⁺)
to form NADH + H⁺.
Next, GAPDH adds a phosphate from the cytosol
to the oxidized GAP to form 1,3-
bisphosphoglycerate (BPG).

14. Step 7
The enzyme phosphoglycerokinase transfers a
phosphate from BPG to a molecule of ADP to form
ATP. This happens to each molecule of BPG. This
reaction yields two 3-phosphoglycerate (3 PGA)
molecules and two ATP molecules.

15. Step 8
The enzyme phosphoglyceromutase relocates
the P of the two 3 PGA molecules from the third
to the second carbon to form two 2-
phosphoglycerate (2 PGA) molecules.

16. Step 9
The enzyme enolase removes a molecule
of water from 2-phosphoglycerate to form
phosphoenolpyruvate (PEP). This happens for
each molecule of 2 PGA.

17. Step 10
Phosphenolpyruvate is converted into pyruvate
by pyruvate kinase, which yields second molecule of
ATP. This is irreversible, and is therefore another key
regulatory step.
The enzyme pyruvate kinase transfers a P from PEP to
ADP to form pyruvate and ATP.

18. • Only pathway that is taking place in all the cells of the body.
• Only source of energy in erythrocytes as they do not have mitochondria and are not
capable of aerobic respiration.
• During strenuous exercise, when muscle tissues lack enough oxygen, anaerobic
glycolysis forms the major source of energy for muscles.
• Considered as the preliminary step before complete oxidation
it provides carbon skeleton for synthesis of non essential amino acids as well as
glycerol part of fat.
Significance of Glycolysis